ORGANOMETALLIC GROUP 4 BIS(BORYLAMIDE) COMPLEXES AS
TEMPLATES FOR ZIEGLER-NATTA CATALYSIS
by
TIMOTHY HAROLD WARREN
B.S. in Chemistry, summa cum laude
University of Illinois at Urbana-Champaign
(1992)
Submitted to the Department of Chemistry
in Partial Fulfillment of the Requirements
for the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1997
© Massachusetts Institute of Technology, 1997
Signature of Author
Department of Chemistry
May 27, 1997
Certified by
,
Richard R. Schrock
Thesis Supervisor
Accepted by
/
Dietmar Seyferth
Chairman, Departmental Committee on Graduate Students
JUL 1 41997 :;iience
L ' RAR•"
This doctoral thesis has been examined by a Committee of the Department of Chemistry as
follows:
Professor Christopher C. Cummins
Chairman
Professor Richard R. Schrock
I
,,
-
S
Thesis Supervisor
Professor Alan Davison
V&U
To mom and dad
for all your love, support, and encouragement
ORGANOMETALLIC GROUP 4 BIS(BORYLAMIDE) COMPLEXES AS
TEMPLATES FOR ZIEGLER-NATTA CATALYSIS
by
TIMOTHY HAROLD WARREN
Submitted to the Department of Chemistry, June 1997,
in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy in Chemistry
ABSTRACT
Chapter 1
Motivated by the isolobal and isoelectronic relationship between the do CpW(CR) fragment
and the do bent metallocene core, the synthesis and reactivity of alkyl complexes of the type
CpW(CAd)R 2 (Ad = 1-adamantyl; R = alkyl, Cl, NMe 2 ) are presented. AdCN reacts with
W2 (OCMe3 )6 to provide W(CAd)(OCMe 3 )3 , from which CpW(CAd)C12 may be prepared in three
steps. Alkylation of CpW(CAd)Cl 2 with Grignard reagents at -30 'C provides CpW(CAd)R 2 (R =
CH 2 Ph, CH 3 , CH 2 CMe 3 ) and CpW(CAd)(CH 2 CMe 3 )C1 in 70-80 % isolated yield. In contrast to
the benzyl and methyl complexes, the mono- and dineopentyl derivatives partially tautomerize in
benzene-d6 by migration of the alkyl ax-H atoms to the alkylidyne a-C atom forming equilibrium
mixtures of CpW(CCMe 3 )(CH 2 Ad)Cl and CpW(CCMe 3 )(CH 2 Ad)(CH 2 CMe 3 ), respectively.
Each neopentyl and 1-adamantylmethyl complex displays 1 H and 13 C NMR parameters indicative
of a-agostic alkyl C-H bonds. An X-ray study of CpW(CAd)(CH 2 CMe 3 ) 2 reveals a bent
metallocene-like structure in which the orientation of the neopentyl groups places one set of C-H
bonds in between the two neopentyl ligands and just out of the Calkyl-W-Calkyl plane. The amido
lone pair in complexes CpW(CAd)(NMe 2 )X (X = Cl, CH 2 CMe 3 ) (prepared in one and two steps
from CpW(CAd)C12 ) strongly interacts with a metal acceptor orbital as demonstrated by a high
barrier to rotation about the W-NMe 2 bond. CpW(CAd)(NMe 2 )(CH 2 CMe 3) shows no evidence of
o-agostic interactions and does not tautomerize by alkyl to alkylidyne ax-H migration. The primary
amide CpW(CAd)(NHCMe 3 )Cl irreversibly undergoes ox-N-H migration to the alkylidyne a-C
atom, forming a kinetic isomer of the alkylidene CpW(NCMe 3 )(CHCAd)Cl which slowly converts
to its opposite, thermodynamic rotamer. The factors promoting ac-H transfer in the CpW(CAd)R 2
(R = alkyl, chloride) system are discussed in relation to o-H abstraction reactions in dx chemistry.
Chapter 2
The synthesis of do borylamido and borylimido complexes is presented along with an
investigation into the electronic nature of the M-N-B linkage. MC14 (M = Zr, Hf) react with 2 eq.
[Li(OEt 2 )NHBMes 2 ]2 to provide the homoleptic M(NHBMes 2 )4 whereas TiC14 reacts with 3/2 eq.
[Li(OEt 2 )NHBMes 2 ]2 to afford Ti(NHBMes 2 )3 C1. Variable temperature 1H NMR studies show
that these do borylamides possess barriers to N-B bond rotation (AGtrot = 15 - 16 kcal/mol)
significantly lower than in related borylamines. a-Abstraction reactions lead to borylimido
complexes. Heating M(NHBMes 2 )4 (M = Zr, Hf) in pyridine at 85 'C produces
M(NBMes2)(NHBMes 2 ) 2py 2 and 1 eq. H2 NBMes 2 . The addition of 5/2 eq.
[(Et 2 0)LiNHBMes 2 ]2 to TaC15 in toluene similarly results in H2 NBMes 2 formation, yielding
Ta(NBMes 2 )(NHBMes 2 )3 . Addressing the origin of the reduced barriers to N-B rotation in the dý
borylamides, Sn(NHBMes 2 )3 C1 is prepared from SnC14 and 3/2 eq. [(Et 2 0)LiNHBMes 2 ] 2. In
contrast to the analogous titanium complex, the barrier to N-B bond rotation for the tin
tris(borylamide) is greater than 22 kcal/mol, suggesting that the reduced values of AG rot in the do
complexes result from competition between the do metal and boron for acceptance of the nitrogen
lone pair.
Chapter 3
Titanium and zirconium derivatives of the new chelating bis(borylamido) ligand,
[Mes 2 BNCH 2 CH 2 NBMes 2 ]2- ([Ben] 2 -), are prepared by treating MC14 (THF)2 (M = Ti, Zr) with
(Ben)Mg(THF) 2 . Nitrogen-boron x-interactions in (Ben)TiC12 and (Ben)ZrC12 (THF) result in one
mesityl group in each BMes2 unit occupying space roughly above and below the MC12 plane.
(Ben)TiC12 is smoothly alkylated by Grignard reagents in dichloromethane to give (Ben)Ti(R)C1
(R = CH 2 Ph, CH 2 CMe 3 ) and (Ben)TiR 2 (R = Me, CH 2 Ph), while unstable (Ben)ZrMe 2 can be
prepared from (Ben)ZrCl 2 (THF) and methyllithium in toluene. An X-ray study of
(Ben)Ti(CH 2 Ph)Cl confirms the proposed ligand conformation and features a highly distorted "r12"
benzyl ligand with Ti-Cla-Cipso angle of only 87.0 (5)o. (Ben)MMe 2 complexes cleanly
decompose by metallation of the ortho methyl groups from mesityl rings on different boron atoms
at room temperature (for Zr) or upon heating (for Ti). An X-ray crystal structure of (TwistBen)Zr
shows it to be a dimer in which the two zirconium centers are bridged by two mesityl o-methylene
groups. B(C 6 F5 )3 binds to a methyl group in (Ben)MMe 2 complexes in dichloromethane, but
such compounds show little polymerization activity towards ethylene at 25 0 C and 1-2 atm as a
consequence of strong anion binding.
Chapter 4
The effect of ligand sterics on ion pairing and reactivity with olefins in cationic
bis(borylamide) alkyl complexes is investigated utilizing the sterically demanding
[Trip 2 BNCH 2 CH 2 NBTrip2] 2 - ([BigBen] 2 -) (Trip = 2,4,6 - triisopropylphenyl) ligand.
(BigBen)ZrC12 is obtained from the reaction of Li2 (BigBen)o4THF with ZrC14 (THF 2 )2 in toluene.
Reaction of (BigBen)ZrC12 with Grignard reagents provides (BigBen)ZrMe 2 as well as the 3-H
containing (BigBen)ZrR2 (R = Et, Bu), all which possess reasonable thermal stability at room
temperature. The X-ray structure of (BigBen)ZrMe2 reveals a sterically congested coordination
wedge in which the methyl groups are laterally shielded by the ligand o-isopropyl groups.
Reaction of (BigBen)ZrMe2 with B(C6F 5 ) 3 in pentane allows the isolation of
[(BigBen)ZrMe][MeB(C 6 F5)3] whereas (BigBen)ZrR2 (R = Me, Et, Bu) react with
[HNMe 2 Ph][B(C 6 F 5 )4] in toluene-d8 to give the spectroscopically characterized
[(BigBen)ZrR][B(C 6 F5 )4 ] which do not bind NMe 2 Ph. The thermal stability of these cations
allows upper limits on the barriers to anion dissociation/reassociation (15.5 - 17.0 kcal/mol) to be
placed by variable temperature 1H NMR spectroscopy. Whereas [(BigBen)ZrMe][B(C 6 F5 )4 ] does
not react with C2 H4 , CO,or H2 at 1-3 atm, [(BigBen)Zr(Me)(NH 3 )][B(C 6F5 )4 ] may be prepared
with 1 eq. NH 3 (g). The stability of these alkyl cations towards C2H 4 and CO is discussed in terms
extensive shielding of the coordination wedge due to the steric demands of the ligand.
Chapter 5
Zirconium and hafnium derivatives of the less sterically demanding bis(borylamido) ligand
[Cy 2 BNCH 2 CH 2 NBCy 2 ]2 - ([CyBen] 2- ) are prepared by treating in situ prepared solutions of
M(CH 2 SiMe 3 )2 C12 (OEt 2 )2 (M = Zr, Hf) in ether with Li 2 (CyBen)*OEt 2 . Reaction of
(CyBen)M(CH 2 SiMe 3 )2 with 2 eq. 12 in dichloromethane provides the synthetically versatile
(CyBen)MI2.
(CyBen)MI 2 undergoes smooth reactions with Grignard reagents in
dichloromethane to afford the f-H containing primary dialkyls (CyBen)MR 2 (R = Et, 'Bu, nhexyl; M = Zr, Hf) and secondary dialkyl (CyBen)Zr(CHBu 2 )2 , all of which possess a high
degree of thermal stability. The X-ray structure of (CyBen)Zr(CH 2 CH 3 )2 reveals a symmetric,
laterally open structure in which the ethyl groups lie sandwiched between two cyclohexyl groups
flanking the coordination wedge. Reaction of the dialkyls (CyBen)ZrR2 (R = Et, iBu) with
[Ph 3 C][B(C 6 F5 )4 ] in toluene-d8 produces the cations [(CyBen)M(R)][B(C 6 F5 )4 ] which display
sharp AA'BB' backbone 1H NMR resonances, indicating the presence of a relatively non-labile
ligand adjacent to the Zr-alkyl group. Toluene binds to the cations [(CyBen)M(R)][B(C 6 F5 )4 ] (R
= Et, iBu, n-hexyl; M = Zr, Hf) similarly prepared in chlorobenzene-d5 at -30 'C, which show
broadbackbone resonances at low temperature whose coalescence temperature (0 - 70 'C) strongly
depends on the amount of added toluene. Chlorobenzene solutions of [(CyBen)M(R)][B(C 6 F5 )4 1
(R = Et with M = Zr, Hf; R = Bu' with M = Zr) polymerize 225 eq. 1-hexene at -30 'C to nearly
complete conversion producing a regioregular polymer with a substantial degree of isotacticity, but
which is of high molecular weight and polydispersity (Mn = 0.4 - 1.3 x 105 g/mol, PDI = 5 - 6).
In dichloromethane 225 eq. 1-hexene is polymerized to 50 - 76% conversion at -30 - 0 'C by
(CyBen)Zr(CH 2 CHMe 2 )2 activated with [Ph 3 C][B(C 6F 5 )4 ] in the presence of 5 - 15 eq. toluene
to give polymers of lower molecular weight (Mn = 1.5 - 2.4 x 104 g/mol) and much lower
polydispersity (PDI = 1.08 - 1.53), but these polymers are highly regioirregular. The
polymerization results are discussed in terms of a "secondary" activation of the CyBen based alkyl
cations, producing the active catalyst(s) responsible for the 1-hexene polymerization activity
observed.
Thesis Supervisor:
Title:
Dr. Richard R. Schrock
Frederick G. Keyes Professor of Chemistry
TABLE OF CONTENTS
Title Page
1
2
Signature Page
3
Dedication
Abstract
4
Table of Contents
List of Figures
List of Tables
List of Schemes
List of Abbreviations Used in Text
General Introduction
7
10
12
13
14
16
CHAPTER 1: a-H Migration Reactions Assisted by a-Agostic Interactions in
Cyclopentadienyl Alkylidyne Complexes of Tungsten(VI).
Introduction
Results
Synthesis of CpW(CAd)X 2 .
Synthesis of Alkyl Derivatives of the CpW(CAd) Fragment.
NMR Parameters of the Alkyl Derivatives: a-Agostic Interactions.
X-ray Structure of CpW(CAd)(CH 2 CMe3) 2
Agostic Interactions and cx,a-H Migration.
Related a-Hydrogen Shift from a Primary Amide.
17
18
20
20
23
24
28
34
38
Discussion and Conclusions
40
Experimental Section
References
44
52
CHAPTER 2: Synthesis of Group 4 and 5 dO Primary Borylamido and
Borylimido Complexes.
Introduction
Results
Synthesis and NMR Spectra of Group 4 Borylamide Complexes.
Preparation of Group 4 and 5 Borylimido Complexes.
Origin of the Decreased Barriers to Rotation.
55
56
58
58
61
62
Discussion and Conclusions
64
Experimental Section
References
66
CHAPTER 3: Synthesis of Group 4 Organometallic Complexes that Contain the
Bis(borylamide) Ligand, [Mes 2 BNCH 2 CH 2NBMes 2 ].
Introduction
Results
Entry into Titanium and Zirconium Chemistry.
Preparation of Alkyl Derivatives.
X-ray Structure of (Ben)Ti(CH 2Ph)C1.
Intramolecular C-H Bond Activation.
X-ray Structure of [(TwistBen)Zr] 2 .
Conversion to Zwitterionic Complexes.
Discussion and Conclusions
Experimental Section
References
CHAPTER 4: Neutral and Cationic Organozirconium Complexes of the Sterically
Demanding Bis(borylamide) Ligand, [Trip 2 BNCH 2 CH 2 NBTrip 2 ].
Introduction
Results
71
74
75
76
77
78
79
83
84
88
88
90
100
103
104
105
Ligand Synthesis and Entry into Zirconium Chemistry.
Preparation of Dialkyl Derivatives.
105
106
X-ray Structure of (BigBen)ZrMe2.
Generation of Alkyl Cations.
108
Variable Temperature NMR Studies.
Reactivity of Alkyl Cations.
115
118
119
Discussion and Conclusions
Experimental Section
References
112
122
128
CHAPTER 5: Neutral and Cationic f-H Containing Alkyls Supported by
[Cy 2 BNCH 2 CH 2NBCy 2 ]2- Ligated Zirconium and Hafnium
and their Use in the Polymerization of 1-Hexene.
Introduction
130
131
Results
132
Ligand Synthesis and Preparation of Group 4 Derivatives.
Synthesis of Dialkyl Complexes Containing P-H Atoms.
132
134
X-ray Structure of (CyBen)ZrEt 2 .
Cation Formation and Solution Structure:
136
The Example of [(CyBen)Zr(CH2CHMe2)]+.
140
Generation and Characterization of Primary Alkyl Cations
in Chlorobenzene-d5.
144
Polymerization of 1-Hexene in Chlorobenzene.
Polymerization of 1-Hexene in Dichloromethane.
Discussion
Comparison with Other Bis(amido) Systems.
Conclusions
147
Experimental Section
References
158
168
ACKNOWLEDGMENTS
148
151
155
157
170
List of Figures
Chapter 1
Figure 1.1.
Figure 1.2.
Figure 1.3.
Figure 1.4.
Figure 1.5.
Figure 1.6.
Figure 1.7.
Figure 1.8.
Figure 1.9.
Figure 1.10.
Filled molecular orbitals of the cyclopentadienyl fragment.
Filled orbitals of linear alkylidyne, imido, or alkoxide ligands.
Isolobal and isoelectronic members of the bent metallocene family.
1H NMR (300 MHz) spectrum of an equilibrium mixture of 9 and 11.
Depiction of 1H- 1H coupling through four a-bonds.
Chem-3D drawing of the X-ray structure of 9 viewed down the
crystallographic mirror plane.
Chem-3D drawing of the X-ray structure of 9 viewed from the
side highlighting its bent metallocene-like structure.
page
18
18
19
27
28
30
31
Frontier orbitals of the Cp2 M fragment.
Depiction of c-bonding and ao-agostic interactions in 9.
Qualitative rates of a-H migration in neopentyl derivatives of CpW(CAd).
35
35
40
Electronic and structural analogy between borylamine and ethylene.
Two- and quasi-two-coordinate first row bis(borylamido) complexes.
nr-Orbital interactions available to do metalloborylamides and
56
metalloborylimides.
57
Chapter 2
Figure 2.1.
Figure 2.2.
Figure 2.3.
Figure 2.4.
Figure 2.5.
Figure 2.6.
Figure 2.7.
1H
NMR (300MHz) spectra of 3 at -15 'C (bottom) and 80 'C (top)
in bromobenzene-d5.
Proposed structures of borylimido complexes 4 - 6.
Orbital analogies between
bis(amino)boranes.
do
60
62
borylamides, bis(boryl)amines, and
63
Newman projection down one B-N bond in metalloborylamide
complexes 1 - 3.
Chapter 3
Figure 3.1.
Figure 3.2.
Figure 3.3.
57
Proposed geometry of (Ben)MC12 complexes.
Chem-3D drawing of the X-ray structure of (Ben)Ti(CH 2 Ph)CI (3a).
Chem-3D drawing of the X-ray structure of [(TwistBen)Zr] 2 (6).
63
Chapter 4
Figure 4.1.
Steric interactions between opposing o-isopropyl groups
in H2 (BigBen) and 1.
105
Figure 4.2.
1H
NMR spectra of 2 at 25 'C (bottom) and 75 'C (top).
Chem-3D drawing of the X-ray structure of (BigBen)Zr(CD 3 )2 (3-d 6 ).
107
1H
NMR (300 MHz, 25 °C) spectrum of 8.
Symmetrization resulting from anion dissociation/reassociation
114
in cations 6 - 8.
116
Unfavorable steric interactions between opposing o-CHMe 2 groups
in three-coordinate alkyl cations.
117
Figure 4.3.
Figure 4.4.
Figure 4.5.
Figure 4.6.
Chapter 5
Figure 5.1.
Figure 5.2.
Figure 5.3.
Figure 5.4.
109
Chem-3D drawing of the X-ray structure of (CyBen)Zr(CH 2 CH 3 )2 (5a).
Variable temperature 1H NMR (300 MHz) spectra of 6a+
in chlorobenzene-d5.
137
Symmetrization of 6a+ in chlorobenzene-d 5 .
13 C NMR
(75.4 MHz) spectra of poly-1-hexene
143
150
142
List of Tables
Chapter 1
page
26
Table 1.1.
Table 1.2.
W-CH 2 NMR Parameters for Complexes 7 - 12.
Selected Bond Lengths (A) and Angles (0)for
Table 1.3.
CpW(CAd)(CH 2 CMe3) 2 (9).
Crystallographic Data, Collection Parameters, and Refinement
Table 1.4.
Table 1.5.
Parameters for CpW(CAd)(CH 2 CMe 3 )2 (9).
W-CH2 CMe 3 NMR Parameters for 14.
Selected NMR Parameters for Complexes 15 - 17.
Chapter 2
Table 2.1.
Barriers AGtrot for N-B Bond Rotation in Complexes 1 - 3.
59
Chapter 3
Table 3.1.
Table 3.2.
Selected Bond Distances (A) and Angles (0) for (Ben)Ti(CH 2 Ph)Cl (3a).
Crystallographic Data, Collection Parameters, and Refinement
81
Parameters for (Ben)Ti(CH 2 Ph)Cl (3a).
Selected Bond Distances (A) and Angles (o)for [(TwistBen)Zr]2 (6).
Crystallographic Data, Collection Parameters, and Refinement Parameters
82
86
for [(TwistBen)Zr] 2 * 3 C 6 H 6 (6 * 3 C 6 H 6 ).
87
Table 3.3.
Table 3.4.
32
Chapter 4
Table 4.1.
Table 4.2.
Table 4.3.
Chapter 5
Table 5.1.
Table 5.2.
Table 5.3.
Selected Bond Distances, Intramolecular Contacts (A) and Angles (0)
for (BigBen)Zr(CD 3 )2 (3-d 6 ).
110
Crystallographic Data, Collection Parameters, and Refinement Parameters
for (BigBen)Zr(CD 3 )2 * C5H12 (3-d 6 * C5 H 12 ).
Estimated Values of AGt for Symmetrization in Complexes 6 - 8.
111
117
Selected Bond Distances (A) and Angles (0) for
(CyBen)Zr(CH 2 CH 3 )2 (3a).
Crystallographic Data, Collection Parameters, and Refinement Parameters
for (CyBen)Zr(CH 2 CH 3) 2 (3a).
1H NMR Parameters for Alkyl
Cations 5a+ - 7a+.
138
139
145
List of Schemes
Chapter 1
Scheme 1.1.
Scheme 1.2.
Tautomerizations of 9 - 12 Mediated by a Bis(alkylidene).
3ag4
34
Tautomerization of 15 to 16 Followed by Alkylidene
Rotamerization to 17.
Chapter 4
Scheme 4.1.
Summary of the Reactivity of 7 with Small Molecules.
119
Chapter
Scheme
Scheme
Scheme
Synthesis of H2 (CyBen) (1) and Li 2 (CyBen)*OEt 2 (2).
Preparation of Zirconium and Hafnium CyBen Derivatives.
Synthesis of P-H Containing Dialkyl Complexes.
133
134
135
Scheme 5.4.
Competitive B vs. Zr Alkylation
135
Scheme 5.5.
Scheme 5.6.
Generation of Primary Alkyl Cations.
Bis(amides) as Ziegler-Natta Catalysts.
144
5
5.1.
5.2.
5.3.
154
Abbreviations Used in Text
[(Ben)] 2 [(BigBen)] 2 br
a-C or Coa
3-C or Cp
[Mes 2 BNCH 2 CH 2 NBMes 2] 2[Trip 2BNCH 2 CH 2 NBTrip 2 ]2broad
carbon bound to metal
carbon bound to Ca, etc.
Cp
Cp*
Cp'
Cy
C5 H5
C5 Me5
C5 H4 Me
cyclohexyl
[(CyBen)] 2 d
[Cy 2 BNCH 2 CH 2NBCy 2 ]2doublet
dme
Bu
1,2-dimethoxyethane
n-butyl
'Bu
isobutyl
eq
equation
eq.
Et
equivalents
ethyl
h
hex
a-H or Hax
hour(s)
n-hexyl
hydrogen (proton) bound to Ca
hydrogen (proton) bound to Cp, etc.
3-H or Hp, etc.
Hz
IR
J
m
Me
Mes
NMR
OTf
Ph
Hertz
Infrared
coupling constant in Hertz
multiplet
methyl
2,4,6-trimethylphenyl
nuclear magnetic resonance
s
0 3 SCF 3 , triflate, trifluoromethanesulfonate
phenyl
parts per million
pyridine
singlet
t
triplet
ppm
py
THF
tetrahydrofuran
TMS
Trip
tol
5
Me3 Si, trimethylsilylmethyl
2,4,6-triisopropylphenyl
toluene
chemical shift downfield from tetramethylsilane
General Introduction
The tremendous industrial success of the group 4 metallocenes in Ziegler-Natta catalysis
motivates the development of other metal fragments which emulate structural and/or electronic
features of the bent metallocene core. Numerous important issues in Ziegler-Natta catalysis such
as mode of olefin insertion, rates of chain growth and termination, extent of ion-pairing, and
degree of stereocontrol are all factors which depend intimately on the structure of the catalyst in
ways that are not yet fully understood. The synthesis of new, non-metallocene group 4 systems
thus offers not only the opportunity to better understand the above important issues in the
Ziegler-Natta polymerization of a-olefins but also the prospect of uncovering modes of behavior
exclusive to the new system at hand.
This thesis describes the synthesis and reaction chemistry of two different basic structural
units designed to emulate the group 4 bent metallocenes.
The isolobal analogy identifies
cyclopentadienyl-alkylidyne complexes of tungsten(VI) as electronic relatives of the dO group 4
metallocenes.
Instead of serving as Ziegler-Natta pre-catalysts, however, certain alkyl
complexes of the type CpW(CAd)R 2 (R = alkyl, chloride) partially tautomerize by shift of the
alkyl a-H atoms to the alkylidyne a-carbon and afford the opportunity to explore factors
important in promoting this a-process.
The remainder of this thesis utilizes borylamides
([NRBR' 2 ]-) as ancillary ligands for the first time in do chemistry, beginning with studies
directed toward understanding the electronic nature of the M-N-B linkage in group 4 complexes
of the [NHBMes 2 ]- ligand. N-B it-interactions are then utilized to produce an electrophilic, welldefined metallocene-like coordination environment in group 4 chelating bis(borylamide)
complexes. Comparison of neutral and cationic complexes in the first two generations of ligand
systems bearing sterically demanding diarylboryl groups suggests features crucial to achieve
thermal stability as well as olefin polymerization activity. On the basis of these findings, this
thesis concludes with a family of 3-H containing dialkyl complexes of zirconium and hafnium,
their conversion to spectroscopically well-characterized cations, and their use in the
polymerization of 1-hexene.
CHAPTER 1
ox-H Migration Reactions Assisted by a-Agostic Interactions
in Cyclopentadienyl Alkylidyne Complexes of Tungsten(VI)
Introduction
The isolobal analogy is a powerful approach which may be used to relate metal fragments
based on the spatial arrangements of orbitals used for metal-ligand bonding.' In order to identify
electronic analogs of the d o group 4 bent metallocenes based on ligands other than the
cyclopentadienyl group,
it is instructive to consider the orientations of the filled
cyclopentadienyl orbitals used for bonding with dO transition metal centers (Figure 1.1).
Figure 1.1. Filled molecular orbitals of the cyclopentadienyl fragment (viewed from one face of
the nt-system).
0
~gC9
o-orbital
nt-orbitals
Other ligands based on second period donor atoms offer a similar spatial arrangement of orbitals
used for bonding with transition metal fragments. Shown in Figure 1.2 are the filled orbitals
used for bonding in a strictly linear fragment such as an alkylidyne (3-), imido (2-), or alkoxide
ligand (I-).
Figure 1.2. Filled orbitals of linear alkylidyne, imido, or alkoxide ligands (as viewed down the
metal-ligand bond).
III
E
O
R
E = C, N, O
o-orbital
7t-orbitals
Since the lo, 27t ligands appearing in Figure 1.2 offer the same arrangement of filled orbitals for
bonding to a transition metal, the isolobal analogy suggests that in some cases an alkylidyne,
imido, or alkoxide ligand may be substituted for a cyclopentadienyl ring to produce metal
fragments similar in geometric and electronic structure to the well-studied do Cp 2 MX 2 (M = Ti,
Zr, Hf; X = alkyl, halide) 2 complexes.
Several examples of group 5 and 6 do complexes of the type (L)(L')MX 2 (L, L' =
cyclopentadienyl or cyclopentadienyl analog) isolobal with the group 4 bent metallocenes exist,
and illustrate sets of ligands compatible with a particular triad of metals (Figure 1.3). For
instance, the combination of a cyclopentadienyl ring and an imido ligand gives rise to a family of
group 5 do half-sandwich imido complexes CpM(NR)X 2 (M = V, 314 Nb, Ta; 5 .6 R = aryl, tbutyl). Due to their higher valency, related imido complexes of the group 6 metals possess two
87
imido ligands exemplified by the series of bis(imido) complexes (RN) 2 MX 2 (M = Cr. Mo. 11
W; 8 ,12 R = aryl, t -butyl; X = halide, alkyl).
Figure 1.3. Isolobal and isoelectronic members of the bent metallocene family.
Ar
M
X
.M""•\
.11I
,X
,;•\ X
..
,x
AN\ \..1\ X
M
N
Ar
M = Ti, Zr, Hf
M = V, Nb, Ta
M = Cr, Mo, W
Ar = 2,6 - diisopropylphenyl
X = halide or alkyl
Half-sandwich complexes of the group 6 metals containing a cyclopentadienyl ring in
combination with a trianionic lo, 27r donor such as a nitride or an alkylidyne ligand may also be
prepared.
Whereas the fragment [Cp*Mo(N)C12 ] may be identified in several oligomeric
structures, 13 CpW(CCMe 3 )C12 14 and (rl5 -C5 Me 4 -t-Bu)W(CCMe 3 )X2 (X = Cl, I, Me) 15 .16 are
isolable and presumably bear monomeric, bent metallocene-like structures.
The great interest in dialkyl complexes of the type Cp 2 MR 2 (M = Ti, Zr, Hf) as precatalysts for the cationic Ziegler-Natta polymerization of oa-olefins 17 motivates the exploration
of alkyl complexes of the CpW(CR) fragment. Due to the strong effect on the stereochemical
course of olefin polymerization that the cyclopentadienyl or cyclopentadienyl-like ligands have
in the group 4 systems, 17 a versatile synthetic route to do CpW(CR) complexes was sought
which offers some synthetic control over the alkylidyne substituent. In this regard, the reported
preparations of CpW(CCMe3)CI2 and (fT5 -C5 Me4-t-Bu)W(CCMe3)X2 are not as flexible as
desired since they ultimately derive from W(CCMe 3 )(CH 2 CMe 3 )3.
Finally, alkyl derivatives of the CpW(CR) fragment may be of fundamental interest in the
continued understanding of a-H processes in do alkyl complexes. Related to the scrambling of
ac-H atoms about the alkylidene and alkyl ligands in Ta(CHCMe 3 )(CH 2 CMe 3 )3 ,18 Chisholm et
al. observed that some alkyl-alkylidyne complexes may not be stable towards tautomerization by
double a,a-H atom migration from the alkyl to alkylidyne a-C atoms. For example, heating a
solution of W(CSiMe 3 )(CH 2 CMe3)3
results
in
an
equilibrium
mixture
of
W(CSiMe 3 )(CH 2 CMe 3 )3 and W(CCMe 3 )(CH 2 CMe 3 )2 (CH 2 SiMe 3 ) (eq 1).19
A
W(CSiMe 3 )(CH 2 CMe 3 )3
W(CCMe 3 )(CH2SiMe 3)(CH2CMe 3)2
(1)
Thus the synthesis of a new family of alkyl-alkylidyne complexes would not only offer the
opportunity to explore the generality of such ca-H migration reactions, but perhaps also factors
important in promoting this process.
Results
Synthesis of CpW(CR)X 2.
The most versatile route to W(VI) alkylidynes is through the elegant metathetical reaction
of W2 (OCMe 3 )6 (1) and a nitrile or disubstituted acetylene (eq 2).20,21
RC E
+
(Me 3CO) 3 W - W(OCMe 3 )3
CR or N
O
(Me 3CO) 3W - CR
+
(2)
(Me 3CO) 3W 2 E
This reaction is wide in scope and provides access to a remarkable variety of alkylidyne
substituents.
Two alkylidyne substituents were targeted for this study which were anticipated to
provide a significantly different steric and electronic profile to the metal center. The new
alkylidyne W(CAd)(OCMe3)3 (Ad = 1-adamantyl) (2) may be isolated from the reaction of 1 and
1-adamantanecarbonitrile in pentane in good yield as colorless cubes (eq 3).
pentane
W 2 (OCMe 3 )6 + 1-AdC -N
W(CAd)(OCMe 3)3
p
- W(N)(OCMe 3 )3
1
(3)
2
The insolubility of the nitrido byproduct W(N)(OCMe 3 )3 in pentane conveniently allows its
separation from 2 by simple filtration of the reaction mixture.
The previously reported
W(CPh)(OCMe 3 )3 (3) originates from the reaction of 1 with two equivalents of PhCCMe (eq
4).21
pentane
W 2 (OCMe 3)6 + 2 PhCE CMe
1
pentane 0
-MeC=CMe
W(CPh)(OCMe3 )3
3
(4)
Though complexes 2 and 3 do not react directly with cyclopentadienyl reagents such as
NaCp, a handle for subsequent reaction with an anionic cyclopentadienyl source may be
introduced by the reaction of 2 or 3 with trimethylsilyl triflate in pentane in the presence of dme
(eq 5).
R
C
R
C
III
Me 3 CO'
III
,OCMe3
pentane
W.,,IOCMe 3 + TMS-OTf + dme
,OCMe 3
-
TMS-OCMe 3
TfO-W--
OCMe 3
(5)
O
4a R = 1-Ad
4b R = Ph
The W(CR)(OCMe 3 )2 (OTf)(dme) complexes precipitate from the reaction mixture as yellow
microcrystals which are isolated in 85-95% yield by filtration of the pentane reaction solution.
Complexes 4 exhibit broad, inequivalent t-butoxide resonances in room temperature 1H NMR
spectra (benzene-d6), consistent with the structure shown in eq 5 in which the arm of the dme
ligand cis to the alkylidyne reversibly dissociates on the 1H NMR timescale. The selectivity of
this reaction with trimethylsilyl triflate may be contrasted with that of reaction of protic acids.
For example, pyridinium chloride or carboxylic acids react with W(CCMe 3 )(OCMe 3 ) 3 to give
alkylidene complexes of the type W(CHCMe 3 )(OCMe 3 )2 X2 py
(X
= halide)
or
W(CHCMe 3 )(OCMe 3 )2 (O2 CR) 2 , respectively. 22
Triflates 4 are precursors to the CpW(CR) fragment, reacting cleanly with sodium
cyclopentadienide in THF at -40 "C to give complexes CpW(CR)(OCMe 3 )2 (5a, R = 1-Ad; 5b. R
= Ph) which are isolated high yield as yellow crystals from pentane (eq 6).
R
C
I .OCMe
3
THF
TfO-W-OCMe 3 + NaCp
R
C
W
.,\ OCMe
OCMe
(6)
, ,OCMe 3
5a R = l-Ad
5b R = Ph
The presence of two t-butoxide groups which may engage in it-donation to the metal center
appears to confer electronic protection against reduction of the W(VI) center as reaction of
numerous cyclopentadienyl sources with the related halide complexes W(CCMe 3 )C13 (dme) or
[NEt 4 ][W(CCMe 3)C14 ] do not afford CpW(CCMe 3 )C12. 23
Conversion of 5a to CpW(CAd)C12 (6) occurs upon addition of 2 eq. HCl in ether, but a
persistent, unidentified impurity thwarted attempts to isolate 6 by this route. Clean formation of
6 could be effected by dissolving 5a in highly concentrated dichloromethane or even neat
solutions of trimethylsilyl chloride. This procedure, however, proved irreproducible.
hydrolysis of trimethylsilyl chloride produces hexamethyldisiloxane and HC1,
Since
HCI is likely to
be an impurity in trimethylsilyl chloride, one which could catalyze the formation of 6 (vide
infra). Accordingly, clean, reproducible conversion of 5a to 6 occurs in dichloromethane in the
presence of 5 eq. TMS-Cl and a catalytic amount of 2,6-lutidinium chloride (eq 7).
Ad
C
W .'%\ OCMe 3
OCMe 3
Ad
excess TMS-Cl
cat. 2,6-LutHCI
C
CH 2C2
- 2 TMS-OCMe
3
1
(7)
li"* *VCl
The reaction proceeds in 80-85 % yield and 6 crystallizes
readily from ether/pentane solutions to
yield brilliant purple shards. Unfortunately, similar
conditions do not succeed in preparing the
presumably analogous CpW(CPh)C1 from 5b.
2
Synthesis of Alkyl Derivatives of the CpW(CAd)
Fragment.
CpW(CAd)Cl 2 proves to be a useful precursor
to the targeted alkyl derivatives of the
CpW(CAd) fragment. Treatment of 6 with
Grignard reagents in ethereal solvents at -40
'C
smoothly provides the dialkyl complexes CpW(CAd)R
2 7 - 9 as shown in equation 8. Careful
addition of 1.0 eq. of neopentyl Grignard allows
the isolation of the mononeopentyl complex 10
(eq 9).
Ad
C1
+ 2 RMgCl
-
Et20, -40 oC
Ad
C
O
IR
(8)
7 R = CHPh
8 R=CH3
9 R = CH 2 CMe
3
Ad
C1
W'Cl
+ Me 3 CCH 2 MgCI
Et20, -40 `C
0s
Ad
C
W .,\ CH 2 CMe 3
SIC1
(9)
The bis(benzyl) complex 7 is isolated as bright yellow crystals from ether / dichloromethane and
the more soluble aliphatic alkyl complexes 8 - 10 are isolated as deep red crystals from pentane
at -40 'C (70 - 80% yield).
Whereas solutions of 7 and 8 possess moderate stability, 9 and 10 partially tautomerize
over a period of days in benzene-d6 at room temperature by a double a-H shift from the
neopentyl a-carbon to the alkylidyne a-carbon.
Ad
Me 3 CC
W
W.,,\CH2CMe3
'VCH 2CMe 3
K = 2.1 (1).
RT
W ..,\CH2CMe 3
',CHzAd
W
9
(10)
11
Ad
C
W.,, CHCMe3
'Cl
K=1.05(4)
K= 1.05 (
Me 3C
\
RT
10
,C
..
*CH
2 Ad
12
At room temperature equilibrium (Keq = 2.1 (1)) is reached between 9 and 11 after ca. 48 hours,
whereas the analogous equilibration of 10 with 12 occurs considerably more slowly, reaching
equilibrium (Keq = 1.05 (4)) only after two weeks. In addition to characterization by H and
13 C
NMR spectroscopy, the species shown in equilibrium in eqs 10 and 11 are further corroborated
by the addition of these solutions to a slight excess of 2,6-lutidinium chloride in
dichloromethane, which protolytically cleaves the W-alkyl groups and produces corresponding
mixtures of CpW(CAd)C12 (6) and CpW(CCMe 3 )C12 14 identified by 1H NMR spectroscopy.
NMR Parameters of the Alkyl Derivatives: a-Agostic Interactions.
Inspection of the NMR parameters for the W-CH 2 R units in the alkyl derivatives
described above reveals that the neopentyl and 1-adamantyl derivatives 9 - 12 exhibit three
classes of features indicative of oa-agostic interactions (Table 1.1).
Each symmetry independent W-CH 2 R group in complexes 9 - 12 displays two sets of 1H
resonances centered around 8 3.5 and -1.0 ppm due to the chemically inequivalent environment
of the geminal C-H bonds. Although upfield shifted cx-C-H resonances have been correlated
with agostic structures, 24 the chemical shift anisotropy of the W-alkylidyne triple bond may
also contribute to the disparity in chemical shifts between the geminal hydrogen resonances. 25
The upfield shifted o-C-H signals in 9 - 12 thus may not a reliable guide to a-agostic
interactions. More indicative, however, are the differences in the magnitude of the 1JWH values
within a pair of geminal C-H resonances. In each case the upfield resonance is most strongly
coupled to 183 W with 2JWH ranging from 12.0 - 13.0 Hz whereas each corresponding downfield
resonance exhibits a 2 JWH less than 4.5 Hz. Figure 1.4 presents the two distinct methylene
regions of an 1H NMR spectrum of an equilibrium mixture of 9 and 11 and illustrates the
disparity between the downfield and upfield 2JWH values.
Additionally, the geminal ca-hydrogens in each symmetry independent W-CH 2 R group in
complexes 9 - 12 are coupled inequivalently to their corresponding alkyl cx-C atom. Each WCH 2 R
13 C
resonance (ca. 50 ppm downfield from those in 7 and 8) shows a four line pattern
resulting from one 1JCH of ca. 120 Hz and one diminished 1JCH of ca. 100 Hz. Selective
13 C{ 1H}
NMR decoupling experiments allow the assignment of the high and low 1JCH values in
complexes 9, 10, and 12 to the downfield and upfield 1H resonances, respectively. It should be
noted that the observation of one low 1JCaH coupling constant is not in itself a rigorous criterion
for the identification of a-agostic interactions, since one-bond C-H coupling constants generally
decrease with decreasing electronegativity of the geminal substituents of the C-H bond at
hand. 26 The segregation of the 1JCH values found for 9 - 12 into distinct low and high ranges.
27-29
however, provides better evidence for a-agostic interactions.
Also consistent with the assignment of the upfield 1H resonances observed in 9 - 12 as Uagostic, an unusual 4 JHH of 2.6 Hz between the upfield W-CH2 CMe3 and W-CH 2 Ad resonances
in the mixed alkyl tautomer 11 is observed (Figure 1.4). For comparison, a related 4 JHH of
Table 1.1. W-CH2 R NMR Parameters for Complexes 7-12 (in benzene-d
6 unless noted
otherwise).
Complex
G 1H (ppm)
2.422
CpW(CAd)(CH 2 Ph)2a
(7)
(8)
CpW(CAd)(CH2 CMe3 )2
(9)
8.1
1.133
5.8
-1.147c
(ppm)
1JCH
(Hz)
134.1 b
121.9
90.67
121.0
98.8
b
120.2
94.98g
-0.886ef
12.9
99.6
101.2
87.78g
<4
4.1
-0.454
13.0
4.715
4.4
-0.631
40.25
<4
12.0
4.790
137.1
12.6
-1.502d.e
3.299f
CpW(CCMe 3 )(CH 2 Ad)CI
(12)
13C
4.8
1.484
3.557d
CPW(CAd)(CH2 CMe 3 )C1
(10)
(Hz) 6
32.37
CpW(CAd)(CH 3 )2
CpW(CCMe3)(CH2 CMe3 )(CH2 Ad)
(11)
2JWH
119.3
98.08
125.2
98.4
100.58
12.9
125.0
97.1
a recorded in dichloromethane-d 2.
b arbitrary assignment; 1JCH not
assigned to
1
corresponding H resonance by selective heteronuclear
decoupling. c Jxx' - 4 JHH = 2.1 Hz.
2
4
d JHH = 11.7 Hz. e JHH = 2.6 Hz.
f 2
JHH = 12.4 Hz. g arbitrary assignment of
resonance (with corresponding 1JCH values) to 1
H resonances.
downfield and upfield 1H resonances in analogy to
compounds 9,
1JCH
13
C
values assigned to
10, and 11.
I
Ad
C
11X
1IA
Cý,W..,,\CH2CMe3
"*CH
2CMe 3
.,%\%CH 2CMe
9AA'
9
I
'
'
I
-
'
'
-
' I
3.8.3.
I
I
I
3.6.
--
I I I I I I I I
I
I
3.4
3..5
I
I I I
I
3.3
i
I
I
35.2
1
I
I
p
3.5
A.pp
-
I
II
I
-0.9
I
I
1
I
-1.0
I
V
111111111111
I
I
-1.1
I
I
!
!
t
-1.2
!
w
"V
i1111111(1111
v
I
-1.3
•
• •
.
.
-1.4
.
.
.
.
•
-1.5
T
--
I
I
p
igure 1.4. 'H NMR (300 MHz) spectrum of an equilibrium mixture of 9 and 11. Highlighted are the downfield (top) and upfiel
bottom) methylene resonances. The pairs of coupled resonances (9AA', 9XX'), (11A, 11B), and (11X, 11Y) correspond to th
ree sets of diastereotopic geminal oc-CH 2 R groups. Note the ' 83W satellites exclusively on the upfield resonances.
3.1 Hz has been observed by Berry et al. in the zirconocene silylamide complex
30 It
Cp 2Zr(H)[N(tBu)(SiMe 2 H)] crystallographically shown to possess a 3-agostic Si-H bond.
should be noted that 1H- 1H couplings through four c-bonds are generally only observed in
bicyclic systems in which the H-C-C-C-H linkage is locked in a "W" configuration 26 ,3 1,32 such
33-35
as those shown in Figure 1.5.
Figure 1.5. Depiction of 1 H- 1H coupling through four a-bonds. "W"' coupling pathways
present in the bicyclic alkanes are shown in bold.
H2
C
C.
H
H
4 JHH = 7
Hz
H
4JHH = 3 - 4 Hz
H
n
n
4JHH < 0.5 Hz
X-ray Structure of CpW(CAd)(CH 2 CMe 3 )2 (9).
An X-ray study of 9 was performed to identify key structural aspects in this family of
cyclopentadienyl alkylidyne complexes as well as metrical data relevant to the Ua-agostic
interactions borne out by the NMR parameters of 9 - 12. Figures 1.6 and 1.7 show two drawings
of the solid state structure of 9 in which the two neopentyl ligands are related by a
crystallographic mirror plane passing through the alkylidyne Ca-W-Cp(centroid) plane. The
pseudotetrahedral structure bears strong resemblance to a bent metallocene-like structure 2 in
which the C(1)-W-Cp(centroid) angle (121.5 (5)°; Table 1.2) is considerably larger than the
angle between the two neopentyl ligands (106.2 (4)'). The W-alkylidyne linkage is nearly linear
(< W-C(1)-(2) = 166.2 (6)0) with a normal 3 6 W-Calkylidyne distance of 1.746 (9). Although
clearly bound in an T15 manner, some slippage of the Cp ring is apparent with W-C distances
ranging from 2.328 (7) (C(5)) to 2.512 (10) A (C(7)). This asymmetry may reflect the influence
of the triply bound alkylidyne ligand, as the longest distance is to the Cp carbon atom (C(7))
roughly trans to the W-alkylidyne linkage (C(1)-W-C(7) = 149°).
The X-ray structure of 9 exhibits several features that may be viewed in the context of taagostic interactions. Although the a-H atoms were not located in a difference map, they were
refined by placing them in idealized positions, fixing the C(3)-H(3A) and C(3)-H(3B) distances
at 0.96 A, and then allowing the orientations of these C-H bonds to refine freely. 6 Although the
uncertainty in these distances is due to the nature of the technique, the resulting Woo*H(3A) and
W***H(3B) contacts of 2.42 (7) and 2.61 (7) A suggest that one of the neopentyl a-H atoms in 9
lies closer to the tungsten center than the other. The a-H atom (H(3A)) nearest to W also makes
the smallest W-C(3)-H(3) angle (93 (4) vs. 109 (4)') and rests just above the C(3)-W-C(3') plane
(Figure 1.6).
The W-C(3) distance of 2.159 (7) A lies slightly on the low end of the range (2.096 (5) 2.258 (8) A) found in other crystallographically characterized four- and five-coordinate tungsten
neopentyl complexes.
12 .37 .38
Of note, however, is the wide W-C(3)-C(4) angle of 134.8 (4) ° .
which is outside of the range in the neopentyl complexes described above (122 - 131 ). Similar
Nb-Ca-Cp angles (131.2 (2), 132.5 (3)°), however, have been found in the a-agostic
CpNb(NAr)(CH 2 CMe 3 )2 (Ar = 2,6-diisopropylphenyl). 6 The neopentyl t-butyl group lays not in
the plane of the C(3)-W-C(3') vector, but is instead directed "up" toward the alkylidyne ligand
by 38.5
°
(the torsion angle between the C(3)-W-C(3') and W-C(3)-C(4) planes). In accord with
the discussion of the H(3) atoms above, the W-C(3)-C(4) angle and its torsion angle with the
C(3)-W-C(3') plane suggest that the a-H atom lying nearest the C(3)-W-C(3') vector, H(3A).
should be located nearest tungsten, directed there by orientation of the neopentyl group. While
these parameters do not in themselves identify at-agostic interactions, the structural data may be
viewed as consistent with the a-agostic NMR parameters discussed above.
Figure 1.6. Chem-3D drawing of the X-ray structure of 9 viewed down the crystallographic
mirror plane. The "atom" in the center of the Cp ring is its centroid.
C(7)
Figure 1.7. Chem-3D drawing of the X-ray structure of 9 viewed from the side highlighting its
bent metallocene-like structure. The neopentyl ct-H atoms are omitted for clarity.
C(6')
Table 1.2. Selected Bond Lengths (A) and Angles (0)for CpW(CAd)(CH 2 CMe 3 )2 (9).
Distances
W-C(1)
1.746 (9)
W-C(3)
2.159 (7)
W-C(5)
2.328 (7)
C(3)-H(3A)
0.98 (7)
W-C(6)
2.444 (7)
C(3)-H(3B)
0.93 (7)
W-C(7)
2.512 (10)
Wo**H(3A)
2.42 (7)
W-centroid
2.098 (8)
W***H(3B)
2.61 (7)
134.8 (4)
Angles
W-C(1)-C(2)
166.2 (6)
C(3)-W-C(3')
106.2 (4)
W-C(3)-C(4)
C(1)-W-centroid
121.5 (5)
W-C(3)-H(3A)
93 (4)
C(1)-W-C(3)
102.1 (2)
W-C(3)-H(3B)
109 (4)
C(3)-W-centroid
111.8 (5)
Table 1.3. Crystallographic Data, Collection Parameters, and Refinement Parameters for
CpW(CAd)(CH 2 CMe 3 )2 (9).
Empirical Formula
Formula Weight
Diffractometer
Crystal Color, Morphology
Crystal Dimensions (mm)
Crystal System
a
b
c
C30 H5 8B2 N2 Zr
559.62
Siemens SMART/CCD
red, plate
0.18 x 0.24 x 0.24
Monoclinic
15.7061 (8) A
12.4619 (7) A
12.1803 (6) A
900
97.5680(10)0
900
V
Space Group
2363.3 (2) A3
C2
Dcalc
1.513 g/cm 3
Fooo
1088
4.896 mm-1
(o scans
g(MoKra)
Scan Type
Temperature (°C)
Total No. Unique Reflections
No. Variables
293 (2) K
1797
R
140
0.0375
RwG
GoF
0.0906
1.011
Agostic Interactions and a,ax-H Migration.
The mutual observations of NMR parameters indicative of a-agostic interactions and
tautomerization by alkyl to alkylidyne a-H transfer in 9 - 12, but not in 7 and 8, suggests that
these two phenomena may be related. Through labeling studies and kinetic analysis, the related
tautomerization of W(CSiMe 3 )(CH 2 CMe 3 )3 to W(CCMe 3 )(CH 2 SiMe 3 )(CH 2 CMe 3 )2 has been
proposed to proceed through a bis(alkylidene) intermediate. 19
Although no tungsten
bis(alkylidenes) are known, the tantalum complexes Ta(CHCMe 3 )2 X(PMe 3 )2 (X = Cl, Me) and
CpTa(CHCMe 3 )2(PMe 3 ) provide support for such an intermediate. 39 In analogy, the conversion
of neopentyl complexes 9 and 10 to their tautomers 11 and 12 is also likely to involve the
intermediacy of bis(alkylidene) species (Scheme 1.1).
Scheme 1.1. Tautomerizations of 9 - 12 Mediated by a Bis(alkylidene).
H
W..,I CH CMe
2
Ad -C
/
H
/e
.. R,I
3
"R
'CHAd
CMe3
I
R = CH 2CMe 3 (9)
R = CH 2 CMe3 (11)
R = Cl (10)
R = Cl (12)
Since the stepwise shift of alkyl a-H atoms to the alkylidyne a-C atom must involve alkyl a-CH bond breaking, any weakening of an alkyl a-C-H bond could accelerate the tautomerizations
described in eqs 10 and 11. Thus the alkyl a-agostic interactions present in 9 - 12 may provide a
low energy pathway for H-atom migration from the alkyl to alkylidyne ligands, facilitating the
room temperature tautomerization reactions observed here.
The isolobal analogy offers a model for understanding the orbital interactions responsible
for a-agostic interactions in 9 - 12. Since the do CpW(CAd) and Cp2M fragments are isolobal
and isoelectronic,
the isolobal analogy predicts that the frontier
molecular orbitals of the
CpW(CAd) unit will be similar in form to those
of the Cp2M fragment. This set of molecular
orbitals has been previously described by Lauher
and Hoffmann 40 and is reproduced in Figure
1.8.
Using frontier molecular orbitals similar in form
to those of the Cp 2 M unit, Figure 1.9
presents the orbital interactions that constitute the
a-bonds between the tungsten center and the
two neopentyl ligands in complex 9 as a model
for understanding bonding in the ca-agostic
complexes 9 - 12. One symmetric and one anti-symmetric
metal based orbital combine with the
symmetric and anti-symmetric combinations of
the alkyl a-C atom based o-orbitals. One
symmetric metal based orbital remains unoccupied.
Consistent with the X-ray structure of 9. it is
this unoccupied orbital which may accept electron
density contained in suitably oriented cx-C-H
bonds giving rise to ca-agostic interactions.
Figure 1.8. Frontier orbitals of the Cp M fragment
(reproduced from ref. 40).
2
moZD
lal
lb1
2a 1
Figure 1.9. Depiction of a-bonding and ao-agostic
interactions in 9.
H CMe3
C-'L2-Me3
C
W
CHWCMe
2
Symmetric
a-bond
3
L r2%_lvie 3
Anti-symmetric
a-bond
CI
H
A
H CMe 3
Agostic Interactions
AgosticInteractions
A monoanionic ligand capable of strong it-donation to the acceptor orbital of the
CpW(CAd) fragment not used in metal-alkyl or metal-chloride o-bonding in complexes 9 - 12
would be very likely to favorably compete with the C-H bonds involved in ox-agostic
interactions.
Accordingly, the dimethylamido derivative CpW(CAd)(NMe 2 )Cl (13) may be
prepared by the addition of N,N-dimethylaminotrimethylsilane to a dichloromethane solution of
6 (eq 12) and is isolated as yellow crystals in good yield by crystallization from ether.
Ad
Ad
C+ .M C1M
dichloromethane
+ TMS-NMe 2
W
*C1
.,,,(
W
W.I
- TMS-C1
6
1H
CI
(12)
N Me
Me
13
NMR spectra of 13 recorded at room temperature in toluene-d8 display two well-separated
methyl resonances for the dimethylamido group. Heating to 110 'C causes these resonances to
broaden only slightly allowing the lower limit of 18 kcal/mol to be placed on the activation
parameter AGt corresponding to W-NMe 2 bond rotation in 13.
We assume that the
dimethylamido group in 13 is oriented such that methyl groups lie roughly orthogonal to the NW-C1 plane of 13 so that the nitrogen lone pair may engage in 7t-donation to one of the
symmetric metallocene-like acceptor orbitals as shown in Figure 1.8. Though we cannot
completely discount any steric factors which may hinder W-NMe 2 bond rotation in 13 based on
the X-ray structure of 9, strong rt-donation into CpW(CAd) fragment is most likely to be
responsible for this barrier.
The corresponding neopentyl derivative may be prepared by treating 13 with
neopentyllithium in ether and is isolated as orange-yellow crystals from pentane (eq 13). As in
13, the NMe 2 1H resonances are sharp and well-separated, signifying that the W-NMe 2 bond
does not rotate on the NMR timescale at room temperature.
Ad
Ad
C
\\\ ., CH 2CMe 3
W V N Me
Et2 0, -40 oC
SC
N'Me + LiCH 2CMe 3
W
Me
(13)
Me
13
14
The signatures of a-agostic interactions present in the NMR parameters of 9-12 are
absent in 1 H and
13 C
NMR spectra of 14 (Table 1.4). Most informative are the 1JCH values of
113 and 115 Hz recorded for the diastereotopic methylene C-H bonds in 14. These closely
spaced values near the expected range for normal sp 3 C-H bonds may be contrasted with the
JCH values observed for 9-12 which are segregated into two sets of values centered around 100
1
and 120 Hz. The presence of the strongly 7t-donating dimethylamido ligand in 14 appears to
preclude
x -agostic
interactions
which
are
observed
in
the
closely
related
CpW(CAd)(CH 2 CMe 3 )C1 (10) and CpW(CCMe 3 )(CH 2 Ad)CI (12). In further contrast to 10.
CpW(CAd)(NMe 2 )(CH 2 CMe3) is stable in benzene-d6 solution and shows no evidence of
tautomerization to CpW(CCMe 3 )(NMe 2 )(CH 2 Ad) at room temperature over the course of two
weeks (eq 14).
Ad
C
Me
....
31 ,W.,.\ CH 2CMe 3
*N Me
NMe 2
(14)
CH 2Ad
Me
14
Table 1.4. W-CH 2CMe3 NMR Parameters for 14 (in dichloromethane-d 2 ).
Complex
8 1 H (ppm)
1.69
2 JWH
(Hz) 8
13 C
(ppm)
(Hz)
113.6a
11.7
CpW(CAd)(CH 2 CMe 3 )(NMe 2 )
1JCH
48.60
1.53
8.7
a arbitrary assignment of 1JCH to corresponding 1H resonance.
115.9a
Related a-Hydrogen Shift from a Primary Amide.
Reaction of CpW(CAd)C12 with N-t-butylaminotrimethylsilane in dichloromethane gives
CpW(CAd)(NHBut)Cl (15) as yellow crystals in high yield (eq 15). The isolated product is
characterized as the alkylidyne-primary amide tautomer 15 by the observation of an N-H stretch
at 3249 cm - 1 in its IR spectrum.
Ad
C
Ad
C
ý\
.. C1
SC+
C1
TMS-N(H)Bu
t
S.,%%
W~ N(H)Bu
dichloromethane
-
TM-C
(15)
- TMS-C1
12
As expected on the basis of other alkylidyne-primary amide complexes of tungsten, .41
solutions of 15 are not stable towards migration of the amido aI-H to the alkylidyne ac-carbon
atom. After standing 24 hours at room temperature in dichloromethane-d2 the major species is
the imido-alkylidene CpW(NCMe 3 )(CHAd)Cl (16).
A minor third species 17 is also observed
in solution, which becomes the sole product after 9 days. It is an alkylidene as well, but one
displaying NMR parameters for the alkylidyne ca-C-H unit different from those of 16 (Table 1.5).
Table 1.5. Selected NMR Parameters for Complexes 15 - 17 (in dichloromethane-d 2 ).
Complex
5 1H (Cp)
8 1H (ct-H)
6
13 C
(ca-C)
1JCH
(Hz)
CpW(CAd)(NHCMe 3 )Cl
(15)
6.08
9.39
303.5
syn-CpW(NCMe 3 )(CHAd)C1
(16)
6.12
11.15a
264.7
131.3
anti-CpW(NCMe3 )(CHAd)Cl
(17)
6.05
10.40b
274.1
123.5
a
2 JWH =
14.1 Hz. b 2JWH < 4 Hz.
Consistent with the NMR parameters collected in Table 1.5, the products 16 and 17 are
assigned as W=C rotamers of the imido-alkylidene complex CpW(NCMe 3 )(CHAd)C1 which
differ in the relative orientation of the 1-adamantyl alkylidene substituent. We propose that the
first rotamer results from the transfer of the amido hydrogen to the nearest "face" of the WECAd
bond, opposite to that of the Cp ring (Scheme 1.2). This places the 1-adamantyl alkylidene
substituent syn to the Cp ring producing what we assign as syn-CpW(NCMe 3 )(CHAd)Cl (16).
On the basis of the X-ray structure of 9, this would not be the favored orientation of the
alkylidene, as the 1-adamantyl substituent is directed toward the cyclopentadienyl ring rather
than the less sterically demanding t-butylimido group. Accordingly, slow rotamerization of the
W=C alkylidene bond converts all of 16 to anti-CpW(NCMe)(CHAd)C1, the sole product
observed after 9 days. Further corroborating the syn orientation of the Cp and alkylidene ligands.
irradiation of the 1H Cp resonance of 17 results in an NOE enhancement of the alkylidene u-C-H
signal.
Scheme 1.2. Tautomerization of 15 to 16 Followed by Alkylidene Rotamerization to 17.
Ad
C
C•
H
~.a
N(H)Bu t
CC1
CD 2Cl 2 / RT
AdC\\
NBu t
But
N
C1
tl/2 - 16 h
Ad
16
tl/2
rlidene
erization
But
N
N\W ..1l Cl
W C, Ad
I
H
Discussion and Conclusions
In analogy to W(CSiMe 3 )(CH 2 CMe 3 )3 , mono- and dineopentyl derivatives of the
CpW(CAd) fragment 9 and 10 are not stable towards migration of the alkyl a-H atoms to the
alkylidyne a-C atom. The use of the 1-adamantyl alkylidyne substituent which is sterically and
electronically quite similar but not identical to the t-butyl alkylidyne substituent allows a-H
migration in complexes 9 and 10 complexes to be conveniently tracked, a process that would be
degenerate and unobservable in neopentyl derivatives of the known CpW(CCMe 3 )C12 .
Inspection of the qualitative rates of tautomerization in the series of neopentyl derivatives
CpW(CAd)(CH 2 CMe 3 )X (X = CH 2 CMe3 (9), Cl (10), NMe 2 (14)) shows that the rate of
migration of the neopentyl a-H atoms is quite sensitive to the nature of the substituent X (Figure
1.10).
Figure 1.10. Qualitative rates of a-H migration in neopentyl derivatives of CpW(CAd).
Ad
Me 3 CC
*.,, CH 2CMe 3
"_W.,%%%
X
WXVCHAd
Rate: X = CH 2 CMe 3 > C1 >> NMe 2
(9)
(10)
(14)
Tautomerization occurs most readily for X = CH 2 CMe 3 , at a qualitative rate considerably faster
than for X = Cl, even after considering the expected rate enhancement due to the statistical factor
of two. Furthermore, where X = NMe 2 , no sign of tautomerization at all is observed at room
temperature over two weeks. Based on the equilibrium constants in the tautomerization of 9 and
10 which are almost identical to those predicted on statistical grounds,
the absence of
tautomerization in 14 is not likely a reflection of an unfavorable equilibrium, but rather the
result of a substantial kinetic barrier.
The correlation between a-agostic interactions in 9 and 10 and their room temperature
partial tautomerization to 11 and 12 and the absence of both phenomena in 14 is unmistakable.
This relationship suggests that a-agostic interactions assist and are responsible for the migration
of alkyl a-H atoms to the alkylidyne ligand in neopentyl and 1-adamantylmethyl derivatives the
CpW(CAd) unit. The dramatic difference in rates of tautomerization at room temperature
between 9 and 10 relative to 14 (not observed) may be thus be rationalized. The significantly
smaller, yet sizable, difference between the rates of tautomerization observed for 9 and 10.
however, is more subtle and may be related to 7t-donation from the chloride ligand in 10.
As with 14, the dibenzyl and dimethyl complexes 7 and 8 also do not exhibit a-agostic
NMR parameters nor tautomerization by a-H migration. Whereas the value of IJCoH for
dimethyl complex 8 (121.9 Hz) is consistent with a normal sp3 C-H bond, these coupling
constants in the dibenzyl 7 (134.1, 137.1 Hz) are actually higher than expected. The elevated
IJCH values are consistent with a build-up of sp 2 character in the benzylic C-H bonds resulting
from some degree of r12 character of the benzyl ligand. 42 This type of interaction uniquely
available to the benzyl ligand presumably utilizes the same orbital involved in a-agostic
interactions in 9 - 12, and precludes the development of a-agostic interactions in 7. Further
illustrating the difference between dibenzyl 7 and the aliphatic derivatives 8 - 12. in an
admittedly quite qualitative manner, is a comparison of their respective colors. Whereas 8 - 12
are all deep red, 7 is bright yellow-orange similar in color to the amide derivatives 13 - 15 in
which the nitrogen lone pair may interact with the acceptor orbital of the CpW(CAd) fragment.
Furthermore, while the decomposition of 8 into several Cp-containing (and presumably some
NMR silent) products over 36 h clouds the issue of whether a-H migration takes place, it is clear
that no sign of tautomerization in 7 takes place over 4 - 5 days at room temperature. It should
be noted that this negative result for 7 may be a reflection of an unfavorable equilibrium, a
contingency not possible to address in this study due to the inability to prepare analogous alkyl
derivatives from CpW(CPh)(OCMe 3 )2 (Sb).
The irreversible migration of the amide a-H atom to the alkylidyne a-C atom in 15
serves as an imperfect model for the alkyl to alkylidyne a-H migrations observed in 9 - 12. Due
to the orbital differences between the imide and alkylidene ligands, the imido-alkylidene 16
serves not as a model of the proposed bis(alkylidene) intermediate shown in Scheme 1.1, but
instead illustrates the stereospecificity of the ax-H atom transfer.
The preferred pathway in the
transfer of the amido proton to the nearest face of the WEC bond results in a kinetic alkylidene
rotamer sterically destabilized relative to the thermodynamic rotamer. This should be contrasted
with intramolecular a-H abstraction processes which lead to the isolation of only one isomer of
the related CpW(NPh)(CHCMe 3 )Cl through loss of neopentane from W(NPh)(CH 2 CMe3) 2 C1 or
by the addition NaCp to W(NPh)(CH 2 CMe 3 )3 C1. 43 Due to the orbital analogies between the
alkylidyne and imide ligand (Figure 1.2), metal-alkylidene ir-bonding in 16 and 17 presumably
involves a metal-based orbital similar to that responsible for interaction with the amido lone pair
in 13 and 14 and the corresponding high barrier to rotation. Accordingly, a strong M=C IT
interaction in 16 and 17 may be expected, consistent with the slow rotamerization converting the
kinetic rotamer 16 to the thermodynamic product 17 over several days at room temperature.
Finally, these observations on a-H transfer reactions in the CpW(CAd) alkyl complexes
discussed here should be compared with other related a-processes such as a-H abstraction from
alkyl ligands which lead to alkylidene formation. 4 4,45 It has been observed for some time that
rate of alkylidene formation from alkyl complexes strongly depends on the nature of the other
ligands in the metal's coordination sphere 4 4' 4 5 This process is generally accelerated with the
increasing electron deficient nature of the metal center in do complexes. 44 ,45 For example, aabstraction generally occurs faster in bromide than chloride complexes, and often the presence of
suitably oriented it-donors such as t-butoxide ligands can completely shut down a-H abstraction.
Thus in a qualitative sense, a metal's electron deficient nature activates alkyl a-H atoms toward
ca-H abstraction. With respect to the tautomerization reactions discussed above, the NMR
parameters for the a-C-H bonds in complexes 9 - 12 provide a more concrete measure of their
activation toward a-H migration. Furthermore,
the relationship between alkyl-alkylidyne
complexes exhibiting a-agostic NMR parameters as well as alkyl to alkylidyne a-H migration
demonstrates that this activation of c-C-H bonds is due to uc-agostic interactions.
Experimental
General Procedures.
All experiments were performed under nitrogen in a Vacuum
Atmospheres drybox or under argon using standard Schlenk techniques. All solvents were
purified by standard techniques while deuterated NMR solvents were dried and stored over
activated 4 A molecular sieves before use.
W2 (OCMe 3 )6 was prepared by the addition of in situ prepared Na[W 2 C17 (THF) 5 ] to a
THF solution of 6 eq. LiOCMe 3 (prepared from Li metal and t-butanol in hexane).
W(CPh)(OCMe 3 )3 was prepared from W 2 (OCMe 3 )6 and 2 eq. 1-phenylpropyne in pentane. 2 1
1-Adamantanecarbonitrile, trimethylsilyl triflate, N,N-dimethylaminotrimethylsilane, and N-tbutylaminotrimethylsilane were used as received. Solutions of benzylmagnesium chloride and
methylmagnesium chloride were obtained from Aldrich and titrated before use.
Neopentylmagnesium chloride 4 6 and neopentyllithium' 8 were prepared as described in the
literature. 2,6-lutidinium hydrochloride was prepared by the addition of a solution of anhydrous
HCl in ether to 2,6-lutidine in ether.
1H
and
13 C
spectra were recorded at 300 and 75.4 MHz, respectively. Proton spectra
were referenced internally by the residual solvent proton signal relative to tetramethylsilane.
Carbon spectra were referenced internally relative to the
to tetramethylsilane.
13 C
signal of the NMR solvent relative
All J values are reported in Hz; 1JCH values for all alkyl complexes are
collected in Tables 1.1 and 1.4 are not reported below. IR spectra were recorded as Nujol mulls
between KBr plates on a Perkin-Elmer 1600 FT-IR spectrometer. Elemental analyses (C, H. N)
were performed on-site in our laboratories with a Perkin-Elmer PE2400 microanalyzer or by
Oneida Research Services, Whitesboro, New York.
W(CAd)(OCMe 3 )3 (2).
To a solution of W 2 (OCMe 3 )6 (14.58 g, 18.08 mmol) in
pentane (175 ml) was added 1-adamantanecarbonitrile (2.91 g, 18.08 mmol). The deep red color
of W 2 (OCMe 3 )6 soon faded with concomitant formation of a voluminous white precipitate.
After stirring overnight, the [W(N)(OCMe 3 )3 1x was collected by filtration. The resulting pentane
solution was concentrated and cooled to -40 OC to afford 8.0 g (80%) of colorless crystals of
W(CAd)(OCMe 3 )3 in three crops.
An analytical sample was obtained by
double
recrystallization from pentane: 1H NMR(C 6D6 ) 8 2.08 (br, 6, Ad), 2.00 (br, 3, Ad), 1.62 (br, q
6, Ad), 1.503 (s, 27, OCMe3 );
13 C{ 1H}
NMR 8 272.1 (JCW = 294.1, CAd), 79.0 (OCMe 3 ), 52.7
(CAd), 47.0 (Ad), 37.3 (Ad), 33.0 (OCMe3 ), 29.9 (Ad); Anal. Calcd for C23 H42 0 3 W: C, 50.19;
H, 7.69. Found C, 50.03; H, 7.70.
W(CAd)(OCMe 3 )2 (OTf)(dme) (4a). Trimethylsilyl triflate (2.92 g, 13.2 mmol) was
added dropwise to a solution of W(CAd)(OCMe 3 )3 (7.24 g, 3.87 mmol) and dimethoxyethane
(3.62 g) in pentane (125 ml) at -40 oC. Bright yellow microcrystals began to appear after the
addition and the mixture was stirred for 1 h. After standing 1 h at -40 OC, the microcrystals were
collected by filtration and washed with pentane (3 x 15 ml) to afford 9.03 g (96%) of the product.
The product is thermally sensitive and should be stored below room temperature or used
immediately. An analytical sample was recrystallized from toluene at -40 'C:
1H
NMR(C 6 D6 ) 6
3.979 (s, 3, MeOCH2 ), 3.571 (br, 1, MeOCH2 ), 3.077 (s, 3, MeOCH2). 3.022 (br, 1,. MeOCH2).
2.956 (br, 1, MeOCH2 ), 2.035 (br, 9, OCMe3 ), 1.988 (br, 3, Ad), 1.597 (br, 9, OCMe3). 1.533
(br, 6, Ad), 1.488 (br, 6, Ad);
13 C{ 1H}
NMR 5 288.1 (Jcw = 273.5, CAd), 120.6 (JCF = 318.4.
O3 SCF 3 ), 80.13 (br, OCMe 3 ), 80.08 (br, OCMe 3 ), 75.2 (MeOCH 2 ), 73.4 (MeOCH2). 69.5
(MeOCH2 ), 59.2 (MeOCH2 ), 52.2 (Jcw = 40.0, CAd), 46.6 (Ad), 37.1 (Ad), 33.1 (br. OCMe3).
32.9 (br, OCMe 3 ), 29.7 (Ad). Anal. Calcd for C24 H 43 F3 0 7 SW: C, 40.23; H, 6.04. Found: C.
40.27; H, 6.27.
W(CPh)(OCMe 3 )2 (OTf)(dme) (4b). To a solution of W(CPh)(OCMe3)3
2
(1.15 g.
2.34 mmol) and dimethoxyethane (2.5 g) in pentane (30 mL) at -40 OC was added dropwise
trimethylsilyl triflate (0.519 g, 2.34 mmol) in pentane (2 mL). Upon complete addition, a yellow
precipitate began to form. After stirring for 20 minutes, the yellow precipitate was collected by
filtration and washed with pentane (3 x 10 mL) to afford 1.42 g (92%) of product which was pure
enough for further use: 1H NMR(C 6 D6 ) 6 7.468 (d, 2, Pho), 7.247 (t, 2, Phn,), 6.680 (t, 1, Php,).
3.831 (s, 3, MeOCH2 ), 3.41 (br, 1, MeOCH 2 ), 3.055 (s, 3, MeOCH2 ), 2.93 (br, 1, MeOCH 2 ).
2.67 (br, 2, MeOCH 2 ), 1.554 (br, 9, OCMe3 ), 1.453 (br, 9, OCMe3); 13 C{ 1H} NMR 8 273.65
(CPh), 145.72 (Ci), 134.69 (Cp), 127.56, 127.36 (Co and C,,,), 120.72 (JCF = 318.0 Hz, O3 SCF).
74.34, 72.29, 69.75, 59.37 (dme), 32.53 (br, OCMe3 ).
CpW(CAd)(OCMe 3 )2 (5a). To a solution of W(CAd)(OCMe 3 )2 (OTf)(dme) (6.50g.
9.11 mmol) in THF (70 ml) at -40 OC was added NaCp (4.6 mL, 2.0 M in THF, 9.1 mmol) and
the resulting light orange solution was stirred overnight. The volatiles were removed in vacuo
and the residue was extracted with pentane (100 mL). After filtration, the solvent was removed
in vacuo to give a yellow solid which was extracted again with pentane. Filtration and removal
of the solvent in vacuo yielded 4.88 g (99%) of a yellow solid. An analytical sample was
1H
NMR(C 6 D6 ) 6 6.071 (s, 5, Cp), 1.968 (br, 3, Ad), 1.831 (d. 6.
13 C{ I1H}
NMR 8 285.2 (Jcw = 277.2, CAd), 105.7 (Cp), 76.7 (OCMe3).
recrystallized from pentane:
Ad), 1.599 (br, 6, Ad);
51.8 (Jcw = 43.5, CAd), 46.6 (Ad), 37.4 (Ad), 32.8 (OCMe 3 ), 29.9 (Ad). Anal. Calcd for
C24 H 38 0 2 W: C, 53.15; H, 7.06. Found: C, 53.42; H, 7.24.
CpW(CPh)(OCMe 3 )2 (5b). To a solution of W(CPh)(OCMe 3 )2 (OTf)(dme) (1.30 g.
1.97 mmol) in THF (30 mL) at -40 OC was added NaCp (0.93 mL, 2.0 M in THF, 1.97 mmol).
After stirring for 2h the volatiles were removed in vacuo and the residue extracted with pentane
(70 mL).
After filtration the solvent was removed in vacuo to yield 0.865 g of yellow
microcrystals (90%) which were pure by 1H NMR:
1H
NMR(CDC13)
7.350 (t, 2, Phi,,). 7.031
(d, 2, Pho), 6.938 (t, 1, Php), 6.314 (s, 5, Cp), 1.356 (s, 18, OCMe3); 13C{ 1H} NMR 8 270.2
(Jcw = 286, CPh), 147.8 (Ci), 132.0 (Cp), 127.2, 126.2 (Co and C,,,), 106.1 (Cp), 79.2 (OCMe3).
32.1 (OCMe3 ).
CpW(CAd)C1 2 (6). A solution of 2,6-lutidinium chloride (0.500 g, 3.48 mmol) and
trimethysilyl chloride (3.00 g, 27.6 mmol) in dichloromethane (20 ml) was added to
CpW(CAd)(OCMe 3 )2 (3.00g, 5.53 mmol) in dichloromethane (15 mL).
The solution
immediately turned deep red and slowly became purple over ca. four hours. After standing
overnight, the solution was an intense purple. The volatiles were removed in vacuo, and the
purple solid was extracted with 50/50 ether/pentane (125 ml). Purple crystals formed upon
concentrating the solution to ca. 10 ml. After standing at -40 OC overnight, 2.10 g (81%) purple
shards were isolated. An analytical sample was recrystallized from ether/dichloromethane at -40
°C:
1H
NMR(C 6 D6 ) 8 5.719 (s, 5, Cp), 1.920 (br, 3, Ad), 1.615 (br d, 6, Ad), 1.473 (br t, 6. Ad):
13C{ 1H} NMR 6 328.6 (Jcw = 233.4, CAd), 106.1 (Cp), 51.3 (Jcw = 32.9, CAd), 44.7 (Ad).
36.6 (Ad), 29.0 (Ad). Anal. Calcd for C 16 H20 C12 W: C, 41.15; H, 4.31. Found: C, 41.17; H, 4.42.
CpW(CAd)(CH 2 Ph)2 (7). Benzylmagnesium chloride (0.593 ml, 1.11 M in ether, 0.658
mmol) was added to a chilled (-40
oC)
solution of CpW(CAd)C12 (0.150 g, 0.321 mmol) in ether
(10 ml). The solution immediately turned yellow with concomitant formation of precipitate.
After the solution was stirred at room temperature for 1 h, dioxane (0.057 mL, 0.66 mmol) was
added and the volatiles were removed in vacuo. The residue was extracted with toluene (25 mL).
filtered through Celite and concentrated to ca. 10 mL. After filtering again, the extracts were
concentrated to ca. 1 1/2 mL and allowed to stand at -40 OC affording 0.147 g (79%) of yellow
crystals. An analytical sample was recrystallized from ether/dichloromethane at -40 'C:
1H
NMR (CD 2 Cl2) 8 7.272 (t, 1, Ph,), 7.049 (d, 2, Pho), 6.959 (t, 2, Ph,11), 5.510 (s, 5. Cp). 2.422 (d.
2 JHH
= 7.5, 2 JWH = 4.8, 2, CH 2 Ph,), 1.936 (br, 3, Ad), 1.694 (d, 6, Ad), 1.628 (t. 6. Ad). 1.484
(d, 2JHH = 7.5, 2 JWH = 8.1, 2, CH 2 Ph);
13 C
NMR 8 294.5 (WCAd), 135.6 (Ci). 131.4. (CO).
127.5 (Cp), 125.6 (C,,,), 97.7 (Cp), 53.2 (CAd), 42.9 (Ad), 37.2 (Ad), 32.4 (Jcw = 63.3. CH2Ph).
29.2 (Ad). Anal. Calcd for C 30 H 3 4 W: C, 62.30; H, 5.92. Found: C, 62.41; H, 5.86.
CpW(CAd)Me 2 (8). A solution of CpW(CAd)C12 (0.150 g,. 0.321 mmol) in THF (3 ml)
at -40 OC was added to a solution of methylmagnesium chloride (0.228 ml, 2.89 M in THF. 0.658
mmol) in THF (3 ml) at -40 oC. During the initial stages of the addition, the intense purple color
of 6 was quenched and the Grignard-rich solution remained colorless until after ca. 1/2 of the
solution containing 6 was added, when the solution turned deep red. After standing 10 min at
-40 'C, dioxane (0.070 mL, 0.82 mmol) was added and the volatiles were removed in vacuo.
After tituration with pentane, the residue was extracted with pentane (10 mL), filtered, and
concentrated to dryness. The resulting red powder was recrystallized from ether/pentane at -40
'C to yield 0.099 g (72%) of red, grain-like crystals in two crops.
1H
NMR(C 6 D6 ) 6 5.416 (s,
5, Cp), 2.031 (br, 3, Ad), 1.898 (d, 6, Ad), 1.668 (br, 6, Ad), 1.133 (s, 6, Me,);
13 C
NMR 6
300.36 (Jcw = 240, CAd), 100.14 (Cp), 50.79 (CAd), 44.72 (Ad), 40.25 (Jcw = 93, W-CH 3 ).
37.25, 29.37 (Ad). Anal. Calcd for C1 8 H26 W: C, 50.72; H, 6.14. Found: C, 50.97; H, 5.97.
CpW(CAd)(CH 2 CMe 3 )2 (9). Neopentylmagnesium chloride (0.259 ml, 2.97 M in ether.
0.768 mmol) was added to a solution of CpW(CAd)C12 (0.175 g, 0.375 mmol) in ether (10 ml) at
-40 OC.
The solution immediately turned cherry red and turbid.
After stirring at room
temperature for lh, dioxane (0.070 mL, 0.82 mmol) was added and the suspension was filtered
through Celite. The filtrate was concentrated to dryness, titurated with pentane (5 mL), and then
extracted with pentane (10 mL). Following filtration, the volatiles were once again removed in
vacuo to afford a red powder which was recrystallized from pentane at -40 'C to afford 0.143 g
(71%) of small red crystals in two crops.
1H
NMR(C 6 D6 ) 8 5.395 (s,5, Cp), 3.536 (AA"XX'.
JAX +JAX' = 11.3, 2, CH 2 CMe 3 ), 1.985 (br, 9, Ad), 1.641 (br, 6 Ad), 1.302 (s,18, CMe3). -1.136
(AA'XX', Jxx,= 4 JHH = 2.1, 2 JWH = 12.6, 2, CH 2 CMe 3 );
13 C
NMR 6 300.37 (CAd), 100.19
(Cp), 90.67 (Jcw = 94.9, CH 2 CMe3), 51.98 (CAd), 45.18, 37.18 (Ad), 35.32 (CMe3). 34.37
(CMe3), 29.52 (Ad). Anal. Calcd for C26 H42 W: C,58.00; H, 7.86. Found: C. 57.74; H, 7.73.
CpW(CAd)(CH 2 CMe 3 )C1 (10). Neopentylmagnesium chloride (0.108 ml, 2.97 M in
ether, 0.321 mmol) was added to a stirring solution of CpW(CAd)C12 (0.150 g,0.321 mmol) in
ether (10 ml) at -40 OC. The solution immediately turned red and turbid. After warming to room
temperature for 15 min, dioxane (0.028 mL, 0.32 mmol) was added and the volatiles were
removed in vacuo. After tituration with pentane, the residue was extracted with pentane (10
mL), filtered, and concentrated to dryness. The resulting red powder was recrystallized from
pentane at -40 'C to yield 0.123 g (76%) of red microcrystals in two crops. 1H NMR(C 6 D6 ) 6
5.521 (s, 5, Cp), 4.789 (d, 2 JHH = 10.3, 2 JWH= 4.1, 1, CH 2 CMe 3 ), 1.945 (br, 3, Ad), 1.811 (br.
6, Ad), 1.561 (br, 6, Ad), 1.181 (s, 9, CH 2 CMe 3 ), -0.454 (d,
CH2CMe 3 );
13 C
2 JHH
= 10.3, 2 JWH = 13.0. 1.
NMR 6 313.6 (Jcw = 242, CAd), 101.80 (Cp), 98.08 (Jcw = 93.8, CH2 CMe3).
51.74 (CAd), 45.04 (Ad), 36.88 (Ad), 36.49 (CCMe 3 ), 33.51 (CH2 CMe 3 ), 29.25 (Ad). Anal.
Calcd for C2 1H3 1ClW: C, 50.17; H, 6.21. Found: C, 50.24; H, 6.01.
Partial conversion of 9 to CpW(CCMe 3)(CH 2 CMe3 )(CH 2 Ad) (11). 9 (0.050 g, 0.093
mmol) was dissolved in benzene-d6 (0.60 mL) and a small amount of C6 H6 was added as an
internal standard. The solution was transferred to a Teflon-sealed NMR tube and monitored
periodically over 2 1/2 days. After 48 h an equilibrium mixture of 9 and 11 was reached in the
ratio of 1 / 2.1 (1) with minimal total sample decomposition. After 55 h, the solution was added
to a solution of 2,6-lutidinium chloride (0.030 g, 0.21 mmol) in dichloromethane (3 mL). After
standing overnight, the volatiles of the deep purple solution were removed in vacuo and the
resulting residue was extracted with ether and filtered through Celite to remove unreacted 2.6lutidinium chloride. After concentrating to dryness in vacuo for 30 min to remove 2.6-lutidine.
the purple residue was dissolved in benzene-d 6 to give a 2.0 / 1 ratio of CpW(CCMe 3 )C12 and
CpW(CAd)C12 as measured by integrating the corresponding 1H Cp resonances at 8 5.6314 and
5.69 ppm, respectively. NMR data for 11:
1H
NMR(C 6 D6 ) 8 5.375 (s, 5, Cp), 3.559 (d. 2 JHH =
11.7, 1, CH 2 ), 3.279 (d, 2 JHH = 12.3, 1, CH2), 2.113 (m, 3, Ad), (br d of t, 3, Ad), 1.772 (t. 6
Ad), 1.623 (br d of t, 3, Ad), 1.354 (CMe 3 ), 1.273 (CMe3 ), -0.886 (dd, 2 JHH = 12.3 Hz,. 4 JHH =
2.6, 1, CH 2 ), -1.502 (dd, 2 JHH = 11.7, 4 JHH = 2.6, 1, M12 );
13C
NMR 8 299.62 (CAd), 100.39
(Cp), 94.98 (Jcw = 93.5, CH 2 ), 87.78 (Jcw = 95.6, CH 2), 49.74 (CAd), 47.21 (CAd). 37.84
(Ad), 35.57, 33.48 (CMe3 ), 30.42 (Ad).
Partial conversion of 10 to CpW(CCMe3 )(CH 2 Ad)CI (12). 10 (0.072 g, 0.093 mmol)
was dissolved in benzene-d6 (0.60 mL) and a small amount of C6 H6 was added as an internal
standard. The solution was transferred to a Teflon-sealed NMR tube and monitored periodically
over 3 weeks while standing at room temperature. After 14 days an equilibrium mixture of 10
and 12 was reached in the ratio of 1 / 1.05 (4) with minimal sample decomposition. After three
weeks, quenching with 2,6-lutidinium chloride as described for 9 and 11 gave a 1.0 (1) / 1 ratio
of CpW(CCMe 3 )C12 and CpW(CAd)C12 . NMR data for 12: 1H NMR(C 6 D6 ) 8 5.497 (s, 5, Cp).
4.721 (d, 2JHH = 9.9 Hz, 2 JWH = 4.4 Hz, 1, CH 2 Ad), 2.052 (br, 3, Ad), 1.801 (br d of m, 6, Ad),
1.707 (br, 6, Ad), 1.407 (br d of m, 3 Ad), 1.232 (s, 9, CCMe3 ), -0.632 (d, 2 JHH = 9.9, 2 JWH =
12.8, 1, CH 2 Ad);
13 C
NMR 8 312.8 (JCW = 242, CAd), 101.89 (Cp), 100.58 (CH 2 Ad), 49.35
(CAd), 46.15 (Ad), 38.62 (CCMe3), 37.37 (Ad), 32.99 (CH2 CMe3 ), 29.96 (Ad).
CpW(CAd)(NMe 2 )CI (13). TMS-NMe 2 (60 itL, 0.37 mmol) was added to a solution of
CpW(CAd)C1 2 (0.160 g, 0.343 mmol) at -40 OC in dichloromethane (5 mL). The solution turned
from brilliant purple to light yellow over 15 minutes. After stirring for lh, the volatiles were
removed in vacuo and the residue extracted with ether (10 mL) and the extracts were filtered.
Concentrating and cooling to -40 OC afforded 0.142 g (87%) of yellow crystals. An analytical
sample was recrystallized from ether at -40 "C: 1H NMR(C 6 D6 ) 8 5.726 (s, 5, Cp), 4.251 (s, 3.
NMe2 ), 3.040 (s, 3, NMe2 ), 1.933 (br, 3, Ad), 1.776 (d, 6, Ad), 1.565 (t, 6, Ad);
302.3 (Jew = 257.7, CAd),
13 C{ 1H}
NMR 8
102.2 (Cp), 71.1 (NMe2 ), 57.1 (NMe2), 52.5 (JCw = 36.4. CAd).
44.2 (Ad), 37.0 (Ad), 29.0 (Ad). Anal. Calcd for C 18 H2 6 NClW: C, 45.45; H, 5.50; N, 2.95.
Found: C, 45.50; H, 5.37;N, 2.72.
CpW(CAd)(NMe 2 )(CH 2 CMe 3 ) (14). Neopentyllithium (0.012 g, 0.15 mmol) in ether
(1 ml) was added with stirring to CpW(CAd)(NMe2)CI (0.060 g, 0.13 mmol) in ether (3 ml) at
-40 OC. The solution became immediately cloudy. After standing 6h at -40 "C, the mixture was
concentrated to dryness and then titurated with dichloromethane to destroy any excess
neopentyllithium. After tituration with pentane, the residue was extracted with pentane, filtered
and the extracts were concentrated to dryness. Recrystallization from pentane at -40 OC afforded
0.045 g (70%) of orange crystals.
1H
NMR(CD 2 Cl 2 ) 8 5.773 (s, 5, Cp), 4.101 (s, 3, NMe2).
3.156 (s, 3, NMe 2), 1.957 (br, 3, Ad), 1.765 (d, 6, Ad), 1.69 (AB, 2 JWH = 11.7 Hz, 1, CH 2 CMe3).
1.628 (t, 6, Ad), 1.53 (AB, 2 JWH = 8.7 Hz, 1, CH 2 CMe 3 ), 1.016 (s, 9, CH 2 CMe 3 ); 13 C{IH}
NMR(C 6 D6 ): 5 291.72 (Jcw = 254.7 Hz, CAd),
100.63 (Cp), 71.00 (NMe 2), 56.27 (NMe 2 ).
52.27 (CAd), 48.60 (Jcw = 118.6, CH 2 CMe3 ), 45.15, 37.46 (Ad), 35.52 (CH 2 CMe3), 35.11
(CH 2 CMe3 ), 29.63 (Ad). Anal. Calcd for C23 H37 NW: C, 54.02; H, 7.29; N, 2.74. Found: C,
54.18; H, 7.51;N, 2.62.
CpW(CAd)(NHBut)CI (15).
TMS-NHBut (0.195 g, 1.46 mmol) was added to
CpW(CAd)C12 (0.500 g, 1.07 mmol) in dichloromethane (15 ml). After 30 minutes the solution
was yellow at which time the volatiles were removed in vacuo. The residue was extracted with
ether, the extracts were filtered, concentrated and cooled to yield 0.463 g (88%) of yellow
crystals.
1H
NMR(CD 2 Cl 26) 9.395 (br, 1, NH), 6.077 (s, 5, Cp), 1.999 (br, 3, Ad), 1.785 (d, 6.
Ad), 1.636 (t, 6, Ad), 1.386 (s, 9, NHBut);
13 C
NMR 8 303.54 (CAd), 103.01 (Cp), 59.53
(NCMe3), 52.72 (CAd), 44.49 (Ad), 37.05 (CMe 3 ), 33.97, 29.36 (Ad); IR(nujol/ KBr) 3249
v(NH) cm- 1. Anal. Calcd for C20 H30 NClW: C, 47.69; H, 6.00; N, 2.78. Found: C, 48.04; H,
6.05; N, 2.67.
anti-CpW(NCMe 3 )(CHAd)CI (17). A saturated solution of 15 in dichloromethane-d2
(ca. 0.020 g / 0.60 mL) was allowed to stand in a Teflon-sealed NMR tube for 11 days. After 3
days 1H resonances for 15 had completely disappeared resulting in a mixture of 16 and 17. After
10 days complete tautomerization to 17 had occurred with minimal (<5%) sample decomposition
as measured by a CH 2 C12 internal standard: 1H NMR(C 6 D6 ) 8 5.497 (s, 5, Cp), 4.721 (d, 2 JHH =
9.9 Hz, 2 JWH = 4.4 Hz, 1, CH 2 Ad), 2.052 (br, 3, Ad), 1.801 (br d of m, 6, Ad), 1.707 (br, 6. Ad).
1.407 (br d of m, 3 Ad), 1.232 (s, 9, CCMe3 ), -0.632 (d, 2 JHH = 9.9, 2 JWH = 12.8.
1. CH2Ad):
13 C NMR 8 274.08 (Jcw = 154.7 (in CDC13), CAd), 103.82 (Cp), 69.81 (NCMe 3 ). 46.65 (Ad).
45.82 (CAd), 37.40 (Ad). 31.77 (CMe3 ), 30.23 (Ad).
References
(1) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711.
(2) Comprehensive OrganometallicChemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Ed.:
Pergamon Press: Oxford, 1982; Vol. 3.
(3) Devore, D. D.; Lichtenhan, J. D.; Takusagawa, F.; Maatta, E.A. J. Am. Chem. Soc. 1987,
109, 7408.
(4) Preuss, F.; Becker, H.; Hdiusler, H.-J. Z. Naturforsch. 1987, 42b, 881.
(5) Williams, D. N.; Mitchell, J. P.; Poole, A. D.; Siemling, U.; Clegg, W.; Hockless, D. C. R.;
O'Neil, P. A.; Gibson, V. C. J.Chem. Soc., Dalton Trans. 1992, 739.
(6) Poole, A. D.; Williams, D. N.; Kenwright, A. M.; Gibson, V. C.; Clegg, W.; Hockless. D. C.
R.; O'Neil, P. A. Organometallics1993, 12, 2549.
(7) Coles, M. P.; Gibson, V. C.; Clegg, W.; Elsegood, M. R. J.; Porrelli, P. A. J.Chem. Soc.,
Chem. Commun. 1996, 1963.
(8) Nugent, W. A. Inorg. Chem. 1983, 22, 965.
(9) Schoettel, G.; Kress, J.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1989, 1062.
(10) Fox, H. H.; Yap, K. B.; Robbins, J.; Cai, S.; Schrock, R. R. Inorg. Chem. 1992, 31, 2287.
(11)
Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.; O'Regan. M. J. Am.
Chem. Soc. 1990, 1990, 3875.
(12) Schrock, R. R.; DePue, R. T.; Feldman, J.; Yap, K. B.; Yang, D. C.; Davis, W. M.; Park. L.:
DiMare, M.; Schofield, M.; Anhaus, J.; Walborsky, E.; Evitt, E.; Krtiger, C.; Betz, P.
Organometallics1990, 9, 2262.
(13) Herrmann, W. A.; Bogdanovic, S.; Priermeier, T.; Poli, R.; Fettinger, J. C. Angew. Chem.
Int. Ed. Engl. 1995, 34, 112.
(14) Churchill, M. R.; Ziller, J. W.; McCullough, L.; Pedersen, S. F.; Schrock, R. R.
Organometallics 1983, 2, 1046.
(15) Holmes, S. J.; Schrock, R. R. Organometallics1983, 2, 1463.
(16) Churchill, M. R.; Li, Y.-J.; Blum, L.; Schrock, R. R. Organometallics1984, 3,109.
(17) Brintzinger, H. H.; Fischer, D.; Mtillhaupt, R.; Rieger, B.; Waymouth, R. M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1143.
(18) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359.
(19) Caulton, K. G.; Chisholm, M. H.; Streib, W. E.; Hue, Z. J. Am. Chem. Soc. 1991, 113,
6082.
(20) Schrock, R. R.; Listemann, M. L.; Sturgeoff, L. G. J. Am. Chem. Soc. 1982, 1982, 4291.
(21) Listemann, M. L.; Schrock, R. R. Organometallics 1985, 4, 74.
(22) Freudenberger, J. H.; Schrock, R. R. Organometallics 1985, 4, 1937.
(23) Pederson, S. Thesis, Massachusetts Institute of Technology, 1983.
(24) Brookhart, M.; Green, M. L. H.: Wong. L.-L. Prog. Inorg. Chem. 1988, 36. 1.
(25) Xue, Z.; Li, L.; Hoyt, L. K.; Diminnie, J. B.; Pollitte, J. L. J. Am. Chem. Soc. 1994, 116.
2169.
(26) Friebolin, H. "Basic One- and Two-Dimensional NMR Spectroscopy"; 2nd ed.; VCH
Publishers: New York, 1993.
(27) Etienne, M. Organometallics1994, 13, 410.
(28) Etienne, M.; Biasotto, F.; Mathieu, R.; Templeton, J. L. Organometallics1996, 15. 1106.
(29) Etienne, M.; Mathieu, R.; Donnadieu, B. J. Am. Chem. Soc. 1997, 119, 3218.
(30) Procopio, L. J.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 1994, 116, 177.
(31) Barfield, M. J. Chem. Phys. 1964, 41, 3825.
(32) Jackman, L. M.; Sternhell, S. "Applications of Nuclear Magnetic Resonance Spectroscopy
in Organic Chemistry"; 2nd ed.; Pergamon Press: Oxford, 1969.
(33) Wiberg, K. B.; Lowry, B. R.; Nist, B. J. J. Am. Chem. Soc. 1962, 84, 1594.
(34) Meinwald, J.; Lewis, A. J. Am. Chem. Soc. 1961, 83, 2769.
(35) Meinwald, J.; Meinwald, Y. C. J. Am. Chem. Soc. 1963, 85, 2514.
(36) Nugent, W. A.; Mayer, J. A. "Metal-Ligand Multiple Bonds"; Wiley-Interscience: New
York, 1988, p 188.
(37) Churchill, M. R.; Youngs, W. J. Inorg. Chem. 1979, 18, 2454.
(38) Chisholm, M. H.; Huffmann, J. C.; Klang, J. A. Polyhedron 1990, 9, 1271.
(39) Fellmann, J. D.; Rupprecht, G. A.; Wood, C. D.; Schrock, R. R. J. Am. Chem. Soc. 1978.
100, 5964.
(40) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729.
(41) Rocklage, S. M.; Schrock, R. R.; Churchill, M. R.; Wasserman, H. J. Organometallics
1982, 1,1332.
(42) Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell, I. P.; Huffman, J. C.
Organometallics1985, 4, 902.
(43) Pedersen, S. F.; Schrock, R. R. J. Am. Chem. Soc. 1982, 104, 7483.
(44) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98.
(45) Schrock, R. R. in "Reactions of Coordinated Ligands"; P. S. Braterman, Ed.: Plenum Press:
New York, 1986; Vol. 1; p 221.
(46) Schrock, R. R.; Sancho, J.; Pedersen, S. F. Inorg. Syn. 1989, 26, 44.
CHAPTER 2
Synthesis of Group 4 and 5 dO Borylamido and Borylimido Complexes
Introduction
Since Wiberg first proposed in 1948 that the isoelectronic relationship between borylamines
(R2 N-BR' 2 ) and alkenes could lead to partial double bond character in the N-B linkage,' a variety
The parent
of studies have conclusively demonstrated pi - px bonding in borylamines.
borylamine, H2 N-BH 2 , is a planar molecule structurally resembling ethylene, possessing an N-B
distance (1.39
A)2
only 0.06 A longer than the C-C distance (1.33 A)3 found in ethylene itself
(Figure 2.1). Consistent with it-donation of the nitrogen lone pair into the empty boron acceptor
orbital, structural studies of substituted monoborylamines also show planar nitrogen and boron
units with small twist angles between the N and B planes, provided that severe steric crowding is
not present.4 -6 In further analogy with alkenes, unsymmetrically tetrasubstituted borylamines
exhibit cis-trans isomerism. 7 .8 Thermodynamic mixtures, however, are normally observed in
solution at room temperature since the barrier to isomerization, or rotation about the N-B bond
(AG-rot), generally lies in the range of 18 - 25 kcal/mol as established by variable temperature
NMR studies. 9- 12 The availability of a low-laying acceptor orbital on the N-boryl substituent thus
distinguishes borylamines as a special class of amines.
Figure 2.1. Electronic and structural analogy between borylamine and ethylene.
I
H
H
H1.39
H
N
B
borylamine
C
C
ethylene
The dative N->B n-interaction present in borylamines results in important structural
consequences in divalent first row transition metal complexes based on borylamides. Whereas
homoleptic, divalent first row complexes of the voluminous monoanionic [N(SiMe 3 )2 ]- ligand are
dimeric, 13- 15 Power et al. have prepared a series of two-coordinate and quasi two-coordinate
complexes of the first row transition metals utilizing sterically demanding borylamides of the type
[NArBMes 2 ]- (Figure 2.2).16 - 18
The lower availability of the nitrogen lone pair in these
borylamido complexes is believed to disfavor dimeric structures bridged through the amido
nitrogen donors, leading to a family of crystallographically characterized monomeric complexes.
Figure 2.2. Two- and quasi-two-coordinate first row bis(borylamido) complexes.
Mes
Mes-
M = Cr, Mn, Fe, Co, Ni
Ar
B
N-M-N
Ar = Ph, Mes
Ar
B-
Mes
(Mes = 2,4,6-Me 3C6H2)
Mes
In the arena of dO chemistry, not only would borylamido and borylimido complexes be
expected to exhibit a reduced tendency towards oligomerization, but the presence of the boronbased acceptor orbital should attenuate the ability of a nitrogen lone pair to engage in x-donation to
an empty metal orbital of appropriate symmetry (Figure 2.3). Early transition metal complexes
based on borylamide and borylimido ligands may thus result in metal centers more electrophilic
than those supported by more traditional organoamido and imido ligands. Furthermore, variable
temperature NMR spectroscopy may offer the opportunity to qualitatively judge the extent of the
borylamido N->M ix-interaction. Such a competition for the nitrogen lone pair between the metal
and boron acceptor orbitals as shown in Figure 2.3a might be expected to lower the barrier to N-B
bond rotation, analogous to the reduction in AG-rot observed for diborylamines [(R2 B) 2 NR']
which possess two competing acceptor orbitals. 19
Figure 2.3. in-Orbital interactions available to do metalloborylamides and metalloborylimides.
M
N
B
(a) do metalloborylamide
R
\\R
R
R
M
N
B
(b) do metalloborylimide
A small number of do borylamide complexes have been previously reported, all prepared
by the interaction of borylimines, RB=NR',20 ,21 with reactive group 4 and 5 functionalities. For
example, hydrozirconation of tert-butylimino-2,2,6,6-tetramethylpiperdinoboraneresulted in
zirconium-nitrogen bond formation as well as a P-agostic B-H-Zr interaction. 22 Addition of tBu(Me 3 Si)N-BEN-t-Bu across one M-C bond of the group 4 benzyne complexes Cp 2 M(C6H4) (M
= Ti, Zr) gave five-membered metalacycles containing the M-N-B unit, 23 while a related fourmembered metalacycle resulted from the 2+2 cycloaddition of i-BuNEB-i-Bu with the Ta=C double
bond of CpTaC12 (CHCMe 3 ).2 4
Due to the importance of amides 25-27 and imides 28 as ancillary ligands and imides as
reactive functionalities 29 -34 in early transition metal organometallic chemistry, do group 4 and 5
complexes based on the [NHBMes 2 ]- ligand were targeted in the broader goal of determining the
suitability of borylamides and borylimides as ancillary ligands in dO chemistry. In addition to
possessing simple but sterically demanding mesityl boryl substituents that were expected to
facilitate the isolation of these complexes and their study by NMR spectroscopy, this primary
borylamide ligand was also chosen for its N-H functionality which may allow access to borylimido
complexes through ox-H abstraction reactions.28
Results
Synthesis and NMR Spectra of Group 4 Borylamide Complexes.
The homoleptic borylamides M(NHBMes 2 )4 (M = Zr (1), Hf (2)) may be prepared from MC14 and
2 eq. of the dimeric [(Et 2 0)LiNHBMes 2 ]2 35 in toluene as shown in eq. 1. Recrystallization from
ether affords complexes 1 and 2 as colorless crystals in 60 - 70 % yield.
MC14 + 2 [(Et 2 0)LiNHBMes 2] 2
toluene
M(NHBMes 2) 4
1 M=Zr
2 M=Hf
(1)
The smaller size of titanium relative to its heavier congeners results in the isolation of the
tris(borylamide) Ti(NHBMes 2 )3 C1 (3) as pale yellow microcrystals in 63 % yield as shown in eq.
2.
TiCl4 + 3/2 [(Et 2O)LiNHBMes 2] 2
1H and
13 C
toluene
Ti(NHBMes 2) 3C1
3
(2)
NMR spectra of 1-3 in bromobenzene-d5 at low temperature (-15 'C) display
resonances indicative of two inequivalent mesityl groups as illustrated by the 1H NMR spectra of 3
which appear in Figure 2.4. In analogy to other structurally characterized borylamine 4 -6 and
metalloborylamide complexes, 16- 18 complexes 1-3 are expected to contain planar M-NHBMes2
units in which one mesityl group is cis and the other trans to the M-N linkage (Figure 2.4; inset).
The most downfield o-Me resonance shown in Figure 2.4 exhibits considerable broadening at low
temperature, consistent with hindered rotation about the B-Cipso bond of this mesityl group.36
Presumably this is the mesityl group cis to the Ti-N bond which experiences a more crowded
environment than the one trans to the metal center. Upon warming (70 - 80 'C),
these six
resonances coalesce into three, demonstrating that fast rotation about the N-B bond occurs on the
NMR timescale at elevated temperatures. Table 2.1 collects the activation parameters AG rot for
N-B bond rotation for compounds 1-3 obtained by monitoring the coalescence of the mesityl nm-H
1H
NMR signals.
Table 2.1. Barriers AG rot for N-B Bond Rotation in Complexes 1-3 (bromobenzene-d5).
Complex
AGtrot (kcal/mol)
Tc (K)
Zr(NHBMes 2 )4
15.1 (2)
315 (1)
15.8 (2)
330 (1)
15.0 (1)
302 (1)
(1)
Hf(NHBMes 2 )4
(2)
Ti(NHBMes 2 ) 3C1
(3)
Figure 2.4. 1H NMR (300 MHz) spectra of 3 at -15 OC (bottom) and 80 'C (top) in
bromobenzene-d5. Included is an inset showing a structural representation of 3.
o-Me --
m-H
p-Me
N-H
I
k-- 9
7
B
j
6
4
5
J
3
T-7
22 PPM
Cl
T
Mes 2BHN\
Mes 2BHN
o-Me -
(3)
p-Me
m-H
N-H
--I
o-Me
*i
A
a-
/\
1A
I _ 1 1 _ Ir
_ _r_
5
_ I I
i
T r 1_ _I
* denotes residual proteo impurities of bromobenzene-d 5 .
60
1 I r_ _ _
4
ii, I
k
3
2 PPM
Preparation of Group 4 and 5 Borylimido Complexes.
The primary borylamido complexes 1 and 2 eliminate H2 NBMes 2 upon reaction with
excess pyridine under forcing conditions to provide the yellow borylimido complexes
M(=NBMes 2 )(NHBMes2) 2py 2 (M = Zr (4), Hf (5)) (eq 3).
M(NHBMes 2) 4
pyridine
pyridine
90 oC, 24 h
M(=NBMes2)(NHBMes2)2PY 2 + H2NBMes 2
(3)
4 M=Zr
5 M=Hf
Although the reactions proceed reasonably cleanly as monitored by 1H NMR in pyridine-d5.
isolation of the borylimido complexes from H2 NBMes 2 by crystallization limits the recovery of 4
and 5 in pure form to 45 - 60%. In addition to two equivalent pyridine ligands, 1H NMR spectra
of 4 and 5 show three sets of mesityl signals of equal intensity at room temperature. Warming
through 80 oC results in the coalescence of two of the three sets of mesityl resonances resulting
from free rotation about the borylamide N-B bonds. Consistent with their NMR spectra and in
analogy to the previously structurally characterized M(=NAr)(NHAr) 2 (py') 2 (M = Zr. Hf: Ar =
2,6-diisopropylphenyl; py' = 4-pyrrolidinopyridine),
37 ,3 8
the proposed structure of 4 and 5 is
shown in Figure 2.5a.
oa-H abstraction also produces a related tantalum imido complex. Reaction between TaCl 5
and 5/2 eq [(Et 2 0)LiNHBMes 2 ]2 in toluene produces a mixture of Ta(=NBMes 2 )(NHBMes2) 3 (6)
and H2NBMes 2 (eq 4) from which 6 may be isolated as colorless crystals in 64% yield from ether.
TaCl 5 + 5/2 [(Et20)LiNHBMes 2] 2
Although the room temperature
1H
toluene
t
- H 2NBMes 2
Ta(=NBMeS2)(NHBMes 2 )3
(4)
NMR (300 MHz) spectrum of 6 shows several broad
resonances and not is not readily interpretable, warming to 90 'C significantly simplifies its
appearance. At this temperature two sets of mesityl resonances in a 3:1 ratio are observed along
with a resonance of relative area 3 corresponding to the borylamide ca-N-H units. Consistent with
these observations and the spectra of 1 - 5 above, the proposed low temperature structure of 6 is
shown in Figure 2.5b.
Figure 2.5. Proposed structures of borylimido complexes 4 - 6.
Mes
BMes
H
N
Mes
I
\
/
Mes
I
H
],P
N
II
N-M-N
\
Mes-B PY
Mes
B
B-
Mes
,
Mes 2BHN
Mes 2BHN
N/Ta
N
/B
Mes
Mes
(b)
(a) M=Zr(4)
M = Hf (5)
H
Mes
6
Origin of the Decreased Barriers to Rotation.
The do borylamide complexes 1-3 exhibit barriers to rotation AG rot significantly (ca. 10
kcal/mol) lower than reported for similar, non-coordinated borylamines such as HNRBMes 2 (R =
Me, Ph) 3 7 '38 and Mes 2 BNHCH 2 CH 2 NHBMes 2 39 which were found to be in the vicinity of 25
kcal/mol. Interaction with a group 4 metal thus causes important effects on the nature of the N-B
linkage, and we considered two possible explanations.
Within the M-NH-BMes 2 unit, overlap between an empty metal d orbital and the nitrogen
lone pair may result in a competition for the lone pair between the metal and boron (Figure 2.3.
2.6a). This competition would weaken the ir-interaction between the N and B centers and
consequently would lead to a decrease in the barrier to N-B bond rotation. This situation would be
analogous to that in di(boryl)amines,
19
where two boron atoms compete for the lone pair of the
central nitrogen (Figure 2.6b). In the opposite sense, similarly reduced barriers (AG rot = 10 - 12
kcal/mol) are also observed in non-sterically encumbered di(amino)boranes where two nitrogen
lone pairs compete for donation into a central boron acceptor orbital (Figure 2.6c). 40,4 1
Figure 2.6. Orbital analogies between do borylamides, bis(boryl)amines, and
bis(amino)boranes.
M
N
B
B
N
(a)
B
N
B
(b)
N
(c)
The reduced barriers to N-B bond rotation observed in the do metalloborylamides 1 - 3
may also be a reflection of a more sterically crowded environment in the borylamide complexes
than exists in the free borylamines they are being compared against. Excessive steric interactions
between nitrogen and boron substituents favor twisting the N-B bond out of planarity to relieve
steric strain, thus reducing the N-B pin-prt overlap. 4 0 .4 1 Figure 2.7 illustrates how steric
interactions in complexes 1 - 3 could lead to both a destabilization of planar N-B units and a
corresponding reduced barrier to N-B rotation.
Figure 2.7. Newman projection down one B-N bond in metalloborylamide complexes 1 - 3.
e•MVIfe
Hý4
B
H
Mes
N
M(NHBMes 2 )xC13-x -
-I
AG+rot
4.oC
H-
M(NHBMes2)xCl 3-,
es
The nearly identical ionic radii of titanium(IV) and tin(IV) coupled with the many
similarities in their coordination chemistry involving simple a-type interactions 42 suggests that the
tin borylamide Sn(NHBMes 2 )3 C1 could serve as a model of the titanium complex 3 which has no
low-lying empty d orbitals available to interact with the borylamide it-system. Analogous to the
synthesis of 3, Sn(NHBMes 2 )3 CI (7) may be prepared as shown in eq 5 below and is isolated as
colorless crystals in 82 % yield from ether.
SnCl 4 + 3/2 [(Et20)LiNHBMes 2] 2
toluene
-
(5)
Sn(NHBMes2) 3 CI
7
In contrast to variable temperature 1H NMR spectra of 1-3, no considerable broadening of the six
mesityl resonances of 7 are observed up to 110 'C, allowing a lower limit of 22 kcal/mol for
AG rot to be estimated. The significantly higher barrier to rotation in 7, on par with those
observed for related free borylamines, strongly suggests that the lower values of 15 - 16 kcal/mol
observed for 1 - 3 result not from steric destabilization, but rather from competition between empty
metal d orbitals and boron for acceptance of the nitrogen lone pair.
Discussion and Conclusions
The
homoleptic
M(NHBMes 2 )4 (M = Zr (1), Hf (2)) and tris(borylamide)
Ti(NHBMes 2 )3 C1 (3) may be prepared through straightforward metathetical reactions between the
lithium borylamide [Li(OEt 2 )NHBMes 2 ]2 and the appropriate metal halide. Furthermore. the
primary borylamides 1 and 2 serve as precursors to group 4 borylimido complexes through uc-H
abstraction reactions.
In contrast to the very reactive group 4 [Cp 2 Zr(NR)] 3
1- 34
and
[Zr(=NSiBut 3 )(NHSiBut 3 )2 ]29 systems which are active towards C-H bond activation, the
reactivity of this borylamide based system is more closely related to systems based on bulky
arylimido ligands. 37,38,43 For example, no significant elimination of H2 NBMes 2 nor scrambling
29 ,3 1
of deuterons into the M-NHBMes 2 units occurs upon prolonged heating 1 or 2 in C6 D 6 .
Inducing elimination of H2NBMes 2 from 1 or 2 instead requires prolonged heating in pyridine.
particularly for the Hf derivative 2, representing far more forcing conditions than necessary for the
related arylimido system based on the [NHAr]- (Ar = 2,6-diisopropylphenyl) ligand. 37 ,38.4 3 On
the other hand, a-N-H abstraction in the tantalum based system proceeds much more readily, 28
affording Ta(=NBMes 2 )(NHBMes 2 ) 3 and H2 NBMes 2 upon reaction of TaC15 with 5/2 eq.
[Li(OEt 2 )NHBMes 2 ]2 .
The observation that 1 - 3 exhibit substantially decreased barriers to N-B bond rotation
than found in free borylamines and the presumably structurally related tin derivative 7
demonstrates the competition between empty metal based orbitals and boron for the acceptance of
the nitrogen lone pair in these do metalloborylamides. This finding serves to emphasize N->M 7tdonation as an important part of the do metal-amide bond,4 4-46 and further suggests that do early
transition metal complexes based on borylamide and borylimide ancillary ligands would be
considerably more electrophilic than their more traditional alkyl and aryl N-substituted
counterparts. Imide formation from 1 and 2 as well as in 6 demonstrates, however, that the
development of such an ancillary ligand system utilizing borylamide ligands requires the use of
secondary borylamides lacking the reactive N-H functionality.
Experimental
General Procedures. All experiments were performed under nitrogen in a Vacuum
Atmospheres drybox or under argon using standard Schlenk techniques.
All solvents were
purified by standard techniques while deuterated NMR solvents were dried and stored over
activated 4 A molecular sieves before use. [Li(OEt 2 )NHBMes 2 ]2 was prepared as described in the
literature. 35 HfC14 (<0.2 % Zr) was obtained from Cerac and all other metal halides were obtained
from Strem or Aldrich and used as received.
1H
and
13 C
spectra were recorded at 300 and 75.4 MHz, respectively. Proton spectra were
referenced internally by the residual solvent proton signal relative to tetramethylsilane. Carbon
spectra were referenced internally relative to the
tetramethylsilane.
13 C
signal of the NMR solvent relative to
I11B NMR spectra of all compounds were extremely broad and therefore not
reported. Coalescence temperatures in 1 - 3 were determined by carefully monitoring the onset of
coalescence and followed by measuring the probe temperature using a neat ethylene glycol
standard. IR spectra were recorded as Nujol mulls between KBr plates on a Perkin-Elmer 1600
FT-IR spectrometer.
Elemental analyses were performed on a Perkin-Elmer PE2400
microanalyzer in our laboratories.
Zr(NHBMes 2 )4 (1). Powdered ZrC14 (0.506 g, 2.17 mmol) was added to a chilled (-40
'C) solution of [(Et 2 0)LiNHBMes 2 ]2 (3.00 g, 4.34 mmol) in toluene (85 mL). The ZrCl 4
dissolved within 5 min as the mixture warmed to room temperature and after 15 min the reaction
mixture became cloudy again. After stirring overnight, the mixture was filtered through Celite and
the volatiles were removed in vacuo. Following titration with ether (2 x 15 mL), the powdery
residue was extracted with ether (200 mL) and filtered through Celite. Concentration and cooling
of this solution afforded colorless crystals of the product in two crops (1.89 g, 76% yield) which
were isolated by filtration. An analytical sample was obtained by recrystallization from ether: 1H
NMR(C 6 D5 Br, -15 OC) 6 6.717 (s, 3, NH and m-H), 6.394 (s, 2, m-H), 2.347 (s, 6, o-Me).
2.222 (s, 3, p-Me), 2.136 (br, 6, o-Me), 1.833 (s, 3, p-Me);
13 C
NMR (+80 oC) 8 139.71 (Co),
137.36 (Cp), 128.42 (Cm), 23.33 (o-Me), 20.81 (p-Me); IR(Nujol/KBr) 3268 cm- 1 v(NH); Anal.
Calcd for C72 H92 B4 N4 Zr: C, 75.33; H, 8.08; N, 4.88. Found C, 75.14; H, 8.18; N, 5.07.
Hf(NHBMes 2 )4 (2). Solid [(Et20)LiNHBMes 2 ]2 (0.800 g, 1.16 mmol) was added to
a chilled (-40 °C), stirring suspension of powdered HfC14 (0.186, 0.579 mmol) in toluene (20
mL). After ca. 10 min the HfC14 went into solution and after ca. 30 min the reaction mixture
became cloudy again. After stirring overnight, the mixture was filtered through Celite and the
volatiles were removed in vacuo. Following titration with ether (2 x 5 mL), the powdery residue
was extracted with ether (70 mL) and filtered through Celite. Concentration and cooling of this
solution afforded colorless crystals of the product in two crops (0.356 g, 64% yield) which were
isolated by filtration. An analytical sample was obtained by recrystallization from ether:
1H
NMR(C 6 D5 Br, -25 OC) 8 6.735 (br, 2, in-H), 6.394 (s, 2, m-H), 6.185 (br, 1, NH), 2.319 (s, 6.
o-Me), 2.244 (s, 3, p-Me), 2.18 (br, 6, o-Me), 1.815 (s, 3, p-Me);
13 C
NMR (+80 oC) 8 139.61
(Co), 137.34 (Cp), 128.40 (Cm), 23.35 (o-Me), 20.83 (p-Me); IR(Nujol/KBr) v(NH) 3273 cm- 1:
Anal. Calcd for C72 H 92 B 4 N4 Hf: C, 70.01; H, 7.50; N, 4.54. Found C, 70.28: H. 7.33: N.
4.38.
Ti(NHBMes 2 )3 CI (3). Solid [(Et20)LiNHBMes 2 ]2 (2.00 g, 5.79 mmol) was added to
a chilled (-40 °C), stirring solution of TiC14 (0.366 g, 1.931 mmol) in toluene (50 mL). As the
lithium amide dissolved, the initially orange solution gave rise to a yellow suspension. After
stirring overnight, the solution was filtered through Celite and concentrated to dryness. The
yellow residue was titurated with ether (5 mL), extracted with ether/dichloromethane (75 mL.
50/50), and the extracts were filtered through Celite.
Pale yellow crystals formed upon
concentrating (to ca. 10 mL) and cooling the filtrate to -40 OC which were isolated by filtration.
washed with pentane (5 mL) and dried in vacuo to give 1.06 g (63%) of the product. An analytical
sample was obtained by recrystallization from ether:
1H
NMR(C 6 D5Br, -15 OC) 8 9.374 (s, 1.
NH), 6.680 (s, 2, m-H), 6.547 (s, 2, m-H), 2.453 (br, 6, o-Me), 2.216 (s, 3, p-Me), 2.155 (s,
6, o-Me), 1.819 (s, 3, p-Me);
13 C
NMR (+80 oC) 8 140.93 (Co), 138.44 (Cp), 128.65 (Cm),
22.97 (o-Me), 20.79 (p-Me); IR(nujol/KBr):
v(NH) 3247, 3225 cm- 1 ; Anal. Calcd for
C54 H69 B 3 CIN 3 Ti: C, 74.04; H, 7.94; N, 4.80. Found C, 74.27; H, 7.71; N, 4.66.
Zr(NBMes 2 )(NHBMes 2 )2py2 (4).
Zr(NHBMes 2 )4 (0.237 mg, 0.228 mol) was
dissolved in pyridine (5 mL) and heated to 85 OC in a Teflon-sealed vessel. After 15 m.in the
solution turned light yellow. After 19 h the solution was removed from the heating bath and the
yellow-orange solution was concentrated to < 1 mL. Ether (10 mL) was added, and the resulting
yellow solution was filtered, concentrated to ca. 2 ml, and allowed to stand at -40 "C to afford
0.135 g (63%) of yellow crystals in 2 crops. An analytical sample was doubly recrystallized at
-40 'C from ether containing a few drops of pyridine:
1H
NMR(C 6 D6 ) 8 8.402 (d, 4. o-py).
6.793 (s, 4, in-H), 6.729 (s, 4, mn-H), 6.690 (t, 2. p-py), 6.511 (s, 4, m-H). 6.471 (s. 2. NH).
6.290 (t, 4, mn-py), 2.554 (s, 12, o-Me), 2.417 (br s, 12, o-Me), 2.248 (s, 6, p-Me), 2.176 (br.
12, p-Me), 2.147 (br, 12, p-Me);
13 C
NMR 8 151.23 (pyo), 143.95, 143.63 (br. amide Co).
140.32 (imide Cp), 140.07 (imide Co), 137.63, 136.18 (br, amide Cp), 135.02 (pyp), 128.63 (br.
amide Cm), 128.15 (imide Cm), 123.68 (pym), 23.63 (br, amide o-Me), 23.48 (imide o-Me).
21.22 (imide p-Me), 21.05 (amide p-Me). Anal. Calcd for C64 H78 N5B 3 Zr: C, 73.85; H, 7.55: N.
6.73. Found C, 73.75; H, 7.62; N, 6.54.
Hf(NBMes 2 )(NHBMes
2 )2 py 2
(5). Hf(NHBMes 2 )4 (0.238 mg, 0.193 mol) was
dissolved in pyridine (5 mL) and heated to 85 oC in a Teflon-sealed vessel. After ca. 3-4 h the
solution turned light yellow. After 42 h the solution was removed from the heating bath and the
yellow-orange solution was concentrated to < 1 mL. Ether (10 mL) was added, and the resulting
yellow solution was filtered, concentrated to ca. 2 ml, and allowed to stand at -40 'C to afford
0.097 g (45%) of yellow crystals in 2 crops. Subsequent crops consisted largely of H2NBMes 2 .
An analytical sample was doubly recrystallized at -40 'C from ether containing a few drops of
pyridine:
1H
NMR(C 6 D6 ) 8 8.476 (d, 4, o-py), 6.785 (s, 8, m-H), 6.636 (2, t, p-py), 6.461 (s.
4, mn-H), 6.263 (t, 4, m-py), 6.004 (s, 2, NH), 2.509 (s, 12, o-Me), 2.426 (s, 12, o-Me), 2.279
(s, 6, p-Me), 2.188 (s, 12, p-Me), 2.143 (s, 6, p-Me), 2.1159 (s, 6, p-Me).
13 C
NMR 8 151.31
(pyo), 146.55, 143.65 (br, amide Co), 140.27 (imide Cp), 140.07 (imide Co), 137.94, 136.40.
136.04 (amide Cp and imide Ci), 134.71 (pyp), 128.74, 128.48 (amide Cm), 128.21 (imide Cm),
123.79 (pym), 23.83, 23.601 (amide o-Me), 23.47 (imide o-Me), 21.28 (imide p-Me), 21.03
(amide p-Me). Anal. Calcd for C64 H78 N 5 B3 Hf: C, 68.13; H, 6.96; N, 6.21. Anal. Calcd for
C64 H 78 N5 B 3 Hf: C, 68.13; H, 6.96; N, 6.21. Found C, 68.43; H, 7.11; N, 6.15.
Ta(NBMes 2 )(NHBMes2)3 (6).
TaC15 (0.116 g, 0.324 mmol) was added to a
solution of [(Et 2 0)LiNHBMes212 (0.560 g, 0.811 mmol) in toluene (20 ml) chilled to -40 OC. The
solution was stirred for 16 hours over which time the initial yellow color faded. The solution was
filtered through Celite and the volatiles were removed in vacuo to leave a pale yellow solid. The
residue was extracted with ether / dichloromethane (30 mL, 50/50) which was then filtered through
Celite and concentrated. Cooling to -40 OC yielded 0.228 g (64%) of colorless crystals of the
product. An analytical sample was recrystallized from ether:
1H
NMR(C 6 D5 Br, +90 OC): 7.17
(br, 3, NH), 6.582 (s, 4, imidem-H), 6.544 (s, 12, amidem-H), 2.262 (s, 36, amideo-Me), 2.197
(s, 6, imidep-Me), 2.145 (s, 18, amideo-Me), 2.098 (s, 12, imideo-Me);
13 C
NMR (+90 `C) 8
140.57 (amide Co), 140.19 (imide Co), 128.65 (Cm), 137.75 (Cp), 128.68 (amide Cm), 128.06
(imide Cm), 23.52 (amide o-Me), 23.12 (imide o-Me), 21.11 (imide p-Me), 21.03 (amide p-Me):
IR(nujol/KBr): v(NH) 3213, 3172 cm-1; Anal. Calcd for C 72 H9 1 B4 N4 Ta: C, 69.93: H, 7.41; N.
4.53. Found C, 68.99; H, 7.55; N, 4.36.
Sn(NHBMes 2 )3 CI (7). Solid [(Et 2 0)LiNHBMes 2 ] 2 (0.819 g, 2.372 mmol) was
added to a chilled (-40 'C), stirring solution of SnC14 (0.206 g, 0.791) in toluene (50 ml). The
flocculent lithium amide soon dissolved to give a colorless fine suspension which was allowed to
warm to room temperature. After stirring overnight, the solution was filtered through Celite and
concentrated to dryness. The resulting white residue was titurated with ether (5 mL). extracted
with ether/dichloromethane (70 mL, 50/50), and the extracts filtered through Celite. Colorless
crystals formed upon concentrating (to ca. 10 mL) and cooling the filtrate to -40 'C which were
isolated by filtration, washed with pentane (5 mL), and dried in vacuo gave 0.616 g (82%) of the
product. An analytical sample was obtained by recrystallization from ether:
1H
NMR(C 6 D5 Br, 25
OC) 6 6.705 (s, 2, in-H), 6.558 (s, 2, mn-H), 3.641 (2 JSnH = 30.0 Hz. 1, NH), 2.382 (s, 6. o-
Me), 2.205 (s, 3, p-Me), 2.169 (s, 6, o-Me), 1.782 (s, 3, p-Me);
13 C
NMR 8 141.03, 140.26
(Co), 139.29 (Ci), 138.85 (Cp), 137.86 (Ci), 137.67 (Cp), 128.78, 128.40 (Cm), 22.98, 22.90
(o-Me), 21.19, 20.61 (p-Me); IR(nujol/KBr):
v(NH) 3329, 3324 cm-1; Anal. Calcd for
C54 H 69 N3 B 3 C1Sn: C, 68.51; H, 7.35; N, 4.44. Found C, 68.53; H, 7.27; N, 4.54.
References
(1) Wiberg, E. Naturwissenschaften 1948, 35, 182, 212.
(2) Sugie, M.; Takeo, H.; Matsumara, C. J. Mol. Spectrosc. 1987, 123, 286.
(3) Herzberg, G. "Molecular Spectra and Molecular Structure III. Electronic Spectra and
Electronic Spectra of Polyatomic Molecules"; Van Nostrand: New York, 1966.
(4) Bartell, L. S.; Clippard, F. B. Inorg. Chem. 1970, 9, 2439.
(5) Zettler, F.; Hess, H. Chem. Ber. 1975, 108, 2269.
(6) Mdinnig, D.; N6th, H.; Prigge, H.; Rotsch, A.-R.; Gopinathan, S.; Wilson, J. W. J.
Organometal. Chem. 1986, 310, 1.
(7) Niedenzu, K.; Dawson, J. W. J. Am. Chem. Soc. 1960, 82, 4223.
(8) Watanabe, H.; Nagasawa, K.; Totani, T.; Yoshizaki, T.; Nakagawa, T. Adv. Chem. Ser.
1964, 42, 108.
(9) Barfield, P. A.; Lappert, M. F.; Lee, J. Proc. Chem. Soc. 1961, 421.
(10) Friebolin, H.; Rensch, R.; Wendel, H. Org. Magn. Reson. 1976, 8, 287.
(11)
Brown, N. M. D.; Davidson, F.; Wilson, J. W. J. Organometal. Chem. 1981, 210, 1.
(12) Brown, C.; Cragg, R. H.; Miller, T. J.; Smith, D. O. N. J. Organomet. Chem. 1988, 342.
153.
(13)
BUirger, H.; Wannagat, U. Monatsh. Chem. 1963, 94, 1007.
(14) Btirger, H.; Wannagat, U. Monatsh. Chem. 1964, 95, 1099.
(15)
Olmstead, M. M.; Power, P. P.; Shoner, S. C. Inorg. Chem. 1991, 30, 2547.
(16) Chen, H.; Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Shoner, S. C. J. Am. Chem.
Soc. 1990, 112, 1048.
(17) Bartlett, R. A.; Feng, X.; Olmstead, M. M.; Power, P. P.; Weese, K. J. J. Am. Chem. Soc.
1987, 109, 4851.
(18) Bartlett, R. A.; Chen, H.; Power, P. P. Angew. Chem. Int. Ed. Engl. 1989, 28, 316.
(19)
Nath, H.; Vahrenkamp, H. J. Organomet. Chem. 1968, 16, 357.
(20) Paetzold, P. Adv. Inorg. Chem. 1987, 31, 123.
(21) N6th, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1603.
(22) Minnig, D.; N6th, H.; Schwartz, M.; Weber, S.; Wietelmann, U. Angew. Chem., Int. Ed.
Engl. 1985, 24, 998.
(23) Hebner, B.; Manners, I.; Paetzold, P. Chem. Ber. 1987, 120, 1065.
(24) Braunschweig, H.; Paetzold, P.; Boese, R. Chem. Ber. 1990, 123, 485.
(25) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(26) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997, 275, 1445.
(27) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
(28) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.
(29) Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T. J. Am. Chem. Soc. 1988, 110, 8731.
(30) Bennet, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1994, 116, 2179.
(31) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110, 8729.
(32) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc. 1992, 114. 1708.
(33) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1994, 116, 2669.
(34) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1995, 117, 974.
(35) Bartlett, R. A.; Chen, H.; Dias, H. V. R.; Olmstead, M. M.: Power, P. P. J. Am. Chem.
Soc. 1988, 110, 446.
(36) Gragg, B. R.; Layton, W. J.; Niedenzu, K. J. Organometal. Chem. 1977, 132, 29.
(37) Profilet, R. D.; Zambrano, C. H.; Fanwick, P. E.; Rothwell, I. P. Inorg. Chem. 1990. 29.
4362.
(38) Zambrano, C. H.; Profilet, R. D.; Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Polyhedron
1993, 12, 689.
(39) See Chapter 3
(40) Nelson, R. H.; Wells, R. L. Inorg. Chem. 1977, 16, 7.
(41) Beswick, Y. F.; Wisian-Neilson, P.; Neilson, R. H. J. Inorg. Nucl. Chem. 1981, 43.
2639.
(42) Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry"; Wiley-Interscience: New
York, 1988, p 652.
(43) Arney, D. J.; Bruck, M. A.; Huber, S. R.; Wigley, D. E. Inorg. Chem. 1992, 31, 3749.
(44) Andersen, R. A.; Beach, D. B.; Jolley, W. L. Inorg. Chem. 1985, 24, 4741.
(45) Hillhouse, G. L.; Bulls, A. R.; Santarsiero, B. D.; Bercaw, J. E. Organometallics1988, 7.
1309.
(46) Giolando, D. M.; Kirschbaum, K.; Graves, L. J.; Bolle, U. Inorg. Chem. 1992, 31, 3887.
CHAPTER 3
Synthesis of Group 4 Organometallic Complexes That Contain the
Bis(borylamide) Ligand [Mes 2 BNCH 2 CH 2 NBMes2] 2-
Much of the material presented in this chapter has already appeared in print:
Warren, T. H.; Schrock, R. R.; Davis, W. M. Organometallics1996, 15, 562-569.
Introduction
The tremendous industrial importance of the group 4 metallocenes in the Ziegler-Natta
polymerization of a-olefins elicits the development of new coordination environments inspired by
the basic structural features of do bent metallocene complexes. Initial studies largely focused on
cationic group 4 alkyl complexes supported by two cyclopentadienyl ligands or direct
cyclopentadienyl analogs such as indenyl or fluorenyl. Careful study of these first systems by
numerous workers has led to the identification of metallocene alkyl cations [Cp 2 M-R] + as the chain
propagating species as well as the delineation of important chain termination and transfer pathways
in Ziegler-Natta catalysis.1, 2 These findings form a sound framework on which the development
of new, non-metallocene group 4 alkyl cations for the polymerization of ca-olefins may be based.
Accordingly, complexes containing only one cyclopentadienyl ring (or cyclopentadienyl
analog) tethered to an amide have received increasing attention, 3-7 particularly since they offer the
prospect of producing syndiotactic polymers. 8 Focus on group 4 diamide complexes as templates
for Ziegler-Natta catalysis, on the other hand, has only recently commenced. 9 - 15 despite the
preparation of potential dialkyl pre-catalysts by Andersen almost 20 years ago.16-18
The chelating di(borylamido) ligand [Mes 2 BNCH 2 CH 2NBMes 2 ]2- would be a structurally
and electronically unusual bidentate diamido ligand, since two mesityl groups should define and
sterically protect two or three coordination sites in a plane roughly perpendicular to the NCCN
ligand backbone as a consequence of N-B 7r bonding. Furthermore, the strongly n-accepting boryl
groups should attenuate the ability of the nitrogen atoms to donate
in
electron density to the metal
center and therefore should yield complexes in which the metal is more electrophilic than in more
traditional alkyl and arylamide complexes. 5,6 ,16 ,17,19- 28 Therefore a combination of N-B
it
bonding and the steric bulk of the BMes 2 group should play a significant role in the chemistry of
group 4 complexes containing the [Mes 2 BNCH 2 CH 2 NBMes 2 ]2- ligand.
Results
Entry into Titanium and Zirconium Chemistry.
The di(borylamine), Mes 2 BNHCH 2 CH 2 NHBMes 2 [H2(Ben)], was prepared from lithium
ethylenediamide (generated in situ ) and two equivalents of dimesitylboron fluoride (eq 1).
Deprotonation of H2 (Ben) with butyllithium in the presence of MgBr 2 (ether) affords
(Ben)Mg(THF) 2 (eq 2). Both H2 (Ben) and (Ben)Mg(THF) 2 are white crystalline solids that were
prepared in 80 - 90% yield on a scale of typically 30 and 10 g, respectively.
H 2NCH 2 CH 2 NH 2
H2 Ben
1) 2 LiBu / THF
o2) 2 Mes 2BF
Mes 2 BNHCH 2 CH 2 NHBMes,
> 2LiBu, MgBr 2(ether)
N
THF
(1)
(Ben)Mg(THF) 2
(2)
Addition of MC14 (THF) 2 (M = Ti, Zr) to a dichloromethane solution of (Ben)Mg(THF)2
produces yellow-orange (Ben)TiCl2 (1) or colorless (Ben)ZrC12(THF) (2) (eq 3) in 81% and 71%
yields, respectively.
Room temperature proton NMR spectra of both 1 and 2 exhibit a
BenMg(THF) 2 + MC14 (THF) 2
CH2C12
BenTiCl,
1 M = Ti
2
(3)
(3)T
BenZrCl 2 (THF)
2 M=Zr
single sharp resonance for the methylene protons in the backbone as well as two sets of resonances
due to inequivalent mesityl groups. Inequivalent mesityl groups within each BMes2 unit are to be
expected since N-B ir-bonding 29,30 should favor a structure in which one mesityl ring of each
dimesitylboron unit lies roughly parallel to the MC12 plane and the other flanks the NCCN
backbone (Figure 3.1)
Figure 3.1. Proposed geometry of (Ben)MC12 complexes.
Upon warming a sample of (Ben)TiC12 to 90 0 C, the six mesityl proton resonances (17-H.
o-Me, p-Me) coalesce to give three as a consequence of rotation about the N-B bond. This
coalescence behavior allows the determination of AGtrot in 1 which was found to be 17.4 (2)
kcal/mol, significantly lower than the value of -25 kcal/mol found for other dimesitylborylamines
such as MeNHBMes2, PhNHBMes 2 ,31 and H2(Ben). (The fact that no coalescence of the closely
spaced mn-H, o-Me, and p-Me resonances of H2 (Ben) was observed by 1H NMR spectroscopy in
C6 D5Br at 150 0 C allowed us to estimate a lower limit of 23 kcal/mol for the barrier to N-B bond
rotation in H2(Ben).) The barrier to N-B bond rotation found for 1 is slightly higher than the
range of 15 - 16 kcal/mol observed for the related primary borylamide complexes
Ti(NHBMes 2 )3 C1 and M(NHBMes 2 )4 (M = Zr, Hf), 32 but otherwise consistent with competition
between the titanium center and boron for acceptance of the nitrogen lone pairs.
In contrast, free rotation of the N-B bond in 2 is not observed up to 100 "C, perhaps
reflecting the more crowded coordination environment of the THF adduct. The isolation of 2 as a
THF adduct, whereas 1 is isolated base-free, is consistent with the larger size of Zr and its often
higher coordination number in otherwise analogous Ti and Zr complexes. On the basis of X-ray
structures of Ben derivatives to be described later we assume that the THF in 2 is bound to the
metal in the ZrC12 plane between the two chloride ligands.
Preparation of Alkyl Derivatives.
Monoalkyl and dialkyl titanium derivatives can be prepared by treating (Ben)TiC12 in
dichloromethane at -40 0 C with Grignard reagents (eqs 4 and 5). Proton NMR spectra of
(Ben)TiC12 + RMgC1
CHzC12
(4)
(Ben)Ti(R)C1
CH22--
3a R = CH2Ph
3b R = CH 2 CMe 3
(Ben)TiC 12 + 2 RMgC1
CH,Cl2
O-
(5)
(Ben)TiR 2
4a R = CH 2 Ph
4b R = Me
unsymmetrically substituted 3a and 3b show four meta and six mesityl methyl resonances.
consistent with mirror symmetry and no rotation about B-N or mesityl B-Cipso bonds on the NMR
timescale. The AA'BB' pattern ascribed to the ethylene backbone is also indicative of two different
ligands in the coordination wedge opposite and orthogonal to this linkage. In contrast, proton
NMR spectra of 4a and 4b are identical in form to those of 1 and 2, exhibiting a sharp singlet for
the ligand backbone and are otherwise consistent with C2v symmetry.
The ortho proton
resonances of the benzyl ligands in both 3a and 4a are shifted upfield (to 5.756 and 6.101 ppm,
respectively, in C6 D 6 ), which suggests that the phenyl 7t systems may be interacting with the
titanium center. 33,34 The inference of such a it-interaction with the titanium center based solely on
chemical shifts, however, should be taken with caution, as the o-Ph 1 H resonances of the benzyl
ligands in 3a and 4a could be shifted upfield also as a consequence of the ring current of the
mesityl rings flanking the coordination wedge. Rotation of the N-dimesitylboryl units in 4a and
4b is qualitatively slower than in 1, presumably as a consequence of both the increased steric
demands of al
ligands in the coordination wedge and the less electrophilic nature of the metal in
4a or 4b and consequently reduced competition for the nitrogen's ni electron pairs.
X-ray Structure of (Ben)Ti(CH 2Ph)C1 (3a).
A drawing of the X-ray structure of 3a can be found in Figure 3.2 and relevant distances
and angles in Table 3.1. The Ben ligand nearly symmetrically chelates the titanium center (Ti-N
distances = 1.894 (6) and 1.912 (6) A) to form a puckered five-membered ring with a "bite" angle
(N-Ti-N) of 89.2 (2)0. The Ti-N distances are comparable to those found in other four-coordinate
, 2,6 3 5 which are in the 1.882 - 1.940 A range. The coordination
amido complexes, 1 9'20 ,22
geometry at both B and N is trigonal planar and the dihedral angle between the planes containing B
and N is small (e.g., the dihedral angle C(2)-N(1)-B(2)-C(21) is 130), consistent with substantial
B-N it bonding. A significant degree of N-B rt bonding is also suggested by N-B distances of
1.419 (9) and 1.43 (1) A. Note, however, that these N-B distances are longer than N-B distances
(1.36-1.41
A) generally found in structurally characterized monoborylamines and
metalloborylamides, 29 ,30 ,35-4 1 perhaps as a consequence of titanium's competing with the boryl
groups for the nitrogen lone pairs. Puckering of the ethylene backbone and maintenance of the NB xt interactions causes the mesityl rings not to be symmetrically oriented directly above and below
the remaining two coordination sites, but "off-center". The chloride and benzyl ligands complete
the pseudotetrahedral coordination geometry in 3a. It is clear from the acute Ti-C(27)-C(28) angle
( 87.0 (5)0) as well as the short Ti-C(28) distance (2.500 (8) A) that the rt-system of the benzyl
ligand is interacting in a symmetrical manner with the titanium center. For comparison, the most
distorted benzyl ligand in Ti(CH 2 Ph) 4 has a Ti-CQc-Cipso angle of 88 (2)' and a Ti-Cipso distance of
2.61 (3) A.4 2,43
Close examination of 1H NMR spectra of 3a suggests that a second isomer is present in
small amounts (<10%) in solution. Two sets of benzyl o-Ph resonances at 5 5.756 and 5.363 ppm
and two sets of backbone resonances centered at 8 4.063 and 4.086 ppm are observed in C6 D6 for
the major and minor isomers, respectively. In view of the solid state structure of 3a we speculate
that the minor isomer may be one in which the benzyl group is rotated by 1800, i.e., the Ti-CH2
bond is now in the "central" coordination position, and the phenyl ring is bound to the metal in an
"outside" position. In this orientation the phenyl group of the benzyl ligand would interact to a
Figure 3.2. Chem-3D drawing of the X-ray structure of (Ben)Ti(CH 2 Ph)C1 (3a).
C(27)
C(2)
C(1)
Table 3.1. Selected Bond Distances (A) and Angles (0) for (Ben)Ti(CH 2 Ph)Cl (3a).
Distances
Ti-N(1)
1.894 (6)
Ti-N(2)
1.912 (6)
Ti-C(27)
2.106 (8)
Ti-C(28)
2.500 (8)
Ti-Cl
2.325 (3)
C(27)-C(28)
1.46 (1)
N(1)-B(2)
1.43 (1)
N(2)-B(1)
1.419 (9)
Angles
N(1)-Ti-N(2)
89.2 (2)
Ti-C(27)-C(28)
87.0 (5)
C(27)-Ti-C(28)
35.8 (3)
C(27)-Ti-C1
125.5 (2)
N(1)-Ti-C(27)
110.7 (3)
N(2)-Ti-C(27)
113.9 (3)
N(1)-Ti-Cl
110.3 (2)
N(2)-Ti-Cl
100.9 (2)
N(1)-C(2)-C(1)
109.2 (6)
N(2)-C(1)-C(2)
107.8 (6)
Ti-N(1)-B(2)
127.4 (5)
Ti-N(2)-B(1)
137.6 (5)
Ti-N( 1)-C(2)
108.8 (2)
Ti-N(2)-C(1)
100.2 (4)
B(2)-N(1)-C(2)
122.3 (6)
B(1)-N(2)-C(1)
121.0 (6)
N(2)-B(1)-C(9)
121.0 (7)
N(1)-B(2)-C(2 1)
118.7 (7)
N(2)-B(1)-C(3)
117.7 (6)
N(1)-B(2)-C(15)
118.4 (7)
C(9)-B(1)-C(3)
121.3 (6)
C(21 )-B(2)-C(15)
122.9 (7)
Table 3.2. Crystallographic Data, Collection Parameters, and Refinement Parameters for
(Ben)Ti(CH 2 Ph)Cl 2 (3a).
Empirical Formula
Formula Weight
Diffractometer
Crystal Color, Morphology
Crystal Dimensions (mm)
Crystal System
C43 H5 1B2 N2 CITi
700.86
Rigaku AFC6S
red, plate
0.25 x 0.12 x 0.50
Triclinic
a
b
13.949 (6) A
16.633 (7) A
c
9.136 (4) A
106.00 (6)
96.84 (3)
(X
88.38 (3)
V
Space Group
2023 (3) A•
PI
Dcalc
1.150 g/cm 3
Fooo
744
3.04 cm- 1
ýp(MoKQa)
Scan Type
Temperature (oC)
Total No. Unique Reflections
No. Variables
R
RwG
GoF
o -20
193 (2) K
7132
460
0.079
0.086
2.48
greater degree sterically with the mesityl groups flanking the coordination wedge, and this isomer
therefore should be significantly less favored than the one found in the solid state.
Intramolecular C-H Bond Activation.
Thermolysis of (Ben)TiMe 2 (4b) in benzene follows first-order kinetics (kobs = 1.8 (2) x
10-4 s-1 at 79 0 C) and yields deep red, C2-symmetric (TwistBen)Ti (5) and methane (eq 6). We
believe two pathways for this reaction to be most plausible. The first is a stepwise C-bond
metathesis 27 ,44 -48 involving opposing o-Me C-H bonds and Ti-Me groups. The second is initial
loss of methane via a-abstraction followed by 1,2-addition of a C-H bond in one ortho methyl
V
BO
B
B
toluene / 70 - 100 0C
K
- 2 CH 4
(6)
5
group across the Ti=CH 2 bond; a second a abstraction followed C-H addition to the resulting
Ti=C bond would yield 5.49 ,5 0 Thermolysis of (Ben)Ti(CD 3 )2 (4b-ds) yields only CD3H.
according to 2 H NMR, consistent with the a bond metathesis pathway. (a-Abstraction requires
that both CD 4 and CD 2 CH 2 are formed.) A small inverse secondary isotope effect (kH/kD = 0.84
(3)) is found at 79 0 C.
(Ben)ZrMe 2 may be prepared from 2 and two equivalents of methyllithium in toluene at
-78 0 C. However, it is much less stable thermally than (Ben)TiMe 2 and converts readily to the
yellow dicyclometalated (TwistBen)Zr (6) and methane. Under conditions where (Ben)ZrMe2 is
prepared, only mixtures of (Ben)ZrMe 2 and 6 can be obtained; significant conversion of
(Ben)ZrMe 2 to 6 occurs even during crystallization of (Ben)ZrMe 2 at -40'C. A rate constant could
be obtained for conversion of (Ben)ZrMe 2 to 6 in mixtures of the two (kobs = 8.4 (2) x 10-4 s-I at
43 0 C). Derivatives that contain more sterically demanding alkyl derivatives are even less stable
towards metalation. For example, the reaction between (Ben)ZrC12 (THF) and two equivalents of
LiCH 2 SiMe 3 yields only 6 (62% yield) and tetramethylsilane (eq 7).
BenZrC12 (THF) + 2 LiCH 2SiMe 3
IH and
13 C
toluene
toluene
- 2 SiMe 4
6
(7)
NMR spectroscopy demonstrate that the solution structures of 5 and 6 are
significantly different. Whereas the titanium complex is C2-symmetric, the zirconium analog
appears to lack any symmetry on the NMR timescale, even at 100 0 C. Furthermore, the four ZrCH 2 IJCH coupling constants range from 112 to 145 Hz, suggesting there are significant
differences between the nature of the two Zr-methylene bonds.
X-ray Structure of [(TwistBen)Zr] 2 (6).
An X-ray diffraction study shows that 6 is dimeric and possesses crystallographically
imposed C2 symmetry (Figure 3.3; Table 3.3). Two methylene groups nearly symmetrically
bridge the two zirconium centers (Zr-C(121) = 2.464 (8), Zr-C(121*) = 2.449 (8) A), Zr ..... Zr =
3.251 (2) A). The ipso carbon from one mesityl group (e.g., C(12)) also strongly interacts with
the zirconium center from which it originates (Zr-C(12) = 2.549 (8) A). These structural
parameters should be compared with the non-bridging mesityl methylene group in which the ZrC(221) distance is 2.252 (8) A. However, the Zr-C(22) distance of 2.703 (8) A and the ZrC(221)-C(22) angle of 90.6 (5)0 demonstrate that the ipso carbon of even the non-bridging mesityl
ring also weakly interacts with the zirconium center. The N(1)-Zr-N(2) angle (73.3 (2) ° ) for the
five-membered ring is more acute than that (89.2 (2)0) found in the structure of 3a. The
contraction is due in part to the larger size of zirconium relative to titanium as well as perhaps to the
decreased steric demands of the mesityl rings upon metalation. The Zr-N distances (2.088 (6) and
2.106 (6) A) are comparable to those found in other four and pseudo four-coordinate zirconiumamido complexes. 5 ,6 ,19- 2 1,25 ,2 7,28,40 Borylamide N-B i7-interactions are maintained in the
Figure 3.3. Chem-3D drawing of the X-ray structure of [(TwistBen)Zr] 2 (6) (viewed almost
down the crystallographic C2 axis). The three benzene solvent molecules are omitted for clarity.
B (2) C,•
C(22)(
N(2) C(121*
C(221)a
C(1)
B(1)
Table 3.3. Selected Bond Distances (A) and Angles (0) for [(TwistBen)Zr] 2 (6).
Distances
Zr-N(1)
2.106 (6)
Zr-N(2)
2.088 (6)
Zr-Zr*
3.251 (2)
Zr-C(121*)
2.449 (8)
Zr-C( 121)
2.464 (8)
Zr-C(12)
2.549 (8)
Zr-C(221)
2.252 (8)
Zr-C(22)
2.703 (8)
N(1)-B(1)
1.38 (1)
N(2)-B(2)
1.43 (1)
Angles
N(1)-Zr-N(2)
73.3 (2)
C(121)-Zr-C(121*)
82.8 (2)
Zr-C(121)-C(12)
69.7 (4)
Zr-C(221)-C(22)
90.6 (5)
N(1)-Zr-C(221)
108.0 (3)
N(2)-Zr-C(221)
96.1 (3)
Zr-N(1)-B(1)
109.3 (5)
Zr-N(2)-B(2)
107.4 (5)
Zr-N(1)-C(1)
119.6 (5)
Zr-N(2)-C(2)
123.0 (5)
B(1)-N(1)-C(1)
128.8 (7)
B(2)-N(2)-C(2)
122.9 (7)
N(1)-B(1)-C(1 1)
111.4 (7)
N(2)-B(2)-C(21)
115.0 (7)
N(1)-B(1)-C(31)
124.8 (8)
N(2)-B(2)-C(41)
123.1 (8)
C(11)-B(1)-C(31)
123.8 (7)
C(21)-B(2)-C(41)
121.9 (8)
Table 3.4. Crystallographic Data, Collection Parameters, and Refinement Parameters for
[(TwistBen)Zr]2 * 3 C6 H6 (6 * 3 C6 H6 )-
Empirical Formula
Formula Weight
Diffractometer
Crystal Color, Morphology
Crystal Dimensions (mm)
Crystal System
a
b
c
oX
C76 H9 2B4 N4 Zr2 *3C6 H6
1521.60
Enraf-Nonius CAD-4
pale yellow, parallelepiped
0.32 x 0.38 x 0.21
Monoclinic
30.775 (2) A
11.935 (1) A
27.435 (3) A
900
125.34 (3) o
Y
V
Space Group
Dcalc
Fooo
ýp(MoKca)
Scan Type
Temperature
Total No. Unique Reflections
No. Variables
R
Rw.
GoF
900
8220 (5) A3
C2/c
1.229 g/cm 3
6416
2.94 cm- 1
0)
186 (2) K
7601
470
0.076
0.069
1.62
metalated structure according to N-B distances (1.38 (1) and 1.43 (1) A) and small dihedral
angles C(2)-N(2)-B(2)-C(41) (9.20) and C(1)-N(1)-B(1)-C(31) (3.1 ).
Conversion to Zwitterionic Complexes.
The (Ben)MMe 2 complexes react with the strong Lewis acid B(C 6 F5 )3 in dichloromethane
to give what we presume on the basis of analogous studies in metallocene chemistry 5 1- 54 to be the
"zwitterionic" complexes [(Ben)MMe][MeB(C 6 F 5 ) 3 ] in which a methyl group is partially
abstracted by the triarylborane (eq 8).
(Ben)MMe 2 + B(C 6F5 )3
M = Ti, Zr
1H
NMR spectra of 7 and 8 are similar in form to
CD 2 C12
CD2 12 O
[(Ben)MMe][MeB(C 6 FS) 3 ]
(8)
7 M = Ti
8 M = Zr
those of the unsymmetrically substituted monoalkyl complexes 3a and 3b, featuring a wellresolved AA'BB' multiplet for the ligand backbone. All of these data, along with the demonstrated
strong B-N 7t bonding, would seem to eliminate the possibility that B(C 6 F5 )3 binds to a lone pair
on nitrogen. We have also eliminated the possibility that (Ben)MMeCl complexes form (along
with free [MeB(C 6F 5 )3 ]- ion) by chloride abstraction from dichloromethane by showing that
characteristic resonances for (Ben)MMeCl can be observed by NMR in the presence of
[MeB(C 6 F5 )3 ]- (as a tetraalkyl ammonium salt). Therefore it would appear that the interaction
between "[MeB(C 6 F5 )3 ]-" and the metal is relatively strong. Unfortunately, we have not been able
to isolate either of these adducts, and so have not been able to confirm their identity via X-ray
studies. Strong binding of [MeB(C 6 F5 ) 3]- to the metal would help explain why dichloromethane
solutions of 7 and 8 do not polymerize ethylene readily at 250 C and 1-2 atm.
Discussion and Conclusions
On the basis of the results presented here we believe that the concept of using N-B ntbonding to enforce a desirable orientation of the mesityl groups in complexes that contain the Ben
ligand is a valid one, and that Ben complexes have some characteristics that are reminiscent of
metallocenes. The N-B In-interaction also is likely to be responsible for what appears to be a
relatively electron-deficient nature of the metal in Ben complexes. The short M-Cipso contacts and
acute M-Co-Cipso angles seen in the solid state structures of 3a and 6 are of the magnitude
commonly seen in actinide 55 and cationic group 4 complexes. 33 ,34,56 The electrophilic nature of
the metal is also evidenced by facile conversion of (Ben)MMe2 complexes to (TwistBen)M
complexes, one that is consistent with attack by a coordinatively unsaturated, electrophilic metal
4 45
center on a CH bond with concerted abstraction of a proton by the leaving methyl group. 4.
The fact that 7 and 8 do not react with ethylene is also consistent with the relatively high
electrophilicity of the metal center and strong binding of the [MeB(C 6F5 )3 ]- ion to the metal. 51 .
A disadvantage of the present Ben ligand, however, is the lack of sufficient steric
protection to prevent dimerization via bridging methylenes (to give 6) or to expel the
[MeB(C 6 F5 )3 ] anion in 7 and 8 in the presence of ethylene. Some flexibility in the ethylene bridge
between the two amido nitrogens may exacerbate to some degree what we perceive to be
insufficient "lateral" steric protection. Nevertheless, the similarity between the basic coordination
geometry of the Ti and Zr Ben complexes and typical metallocene coordination geometries. in
particular the presence of three coordination sites in a plane perpendicular to the MN 2 plane. is
unmistakable. Future efforts will be aimed at a further exploration of these similarities and
differences, and the construction of a more crowded Ben ligand that would not only resist
metalation but would provide a more crowded environment in which a boron-based anion would
be bound more weakly than observed here.
Experimental
General Procedures. All experiments were performed under nitrogen in a Vacuum
Atmospheres drybox or under argon using standard Schlenk techniques. All solvents were
purified by standard techniques while deuterated NMR solvents were dried and stored over 4 A
molecular sieves before use.
Magnesium bromide etherate,
butyllithium, methyllithium, methyllithium-d3,
methylmagnesium chloride, and benzylmagnesium chloride were used as received.
Ethylenediamine containing 0.6% water was stored over 4-A molecular sieves before use.
MC14 (THF) 2 (M = Ti, Zr),57 Mes2BF, 39 B(C 6 F 5 ) 3 ,58 and neopentylmagnesium chloride in
ether 59 were prepared according to literature procedures.
1H, 13 C, 19 F,
and
11B
NMR spectra usually were recorded at 300, 75.4, 282, and 96.2
MHz, respectively. Proton spectra were referenced internally by the residual solvent proton signal
relative to tetramethylsilane. Carbon spectra were referenced internally relative to the
13 C
signal of
the NMR solvent relative to tetramethylsilane. Fluorine and boron spectra were referenced
externally to neat CFC13 and BF 3 etherate, respectively. IR spectra were recorded as Nujol mulls
between KBr plates on a Perkin-Elmer 1600 FT-IR spectrometer. Elemental analyses were
performed on a Perkin-Elmer PE2400 microanalyzer in our laboratories.
Mes 2 BNHCH 2 CH 2 NHBMes 2 [H 2 (Ben)].
Butyllithium (52 mL,. 131 mmol. 2.5 M
in hexane) was added to a solution of ethylenediamine (3.74 g, 62.1 mmol) in THF (500 mL) at
-40'C. A white flocculent precipitate formed almost immediately and the color of the solution
changed to dark purple. After 5 hours the purple color had faded and the slurry was chilled to
-40 0 C. Solid dimesitylboron fluoride (35.0 g, 131 mmol) was added to the chilled solution over
10 minutes. The flocculent white precipitate dissolved almost immediately to yield a light yellow
solution with suspended LiF. After the solution was stirred overnight, the solvents were removed
in vacuo and the residue was extracted with warm dichloromethane (800 mL). The extract was
filtered through Celite, concentrated, and cooled to -40 0 C. Fluffy white needles were collected by
filtration, washed liberally with pentane, and dried in vacuo for 12 hours to afford 30.50 g (84 %
yield) of the product as a dichloromethane solvate (1 part dichloromethane to 3 parts H2 (Ben)). An
analytical sample of H2 (Ben).(as a solvate containing 0.5 equiv of benzene) was obtained by
1H
recrystallization from warm benzene:
NMR (CDC13 ) 6 6.756 (s, 4, m-H), 6.735 (s, 4, mn-H).
4.478 (br s, 2, NH), 3.092 (pseudo t, 4, NCH2 ), 2.267 (s, 6, p-Me), 2.236 (s, 6, p-Me), 2.205
(s, 12, o-Me), 2.109 (s, 12, o-Me);
13 C
NMR 8 140.8, 140.1 (Co), 137.3, 136.9 (Cp), 128.3.
127.6 (Cm), 46.4 (NCH 2 ), 22.8, 22.1 (o-Me), 21.1, 21.0 (p-Me);
11 B
NMR 5 45.3 (width =
1170 Hz); IR (Nujol/KBr) 3367, 3356 v(NH) cm- 1 . Anal. Calcd for C4 1H53 N2 B2 : C, 82.70; H.
8.96; N, 4.71. Found: C, 83.03; H, 9.02; N, 4.70.
(Ben)Mg(THF) 2 . Butyllithium (22.5 mL, 56.2 mmol, 2.5 M in hexane) was added to a
solution of H2 (Ben) (8.93 g, 16.1 mmol) and MgBr2 (ether) (4.75 g, 18.4 mmol) in THF (250
mL) at -40 0 C.
The solution was warmed to room temperature and stirred overnight.
Dichloromethane (100 mL) was added and the mixture was stirred for 6 hours after which the
solvents were removed in vacuo to give a yellow oil. The oil was extracted with CH2C12/pentane
(150 mL, 50/50). The extract was filtered and concentrated to an oil. Upon stirring the residue
with a mixture of pentane (100 mL) and THF (10 mL) a white solid formed and was collected by
filtration. The white solid was redissolved in dichloromethane (100 mL) and the mixture was
filtered and concentrated again to an oil. The oil was triturated with pentane (100 mL) to give
white crystals. The mixture was chilled to -40 0 C for two hours and the crystals were collected by
filtration, washed liberally with pentane, and thoroughly dried in vacuo to afford 9.51 g (77%) of
the product as a 2/3 pentane clathrate which contained -5% H2 (Ben). Repeated recrystallization
failed to remove the H2(Ben) impurity:
1H
NMR (pyridine-d5)
6.897 (s, 4, mi-H), 6.551 (s, 4,
m-H), 3.782 (s, 4, NCH 2 ), 3.647 (m, 8, THF), 2.592 (s, 12, o-Me), 2.375 (s, 12, o-Me), 2.262
(s, 6, p-Me), 2.123 (s, 6, p-Me), 1.609 (m, 8, THF);
13 C
NMR 6 141.0, 140.5 (Co), 134.6 (Cp).
128.3, 127.9 (Cm), 67.9 (THF), 54.8 (NCH 2 ), 34.2 (CH 2 ), 23.6, 23.4 (o-Me), 22.5 (CH 2).
21.2, 20.9 (p-Me), 14.2 (THF).
(Ben)TiCl2 (1). TiC14 (THF) 2 (3.40 g, 10.2 mmol) was added slowly to a solution of
(Ben)Mg(THF)2(pentane) 2/ 3 (7.84 g, 10.2 mmol) in dichloromethane (100 mL) at -40 0 C. The
solution quickly turned orange. A copious amount of white precipitate had formed after 1 hour.
After three hours the solution was filtered and the extracts were concentrated to dryness. The
residue was crystallized from dichloromethane/ether at -40 0 C to yield 5.52 g (81%) of fluffy
orange solid.
An analytical sample was obtained by double recrystallization from
dichloromethane/pentane:
1H
NMR(CDC13 ) 8 6.802 (s, 4, m-H), 6.745 (s, 4, m-H), 4.409 (s, 4.
NCH 2 ), 2.402 (s, 12, o-Me), 2.333 (s, 12, o-Me), 2.251 (s, 6, p-Me), 2.193 (s, 6, p-Me);
13C{ 1H} NMR 6 145.7 (Co), 140.9 (Co), 140.4 (Cp), 138.3 (Cp), 128.9, 128.3 (Cm), 59.5
(CH 2), 23.6, 22.8 (o-Me), 21.3, 21.0 (p-Me). Anal. Calcd for C38 H48 N2 B2 C12 Ti: C, 67.80: H.
7.18; N, 4.16. Found: C, 67.55; H, 7.13; N, 3.95.
(Ben)ZrC12 (THF) (2). ZrC14 (THF) 2 (3.25 g, 8.61 mmol) was added slowly to a
stirred solution of (Ben)Mg(THF) 2 (pentane)0.22 (6.05 g, 8.61 mmol) in dichloromethane (100
mL) at -40 0 C. The solution was allowed to warm to room temperature. After standing overnight.
the solution was filtered and the extracts were concentrated to dryness in vacuo. The residue was
extracted with toluene (75 mL) and concentrated to -20 mL. Layering with pentane followed by
cooling the solution overnight afforded white crystals, which were collected by filtration, washed
with pentane, and dried in vacuo to give 4.84 g (71%) of the product. An analytical sample of
(Ben)ZrC12 (THF).CH 2 C12 was obtained by recrystallization from dichloromethane/pentane:
1H
NMR (C6 D 6 ) 6 6.816 (s, 4, m-H), 6.618 (s, 4, m-H), 4.129 (s, 4, NCH 2 ), 3.197 (m, 4, THF).
2.694 (s, 12, o-Me), 2.513 (s, 12, o-Me) 2.189 (s, 6, p-Me), 1.945 (s, 6, p-Me), 1.058 (m. 4.
THF);
13 C(1H}
NMR 5 147.2, 140.8, 140.0, 137.3 (Co and Cp), 129.6, 128.6 (Cm). 73.4
(THF), 55.8 (NCH 2 ), 25.4, 25.3, 23.5, 21.1(Ar-Me). Anal. Calcd for C4 3 H58 N2 B 2 C14 OZr: C.
59.12; H, 6.69; N, 3.21. Found: C, 58.84; H, 6.83; N, 2.92.
(Ben)Ti(CH 2 Ph)CI (3a). Benzylmagnesium chloride (0.256 mL, 0.446 mmol, 1.74
M in THF) was added to a solution of (Ben)TiC12 (0.300 g, 0.446 mmol) in dichloromethane (10
mL) at -40 0 C. The solution immediately turned deep red and a fine precipitate formed. The
solution was kept at -400 C for 3 hours and was filtered through Celite. The solvents were removed
from the filtrate in vacuo and the residue was recrystallized from a mixture of toluene and pentane
to afford 0.144 g (32%) of red plates in three crops:
1H
NMR(C 6 D6 ) 8 6.915 - 6.70 (m, Ar).
major isomer: 5.756 (dd, 2, o-Ph), 4.160 (AA'BB', 2, NCH 2 ), 3.965 (AA'BB', 2, NCH 2 ).
3.208 (s, 2, CH 2 Ph), 2.545 (br s, 6, Ar-Me), 2.511 (s, 6, Ar-Me), 2.483 (br s, 6, Ar-Me), 2.320
(s, 6, Ar-Me), 2.172 (s, 6, Ar-Me), 2.122 (s, 6, Ar-Me); minor isomer: 6 5.363 (br d, 2, o-Ph),
4.086 (br m, 4, NCH 2 ), 3.279 (s, 2, CH 2 Ph), 2.584 (s, Ar-Me), 2.496 (s, Ar-Me), 2.390 (s, ArMe) - corresponding integrals and remaining Ar-Me resonances are obscured by the methyl
resonances of the major isomer;
13 C
NMR (major isomer) 8 144.2, 142.5, 140.8, 139.9, 139.6.
138.3, 137.8, 130.7, 129.6, 129.5, 129.3, 128.9, 128.5, 126.8 (Caryl), 100.9 (1JCH = 134.
CH 2 Ph), 54.5 ( 1JCH = 142, NCH 2 ), 24.1, 23.3, 23.1, 21.2, 21.1 (Ar-Me). Anal. Calcd for
C4 5H55 N2 B 2 ClTi: C, 74.15; H, 7.61; N, 3.84. Found: C, 73.91; H, 7.83; N, 3.64.
(Ben)Ti(CH 2 CMe 3 )CI (3b). Neopentylmagnesium chloride (0.495 mL, 1.34 mmol.
2.7 M in ether) was added to a solution of (Ben)TiC12 (0.750 g. 1.11 mmol) in dichloromethane
(20 mL) at -40 0 C. The mixture was warmed to room temperature, allowed to stand overnight at
room temperature, and filtered through Celite. The solvents were removed from the filtrate in
vacuo and the residue was recrystallized from pentane at -40 0 C to afford 0.586 g (74%) of orange
crystals in two crops. An analytical sample was obtained by double recrystallization from
dichloromethane/pentane:
1H
NMR (C6 D6 ) 8 6.834 (s, 2, m-H), 6.808 (s, 2, mn-H). 6.756 (s. 2.
m-H), 6.741 (s, 2, m-H), 3.963 (AA'BB', 2, NCH 2 ), 3.674 (AA'BB', 2, NCH 2), 2.628 (s. 6.
Ar-Me), 2.574 (s, 6, Ar-Me), 2.487 (s, 6, Ar-Me), 2.432 (s, 2, CH 2 CMe3), 2.357 (s, 6. Ar-Me),
2.169 (s, 6, Ar-Me), 2.116 (s, 6, Ar-Me), 0.586 (s, 9, CH 2 CMe 3 ); 13 C{1H} NMR 6 143.24.
141.84, 140.67, 139.89, 139.22, 137.74, 129.63, 129.30, 128.82, 128.56 (Caryl), 53.79
(NCH 2 ), 39.08 (Ti-CH 2 ), 31.65 (CMe3 ), 31.52 (CMe 3 ), 24.08, 23.36, 23.17, 23.04, 21.12 (ArMe). Anal. Calcd for C43 H59 N 2 B2 CITi: C, 72.85; H, 8.39; N, 3.95. Found: C, 72.31; H, 8.55;
N, 3.81.
(Ben)Ti(CH 2 Ph) 2 (4a). Benzylmagnesium chloride (1.11 mL, 1.93 mmol, 1.74 M in
THF) was added to a solution of (Ben)TiC12 (0.620 g, 0.921 mmol) in dichloromethane (20 mL)
at -40'C. The solution immediately turned deep red and a fine precipitate formed. After standing at
-40 0 C for 1.5 hours, 1,4-dioxane (0.100 g, 1.12 mmol) was added and the solution was filtered
through Celite. The solvents were removed in vacuo and the residue was recrystallized from
pentane at -400 C to afford 0.382 g (53%) of red crystals:
1H
NMR (C6 D6 ) 6 7.003 (t, 4, mn-Ph).
6.836 (t, 2, p-Ph), 6.839 (s, 4, m-Mes), 6.788 (s, 4, m-Mes), 6.101 (d, 4, o-Ph), 3.776 (s, 4,
NCH 2 ), 2.790 (s, 4, CH 2 Ph), 2.516 (s, 12, o-Me), 2.249 (s, 12, o-Me), 2.194 (s, 6, p-Me).
2.184 (s, 6, p-Me);
13 C
NMR 5 146.2 (Phi), 142.4, 140.6 (Meso), 138.9, 137.5 (Mesp), 129.3
(Phm), 128.74, 128.65 (Cm), 127.0 (Pho), 124.0 (Phm), 99.7 (CH 2 Ph), 52.1 (NCH 2 ), 23.9.
23.3 (o-Me), 21.13, 21.07 (p-Me). Anal. Calcd for C52 H62 N2 B 2 Ti: C, 79.61; H, 7.96; N. 3.57.
Found: C, 79.30; H, 8.14; N, 3.38.
(Ben)TiMe2 (4b). Methylmagnesium chloride (1.50 mL, 4.51 mmol, 3.0 M in THF)
was added to a solution of (Ben)TiC12 (1.38 g, 2.05 mmol) in dichloromethane (75 mL) at -40 0 C.
The solution immediately turned yellow and a fine precipitate formed. After standing the mixture at
-40 0 C for 1.5 hours, 1,4-dioxane (0.400 g, 4.51 mmol) was added and the solution was filtered
through Celite. The solvents were removed in vacuo and the residue was recrystallized from
dichloromethane/ether at -40 0 C to afford 0.876 g (67%) of fluffy yellow needles in two crops. An
analytical sample was obtained by recrystallization from dichloromethane/pentane:
1H
NMR
(C6 D6 ) 8 6.822 (s, 4, mn-H), 6.744 (s, 4, m-H), 3.860 (s, 4, NCH 2 ), 2.527 (s, 12, o-Me), 2.427
(s, 12, o-Me), 2.193 (s, 6, p-Me), 2.084 (s, 6, p-Me), 0.858 (s, 6, Ti-CH3 );
13 C
NMR 8 144.2.
140.3 (Co), 139.5, 137.5 (Cp), 137.0, 136.2 (Ci), 129.3, 128.6 (Cm), 67.3 ( 1JCH = 121.5. TiCH 3 ), 53.8 (NCH2 ), 23.8. 22.9 (o-Me), 21.2 (p-Me). Anal. Calcd for C40 H54 N2B 2 Ti: C. 75.98:
H, 8.60; N, 4.43. Found: C, 75.71; H, 8.55; N, 4.16.
(Ben)Ti(CD 3 )2 (4b-d6 ) was prepared analogously employing 2 equivalents of LiCD3-Lil in
toluene:
2H
NMR (C6 H6 ) 8 0.738.
(Ben)ZrMe 2 . Methyllithium (1.10 mL, 1.64 mmol, 1.5 M in ether) was added to a
solution of (Ben)ZrC12 (THF) (0.707 g, 0.821 mmol) in toluene at -78'C. After 1 hour, the
solution became cloudy as it was warmed through -40 0 C. The solution was further warmed to 00 C
and the volatile components were removed in vacuo. The oily, yellow reside was extracted with
pentane and the extract was concentrated and cooled to -40 0 C to give 0.212 g (56%) of white
crystals after 1 day. Subsequent crops consisted largely of (TwistBen)Zr. Recrystallization and
1H
elemental analysis of (Ben)ZrMe2 were not attempted due to its thermal instability.
NMR
(C 6 D6 ) 5 6.844 (s, 4, m-H), 6.703 (s, 4, in-H), 3.911 (s, 4, NCH 2 ), 2.461 (s, 12, o-Me), 2.407
(s, 12, o-Me), 2.213 (s, 6, p-Me), 2.020 (s, 6, p-Me), -0.015 (s, 6, Zr-CH 3 );
13 C
NMR
(CD 2 C12 , -40 0 C) 146.48, 141.15, 140.00, 136.88, 129.78, 127.72 (Caromatic), 54.51 (NCH 2 ),
47.57 (Zr-Me), 23.90, 22.34, 21.19, 20.95 (Ar-Me).
(TwistBen)Ti (5). (Ben)TiMe2 (0.200 g, 0.316 mmol) in toluene (4 mL) was heated to
70'C in a Teflon-sealed tube for 36 hours. The solution was filtered and the volatile components
were removed in vacuo. The resulting red solid was triturated with dichloromethane to give 0.148
g (79%) of the product. An analytical sample was crystallized from toluene at -40 0 C:
1H
NMR
(C6 D 6 ) 8 6.951 (br s, 2, ni-H), 6.702 (br s, 2, m-H), 6.628 (s, 2, m-H), 6.568 (s, 2, in-H).
4.344 (AA'BB', 2, NCH 2 ), 4.175 (AA'BB', 2, NCH 2 ), 2.811 (d, 1JHH = 5.7, 2. Ti-CH2).
2.530 (s, 6. Ar-Me), 2.257 (s, 6, Ar-Me), 2.195 (s, 6, Ar-Me), 2.136 (s, 6. Ar-Me), 2.071 (s, 6.
Ar-Me), 2.247 (d, 1JHH = 5.7, 2, Ti-CH 2 ); 13 C{ 1H} NMR 149.18, 148.34, 140.94, 140.78 (br).
137.80, 131.82, 128.60 (br), 128.32 (Caryl), 75.97 (Ti-CH 2 ), 60.00 (NCH 2 ). 23.15. 22.45 (br).
21.18, 21.11 (Ar-Me). Anal. Calcd for C3 8 H46 N2 B2 Ti: C, 76.03; H, 7.72; N, 4.67. Found: C.
76.17; H, 8.04; N, 4.31.
[(TwistBen)Zr] 2 (6). (a) from (Ben)ZrMe 2 . A sample of (Ben)ZrMe 2 was allowed
to stand in benzene-d6 overnight. 6 was formed quantitatively, according to 1H NMR, along with
methane.
(b) from reaction of (Ben)ZrCI2 (THF) with
LiCH 2 SiMe 3 .
A solution of
LiCH 2 SiMe 3 (0.056 g, 0.608 mmol) in toluene (2 mL) at -40 0 C was added to a solution of
(Ben)ZrC12 (THF) (0.240 g, 0.304 mmol) in toluene (10 mL). After 15 minutes the cloudy, light
yellow solution was filtered through Celite and the solvents were removed in vacuo to give a
yellow oil. The oil was extracted with pentane (10 mL) and immediately filtered through Celite.
Yellow needles formed upon standing the filtrate. The mother liquor was removed after one day
and the needles washed with pentane. Concentrating and cooling the mother liquor to -40'C
afforded a second crop; yield 0.122 g (62%).
1H
recrystallization from benzene/pentane:
An analytical sample was obtained by
NMR (CD 2 Cl 2 ) 5 6.982 (s, 1, in-H), 6.807 (s, 1, nm-
H), 6.771 (s, 1, mn-H), 6.712 (s, 2, m-H), 6.683 (s, 1, m-H), 6.251 (s, 1, m-H), 6.154 (s, 1, 7nH), 4.288 (m, 1, NCH 2 ), 3.941 (m, 1, NCH 2 ), 3.657 (m, 1, NCH 2 ), 3.451 (m, 1, NCH 2 ).
2.527 (d, 2 JHH = 9.2, 1, Zr-CH 2 ), 2.481 (d, 2 JHH = 7.0, 1, Zr-CH 2 ), 2.410 (s, 3, Ar-Me), 2.353
(s, 3, Ar-Me), 2.242 (s, 3, Ar-Me), 2.226 (s, 3, Ar-Me), 2.149 (s, 3, Ar-Me), 2.101 (s, 3, ArMe), 2.083 (s, 3, Ar-Me), 1.900 (s, 3, Ar-Me), 1.881 (s, 3, Ar-Me), 1.866 (s, 3, Ar-Me), 1.041
(d, 2 JHH = 9.2, 1, Zr-CH2), -0.121 (d,
2 JHH
= 7.0, 1, Zr-CH 2 );
13 C
NMR 8 151.96, 145.50,
145.23, 143.25, 142.73, 142.06, 140.58, 140.31, 139.53, 137.26, 136.85, 130.71. 130.42.
128.23, 127.77, 127.65, 127.24 (Caryl), 71.78 ( 1JCH = 145 and 133, Zr-CH 2 ). 58.92 ( 1JCH =
135 and 112, Zr-CH2 ), 57.61 (1 JCH = 134, NCH 2 ), 52.54 ( 1JCH = 136, NCH 2 ), 23.67, 23.52.
23.03, 22.56, 22.44, 22.36, 21.48, 21.25, 21.19, 21.16 (Ar-Me).
Anal. Calcd for
C38 H46 N2 B 2 Zr: C, 70.92; H, 7.20; N, 4.35. Found C, 71.18; H, 7.57; N, 3.99.
(Ben)TiMe[MeB(C 6 F 5 )3 ] (7) was generated in solution by dissolving (Ben)TiMe 2
(0.060 g, 0.094 mmol) and B(C 6F 5 )3 (0.060 g, 0.117 mmol) in CD 2 CI2 . The orange solution
was analyzed by 1H NMR. No solid product could be recovered from these solutions:
1H
NMR
(CD 2 Cl 2 ) 8 7.286 (s, 2, mn-H), 7.254 (s, 2, m-H), 6.940 (s, 4, m-H), 4.507 (AA'BB', 4.
NCH 2 ), 2.471 (s, 6, Ar-Me), 2.443 (s, 6, Ar-Me), 2.308 (s, 6, Ar-Me), 2.275 (br, 12, Ar-Me).
2.229 (s, 6, Ar-Me), 0.710 (s, 3, Ti-CH3 ), 0.497 (br, 3, Ti-CH 3 -B(C 6 F5 )3 ); 13 C{ 1H NMR 8
156.6, 148.8 (Co or Cp), 148.8 (m, 1JCF = 234, o-C 6 F5 ), 142.4, 141.5, 140.8 (Co and Cp),
137.9 (m,
1JCF
= 241, p-C 6 F5 ), 137.3 (Cm), 136.8 (m, 1JCF = 241, m-C 6 F5 ), 136.7 (Cm).
129.3 (Cm), 72.0 (Ti-CH 3 ), 64.6 (NCH 2 ), 26.2, 23.1, 22.9, 22.0, 21.3 (Ar-Me and MeB(C 6 F 5 ));
19 F
NMR 8 -131.21 (d,
3 JFF
= 19.8, o-C 6 F5 ), -163.37 (t,
3 JFF
= 17.8, p-C 6 F 5 ).
-165.94 (m, 3 JFF (ave) = 22.5, m-C 6 F5 ).
(Ben)ZrMe[MeB(C 6 F5 )3 ] (8) was generated by dissolving (Ben)ZrMe 2 (0.066 g,
0.098 mmol ) and B(C 6 F5 )3 (0.050 g, 0.098 mmol) in CD 2 C12 : 1H NMR (CD 2 Cl 2 ) 6 7.281 (s,
4, mn-H), 6.903 (s, 4, m-H), 4.095 (AA'BB', 2, NCH 2 ), 3.968 (AA'BB', 2, NCH 2 ), 2.535 (s,
6, Ar-Me), 2.310 (s, 6, Ar-Me), 2.288 (s, 12, Ar-Me), 2.274 (s, 6, Ar-Me), 2.234 (s, 6, Ar-Me).
0.493 (br, 3, Zr-Me-B(C 6 F5 ) 3 ), 0.096 (s, 6, Zr-CH 3 ); partial 13 C{ 1 H} NMR 8 153.76, 148.10
(m, 1JCF = 236, o-C 6 F 5 ), 146.88, 143.18, 141.38 (br), 140.70 (br), 139.68, 137.40 (m, 1JCF =
245, p-C6F5), 136.24 (m, 1JCF = 246, m-C6F5), 135.08, 132.84, 128.53, 131.58;
-131.63 (d,
3 JFF
19 F
NMR 6
= 18.6, o-C 6 F 5 ), -163.80 (t, 3 JFF = 21.2, p-C 6 F 5 ), -166.37 (m, 3 JFF (avg) =
21.3, m-C 6 F5 ).
Kinetic Measurements of Intramolecular C-H Activation.
Teflon-sealed NMR
tubes containing 30 - 90 mg of the dimethyl derivatives in 0.60 mL toluene-d8 (for 4b and 4b-d 6 )
or benzene-d6 (for mixtures of (Ben)ZrMe 2 and 6 ) were placed in a heated NMR probe. The
probe temperature was calibrated before and after the measurements using ethylene glycol (for 4b
and 4b-d 6 ) or methanol (for (Ben)ZrMe2) and remained constant within ± 0.5 0 C. The reactions
were monitored by 1H NMR spectroscopy by integrating the N-CH2 resonances of 4b and 4b-d6
or the Zr-Me resonance in (Ben)ZrMe 2 relative to an internal benzene standard. In all cases the
kinetics followed first order behavior over 3 half-lives and the rate constants obtained were
insensitive to the initial concentration of metal complex.
X-ray Structure of (Ben)Ti(CH 2 Ph)C1. Suitable deep red crystals of 3a were
grown from a concentrated benzene/pentane solution at room temperature. A crystal having
approximate dimensions of 0.25 x 0.12 x 0.50 mm was mounted on a glass fiber. Data were
collected at -80 +± 1C on a Rigaku AFC6S diffractometer with graphite monochromated Mo Ku
radiation (X = 0.71069 A). Cell constants and an orientation matrix for data collection, obtained
from a least-squares refinement using the setting angles of 20 carefully centered reflections in the
range 15.00 < 20 < 30.000, corresponded to a triclinic cell with parameters: a = 13.949 (6) A, b =
16.633 (7) A, c = 9.136 (4) A, a = 106.00 (3)0, 3 = 96.84 (3)0, y= 88.38 (3)0, V = 2023 (3) A3,
Z = 2, F.W. = 700.86, p(calc) = 1.150 g/cm 3 . Based on packing considerations, a statistical
analysis of intensity distribution, and the successful solution and refinement of the structure, the
space group was determined to be P1.
A total of 7458 reflections were collected in the range 20 < 50.30, with 7132 being unique.
An empirical absorption correction was applied, using the program DIFABS, 60 which resulted in
transmission factors ranging from 0.84 to 1.18.
The data were corrected for Lorentz and
polarization effects. The structure was solved by direct methods. The non-hydrogen atoms were
refined anisotropically. The final cycle of full-matrix least-squares refinement was based on 3589
observed reflections (I > 3.00a(I)) and 460 variable parameters and converged with R = 0.079 and
Rw = 0.086. The maximum and minimum peaks on the final difference Fourier map corresponded
to 0.47 and -0.68 e/A3 , respectively.
All calculations were performed using the TEXSAN
crystallographic software package of Molecular Structure Corporation.
X-ray Structure of [(TwistBen)Zr]
2
- 3 C6 H6 . Suitable light yellow crystals of 6
containing three equivalents of benzene were grown by allowing (Ben)ZrMe2 to decompose in
benzene solution at room temperature. A crystal having approximate dimensions of 0.32 x 0.38 x
0.21 mm was mounted on a glass fiber. Data were collected at -86 +± 1C on an Enraf-Nonius
CAD-4 diffractometer with graphite monochromated Mo Ko radiation (X = 0.71069 A). Cell
constants and an orientation matrix for data collection, obtained from a least-squares refinement
using the setting angles of 25 carefully centered reflections in the range 15.00 < 20 < 23.00".
corresponded to a monoclinic cell with parameters: a = 30.775 (2) A, b = 11.935 (1) A. c =
27.435 (3) A, 3 = 96.84 (3)0, V = 8220 (5) A3 , Z = 4, F.W. = 1521.60, p(calc) = 1.229 g/cm 3 .
Based on the systematic absences of hkl: h+k # 2n and h01: h,l # 2n as well as the successful
solution and refinement of the structure, the space group was determined to be C2/c.
A total of 7758 reflections were collected in the range 20 < 49.90, with 7601 being unique.
An empirical absorption correction was applied, using the program DIFABS, which resulted in
transmission factors ranging from 0.80 to 1.28.
The data were corrected for Lorentz and
polarization effects. A correction for secondary extinction was applied (coefficient: 0.57539 x
10-7). The structure was solved by direct methods. The non-hydrogen atoms were refined
anisotropically.
The final cycle of full-matrix least-squares refinement was based on 4130
observed reflections (I > 3.00a(I)) and 470 variable parameters and converged with R = 0.076 and
Rw = 0.069. The maximum and minimum peaks on the final difference Fourier map corresponded
to 0.71 and -0.88 e/A 3 , respectively.
All calculations were performed using the TEXSAN
crystallographic software package of Molecular Structure Corporation.
References
(1) Brintzinger, H. H.; Fischer, D.; Mtillhaupt, R.; Rieger, B.; Waymouth, R. M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1143.
(2) Bochmann, M. J. Chem. Soc., Dalton Trans 1996, 255.
(3) Shapiro, P. J.; Cotter, W. D.; Schaefer, W. P.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1994, 116, 4623.
(4) Okuda, J. Comments Inorg. Chem. 1994, 16, 185.
(5) Okuda, J.; Schattenmann, F. J.; Wocadlo, S.; Massa, W. Organometallics1995, 14, 789.
(6) Hughes, A. K.; Meetsma, A.; Teuben, J. H. Organometallics1993, 12, 1936.
(7) Devore, D. D.; Timmers, F. J.; Hasha, D. L.; Rosen, R. K.; Marks, T. J.; Deck, P. A.:
Stern, C. L. Organometallics1995, 14, 3132.
(8) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. Soc. 1988, 110, 6255.
(9) Canich, J. A.; Turner, H. W., PCT Int. Appl. WO 92/12162, 1991, Exxon.
(10) Gu6rin, F.; McConville, D. H.; Vittal, J. J. Organometallics1995, 14, 3154.
(11)
Aoyagi, K.; Gantzel, P. K.; Kalai, K.; Tilley, T. D. Organometallics1996, 15, 923.
(12) Scollard, J. D.; McConville, D. H.; Vittal, J. J. Organometallics1995, 14. 5478.
(13) Warren, T. H.; Schrock, R. R.; Davis, W. M. Organometallics1996, 15, 562.
(14) Horton, A. D.; de With, J. J. Chem. Soc., Dalton Trans. 1996, 1375.
(15) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg. H. Organometallics1996.
15, 2672.
(16) Andersen, R. A. Inorg. Chem. 1979, 18, 2928.
(17) Andersen, R. A. J. Organomet. Chem. 1980, 192, 189.
(18) Planalp, R. P.; Andersen, R. A.; Zalkin, A. Organometallics1983, 2, 16.
(19) Airoldi, C.; Bradley, D. C.; Chudzynska, H.; Hursthouse, M. B.; Malik, K. M. A.:
Raithby, P. R. J. Chem. Soc., Dalton Trans. 1980, 2010.
(20) Alcock, N. M.; Pierce-Butler, M.; Willey, G. R. J. Chem. Soc., Chem. Comm. 1974, 627.
(21) Bai, Y.; Roesky, H. W.; Noltemeyer, M.; Witt, M. Chem. Ber. 1992, 125. 825.
100
(22) Brauer, D. J.; Btirger, H.; Wiegel, K. J. Organomet. Chem. 1978. 150, 215.
(23) Biirger, H.; Neese, H.-J. J. Organomet. Chem. 1969, 20, 129.
(24) Btirger, H.; Neese, H.-J. J. Organomet. Chem. 1970, 21, 381.
(25) Cummins, C. C.; Duyne, G. D. V.; Schaller, C. P.; Wolczanski, P. T. Organometallics
1991, 10, 164.
(26) Johnson, A. R.; Wanandi, P. W.; Cummins, C. C.; Davis, W. M. Organometallics1994,
13, 2907.
(27) Planalp, R. P.; Andersen, R. A.; Zalkin, A. Organometallics1983, 2, 16.
(28) Woo, T. K.; Fan, L.; Ziegler, T. Organometallics1994, 13, 2252.
(29) Bartell, L. S.; Clippard, F. B. Inorg. Chem. 1970, 9, 2439.
(30) Zettler, F.; Hess, H. Chem. Ber. 1975, 108, 2269.
(31) Brown, N. M. D.; Davidson, F.; Wilson, J. W. J. Organomet. Chem. 1981, 210, 1.
(32) See Chapter 2.
(33) Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell, I. P.; Huffman, J. C.
Organometallics1985, 4, 902.
(34) Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett, R. J. Am. Chem. Soc.
1987, 109, 4111.
(35) Hebner, B.; Manners, I.; Paetzold, P. Chem. Ber. 1987, 120, 1065.
(36) Bartlett, R. A.; Feng, X.; Olmstead, M. M.; Power, P. P.; Weese, K. J. J. Ami. Chem. Soc.
1987, 109, 4851.
(37) Bartlett, R. A.; Chen, H.; Dias, H. V. R.; Olmstead, M. M.; Power, P. P. J. Am. Chem.
Soc. 1988, 110, 446.
(38) Bartlett, R. A.; Chen, H.; Power, P. P. Angew. Chem. Int. Ed. Engl. 1989, 28, 316.
(39) Chen, H.; Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Shoner, S. C. J. Am. Chem.
Soc. 1990, 112, 1048.
(40) Minnig, D.; N6th, H.; Schwartz, M.; Weber, S.; Wietelmann, U. Angew. Chem. Int. Ed.
Engl. 1985, 24, 998.
101
(41) Paetzold, P. Adv. Inorg. Chem. 1987, 31, 123.
(42) Davies, G. R.; Jarvis, J. A.; Kilbourn, B. T. J. Chem. Soc., C7hem. Comm. 1971, 1511.
(43) Bassi, I. W.; Allegra, G.; Scordamaglia, R.; Chioccela, G. J. 41Am. Chem. Soc. 1971, 93,
3787.
(44) Rothwell, I. P. Polyhedron 1985, 4, 177.
(45) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; ]Nolan, M. C.; Santarsiero, B.
D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203.
(46) Latesky, S. L.; McMullen, A. K.; Rothwell, I. P.; Huffman, J. C. J. Am. Chem. Soc.
1985, 107, 5981.
(47) Chamberlain, L. R.; Rothwell, I. P. J. Am. Chem. Soc. 1983, 105, 1665.
(48) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J. C. J. Am. Chem. Soc. 1986, 1986,
1502.
(49) McDade, C.; Green, J. C.: Bercaw, J. E. Organometallics 198 2, 1,1629.
(50) van der Heijden, H.; Hessen, B. J. Chem. Soc., Chem. Comn 1. 1995, 145.
(51)
Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991 ,113, 3623.
(52) Gillis, D. J.; Tudoret, M.-J.; Baird, M. C. J. Am. Chem. Soc. 1993, 115, 2543.
(53) Pellecchia, C.; Pappalardo, D.; Oliva, L.; Zambelli, A. J. Am. Chem. Soc. 1995, 117,
6593.
(54) Quyoum, R.; Wang, Q.; Tudoret, M.-J.; Baird, M. C. J. Am. Chem. Soc. 1994, 116.
6435.
(55) Edwards, P. G.; Andersen, R. A.; Zalkin, A. Organometallics1984, 3, 293.
(56) Jordan, R. F. Adv. Organomet. Chem. 1991, 32, 325.
(57) Manzer, L. E. Inorg. Synth. 1982, 21, 135.
(58) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 24t5.
(59) Schrock, R. R.; Sancho, J.; Pedersen, S. F. Inorg. Syn. 1989 ,26, 44.
(60) Walker, N.; Stuart, D. Acta Cryst. 1983, A39, 158.
102
CHAPTER 4
Neutral and Cationic Organozirconium Complexes of the Sterically Demanding
Bis(borylamide) Ligand [Trip 2 BNCH 2CH 2 NBTrip 2 ]2-
103
Introduction
In contrast to their metallocene analogs, [(Ben)MMe][MeB(C 6 F5 )3 ] (M = Ti, Zr) do not
exhibit appreciable ethylene polymerization activity at 25 'C and 1-2 atm. It was believed that the
high electrophilicity of these metal centers results in the strong binding of the [MeB(C 6 F5 )31anion, thus shunting polymerization activity. Additionally, the relatively facile metalation of
mesityl o-Me substituents in both neutral and cationic organometallic group 4 Ben complexes was a
further impediment to the development of a Ziegler-Natta catalyst system based on well-defined
bis(borylamide) alkyl cations.
In order to render metalation of the boron substituents less facile as well as to promote
looser ion pairing in cationic alkyl derivatives, we felt that placing larger substituents in the ortho
positions of the aromatic steric shields would be desirable. A suitable boron substituent appeared
to be the 2,4,6-triisopropylphenyl group. As compared to o-methyl groups in the original Ben
system, o-isopropyl groups should kinetically better resist metalation as a consequence of the more
sterically hindered C-H bonds that would be subject to metalation. Furthermore, the C-H bonds
of the o-isopropyl groups are expected to be thermodynamically less acidic than their benzylic oMe counterparts. The increased size of the o-isopropyl substituents may also be expected to apply
more steric pressure to a "non-coordinating" anion such as [MeB(C 6 F5 )3]- or [B(C 6 F5 )4 1-.
favoring looser ion-pairing with the bis(borylamide) ligated metal center.
With these considerations in mind, we set out to prepare the sterically demanding
bis(borylamide) ligand [Trip 2 BNCH 2 CH 2 NBTrip 2 ]2- in the aim of preparing thermally stable
group 4 alkyl cations supported by this ligand which could serve as Ziegler-Natta olefin
polymerization catalysts.
104
Results
Ligand Synthesis and Entry into Zirconium Chemistry.
[Trip 2 BNHCH 2 CH 2 NHBTrip 2 ] (Trip = 2,4,6-triisopropylphenyl), H2 (BigBen), may be
substituting Trip 2 B-FI for Mes 2 B-F (Mes = 2,4,6-
prepared analogously to H2 (Ben),
trimethylphenyl). Though the high solubility of H2 (BigBen) in pentane hampers its isolation in
good yield, the free ligand need not be isolated. Deprotonation of the crude ligand with 2 eq.
butyllithium in pentane containing a small amount of THF proceeds cleanly and results in the
formation of colorless crystals of Li 2 (BigBen) &4 THF (1) after standing overnight at -40 'C (eq
1).
H,NCHCH 2 NH 2
1) 2 BuLi / THF
0
2) 2 Trip 2 B-F
Trip2BNHCH,CHNHBTrip 2
[H,(BigBen)]
(1)
2 BuLi
H2 (BigBen)
0
pentane / THF
Li 2 (BigBen) *4 THF
Room temperature 1H NMR spectra of H2(BigBen) and Li2(BigBen) -4 THF (1) consist of two
sets of sharp isopropyl resonances in addition to several other broad resonances. Warming to ca.
70 'C produces spectra in which two chemically inequivalent triisopropylphenyl groups may be
readily identified. This fluxional process is proposed to result from hindered aryl group rotation
about the B-Cipso bonds due to the steric interaction of opposing o-isopropyl groups as shown in
Figure 4.1.
Figure 4.1. Steric interactions between opposing o-isopropyl groups in H2(BigBen) and 1.
~Nn
N ,
M
105
M = H, Li(THF) 2
Reaction of 1 with ZrC14 (THF) 2 in toluene cleanly provides (BigBen)ZrC12 (2) which is
isolated in high yield (85 - 90%) from pentane as colorless prisms containing 1 eq. of pentane /
zirconium (eq 2).
toluene
Li 2(BigBen) 4THF + ZrCl 4 (THF) 2
*
1
(BigBen)ZrC12
(2)
2
Similar fluxional behavior to H2(BigBen) and 1 is observed in variable temperature 1H
NMR spectra of 2 (Figure 4.2). Whereas at room temperature all resonances except for those of
the p-CHMe 2 groups are broadened, rocking of the o-CHMe 2 groups about the B-Cipso bond
becomes fast enough on the NMR timescale at 75 'C to produce a readily interpretable 1H NMR
spectrum.
Of note is the sharp backbone signal, signifying a symmetrically substituted
coordination wedge, and the p-CHMe2 region which consists of six doublets. The two sharp pCHMe2 resonances indicate that rotation about the N-B bonds does not occur on the NMR
timescale, even at elevated temperature. The remaining four o-CHMe2 doublets indicate that within
a given o-isopropyl group, each methyl group is diastereotopic and that complete rotation about
the B-Caryl bonds does not occur (Figure 4.2; inset).
Preparation of Dialkyl Derivatives.
Reaction of 2 in ether at -40 'C with the appropriate Grignard reagent cleanly provides the
dimethyl (3) as well as the diethyl (4) and dibutyl (5) derivatives as shown in eq 3.
(BigBen)ZrC12 + 2.1 RMgCl
ether / -40 'C
(BigBen)ZrR,
3 R=Me
4 R=Et
5 R = nBu
2
106
(3)
Figure 4.2. 1H NMR (300 MHz) spectra of 2 at 25 'C (bottom) and 75 'C (top). A structural
diagram of 2 (inset) is included illustrating hindered rocking about the B-Cipso bonds.
p-CHMe2
/1
r
o-CHMe2
o-CHMe.0
,
I ...
..
m-H
backbone --
ri
o-CHMe2
i;
i:
*//I
ii
I
ii
I;
i:
n iiiii
I!
p-CHMe2
I It
I
!!
r
1
, i
I~;i'
--
-- · ·- · ·
------
1.
p-CHMe2
/l
'k
o-CHMe2 p-CHMe 2
---m-H
\\J
*\
backbone
'`---
~- -''
~ -' '''
~
-"'''
--
''
* denotes pentane of crystallization
107
'
''
1
The dimethyl complex is isolated in high yield by crystallization from pentane as colorless crystals
which contain 1 eq. pentane / Zr. Unlike 2 and 3, however, the longer chain alkyls 4 and 5
crystallize as fine, fibrous needles which are extremely difficult to isolate from their mother
liquors, preventing isolation in reasonable yield by crystallization. 4 and 5 are instead recovered
as off-white powders in good yield from pentane by exhaustive removal of the solvent in vacuo.
IH and
13 C
NMR spectra of 3 - 5 show the same form as those of 2. With the exception
of the distinct p-CHMe 2 groups, broad resonances are observed at room temperature which
sharpen upon heating to 50 - 70 "C. The 1 H resonances for the coordinated alkyl groups are the
most broadened, and in the case of 4 and 5, are partially obscured by the CHMe2 1H NMR
manifold and thus are not unambiguously assignable. To further confirm the identity of these
complexes as dialkyls (rather than olefin complexes). 2 (BigBen)Zr(CD 2 CD3)2 (4-d 10) was
prepared from 2 and CD 3 CD 2 MgI in ether (eq 4).
ether / -40 'C
O
(BigBen)ZrCl 2 + 2.1 (CD 3 CD 2 )MgI
(BigBen)Zr(CD2 CD 3)2
(4)
4-dj 0
2
The 2 H NMR resonances for the ax and P deuterons of 4-d1 o in C6 H6 are observed at 8 0.469 and
0.956, respectively, as broad signals at room temperature which sharpen with increasing
temperature.
X-ray Structure of (BigBen)ZrMe 2 (3).
The X-ray structure of 3 was determined to identify structural features present in this
family of sterically encumbered bis(borylamide) zirconium alkyl complexes. The structure (Figure
4.3) reveals two boron-based triisopropylphenyl groups above and below the coordination wedge
where two methyl groups reside. The remaining two triisopropylphenyl groups are strongly
canted as to avoid unfavorable steric contacts between the opposing o-isopropyl groups flanking
the coordination wedge. The BigBen ligand symmetrically chelates the zirconium center with a
"bite" angle of 78.4(2) ° and Zr-N distances of 2.102(4) and 2.111(4) A (Table 4.1). These
distances are very similar to that found in the metalated [(TwistBen)Zr] 2 (2.088 (6)and
108
Figure 4.3. Chem-3D drawing of the X-ray structure of (BigBen)Zr(CD 3 )2 (3-d 6 ).
C(1)
C(18)
C(2)
109
Table 4.1. Selected Bond Distances, Intramolecular Contacts (A) and Angles (0) for
(BigBen)Zr(CD3)2 (3-d 6 ).
Distances
Zr-N(1)
2.102 (4)
Zr-N(2)
2.111 (4)
Zr-C(1)
2.232 (6)
Ti-C(2)
2.229 (6)
N(1)-B(1)
1.404 (8)
N(2)-B(2)
1.398 (8)
Zr*..C(59)
3.84
Zr**.C(18)
3.85
C(1)***C(59)
3.37
C(2)***C(18)
3.36
Angles
N(1)-Zr-N(2)
78.4 (2)
C(1)-Zr-C(2)
100.6 (2)
N(1)-Zr-C(1)
112.2 (2)
N(2)-Zr-C(1)
127.6 (2)
N(1)-Zr-C(2)
126.5 (2)
N(2)-Zr-C(2)
113.4 (2)
110
Table 4.2. Crystallographic Data, Collection Parameters, and Refinement Parameters for
(BigBen)Zr(CD 3 )2 * C5 H12 (3-d6 * C5 H 12 ).
C69 H11 4 B2 N2 Zr
1084.46
Empirical Formula
Formula Weight
Diffractometer
Siemens SMART/CCD
colorless, plate
0.30 x 0.22 x 0.12
Triclinic
11.6475 (7) A
Crystal Color, Morphology
Crystal Dimensions (mm)
Crystal System
a
b
c
16.4594 (10) A
18.4034 (10) A
101.0020 (10) o
100.0960 (10) o
98.1700 (10) o
3352.9 (3) A 3
V
Pi
Space Group
Z
2
g/cm3
Dcalc
1.074
t(MoKac)
1184
0.202 mm- 1
w scans
Scan Type
Temperature (°C)
Total No. Unique Reflections
No. Variables
R
188 (2) K
7126
643
0.0654
0.0927
1.093
Rw
GoF
111
2.106 (6) A) and lie toward the high range of other Zr-N distances found in other four- and
pseudo-four-coordinate zirconium amido complexes. 3 Borylamide N-B 7t-interactions are
maintained as shown by the N-B distances (1.404(8) and 1.398(8) A). The Zr-Me distances
A) are essentially
(2.231(7) A)4
(2.229(6) and 2.232(6)
(But 3 SiNH) 3Zr-Me
identical to that found in the four-coordinate amide.
Of note are the close contacts made by one methyl group each of diagonally opposing oisopropyl substituents with both the zirconium center and the zirconium-bound methyl groups.
The short Zro**C(18) and Zr***C(59) separations of 3.84 and 3.85 A demonstrate that the sides of
the coordination wedge are effectively blocked by these o-isopropyl substituents. Furthermore.
the short separations between the isopropyl and metal-bound methyl groups (C(1)o*°C(59) = 3.37
A and C(2)*o*C(18)
= 3.36 A) show that the groups in the coordination wedge strongly feel the
presence of the aryl o-isopropyl substituents.
Generation of Alkyl Cations.
In analogy with (Ben)ZrMe 2 , a cation-like derivative, [(BigBen)ZrMe][MeB(C 6 F5) 3 ] (6).
may be prepared by reaction of 3 with B(C 6 F5 )3 (eq 5).
(BigBen)ZrMe 2 + B(C 6 F5)3
3
pentane
pentane
-40 oC
[(BigBen)ZrMe][MeB(C 6 F5 )3 ]
(5)
6
In contrast to (Ben)ZrMe2, the pentane solubility of 3 leads to the isolation of the zwitterion 6.
Mixing pentane solutions of 3 and B(C 6 F5 )3 results in an oil which crystallizes upon standing at
-40 'C to afford colorless crystals of 6 in 87% yield.
Protonolysis of the dialkyls 3 - 5 with [HNMe 2 Ph][B(C 6 F5 )4 ] in toluene represents a
general route to monoalkyl cationic derivatives [(BigBen)ZrR]+ charge-balanced by the more
weakly anion [B(C 6 F5 )4 ]- (eq 6)
(BigBen)ZrR 2 + [HNMe 2 Ph][B(C 6 F5 )4]
toluene / - 40 'C
- RH
o [(BigBen)ZrR][B(C 6F 5) 4]
-Me 2 NPh
112
7
8
9
R = Me
R = Et
R = nBu
(6)
Room temperature IH and
13 C
NMR spectra of 6 - 9 suggest that these cations contain an
unsymmetrically substituted coordination wedge. In addition to a slightly broadened AA'BB'
pattern ascribed to the ligand backbone in cations 6 - 9, 1H NMR spectra of 6 - 8 display 10 - 11
CHMe2 resonances which may be resolved (Figure 4.4). Twelve are to be expected for an
unsymmetrically substituted coordination wedge as all CHMe2 groups within a given BTrip2 unit
are mutually diastereotopic with the "top" and "bottom" BTrip2 groups related by a mirror plane
containing the coordination wedge (Figure 4.4; inset).
The Me 2 NPh generated in the protonolysis of 3 - 5 does not coordinate to the cations 7 9. Although the isolation of 7 - 9 in crystalline form has proved elusive, concentration of a
toluene solution of 7 to an oil followed by exhaustive washing of this oil with pentane provides a
powder of 7.
H NMR spectra of this solid are otherwise identical to those taken of in sittu
prepared 7, with the conspicuous absence of signals corresponding to Me 2 NPh. On the basis of
this finding coupled with the crowded coordination environment revealed in the X-ray structure of
3, strong coordination of toluene solvent appears unlikely. Consistent with this claim, spectra of
7 generated in chlorobenzene-d5 show the same form as those in toluene-d5 discussed above.
The fact that the aniline does not coordinate to cations 7 - 9 suggests that the boron-based
anions may instead be interacting with the alkyl zirconium cations resulting in the unsymmetric
spectra described above. Though
19F
NMR spectroscopy has the potential to assist in determining
details of such an interaction, particularly in the case of the [B(C 6F 5 )4]- anion, low temperature
spectra (-80 'C) of 7 and 8 show only three resonances which are somewhat broadened relative to
the corresponding room temperature signals.
Thus if the [B(C 6 F 5 )4 ]- anion is intimately
interacting with the metal center, it must be exchanging its fluorine donors swiftly on the NMR
timescale at -80 oC.
113
CHMe2 + Zr-CH2
A
3
7
Figure 4.4.
6
5
4
3
2
1
0
'H NMR (300 MHz, 25 'C) spectrum of 8 (from the reaction of 4 with [HMe 2NPh][B(C 6F5 )4 ] in toluene-d 8).
Included is an inset showing one structural representation of 8. Note the high-field resonance corresponding to the 3-CH 3 group.
* denotes residual proteo impurities of toluene-d8 .
|
ppm
Room temperature 1H NMR spectra of 8 and 9 in toluene-d8 exhibit an additional feature
not present in spectra of 6 and 7. Each displays one slightly broadened upfield resonance, which
in the case of 8 is a broadened triplet at 8 -0.64 corresponding to the P-H atoms of the Zr-CH 2 CH3
ligand (Figure 4.4). The assignment of this upfield resonance is corroborated by the absence of
this signal in 8-d 5 prepared from 4-do0. In analogy to that observed for [Cp 2 M(R)(PMe 3 )][BPh4]
(R = Et, nBu, iBu), 5 ,6 the significant upfield shift of the 3-H 1H NMR resonance upon cation
formation in 8 and 9 suggests the presence of 3-agostic interactions. The dense
13 C
NMR
spectrum of 8 unfortunately prevents unambiguous assignment of the Zr-CH 2 CH 3 resonances and
comparison their respective 1JCH values which could further corroborate assignment of the ethyl
cation 7 as 3-agostic.
Variable Temperature NMR Studies.
Unlike 1H NMR spectra of the symmetrically substituted 2 - 5 which sharpen upon
warming, spectra of cations 6 - 9 broaden with increasing temperature. Most informatively, the
AA'BB' pattern of the ligand backbone coalesces into a broad signal in the temperature range 60 80 OC, the exact coalescence temperature dependent on the nature of the complex. Further
sharpening of the backbone resonance may be observed with increasing temperature. At these
elevated temperatures, however, new additional resonances also grow in signaling the onset of
decomposition.
On the basis of studies in metallocene chemistry, 7- 10 one plausible description of this
symmetrization process which relates the backbone AA'BB' resonances is dynamic anion
dissociation/reassociation shown in Figure 4.5.
115
Figure 4.5. Symmetrization resulting from anion dissociation/reassociation in cations 6 - 8.
Trip
B
I
N
/
N
I
Trip
ZrT.<.BI
Trip,
Trip B Trip
I
N
, Tri p
R
Zr--- R
[B]
N
[B]
,-•
KN
N
I
Trip
Trip
,Trip
B
I
r
-[B]
R
I
Trip
.
Trip
B
Trip
R = Me (6,7) or Et (8)
[B] = [MeB(C 6F5) 3] (6) or [B(C 6F5 )]-4 (7,8)
Reasonably accurate estimates for the activation parameter AG! corresponding to symmetrization in
6 - 8 may be calculated based on the frequency separation of the AA'BB' subspectra (Av) at low
temperature (-15 - 0 'C) and their corresponding coalescence temperatures (Tc) (Table 4.3). It
should be noted that rigorous application of the simple equally-populated, two-site exchange
coalescence temperature approach to the determination of AGt requires a modification to the
frequency separation term (Av) in cases where the exchanging resonances are coupled (e.g., in an
AB system Av' = (Av2 + 6J2AB) 1/2).11 Although the determination of all the coupling constants
within the AA'BB' framework in 6 - 8 is not possible due to the breadth of the signals at low
temperature, reasonable 2JHH or 3JHH values of 0 - 15 Hz are quite small in comparison to the
frequency separation of the AA'BB' subspectra (120 - 150 Hz). These AA'BB' couplings are thus
not expected to make a significant contribution to the "effective" frequency separation required in
the coalescence temperature approach to the determination of AG*, which is anyhow not very
sensitive to small changes in Av for most cases in which this method is applicable.11
116
Table 4.3. Estimated Values of AGt for Symmetrization in Complexes 6 - 8
(measured in toluene-d8 unless noted otherwise).
Complex
AGt (kcal/mol)
[(BigBen)ZrMe][MeB(C 6 F5 )3 1
Tc (K)
16.6 (3)
354 (3)
16.7 (3)
16.3 (2)*
353 (3)
342 (2)
15.7 (3)
328 (3)
(6)
[(BigBen)ZrMe][B(C 6 F5 )4 1
(7)
[(BigBen)ZrEt][B(C 6 F5 )4 1
(8)
* measured in chlorobenzene-d 5 .
An alternative explanation for the fluxional behavior exhibited by 6 9 is related to the
hindered rocking process uncovered in spectra of the symmetrically substituted
neutral complexes
2 - 5. If 6 - 9 exist in solution as well-separated ion pairs, the corresponding
three-coordinate
cations may exhibit enantiomeric C2-symmetric structures as shown in
Figure 4.6. Such a static
structure would minimize the steric contact between o-isopropyl groups
which presumably could
hinder interconversion between the two enantiomeric forms in solution.
Figure 4.6. Unfavorable steric interactions between opposing
o-CHMe2 groups in threecoordinate alkyl cations (viewed down the M-R bond (R = alkyl)).
Me 2 HC
Me 2HC
Me2HC
CHMe 2
N
CHMe2
H--H
R
Me2 HC
Me2HC
MeHC
AGsymm
Me 2HC
N
N
H--
CHMe 2
H
R
H-I-H
Me 2 HC
CHMe 2
H-I-H
CHMe 2
Me2HC
CHMe 2
Me 2 HC
117
N
CHMe 2
CHMe,
Reactivity of Alkyl Cations.
Although the alkylzirconium cations 6 - 8 may be readily prepared in toluene, solutions of
these cations are not active for the polymerization of ethylene. Only a small and variable amount
(<< 100 mg) of polyethylene may be recovered from solutions exposed to 1 atm of ethylene over
15 min to lh, strongly suggesting that the well-defined cations 6 - 8 are not themselves
responsible for polymerization. Further illustrating the relative inertness of 7 towards small
molecules, 7 resists reaction with both CO and H2 . Toluene solutions of 7 remain largely
unchanged after 1 day under 2-3 atm of either reagent (Scheme 4.1).
Although 7 is quite unreactive towards the reagents outlined above, one somewhat
surprising reaction was observed. The addition of 1 eq of gaseous NH 3 to a toluene solution of 7
results in the formation of an ammonia adduct 12 of a cationic zirconium methyl complex (eq 7).
[(BigBen)ZrMe][B(C 6F5 )4] + NH 3
toluene
-
7
[(BigBen)ZrMe(NH 3 )][B(C 6F5)4]
(7)
10
Reaction of [(BigBen)Zr(CD 3 )] [B(C 6 F5 )4 ] (7-d 3 ) (prepared from (BigBen)Zr(CD 3 )2 (3-d 6 ) and
[HNMe 2 Ph][B(C 6 F 5 )4]) with 1 eq. NH 3 produces [(BigBen)Zr(CD 3 )NH 3 ][B(C 6 F5 )4 ] (10-d 3 )
allowing the assignments of the Zr-CH 3 and Zr-NH 3 1 H NMR resonances which appear at 6
0.660 and 0.370 ppm, respectively.
No deprotonation of the amine ligand by the cis Zr-Me group occurs, even during heating
for several minutes at 70 'C. The amine ligand does not appear to be labile - warming to 70 'C and
higher results in a slight downfield shift of the NH 3 1 H resonance, away from that expected for
free NH 3 .13 In contrast to 1H NMR spectra of cations 6 - 9, the backbone and o-CHMe2
resonances of 10 do not broaden at 70 - 80 'C, demonstrating that the amine and methyl ligands
do not exchange sites within the coordination wedge. The analogous reaction with 1 eq. water
vapor results in the protonation of the bis(borylamide) ligand, liberating H2(BigBen) (Scheme
4.1).
118
Scheme 4.1. Summary of the Reactivity of 7 with Small Molecules (in toluene).
No reaction
No reaction
H2
2-3 atm.
CO
2-3 atm.
[(BigBen)ZrMe][B(C 6F 5 )4]
H20(g)
(7)
1 eq.
H 2 (BigBen)
1 eq.
NH3(g )
[(BigBen)ZrMe(NH 3 )][B(C 6 F5 )4]
(10)
Discussion and Conclusions
The more sterically demanding o-isopropyl groups in the BigBen system appear to confer
an added degree of stability against metalation of the o-substituent as compared to the o-methyl
groups in the original Ben ligand.
1H
and
13C
spectra of dialkyls 3 and 4 may be recorded at 50 -
70 'C without appreciable decomposition (the dibutyl 5 is somewhat more thermally sensitive). As
qualitatively observed in Chapter 3, the thermal stability of 3 - 5 decreases as the size of the metal
bound alkyl group increases, suggesting that decomposition may also occur by metalation of an
aromatic o-substituent.
The isolation of relatively stable dialkyl complexes 4 and 5 containing 3-H atoms was
unexpected, but not without precedent. In studies with the [N(TMS) 2 ]- ligand, Andersen was able
to prepare 3-H containing (TMS 2 N) 2 MR 2 (M = Zr, Hf; R = Et, Bu). 14 , 15 Though these
molecules are unstable towards loss of alkane on prolonged heating, it was reported the hafnium
diethyl derivative could be melted without decomposition at 68
oC.1 5
The blocking of 3-H
pathways was attributed to steric considerations of the voluminous [N(TMS) 2 ]- ligand. Hafnium
119
dialkyls generally possess greater thermal stability towards 1-H processes than do their zirconium
counterparts. For example, both (MeCp)2HfEt2 1 6 and (MeCp) 2 HfBu 2 6 (MeCp = C5 H4 Me) are
isolable as oils which possess moderate stability at room temperature, whereas analogous
zirconium derivatives may be considered as synthetic intermediates to Zr(II) chemistry due to facile
1-H abstraction. 2 This difference may be a reflection of greater M-C bond strengths of the third
row metals relative to their first and second row congeners.
The dialkyls 3 - 5 serve as precursors to cations 6 - 9 which cleanly form and possess a
moderate to considerable degree of thermal stability. This enhanced stability relative to the Ben
system allows the isolation and characterization of bis(borylamide) based alkyl cations, including
those containing 3-H atoms whose 1H NMR spectra are suggestive of 1-H agostic interactions.
Furthermore, these findings should be contrasted with recent reports of the generation of related
bis(amide) alkyl cations. For example, { [(TMS) 2 N] 2 Zr(CH 2 Ph)}+ rapidly loses toluene upon
formation due to intramolecular metalation of one of the TMS groups. 17 It thus appears that the
sterically demanding 2,4,6-triisopropylphenyl substituent confers a significant degree of stability
against metalation relative to the original Ben system as well as other alkyl cations based on related
bis(amide) ligand environments.
The steric demands of the boryl substituents in the BigBen system appears to influence the
strength of anion binding as well. The variable temperature studies place an upper limit of 16.7 (3)
kcal/mol on the strength of anion binding in 6 - 8, if bound at all (Figure 4.6). This strength of
association would be quite comparable to that determined for several metallocene cations with the
[Me-B(C 6 F 5 )3 ] and [B(p-R 3 SiC 6 F4 )4 ] anions (AGt = 14.4 - 18.3 kcal/mol). 7- 10 In contrast,
however, to these metallocene complexes which are very active ethylene polymerization catalysts
(typically producing from 0.5 - 1 g polyethylene / min at concentrations of 0.2 - 0.3 mM under
1 atm of ethylene), 9 ,10 cations 6 - 8 do not polymerize ethylene.
The further unreactivity of 7 with other small molecules such as H2 and CO, normally quite
reactive towards zirconocene and other amide supported Zr-alkyl bonds, was completely
unanticipated. For example, carbon monoxide rapidly and reversibly reacts with the neutral
120
Cp2ZrMe2 to give Cp 2 Zr(r1 2 -COMe)Me 18-20 and in the presence of Cp* 2 HfMe 2 and B(C 6 F5 )3 to
give the crystallographically characterized [Cp* 2 Hf(rl2-COMe)(CO)][MeB(C 6 F5 )4 ]2 1 which exists
as well-separated ions. Hydrogenation of the Zr-Me bond in cationic zirconocene derivatives is
also facile - [Cp 2 Zr(Me)(PMe3)][BPh4] undergoes complete hydrogenolysis under 1 atm H2
within 1 second.22 Furthermore, in the diamide system Andersen established that 2 eq. of tbutylisocyanide readily inserts into the Hf-R bonds of [(TMS) 2 N]2 HfR 2 (R = Me, Et). 14
The lack of reactivity of the cations 6 - 9 may be related to the very crowded coordination
environment of these molecules inferred from the X-ray structure of (BigBen)ZrMe2 (3).
Polymerization activity is not only sensitive to the strength of cation-anion interactions, but also to
the steric nature of the cationic center. For example, two metallocene catalysts which lie at
opposites extremes of the cited range of anion binding values above contain similar 1,3disubstituted
Cp rings but with different sized
substituents.
Whereas
[(1.3-
Me 2 C5 H3 )2 ZrMe][MeB(C 6 F5 )3 ] exhibits an ethylene polymerization activity of 6.8 x 106 g mol- 1
h-latm
-1
(AG symm = 18.3 (2) kcal/mol),
the more sterically encumbered [(1.3-
TMS2C 5 H 3 )2 ZrMe][MeB(C 6F 5 )3 ] is roughly only half as active (3.8 x 106 g mol-lh-latm- 1 )
= 14.4 (2) kcal/mol). 8
despite its considerable looser cation-anion interactions (AGsymm+
Although the sterically demanding 2,4,6-triisopropylphenyl group provides access to
bis(borylamide) based alkyl cations which possess considerable stability at room temperature and
reasonably loose cation-anion contacts, the comparisons above suggest that steric considerations
are likely responsible for the lack of x-olefin polymerization activity in this system. The increased
steric bulk of the BigBen system may also be responsible for the inability to prepare titanium
derivatives from a variety of Ti(IV) and Ti(III) starting materials. 23 Attention thus will now be
focused on the development of a less hindered bis(borylamide) system, but one which would also
not be subject to loss of metal-alkyl groups through metalation of the organoboryl substituents.
121
Experimental
General Procedures. All experiments were performed under nitrogen in a Vacuum
Atmospheres drybox or under argon using standard Schlenk techniques.
All solvents were
purified by standard techniques while deuterated NMR solvents were dried and stored over 4 A
molecular sieves before use.
Butyllithium and all Grignard reagents were obtained from Aldrich and titrated immediately
before use. Ethylenediamine containing 0.6% water was stored over 4-A molecular sieves before
use.
[HNMe 2 Ph][B(C 6 F 5 )4 ]2 4 and B(C 6 F 5 ) 3 25 were received as gifts from Exxon:
[HNMe 2 Ph][B(C 6F5 )4 ] was used as received and B(C 6 F5 )3 was sublimed before use. Trip 2 B-FI
and ZrCl 4 (THF)226 were prepared according to literature procedures.
1H, 13 C,
and
19 F
spectra were recorded at 300, 75.4, and 282 MHz, respectively. Proton
spectra were referenced internally by the residual solvent proton signal relative to tetramethylsilane.
Carbon spectra were referenced internally relative to the
13 C
signal of the NMR solvent relative to
tetramethylsilane. Fluorine spectra were referenced externally to neat CFC13 . Elemental analyses
were performed on a Perkin-Elmer PE2400 microanalyzer in our laboratories.
Li2(BigBen) •4 THF (1). Butyllithium (13.9 ml, 35.3 mmol, 2.53 M in hexane) was
added to a solution of ethylenediamine (1.06 g, 17.6 mmol) in THF (150 ml) chilled to -40 'C. A
flocculent white precipitate soon formed. After being stirred for 3 h, the suspension was chilled to
-40 'C and Trip2BF (15.4 g, 35.28 mmol) was added over 5 minutes. The flocculent solid soon
was consumed to give a clear viscous suspension of LiF. After being stirred overnight, the
solution was concentrated to ca. 25 ml , combined with 125 ml of pentane, and filtered through
Celite on a large frit. The frit was washed with pentane (3 x 30 ml) and THF (10 ml) was added to
the filtrate which was then chilled to -40 'C.
Butyllithium (13.9 ml, 35.3 mmol, 2.53 M in
hexane) was added to the chilled filtrate with stirring, and the solution was placed in the freezer
overnight. Colorless crystals were later isolated by filtration and were washed with pentane.
Subsequent concentration and cooling of the mother liquors gave a total of 17.4 g (83%) of the
product in 3 crops.
1H
NMR(C 6 D6 , 70 'C) 6 7.095 (s, 4, m-H), 7.069 (s,4, m-H), 4.042 (br,
122
4, o-CHMe 2 ), 3.681 (s, 4, NCH 2 ), 3.539 (br m, 4, o-CHMe 2 ), 3.347 (br m, 16, THF), 2.878
(sept, 2, p-CHMe2), 2.794 (sept, 2, p-CHMe 2), 1.471 (d, 12, p-CHMe2 ), 1.366 (br, 24, oCHMe2 ), 1.293 (d, 12, p-CHMe2), 1.205 (d, 12, o-CHMe 2 ), 1.171 (br m, 28, THF and oCHMe2 );
13 C
NMR 8 153.70, 151.56 (Co), 147.21, 126.2 (Cp), 121.46, 120.16 (Cm), 67.95
(THF), 59.91 (NCH 2 ), 34.65, 34.57 (p-CHMe 2 ), 33.15, 32.56 (o-CHMe 2 ), 27.11 (CHMe2 ).
25.56 (THF), 25.48, 24.51, 24.46, 24.25 (CHMe2 ). Anal. Calcd for C78 H1 2 8 N2 B 2 Li 2 0 4 : C,
78.51; H, 10.80; N, 2.35. Found: C, 79.08; H, 11.31; N, 2.46. High C and H analyses are
consistent with loss of THF in the solid state.
(BigBen)ZrCl
2
(2). ZrC14 (THF) 2 (0.990 g, 2.62 mmol) was added slowly to a
solution of Li2(BigBen) -4 THF (3.13 g, 2.62 mmol) in toluene (35 mL) at -40 0 C. The solution
was allowed to warm to room temperature and stirred overnight. The volatiles were removed in
vacuo and the residue was titurated with pentane (2 x 5 ml). Subsequent extraction of the residue
with pentane (50 ml) followed by concentration and cooling the extracts to -40 'C produced a first
crop of clear, colorless crystals which were isolated by filtration and washed gently with cold
pentane. Further concentration and cooling of the mother liquors provided two additional crops
yielding 2.27 g (87%) of the product as a pentane solvate containing 1 equiv. pentane / zirconium.
An analytical sample was recrystallized from pentane. 1H NMR(C 6 D6 , 75 "C) 6 7.152 (s. 4. inmH), 7.132 (s, 4, mn-H), 4.276 (s, 4, NCH 2 ), 3.334 (br m, 8, o-CHMe 2), 2.836 (sept. 2, pCHMe2 ), 2.703 (sept, 2, p-CHMe 2), 1.599 (d, 12, o-CHMe2), 1.341 (d, 12. o-CHMe2 ). 1.245
(d, 12, p-CHMe 2), 1.185 (d, 12, o-CHMe 2), 1.120 (d, 12, p-CHMe2 ), 0.910 (br d. 12, oCHMe2 );
13 C
NMR 6 157.37 (Co), 153.52 (Cp), 152.61 (Co), 149.72 (Cp), 136.82, 134.05
(Ci), 124.26, 121.85 (Cm), 56.95 (NCH 2 ), 35.29 (o-CHMe 2 ), 34.75, 34.54 (p-CHMe 2 ), 33.19
(o-CHMe 2 ), 27.37, 26.13 (br), 25.77, 24.16, 24.02, 23.67 (CHMe 2 ). Anal. Calcd for
C67 H1 08 N2 B2 C12 Zr: C, 71.52; H, 9.66; N, 2.49. Found: C, 71.29; H, 9.90; N, 2.28.
(BigBen)ZrMe 2 (3). Methylmagnesium bromide (0.312 mL, 1.02 mmol, 3.26 M in
ether) was added to a solution of 2 (0.546 g, 0.485 mmol) in ether (5 mL) at -40 'C resulting in the
immediate formation of a precipitate. After standing at -40 'C for 30 min, dioxane was added and
123
the mixture was filtered through Celite and concentrated to dryness. After titurating with pentane.
the residue was extracted with pentane (10 mL), filtered through Celite, and concentrated (to ca. 1
mL), and allowed to stand at -40 'C overnight resulting in the formation of 0.439 g (83%) of
colorless crystals containing 1 eq. pentane / Zr.
1H
NMR(C 6 D6 , 50 °C) 6 7.166 (s, 4, mn-H).
7.096 (s, 4, m-H), 4.106 (s, 4, NCH 2 ), 3.422 (m, 8, o-CHMe 2), 2.848 (sept, 2, p-CHMe2 ).
2.711 (sept, 2, p-CHMe2), 1.482 (d, 12, o-CHMe2), 1.375 (d, 12, o-CHMe 2), 1.259 (d, 12, pCHMe2 ), 1.217 (br d, 12, o-CHMe2 ), 1.129 (d, 12, p-CHMe2 ), 0.929 (br, 12, o-CHMe2), 0.012
(br, 6, Zr-CH 3 );
13 C
NMR (70 oC) 5 156.35 (Co), 152.42 (Co), 152.02 (Cp), 149.23 (Cp).
123.17, 121.62 (Cm), 54.72 ( 1 JCH = 137.9 Hz, NCH 2 ), 52.50 ( 1JCH = 114.7 Hz. Zr-CH3).
34.97 (o-CHMe 2 ), 34.72, 34.62 (p-CHMe 2), 32.97 (o-CHMe 2 ), 27.54, 26.05, 25.62. 24.14.
24.10, 23.96 (CHMe 2 ). Anal. Calcd for C69 H 114 N2 B2 Zr: C, 76.42; H, 10.59; N, 2.58. Found:
C, 76.49; H, 10.96; N, 2.50.
(BigBen)Zr(CD 3 )2 (3-d 3 ) was prepared analogously, employing 2.1 eq. LiCD 3 * Lil
in ether with the exception that a total of 4 pentane extract/filter/concentrate to dryness cycles were
performed to completely remove all lithium halides.
(BigBen)Zr(CH 2 CH 3 ) 2 (4). EtMgCl (0.328 mL, 0.692 mmol. 2.11 M in ether) was
added to a solution of 2 (0.371 g, 0.330 mmol) in ether (5 mL) at -40 'C resulting in the immediate
formation of a precipitate. After standing at -40 'C for 30 min, dioxane (0.060 mL, 0.70 mmol)
was added, the solution was filtered through Celite, and the filtrate was concentrated to dryness.
The residue was titurated with pentane (3 x 2 mL) and then extracted with pentane (5 mL). The
pentane extracts were concentrated to dryness, and the residue was again extracted with pentane.
After filtering once again, the filtrate was concentrated to dryness to afford 4 as a fluffy powder
(0.269 g, 78% yield).
1H
NMR(C 6 D6 , 70 °C) 8 7.147 (s, 4, nz-H), 7.081 (s, 4, m-H), 4.127 (s.
4, NCH 2 ), 3.417 (br m, 8, o-CHMe2 ), 2.837 (sept, 2, p-CHMe 2 ), 2.754 (sept, 2, p-CHMe2 ).
1.474 (d, 12, o-CHMe2 ), 1.371 (d, 12, o-CHMe2), 1.249 (d, 12, p-CHMe2 ), 1.175 (d. 24, pCHMe2 ando-CHMe2), 1.061 (br t, 6, Zr-CH 2 CH 3 ), 0.961 (br, 12, o-CHMe2 ), 0.585 (br, 4, ZrCH 2 CH 3 );
13 C
NMR 8 154.71 (Co), 152.17 (Cp), 150.77 (Co), 149.01 (Cp), 138.35 (br. Ci).
124
122.43, 121.60 (Cm), 64.52 (br, Zr-CH 2 CH 3 ), 53.18 (NCH2 ), 34.69 (p-CHMe 2 ), 34.53, 33.00
(o-CHMe 2 ), 27.49, 26.22 (br), 25.52, 24.17, 24.11, 23.99 (CHMe2 ), 11.68 (br, Zr-CH 2 CH3).
Anal. Calcd for C 66 H1 0 6 N2 B2 Zr: C, 76.20; H, 10.26; N, 2.69. Found: C, 76.68; H. 10.45; N.
2.63
(BigBen)Zr(CD2CD3)2 (4-d 1 0 ) was prepared analogously from 2 and 2.5 eq
CD 3 CD 2 MgI with the exception that a total of 4 pentane extract/filter/concentrate to dryness cycles
were performed to completely remove all magnesium halides.
2H
NMR(70
oC)
6 0.938 (Zr-
CD 2 CD 3 ) and 0.482 (br, Zr-CD 2 CD 3 ).
(BigBen)Zr(Bu)
2
(5).
BuMgC1 (0.105 mL, 0.235 mmol, 2.25 M in ether) was added
to a solution of 2 (0.126 g, 0.112 mmol) in ether (5 mL) at -40 OC. After standing at -40 'C for 30
min, dioxane (0.021 mL, 0.25 mmol) was added and the solution was concentrated to dryness.
The residue was titurated with pentane (3 mL) and then extracted with pentane (5 mL). The
pentane extracts were concentrated to dryness, and the residue was again extracted with pentane.
After filtering once again, the filtrate was concentrated to dryness to afford 5 as a fluffy powder
(0.082g, 65% yield).
1H
NMR(C 6 D6 , 50 °C) 8 7.150 (s, 4, m-H), 7.085 (s, 4, m-H), 4.117 (s.
4, NCH2 ), 3.406 (br m, 8, o-CHMe 2 ), 2.832 (sept, 2, p-CHMe 2 ), 2.778 (sept, 2, p-CHMe2).
1.495 (d, 12, o-CHMe 2), 1.385 (d, 12, o-CHMe 2 ), 1.245 (d, 12, p-CHMe2), 1.215 (d, 12. pCHMe 2 ), 1.166 (br d, 12, o-CHMe 2 ), 0.973 (br, 12, o-CHMe 2 ), 0.817 (br m, 10, ZrCH 2 CH 2 CH 3 and Zr-CH 2 CH 2 CH 3 ), 0.654 (br, 4, Zr-CH 2 ).
[(BigBen)ZrMe][MeB(C
6
F5 )3 ] (6). A chilled (-40 "C) solution of B(C 6F 5 )3 (0.047
g, 0.092 mmol) in pentane (5 mL) was added dropwise to a solution of 3 (0.100 g, 0.092 mmol)
in pentane (5 mL) at -40 'C. During addition of the triarylborane the solution became turbid and
after complete addition a colorless oil separated from the pentane solution onto the walls of the
reaction vial. After standing 36 h at -40 OC, the oil crystallized to afford 0.128 g (87%) of
colorless crystals of 6. 1H NMR(C 7 D8 , RT) 6 7.184 (s, 6, m-H), 7.074 (s, 2, m-H), 3.961 (br
AA'BB', 2, NCH 2 ), 3.489 (br AA'BB', 2, NCH2 ), 2.875 (m, 4, o-CHMe 2 ), 2.826 (sept, 2.
CHMe 2 ), 2.726 (br m, 2, CHMe 2 ), 2.590 (sept, 2, CHMe 2 ), 2.436 (br m, 2, CHMe 2 ). 1.413 (d.
125
6, CHMe 2 ), 1.402 (b, 3, B-Me), 1.390 (br d, 6, CHMe2 ), 1.254 (d, 18, CHMe 2), 1.201 (d, 6.
CHMe 2 ), 1.142 (d, 6, CHMe 2 ), 1.109 (d, 6, CHMe 2 ), 0.971 (d, 6, CHMe 2 ), 0.894 (d, 6.
CHMe 2 ), 0.755 (d, 6, CHMe 2 ), 0.387 (d, 6, CHMe 2 ), 0.201 (s, 3, Zr-CH 3 );
-131.77 (d,
3 JFF
= 21.2, o-C 6F 5 ), -164.25 (t, 3 JFF = 21.0, p-C 6 F 5 ), -166.67 (m,
19 F
NMR 8
3 JFF
(ave) =
22.0, m-C6F 5 ). Anal. Calcd for C82 H 102 N2 B 3 F1 5 Zr: C, 64.61; H, 6.74; N, 1.84. Found: C.
64.50; H, 7.06; N, 1.80
[(BigBen)ZrMe][B(C
6
F5 )4 ] (7).
A solution of 7 along with free Me 2 NPh was
generated by stirring [HNMe 2 Ph][B(C 6F 5 )4 ] (0.076 g, 0.095 mmol) with 3 (0.100 g, 0.095
mmol) in toluene-d8 (0.5 mL) resulting in vigorous gas evolution. Although 7 could not be
induced to crystallize, a powder could be obtained by exhaustive removal of toluene in vacuo
which was then washed with pentane (3 x 2 mL) to remove the aniline.
1H
NMR(C 7 D8 . RT) 8
7.188 (s, 2, mn-H), 7.174 (s, 4, m-H), 7.079 (s, 2, m-H), 3.976 (br AA'BB', 2, NCH2 ), 3.503
(br AA'BB', 2, NCH 2 ), 2.891 (m, 4, o-CHMe 2), 2.818 (sept, 2, CHMe 2), 2.732 (br m. 2.
CHMe 2 ), 2.574 (sept, 2, CHMe 2 ), 2.452 (br m, 2, CHMe 2 ). 1.420 (d, 6, CHMe2 ), 1.381 (br d.
6, CHLVfe 2 ), 1.253 (d, 18, CHMe2 ), 1.200 (d, 6, CHMe 2), 1.129 (d, 6, CHMe2 ), 1.116 (d. 6.
CHMe2 ), 0.971 (d, 6, CHMe 2 ), 0.904 (d, 6. CHMe 2 ), 0.756 (d, 6, CHMe 2 ), 0.399 (d. 6.
CHMe 2 ), 0.213 (s, 3, Zr-CH3); 19 F NMR 8 -131.80 (br, o-C 6 F5 ), -163.11 (m, p-C 6 F5 ), -166.85
(br, in-C 6 F5).
[(BigBen)Zr(CH 2 CH 3 )][B(C 6 Fs) 4 ] (8). A toluene-d 8 solution (0.5 mL) of 4 was
prepared from 2 (0.100 g, 0.092 mmol) and EtMgCl (0.072 mL, 0.193 mmol, 2.67 M in ether) by
following the protocol described in the synthesis of 4. A solution of 8 along with free Me 2 NPh
was generated by adding [HNMe 2 Ph][B(C 6 F5 )4 ] (0.074 g, 0.092 mmol) with stirring to the
toluene-d8 solution at -40 'C of freshly prepared 4 resulting in vigorous gas evolution and a slight
yellowing of the solution. 1 H NMR(C 6 D6 , 0 °C) 6 7.225 (s, 2, m-H), 7.200 (s, 2, mn-H), 7.155
(s, 2, m-H), 7.050 (s, 2, m-H), 4.096 (br AA'BB', 2, NCH 2 ), 3.745 (br AA'BB', 2, NCH2).
2.944 (m, 4, o-CHMe 2 ), 2.816 (sept, 2, CHMe 2 ), 2.700 (br m, 6, CHMe 2 ), 1.416 (d, 6,
CHMe2 ), 1.327 (d, 6, CHMe 2), 1.301 (d, 6, CHMe 2), 1.252 (d, 12, CHMe 2 ), 1.205 (d. 6.
126
CHMe2 ), 1.121 (d, 6, CHMe 2 ), 1.085 (d, 6, CHMe 2 ), 1.048 (d, 6, CHMe 2 ), 0.954 (d. 6.
CHMe2 ), 0.878 (d, 6, CHMe2 ), 0.368 (d, 6, CHMe2 ), -0.655 (t, 3 JHH = 8.0 Hz, 3, Zr-CH 2 CH 3 )
-- the Zr-CH 2 CH 3 resonance is obscured by the o-CHMe2 resonances;
[(BigBen)Zr(CH 2 CH 2 CH 2 CH 3 )][B(C 6 F5 )4 ] (9). A toluene-d8 solution (0.5 mL)
of 5 was prepared from 2 (0.100 g, 0.092 mmol) and BuMgCl (0.086 mL, 0.193 mmol, 2.25 M
in ether) by following the protocol described in the synthesis of 5. A solution of 9 along with free
Me 2 NPh was generated by adding [HNMe 2 Ph][B(C 6F 5 )4 ] (0.074 g, 0.092 mmol) with stirring to
the toluene-d8 solution at -40 oC of freshly prepared 5 resulting in a yellowing of the solution. 1H
NMR(C 6 D6 , RT) 8 7.272 (s, 2, m-H), 7.197 (s, 4, m-H), 7.056 (s, 2, m-H), 4.096 (br AA 'BB'.
2, NCH 2 ), 3.659 (br AA'BB', 2, NCH 2 ), 2.98 (m, 8, o-CHMe 2 ), 2.819 (sept. 2, CHMe 2 ).
2.719 (br m, 2, CHMe2), 1.5 - 0.75 (m, CHMe2 and Zr-CH 2 CH2CH 2 CH 3 ), 0.593 (t. 3JHH = 7.0
Hz, Zr-CH 2 CH 2 CH 2 CH 3 ), 0.41 (br d, 6, CHMe 2 ), -0.11 (br, 2, Zr-CH2 CH 2 ).
[(BigBen)ZrMe(NH 3 )][B(C 6 F 5 )4 ] (10). To a toluene solution (3 mL) of 7 freshly
prepared from 3 (0.100 g, 0.095 mmol) and [HNMe 2 Ph][B(C 6 F5 )4 ] (0.076 g, 0.095 mmol) was
added NH 3 (g) (2.35 mL @ 1 atm, 0.095 mmol) by syringe. After stirring for 20 minutes. the
solution was removed to give a solid residue containing 10 and Me 2NPh.
1H
NMR(C 7 D8 . RT) 6
7.230 (s. 2, in-H), 7.203 (s, 2, m-H), 7.101 (s, 2, m-H), 7.033 (s, 2, mn-H), 4.295 (AA'BB'. 2.
NCH 2 ). 3.924 (br AA'BB', 2, NCH 2 ), 3.145 (m, 4, o-CHMe 2 ), 2.920 (sept, 2, CHMe2), 2.814
(sept, 2, CHMe 2 ), 2.655 (br sept, 4, CHMe 2 ), 2.452 (br m, 2, CHMe 2 ), 1.468 (d, 6. CHMei).
1.348 (d, 6, CHMe 2 ), 1.245 (d, 12, CHMe 2 ), 1.184 (d, 6, CHMe 2 ), 1.142 (d, 6, CHMe2).
1.096 (d, 6, CHMe 2 ), 1.048 (d, 18, CHMe 2 ), 0.798 (d, 6, CHMe 2 ), 0.660 (s, 3, Zr-CH 3).
0.370 (s, 3, Zr-NH 3 ), 0.328 (d, 6, CHMe 2 ).
127
References
(1) Trip 2B-F is prepared in an identical fashion to that described for Xyl 2 B-F: Chen, H.; Bartlett,
R. A.; Olmstead, M. M.; Power, P. P.; Shoner, S. C. J. Am. Chem. Soc. 1990, 112, 1048.
(2) Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron Lett. 1986, 27, 2829.
(3) See discussion of the structure of [(TwistBen)Zr]2 in Chapter 3.
(4) Cummins, C. C.; Duyne, G. D. V.; Schaller, C. P.; Wolczanski, P. T. Organometallics
1991, 10, 164.
(5) Jordan, R. F.; Bradley, P. K.; Baenziger, N. C.; LaPointe, R. E. J. Am. Chem. Soc. 1990,
112, 1289.
(6) Guo, Z.; Swenson, D. C.; Jordan, R. F. Organometallics1994, 13, 1424.
(7) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623.
(8) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015.
(9) Jia, L.; Yang, X.; Ishihara, A.; Marks, T. J. Organometallics1995, 14, 3135.
(10) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics1997, 16, 842.
(11) Sandstr6m, J. "Dynamic NMR Spectroscopy"; Academic Press: New York, 1982.
(12)
[Cp 2 Ti(CH 3 )(NH 3 )][PF 6 ] has been prepared through the reaction of Cp 2 TiMe2 +
[NH 4 ][PF 6 ].
See Bochmann, M.; Wilson, L. M.; Hursthouse, M. B.; Short, R. L.
Organometallics1987, 6, 2556.
(13)
Free NH 3 is observed at 8 -0.14 ppm in toluene-d8; Glassman, T. E. Ph.D Thesis (Data
Compilation Appendix), MIT, 1991.
(14) Andersen, R. A. Inorg. Chem. 1979, 18, 2928.
(15) Planalp, R. P.; Andersen, R. A.; Zalkin, A. Organometallics1983, 2, 16.
(16) Erker, G.; Schlund, R.; Krtiger, C. Organometallics1989, 8, 2349.
(17) Horton, A. D.; de With, J. J. Chem. Soc., Chem. Comm. 1996, 1375.
(18)
Fachinetti, G.; Floriani, C.; Marchetti, F.; Merlino, S. J. Chem. Soc., Chem. Commiun.
1976, 522.
(19) Fachinetti, G.; Fochi, G.; Floriani, C. J. Chem. Soc., Dalton Trans. 1977,
128
(20) Erker, G.; Rosenfeldt, F. J. Organometal. Chem. 1980, 188, Cl.
(21) Guo, Z.; Swenson, D. C.; Guram, A. S.; Jordan, R. F. Organometallics1994, 13, 766.
(22) Jordan, R. F.; Bradley, P. K.; LaPointe, R. E.; Taylor, D. F. New. J. Chem. 1990, 505.
(23) Li2(BigBen)*4 THF does not react cleanly with TiC14 , TiC14 (THF) 2 , or TiC13 (THF)3 in a
variety of solvents. Warren, T.H. Unpublished observations.
(24) Turner, H. W.; Hlatky, G. G. PCT Int. Appl. WO 88/05793 (Eur. Pat. Appl. EP 211004.
1988), Exxon.
(25) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
(26) Manzer, L. E. Inorg. Svnth. 1982, 21, 135.
129
CHAPTER 5
Neutral and Cationic 3-H Containing Alkyls Supported by [Cy 2 BNCH 2 CH 2 NBCy 2 ]2
Ligated Zirconium and Hafnium and their Use in the Polymerization of 1-Hexene
130
Introduction
Although group 4 cations based on chelating bis(borylamide) ligands presented in Chapters
3 and 4 exhibit little or no polymerization activity with ethylene, during the course of this work
Scollard and McConville reported the living polymerization of ax-olefins based on titanium
complexes of a related chelating bis(amido) ligand.1 This result suggested that, if suitably
designed, bis(borylamide) based alkyl cations may too exhibit significant a-olefin polymerization
activity. The relative stability of neutral and cationic P-H containing alkyl derivatives presented in
Chapter 4 provides further incentive, since the M-alkyl bond may be stabilized against 3-H
elimination offering the prospect of developing a living a-olefin polymerization system.
Two limitations became apparent, however, in studies of the Ben and BigBen systems.
The high electrophilicity of the neutral and cationic group 4 alkyl complexes of Ben leads to facile
metalation of the ligands o-substituents as well as to strong anion binding. Efforts to combat this
behavior by increasing the size of the o-substituents met with limited success. Whereas neutral and
cationic alkyl complexes could be prepared which exhibit much higher thermal stability, the
extremely crowded nature of the coordination wedge was believed to shunt the reactivity of the
cations with ca-olefins such as ethylene and 1-hexene.
Accordingly, bis(borylamide) ligands based on smaller organoboryl groups were sought
which do not have o-substituents available to be metalated. One targeted ligand was (3,5Me 2 C 6 H3 )2 BNHCH 2 CH 2 NHB(3,5-Me 2 C6 H3 )2 (H2(m-Ben)), an analog of H2(Ben) in which
methyl groups appear in the meta positions of the boryl group. Although this new ligand may be
prepared from Me 2 NBC12 in two steps, 2 clean deprotonation to Li2(m-Ben) does not occur with a
variety of reagents. The Lewis acidity of the more exposed boron center in this borylamine may
lead to competing alkylation at boron with alkyllithium reagents rather than deprotonation at
nitrogen. 3 ,4 It should be noted that in no case during the course of these studies did the interaction
of free borylamines with homoleptic dimethylamides or metal chlorides lead to isolable dO
borylamido complexes. The preparation of a lithium or magnesium borylamide transfer reagent is
thus crucial to the synthesis of do borylamide complexes.
131
The dicyclohexylboryl group was thought to represent a steric compromise between
accessibility of the boron center during deprotonation of the borylamine and accessibility of the
coordination wedge to olefins in cationic group 4 derivatives. An additional attractive feature of a
bis(borylamide) ligand based on this boryl group is the ease of preparation of Cy 2B-Cl (Cy = cC 6 Hll) through hydroboration of H2 B-Cl * SMe 2 with cyclohexene. 5 This chapter reports the
synthesis of neutral and cationic zirconium and hafnium alkyl derivatives of the bis(borylamide)
[Cy 2 BNHCH 2 CH 2 NHBCy 2 ]2 - ([(CyBen)] 2 -) and their polymerization activity with 1-hexene.
Results
Ligand Synthesis and Preparation of Group 4 Derivatives.
Hydroboration of cyclohexene with commercially available H2 BCl*SMe 2 conveniently
provides an ethereal solution of Cy 2 B-C15 which may be added to a suspension of
LiNHCH 2 CH 2 NHLi (generated in situ from butyllithium and ethylenediamine) to prepare
H2 (CyBen) (1) in 30 g or greater quantities. The lithium derivative Li 2 (CyBen)*OEt 2 (2) may be
prepared in 95 % yield by deprotonation of H2(CyBen) with 2.1 eq. tBuLi in pentane containing
-5% ether (Scheme 5.1).
Reaction of ZrC14 or ZrC14 (THF) 2 with Li 2 (CyBen)*OEt 2 (2) in a variety of solvents does
not cleanly lead to the desired dichloro complexes (CyBen)ZrC12 (THF)x (x = 1 or 2). This may be
a result of the anticipated smaller steric extent of the CyBen ligand relative to the earlier
bis(borylamides) based on bulky aromatic substituents, perhaps allowing the formation of metal
centers containing more than one CyBen ligand. 6 .7 An approach involving two metal-based
"protecting" groups was instead employed.
Although preliminary studies showed that 2 reacts with the zirconium dialkyls
ZrR 2 C12(OEt 2 )2 (R = CH 2 Ph, CH 2 CMe 3 )8 to provide the corresponding (CyBen)ZrR 2
complexes, the addition of 2 to in situ prepared solutions of the trimethylsilylmethyl derivatives
M(CH 2 SiMe 3 )2 C12 (OEt 2 )2 9 proved to be the most convenient synthetic entry (Scheme 5.2).
Accordingly, the zirconium (3a) and hafnium (3b) derivatives (CyBen)M(CH 2 SiMe3) 2 may be
isolated in 70-80% yield as colorless crystals from pentane. Complexes 3 are thermally stable in
132
Scheme 5.1. Synthesis of H2 (CyBen) (1) and Li 2 (CyBen).OEt 2 (2).
2
THF
Et20
0OC
NH 2 + 2 BuLi
H2N
+ 2 H2BC1 * SMe 2
- 2 Me 2S
-78 C
1/x LiHN
CB
-2 BuH
NHLi]
C1
i~i
-78
OC
- RT
- 2 LiCl
BN
N
H
H
B
pentane / Et2 O
+ 2 tBuLi
- 40 oC - RT
- 2 tBuH
H2 (CyBen)
(68%)
1
N
LOEt2
Li 2 (CyBen) * OEt 2 (95%)
2
B
133
solution and show no tendency to eliminate SiMe4 at room temperature. The trimethylsilylmethyl
"protecting" groups in 3 may be cleaved by the addition of 2 eq. of 12 to a dichloromethane
solution of 3 affording the diiodo complexes (CyBen)MI 2 (M = Zr (4a), Hf (4b)) which are
isolated as colorless crystals in 80-90% yield (Scheme 5.2).
Scheme 5.2. Preparation of Zirconium and Hafnium CyBen Derivatives.
Et20
MC14 + 2 LiCH 2SiMe 3
Et2
RT
M(CH 2SiMe 3 )2C12 (OEt 2) 2
Li 2(CyBen)*OEt 2 (2)
BI
BI
N
(a) M = Zr
(b) M = Hf
N
,,CHSiMe 3
2 12 / CHC1,
CHSiMe 3
- 2 Me 3SiCH 2
N/
B
N
B
3a,b
4a,b
Synthesis of Dialkyl Complexes Containing 3-H Atoms.
The diiodide complexes 4 are useful precursors to a variety of dialkyl complexes containing
P-H atoms as they undergo swift, clean reactions with Grignard reagents (Scheme 5.3). Whereas
the n-hexyl derivatives 7 are isolated as analytically pure oils, the primary and secondary dialkyl
derivatives 5, 6, and 8 may be isolated as colorless crystals in 70-90% yield from pentane at
-30 `C.
In contrast to the smooth alkylation of 4 with the reagents outlined in Scheme 5.3, 4a does
not react cleanly with MeMgBr or Me 2 Mg. Although (CyBen)ZrMe2 may be identified by 1H
NMR spectroscopy in these reactions (8 3.75 and 0.48 ppm, backbone and Zr-Me, respectively), it
has not been isolated from a significant amount of Cy 2 B-Me (6 0.68 ppm. B-Me) that is always
134
present in its preparations.
(Cy 2 B-Me was prepared straightforwardly from Cy 2 B-Cl and
MeMgBr to confirm this assignment.)
The formation of Cy 2 B-Me here coupled with the
essentially quantitative conversion of 3a to the larger dialkyls 5a - 8 suggests that alkylation of the
Cy2 B unit competes with alkylation at zirconium (Scheme 5.4).
Scheme 5.3. Synthesis of P-H Containing Dialkyl Complexes.a
(CyBen)M(CH 2CH 3)2
5a,b
(CyBen)M(CH 2 CHMe2)2
6a,b
(ii)
B
N
N
4a,b
(CyBen)M(CH2CHBu)2
(a) M = Zr
(CyBen)Zr(CHBu2)2
7a,b
(b) M = Hf
8
a All reactions performed at -30 'C in dichloromethane employing 2.1 eq. of the following
Grignard reagents: (i) EtMgCl; (ii) iBuMgCl; (iii) (n-hexyl)MgBr; (iv) Bu 2CHMgBr.
Scheme 5.4. Competitive B vs. Zr Alkylation.
Zr
.I
(Z
\/
N
•I
+
Me 2Mg or
2 MeMgBr
Me
B
(CyBen)ZrMe2
135
[Zr] ...
Proton NMR spectra of dialkyl complexes 5 - 8 show a symmetric backbone consistent
with the presence of two identical ligands in the coordination wedge. Though the cyclohexyl
groups produce a broad manifold of 1H resonances,
13 C
NMR spectra exhibit eight somewhat
broadened cyclohexyl resonances indicating that N-B bond rotation is not fast on the NMR time
scale at room temperature. N-B bond rotation does occur, however, as pairs of cyclohexyl 13 C
resonances in 5a and 6a coalesce in the range of 50 - 60 'C. The corresponding AG rot values of
16.2 (2) and 16.3 (2) kcal/mol, respectively, demonstrate the sharing of the nitrogen lone pair
within the M-N-B linkage as well as the absence of any sizable steric barrier to N-B rotation in
these dialkyl complexes.
X-ray Structure of (CyBen)ZrEt 2 (5a).
The molecular structure of 5a was determined in an X-ray study and appears in Figure 5.1
and pertinent bond distances and angles are collected in Table 5.1. The structural consequences of
N-B xr-bonding in 5a are unmistakable. The two planar N-B units (N-B distances = 1.392 (4) and
1.398 (4) A) linked by the slightly puckered, symmetrically chelating five-membered ring (Zr-N
distances = 2.067 (2) and 2.070 (2) A; N(1)-Zr-N(2) = 78.87 (2)0) place the boron cyclohexyl
rings directly above and below the coordination wedge resulting in an C(11)-B(1)-B(2)-C(2 1)
dihedral angle of only 9.80. Sandwiched in a plane between these two cyclohexyl groups are the
two ethyl ligands (Zr-C distances = 2.247 (3) and 2.253 (3) A). This conformation is not a result
of f-H atoms interacting with the metal center, however, since the Zr-CU-Cp angles and Zr**'Cp
distances are 114.8 (2), 119.7 (2) ° and 3.20, 3.28 A respectively.
The cyclohexyl rings are bent away from the coordination wedge, reflecting the placement
of the B-C-H unit, rather than the cyclohexyl ring towards the coordination wedge. Furthermore,
the absence of organoboryl o-substituents considerably opens the coordination wedge of
(CyBen)ZrEt 2 from the side relative to that found in the structures of (Ben)Ti(CH 2 Ph)Cl and
(BigBen)ZrMe 2 . It is also clear from the structure of 5a that there are no organoboryl C-H bonds
readily available for metalation, which undoubtedly accounts for the high thermal stability of the
136
Figure 5.1. Chem-3D drawing of the X-ray structure of (CyBen)Zr(CH 2 CH 3 )2 (5a).
137
Table 5.1. Selected Distances (A) and Angles (0) for (CyBen)Zr(CH 2 CH 3 )2 (5a).
Distances
Zr-N(1)
2.067 (2)
Zr-N(2)
2.070 (2)
Zr-C(1)
2.247 (3)
Zr-C(2)
2.253 (3)
N(1)-B(1)
1.392 (4)
N(2)-B(2)
1.398 (4)
Angles
N(1)-Zr-N(2)
78.87 (2)
C(1)-Zr-C(3)
119.53 (12)
Zr-C(1)-C(2)
114.8 (2)
Zr-C(3)-C(4)
119.7 (2)
138
Table 5.2. Crystallographic Data, Collection Parameters, and Refinement Parameters for
(CyBen)Zr(CH 2 CH 3 )2 (5a).
Empirical Formula
Formula Weight
C 30 H5 8B2 N2Zr
559.62
Siemens SMART/CCD
Diffractometer
Crystal Color, Morphology
Crystal Dimensions (mm)
Crystal System
a
b
colorless, prismatic
0.28 x 0.24 x 0.18
Triclinic
9.9911 (2) A
12.23940 (10) A
c
13.82900 (10) A
71.1280 (10) 0
85.0270 (10)
0
77.2990 (10) o
V
1560.80 (4) A-
PI
Space Group
Z
1.191 g/cm 3
Dcalc
F0 00
604
0.372 mm - 1
pt(MoKQc)
Scan Type
0 scans
Temperature (°C)
Total No. Unique Reflections
No. Variables
183 (2) K
4364
317
R
0.0345
Rw
GoF
0.0970
1.234
139
trimethylsilylmethyl complexes 2 relative to the metalation which occurs upon attempted formation
of (Ben)Zr(CH 2 SiMe3)2 from (Ben)ZrCl 2 (THF) and 2 eq. LiCH 2 SiMe 3 Cation Formation and Solution Structure:
The Example of [(CyBen)Zr(CH 2 CHMe2)
+.
Reaction of (CyBen)Zr(CH2CHR2) (R = H (5a), Me (6a)) with [Ph 3 C][B(C 6F 5 )4 110 in
toluene-d8
at
room
gives
temperature
rise
to
the
cationic
complexes
[(CyBen)Zr(CH 2 CHR 2)][B(C 6F 5 )4] (5a+, 6a+) identified by 1H NMR spectroscopy (eq 1).
+
(CyBen)Zr(CH 2CHR 2) 2 + [Ph 3C][B(C 6F
toluene-dg
.
RT
4]
(N\
N/
5a+
6a+
Room temperature
1H
CH
L
(1)
[B(C 6 F5)4]-
R=H
R = Me
NMR spectra of 5a+ and 6a+ in toluene-d8 show sharp, well-resolved
AA'BB' backbone resonances consistent with the presence of a relatively non-labile coordinating
ligand (L) opposite the alkyl ligand in the coordination wedge. It should be pointed out that the
ambiguity of the origin of backbone asymmetry for the cations [(BigBen)Zr-R] does not exist here
(anion coordination vs. hindered organoboryl group rocking), since the symmetrically substituted
3 - 8 each show a sharp, single backbone
1H
resonance, even at low temperature.
19 F
NMR
spectra show three sharp resonances for the [B(C 6 F5 )4 ] anion, suggesting that if the anion is in
fact this ligand L, it must be swiftly rotating in place on the NMR timescale. In contrast to the
upfield-shifted Cp-H 1 H resonances observed for [(BigBen)Zr-R] (R = Et, Bu),
the
corresponding resonances in 5a+ and 6a+ at 6 1.33 and 1.97 ppm (Table 5.3), respectively, do
not suggest O3-agostic interactions. l l
140
Additional 1H resonances are observed in toluene-d8 solution which outline the fate of the
lost metal-bound alkyl groups as well as that of the trityl cation. In the reaction of 5a with
[Ph 3 C][B(C 6 F5 )4 ] in toluene-d8, Ph 3 C-CH 2 CH 3 as well as Ph 3 C-H are observed in a 1.0/1 ratio
(eq 2). Reaction of 6a with [Ph 3 C][B(C 6F 5 )4 ] in toluene-d8 leads to exclusive formation of Ph 3 CH; isobutylene is also present in amounts varying depending on the exact preparative conditions
(eq 3). Although ethylene cannot be directly observed in 1H NMR spectra of solutions of
5a + [Ph3C][B(C6F5) 4]
6a + [Ph3C][B(C6F5) 4
toluene-d 8
RT
[(CyBen)Zr(CH 2CH 3)(L)] [B (C6 F5)4]
(5a+)
+ 0.5 Ph 3C-CH 2 CH 3 + 0.5 Ph 3 C-H
toluene-d 8
[(CyBen)Zr(CH 2 CHMe2)(L)][B(C 6F5 )4]
RT
+ 1.0 Ph C-H + HC=CMe,
-
(6a+)
(2)
(3)
5a+, reaction of 5a with [Ph 3 C][B(C 6 F5 )4 ] produces a small amount of white flocculent solid,
presumably polyethylene, which is not observed in the formation of 6a+. Consistent with this
interpretation, addition of ethylene (ca. 1 atm) to an NMR tube containing 6a+ in toluene-d8
results in the complete consumption and polymerization of ethylene upon mixing.
Scouting experiments with 1-hexene showed that these toluene-d8 solutions, however.
showed little polymerization activity and thus prompted the generation and examination of 5a+ and
6a+ in other solvents. Reaction of [Ph 3 C][B(C 6F 5 )4 ] with 5a or 6a in dichloromethane-d2,
1,1 ,2,2-tetrachloroethane-d2, or bromobenzene-d5 leads to the formation Ph 3 C-Et and/or Ph3C-H.
but no bis(borylamide) based alkyl cation may be identified. Whereas quite persistent in toluened 8 , the cations 5a+ and 6a+ are not stable in these halogenated solvents.
Reaction of 6a with [Ph 3 C][B(C 6F 5 )4 ] at -30 'C in chlorobenzene-d 5 , however, results in
the clean formation of 6a+ which exhibits solution behavior contrasting sharply with that observed
in toluene-d8.
Room temperature 1H NMR spectra of 6a+ in chlorobenzene-d5 exhibit a broad
141
Figure 5.2. Variable temperature 1H NMR (300 MHz) spectra of 6a+ in chlorobenzene-d5.
CHMe2
backbone
Ph 3 C-H
CHMe2
40 "C
,• m , •
r
rT
' I
.
.
.
I
.
.
.
.
.
--
T.I I I . .
I I. .
I I I . I
I
. I I
--
I , . . I
' ' '
. . . . I
CHMe1
25
OC
10 OC
0 oC
H2C--CMe2
Ph 3 C-H
-30 oC
-,A
I
-15 oC
---
p 1 1 1 1
5
4
3
142
2
1
ppm
single resonance for the ligand backbone. Cooling the sample results in the further broadening of
the backbone resonance and at -35 'C two well separated, but slightly broadened AA'BB'
subspectra are observed (Figure 5.2). Thus at low temperature in chlorobenzene-d5, 6a+ also
appears to interact with an additional ligand, but one which is considerably more labile than in
toluene-d8. Low temperature
19 F
NMR spectra of these solutions is virtually superimposable with
that of [Ph 3 C][B(C 6 F5 )4 ] in chlorobenzene and show no appreciable broadening other than that
which may be attributed to increased solvent viscosity at low temperature. As in toluene-d8, it
does not appear that the anion gives rise to the unsymmetric solution structure at low temperature.
The temperature at which the AA'BB' subspectra coalesce (Tc = 4.0 (5) 'C, 300 MHz) may
be used to estimate the barrier to symmetrization (AG.symm = 12.8 (2) kcal/mol) for 6+ in
chlorobenzene-d 5 (Table 5.3).12 This coalescence temperature did not change over a six-fold
concentration range (1.3 - 8.0 x 10-2 M). The addition of small amounts of toluene (0.5 - 2 eq.),.
however, increased the observed coalescence temperature by 10 - 20 'C, increasing with increasing
amount of toluene (2/3 - 2 eq.) added. Since the chemical shift separation between the AA'BB'
subspectra at low temperature does not change appreciably with small amounts of added toluene.
toluene effectively slows this symmetrization process.
The fact that small amounts of added toluene slow the symmetrization process coupled with
the non-labile conformation observed for 6a+ in toluene-d8 suggests that toluene strongly binds to
6a+. In analogy, the asymmetry of 6a+ at low temperature in chlorobenzene is proposed to result
from weak binding of the chlorobenzene solvent (Figure 5.2).
Figure 5.3. Symmetrization of 6a+ in chlorobenzene-d 5 .
+
BI
N
+
AG symm
IlZr".CH2CHMe2
N
N
S
symm
S = toluene or chlorobenzene
B
143
BI
Z
S
N/ ,CH2CHMe2
N
B
2
Generation and Characterization of Primary Alkyl Cations in Chlorobenzene-d 5 .
Reaction of the primary alkyls 5 - 7 with [Ph 3 C][B(C 6 F5 )4 ] in chlorobenzene-d
5
is a
general route to the corresponding alkyl cations 5+ - 7+ (Scheme 5.4). Although the ethyl and
hexyl derivatives 5+ and 7+ were found to be somewhat less thermally stable than the isobutyl
derivatives 6+, in the presence of 5 eq. toluene, 5+ and 7+ could be formed cleanly and exhibited
appreciable thermal stability. The secondary derivative (CyBen)Zr(CHBu 2 )2 (8) did not appear to
form as cleanly by an analogous route, even in the presence of 5 eq. toluene.
Scheme 5.5. Generation of Primary Alkyl Cations (chlorobenzene-d5, -30 'C).
[(CyBen)M(CH 2 CH 3 )(tol)] [B(C 6F5 )4]
- x Ph 3 C-Et, (1-x) Ph 3C-H, CH 2 =CH 2
[Ph 3C][B(C 6F 5) 4]
+
. [(CyBen)M(CH 2CHMe 2)(S)] [B(C 6F5 )4]
[(CyBen)M(CH 2 CHR2)]
- Ph 3 C-H, CH 2 =CMe 2 (S = C 6D5C1)
6a+, 6b+
5-7
(a) M = Zr
(b) M = Hf
[(CyBen)M(CH 2 CH 2Bu)(tol)][B(C 6F5 )4]
- Ph 3C-H, CH 2 =CHBu
Although binding of toluene to 6+ is implied by corresponding increases in Tc for
symmetrization with increasing amount of added toluene,
1H
resonances for both bound (6 2.29 -
2.21 ppm) and free (8 2.18 ppm) Ph-Me are observed at -30 'C in the ethyl (5+) and hexyl (7+)
cations (Table 5.3). Upon warming through -10 'C, the resonance due to bound toluene moves
upfield and is absorbed by the free toluene resonance signifying rapid exchange of toluene.
Furthermore, only in the presence of toluene and at low temperature, a resonance of area 2 may
be found at high field (5 -0.6 - 0.0 ppm) which is assigned to the cyclohexyl B-C-H group which
protrudes into the coordination wedge. Such an upfield shift may be expected due to the ring
current of the bound toluene ligand and the relative orientation and proximity of these B-C-H units
based on the X-ray structure of 5a.
144
Table 5.3. IH NMR Parameters for Alkyl Cations 5+ - 7+ (in chlorobenzene-d5 unless noted otherwise).
Complex Temp (oC) #eq. Base
6a+
6a+
-30
-20
none
5 eq. tol
8(coord)
n.a.
n.r.
8(free)
n.a.
2.14
8 (AA')
3.79
3.79
8 (BB') T, (oC) AG symm
(kcal/mol)
3.46
3.22
4
12.8 (3)
22
(bromobenzene-d5)
6a+
6a+
6b+
25
25
-30
1.1 eq. Me 2 NPh
neat tol-d8
none
2.80
n.o.
n.a.
2.67
n.a.
n.a.
3.99
3.78
3.78
3.48
3.27
3.19
n/a
n.o.
32
14.3 (3)
Other resonances
2.01 (m)
1.07 (d)
0.85 (d)
CHMe 2
Zr-CH2
1.94 (br m)
0.98 (d)
CHMe 2
Zr-CH 2
0.80 (d)
-0.03 (br)
CHMe2
B-C-H
2.08 (m)
1.08 (d)
0.93 (d)
CHMe2
Zr-CH2
-0.44 (br m)
B-C-H
1.97 (m)
0.96 (d)
0.85 (d)
CHMe2
Zr-CH2
2.14 (m)
0.93 (d)
0.88 (d)
CHMe2
CHMe2
Hf-CH2
CHMe2
CHMe2
CHMe2
Table 5.3 (continued).
Complex Temp (oC)
1H
NMR Parameters for Alkyl Cations 5+ - 7+ (in chlorobcnzene-d
#eq. Base
8(coord)
8(free)
8 (AA')
8 (BB')
5
unless noted otherwise).
Tc (°C) AG:symm
(kcal/mol)
5a+
-30
5 eq. tol
2.21
2.17
3.80
3.29
28
5a+
25
neat tol-d 8
n.o.
n.a.
3.75
3.23
n.o.
14.1 (3)
Other resonances
1.39 (t)
-0.47 (br)
Zr-CH 2 CH 3
B-C-H
1.33 (t)
Zr-CH 2 CH3
0.66 (q)
Zr-CH 2
5b+
-30
5 eq. tol
2.26
2.18
3.86
3.35
61
15.8 (3)
1.54 (t)
0.45 (br q)
-0.24 (br)
Hf-CH 2 CH3
Hf-CH 2
B-C-H
7a+
-30
5 eq. tol
2.26
2.18
3.86
3.33
39
14.7 (3)
0.87 (t)
-0.29 (br)
CH 2 CH 3
B-C-H
7b+
-30
5 eq. tol
2.29
2.17
3.87
3.35
72
16.3 (3)
0.86 (t)
0.58 (br m)
-0.21 (br)
CH 2 CH 3
Hf-CH2
B-C-H
n.r. = not recorded; n.o. = not observed; n.a. = not applicable
In accord with the relatively strong binding of toluene, generation of 6a+ in the presence
of
1.1
eq.
NMe 2 Ph
results
in
the
formation
of
the
aniline
adduct
[(CyBen)Zr(CH 2 CHMe 2 )(NMe2Ph)] + which exhibits sharp, well resolved AA'BB' backbone
1H
resonances indicating that the cation is not fluxional. Furthermore, resonances for both bound and
free NMe 2 Ph groups may be observed at room temperature (Table 5.3), indicating exchange is not
fast on the NMR timescale.
Polymerization of 1-Hexene in Chlorobenzene.
Preliminary experiments identified that rapid polymerization of 1-hexene occurred upon
activation of 5a in dichloromethane,
chlorobenzene.
1,1,2,2-tetrachloroethane,
bromobenzene.
and
A more detailed investigation into the polymerization in chlorobenzene was
attractive, given the clean generation of alkyl cations 5+ - 7+ in this solvent coupled with their
known solution behavior revealed by the NMR studies above.
The yield of polymer obtained through a variety of reaction conditions, however, was
generally low (5 - 25 %) and proved to be irreproducible.
GPC analysis of such polymers
generated at 0 - 25 'C with a monomer / cation molar ratio ( 1-hexene / 6+) of 200 - 300 showed
that the molecular weight (Mn = 3 - 7 x 105) and PDI (2.5 - 6) of the these polymers were much
higher than expected for uniform monomer consumption.
Increasing the concentration of 1-hexene and lowering the reaction temperature led to
higher, almost quantitative yields of polyhexene with 5a+, 5b+, or 6a+ (Table 5.4). High
molecular weight polymer resulted in each case, consisting of a broad distribution of chain lengths
biased towards low molecular weight. Though this result is consistent with both incomplete
initiation and gradual catalyst decay, the fact that little polymer (<10%) was produced from the
related 6b+ and that it was irreproducible for 6a+ under these conditions suggests that only a
small fraction of the metal centers initiate chain growth. Where chain growth occurs, it proceeds
with a high degree of regiospecificity. The clean, six-line
13 C NMR
spectra (in chloroform-dl) of
the polymers in entries 1 and 3 additionally attest to some degree of isotacticity (Figure 5.4a) as
147
determined by comparison with the
13 C
NMR resonances reported in the literature for isotactic
poly- 1-hexene.13-16
Table 5.4. Polymerization of 1-Hexene in Chlorobenzene.a
Entry Initiator
Temp
(oC)
Yield
(%)
Mnb
(g/mol)
Mw
(g/mol)
PDI
(1)
5a
-30
96
3.58 x 104
2.07 x 105
5.78c
(2)
5b
-30
96
1.27 x 105
7.19 x 105
5.68c
(3)
6a
-30
97
5.62 x 104
3.09 x 105
5.49
(1.50 mL hexene (225 eq.) / 5.0 mL chlorobenzene / Tp = 60 min, quenched with 1 mL MeOH)
a Each dialkyl was activated by [Ph 3 C][B(C 6F 5 )4 1].b Mn(expected) = 1.89 x 104 for complete.
uniform monomer consumption. CA resolved, low molecular weight peak consisting of less than
7% of sample weight was excluded from analysis.
Polymerization of 1-Hexene in Dichloromethane.
Although NMR studies showed that clean formation of [(CyBen)Zr(CH 2 CHMe 2 )]+ (6a+)
does not occur in dichloromethane, preliminary experiments generating 6a+ in the presence of 1hexene in these solvents reproducibly resulted in very rapid polymerization and heat evolution. In
one such experiment conducted in dichloromethane (Table 5.5; entry 1), roughly half the 1-hexene
was polymerized and GPC analysis showed that although its molecular weight distribution was
trimodal, most of the polymer weight was contained in the low molecular weight peak. Although
Mn for this peak was only somewhat lower than that found for corresponding polymerizations in
chlorobenzene, it was far more narrow with a PDI of 1.18. This observation prompted the further
investigation of polymerization in dichloromethane and the results are collected in Table 5.5.
In accord with the stabilizing effect of toluene on the P-H containing alkyl cations generated
in chlorobenzene, it was thought that toluene could be used as a labile, stabilizing "base" 17 in
these reactions, extending the lifetime of the active catalytic species. Whereas decreasing the
temperature had a modest effect on the yield, the addition of toluene boosted the overall
conversion of 1-hexene to polymer. An optimization of reaction conditions is evidenced by entry
148
7. Although the PDI is quite low (1.08) and Mn is what would be predicted for uniform
consumption at 76% conversion (1.41 x 10 4 g/mol), longer reaction times failed to produce
higher yields of polymer indicating that the species responsible for chain growth dies before the
polymerization is quenched. Furthermore, although the polymer yield is reproducible under these
conditions, the molecular weight (Mn = 5.40 x 10 3 g/mol) and PDI (1.40) varied in a second
polymerization (entry 8).
Table 5.5. Polymerization of 1-Hexene with 6a and 6b Activated by [Ph 3 C][B(C 6 F5 )4 1].
Entry Initiator
Temp
(°C)
Equiv.
Toluene
Yield # modes
Mnb
(%) GPC tracea (g/mol)
Mw
(g/mol)
PDI
(1)c
6a
RTd
0
45
3
3.65 x 104
4.29 x 104
1.18
(2)
6a
RTd
0
50
3
1.24 x 105
1.62 x 105
1.31
(3)
6a
RTd
5
60
2
1.22 x 105
2.28 x 105
1.87
(4)
6a
0
5
58
1
2.42 x 104
3.67 x 104
1.53
(5)
6b
0
5
57
1
5.55 x 104
1.94 x 105
3.50
(6)
6a
-30
5
68
1
2.24 x 104
2.82 x 104
1.26
(7)
6a
0
15
76
1
1.45 x 104
1.56 x 104
1.08
1
4.61 x 105
7.31 x 105
1.59
1
5.40 x 103
7.55 x 103
1.40
1.66 x 105
4.04 x 105
2.44
(8) e
(9)
(10)f
(as above, after heating)
6a
0
15
76
(as above, after heating)
(1.50 mL hexene (225 eq.) / 5.0 mL dichloromethane / Tp = 60 min, quenched with 1 mL MeOH)
a The low molecular weight peak was responsible for most of the polymer sample weight; the
higher molecular weight peaks were not used in the calculations of Mn and Mn. b Mn(expected)
= 1.89 x 104 for complete, uniform polymerization of 1-hexene. c Generated in the presence of
1-hexene and quenched after 10 min. d Polymerization performed at room temperature; exotherm
observed upon addition of 1-hexene. e After heating polymer in entry 7 for 12 h in vacuo at 110
°C. f After heating polymer in entry 9 for 10-15 h in vacuo at 100 OC.
149
(b)
14.18
23.23
28.71
40.24
32.37
34.60
(a)
40
35
30
25
20
15
Figure 5.4. 13C NMR (75.4 MHz) spectra of poly-l-hexene. (a) Polymer from chlorobenzene (Table 5.4, entry 3) is
regioregular and biased toward isotactic (bottom). (b) Polymer from dichloromethane (Table 5.5, entry 8) is regioirregular
(top). Both spectra share the same scale. * denotes spectrometer artifact.
ppm
1H
and
13 C
NMR analysis (chloroform-dl) of the poly-l-hexene produced in
dichloromethane demonstrates it to be a different material than that prepared in chlorobenzene.
Only internal olefinic resonances (5.5 - 5.0 ppm) are identified in 1H NMR spectra and many
broad signals observed in
13 C
NMR spectra (Figure 5.4b) show that the polymer is not
regioregular. Furthermore, extensive drying in vacuo with heating results in what appears to be
cross-linking. Comparison of entries 7 and 8 in Table 5.4 shows that both the molecular weight
and PDI increases after heating for 12 hours at 100 OC. Although cross-linking with heating is
reproducible (entries 9 and 10), the origin of this processes is not clear at this stage. It is to be
noted that all of the polymers formed in dichloromethane are viscous oils, in contrast to the
rubbery solids produced in chlorobenzene.
Discussion
The use of dicyclohexylboryl groups which lack o-substituents to shield the coordination
wedge in group 4 complexes based on the bis(borylamide) [(CyBen)] 2 - leads to a family of
thermally stable 3-H containing dialkyl complexes which do not suffer from ligand metalation.
Furthermore, it appears that 1-H abstraction/elimination pathways are disfavored as well. For
example, (CyBen)Zr(CH 2 CHMe 2 )2 is stable towards heating at 60 'C for at least 20 min as no
discoloration of the benzene-d6 solution occurs. Neither ethane nor ethylene was observed upon
thermolysis of (CyBen)Zr(CH 2 CH3) 2 at 70 oC in the presence of excess PMe3;
rather.
cyclohexane was observed by some undetermined pathway.
Alkyl cation formation readily occurs upon reaction of the 3-H containing dialkyls
[Ph 3 C][B(C 6 F5 )4 ] in chlorobenzene. Although the trityl reagent has seen extensive use in
metallocene chemistry as a methyl and benzyl abstracting reagent, 10,18 Bochmann has recently
reported its use with Cp 2 HfEt 2 which undergoes exclusive 3-H abstraction producing Ph 3 C-H and
ethylene along with the unstable [Cp 2 Hf-Et][B(C 6 F5 )4 ]. 19
The solution behavior of the cations 5+ - 7+ is consistent with labile coordination of the
chlorobenzene solvent. Toluene, however, preferentially binds in chlorobenzene solution and
exchange with free toluene is slow on the NMR timescale at -30 'C for the ethyl (5+) and hexyl
151
(7+) cations. The hexyl cations appear to have the strongest affinity for toluene. Not only do
they exhibit the largest chemical shift difference between the bound and free Ph-Me 1H resonances
at -30 'C, but also require the highest coalescence temperatures for symmetrization relative to the
ethyl and isobutyl derivatives. Another trend is also clear - the hafnium arene complexes are less
labile than their zirconium counterparts, requiring temperatures generally 30 'C higher to effect
symmetrization.
The affinity of cations 5+ - 7+ for toluene may be compared to arene complexes of cationic
group 4 alkyls bearing one cyclopentadienyl ligand. Whereas [CpMMe2(arene)][MeB(C 6F 5 )41
(Cp = Cp*, 20 1,3-(TMS) 2 C 5 H 3 2 1 ) do not undergo exchange with free toluene at room
temperature, the linked Cp-amide cation{ [(Me 4 C5 )SiMe 2 (NCMe 3 )]ZrMe }+ binds toluene in the
coordination site adjacent to the Zr-Me group and only reluctantly undergoes exchange at elevated
temperatures in toluene-d8. 22
Although the 3-H containing primary alkyl cations 5+ - 7+ could be viewed as models of
chain carrying species in oa-olefin polymerization at a cationic group 4 center, several observations
suggest that they are not involved in the 1-hexene polymerizations discussed above.
In
chlorobenzene solvent, initiation of polymerization is not reproducible. One possibility entertained
was that initiation might be slow because CyBen-based secondary alkyl cations might be the actual
chain carrying species, propagating by 2,1-insertion rather than the primary alkyl cations formed
from 5 - 7. Unfortunately 8+, which would serve as a model for 2,1-insertion of 1-hexene.
does not appear to cleanly form upon reaction of 8 with [Ph 3C][B(C 6F5 )4 ], even in the presence of
toluene. Exposure of a chlorobenzene solution of this ill-defined "activated" 8 to 1-hexene does
not result in any significant production of polymer.
Since 6a+ exhibits some stability at 40 - 50 'C where exchange of chlorobenzene is fast,
polymerization of 1-hexene was performed at 45 'C where, due to the greater lability of solvent. 1hexene may better compete with solvent for coordination. Although polymer was formed in ca.
80% yield, GPC analysis showed that it consists of a bimodal distribution of chain lengths.
152
Furthermore, this oil-like polymer more closely resembles the regioirregular polymers produced in
dichloromethane rather than the regioregular polymers prepared in chlorobenzene.
The reproducibility of the polymerizations in chlorobenzene appears to depend most on the
relative stability of the cations 5+ - 7+ in the absence of a strongly coordinating solvent such as
toluene. Qualitatively the least stable cation, 6a+ reproducibly polymerizes 1-hexene to >95%
conversion under the conditions outlined in Table 5.4. This observation strongly suggests that a
species resulting from a second "activation" of the dialkyls 5 - 7 is actually responsible for the
polymerization of 1-hexene.
Further corroborating the need for secondary "activation" to achieve polymerization.
reproducible polymer yields may be obtained in dichloromethane, a solvent in which cations 5+ 7+ cannot be observed, even in the presence of 15 eq. toluene. Whatever unstable species is
actually responsible for the polymerization appears to be well behaved in the sense that
polydispersities under 1.3 could be attained, generally indicative of a single catalytic species. This
species, however, is very active and not very discriminate as evidenced by the regioirregular
polymerization of 1-hexene.
Scouting experiments also suggest that the CyBen-based cations are not themselves
responsible for any polymerization activity which may be observed with other monomers.
Whereas the addition of ca. 2 atm ethylene to an NMR tube containing 6+ in chlorobenzene-d5
results in immediate, complete polymerization of ethylene upon mixing, this activity does not
appear to be long-lived. Little (< 200 mg) polyethylene could be isolated upon prolonged (5-15
min) exposure of chlorobenzene solutions of 6a+ to 1 atm ethylene. Allylbenzene was attractive
as a monomer since it could preferentially pre-coordinate through its phenyl ring to the alkyl
cations described here. Such "pre-binding" of the olefin was in fact observed,
as Tc for
symmetrization went up to 10 OC for a solution of 6a+ in the presence of 5 eq. allylbenzene.
Polymerization of most of the allylbenzene occurred after standing at 35 'C for ca. 10 minutes, as
evidenced by a new manifold of benzylic 1 H resonances between 8 2.5 - 3.0 ppm. Performing
this reaction on a preparative scale, however, did not lead to any appreciable amount of polymer.
153
Scheme 5.6. Bis(amides) as Zicgler-Natta Catalysts. (Anion is [B(C 6 F ) [- for all well defined cations shown
5 4
below.)
SiMe 3
Me 3Si-
SiMe3
N
+ MAO
/
Me 3Si-N
Ti.
"Cl
Cl
N/
N
SiMe 3
SiMe 3
* isotactic polypropylene
+ MAO
10
* long-lived polyethylene catalyst
* ethylene polymerization
propylene oligomerization
NMe 2 Ph
NMePht
t.
Me 3Si\
I
Me 3 C
NII,,-M
/Nve
Me 3 Si I \,
/
\CH 2
Me 3Si
Me Me
C
N
N
Ti
Me
/ NMe
+ B(C6F5) 3
N-
13
Ar = 2,6-Me 2C 6H3 or 2,6-'Pr 2C 6 H
3
* ethylene, propylene polymerization
Living polymerization of ax-olefins
(in the presence of excess olefin)
* Living polymerization of I-hexene,
a-olefin block copolymers
Comparison with Other Bis(amido) Systems.
During the course of this work, several related bis(amide) systems have appeared in the
literature in the context of Ziegler-Natta catalysis (Scheme 5.6). Preceding all such efforts, Canich
and Turner of Exxon laid claim in a 1991 patent to much of bis(amide) chemistry in the context of
a-olefin polymerization by MAO activated systems. 23 Utilizing Andersen's amides based on the
voluminous [(Me 3 Si) 2 N]- ligand, 24 Canich and Turner specifically reported the formation of 90%
isotactic polypropylene in toluene by MAO activated [(Me 3 Si) 2 N] 2 ZrCl 2 (9).24
Since 1995, group 4 bis(amido) systems have received much interest in the open literature
as potential templates for Ziegler-Natta catalysis, and a few exhibit significant olefin polymerization
activity. Related to the silylamide system outlined by Exxon, Tinkler et al. prepared a derivative in
which the amide donors are linked by an ethylene bridge (10) and reported its use in the presence
of MAO as a long-lived polyethylene catalyst which maintains constant activity for over an hour. 25
In a departure from the use of MAO as a cocatalyst, Horton et al. focused attention on the
preparation and characterization of discrete silylamide based alkyl cations charge balanced by the
[B(C 6 F5) 4 ]- anion (1126, 1227). Solutions of 11 and 12 were found to be active for both
ethylene and propylene polymerization. A limitation of the silylamide based systems, however, is
their propensity to metalate SiMe3 substituents, which is particularly acute in the more electron
deficient cationic derivatives. For example, the putative cation {[(Me 3 Si) 2 N] 2 Zr(CH 2 Ph)} +
(obtained from the reaction of [(Me 3 Si) 2 N] 2 Zr(CH 2 Ph)2 and [HNMe 2 Ph][B(C 6 F5 )4 ]) readily
loses toluene to form the spectroscopically characterized 12.27 These finding suggest that similar
metalated species may also be present in the silylamide / MAO polymerizations discussed above.
The last two examples presented in Scheme 5.5 are systems which are active for a-olefin
polymerization, but whose behavior deviates significantly from that of metallocene based catalysts.
When activated by MAO, the system developed by Scollard and McConville (13) produced poly1-hexene in which chain termination appeared to exclusively occur by chain transfer to aluminum:
no olefinic resonances could be observed in the polymers indicative of 3-H elimination. 2 8
Consistent with this observation, activation of the titanium dimethyl complex by B(C 6 F5 )3 resulted
155
in the living polymerization of the a-olefins 1-hexene, 1-heptene, and 1-octene in the presence of
excess ax-olefin. 1 Spectroscopic identification of the species actually responsible for catalysis was
not reported possible due to its instability in the solutions used for polymerization
(dichloromethane and/or toluene) in the absence of excess ax-olefin.
Utilizing a chelating bis(amide) linked with an O-atom donor (14), Baumann and Schrock
have also reported the living polymerization of 1-hexene. 29 Whereas the active catalyst species
responsible for polymerization utilizing 13 could not be observed, olefin insertion into 14 may be
spectroscopically monitored. 3 0 Furthermore, propagating alkyl chains bearing f3-H atoms are
stable in the absence of a-olefin at 0 "C allowing the preparation of 1-hexene and 1-nonene block
copolymers. 30 The presence of the O-atom donor is likely responsible for the greater stability of
these cations relative to those based on 13. Furthermore, this linked O-donor may serve to
attenuate the metal's electrophilicity rendering labile the NMe 2 Ph base, which has been observed to
strongly bind in the metalated bis(amide) 12.27
Comparisons of the structural features present in this class of compounds active for
controlled olefin polymerization (13, 14) and those in the bis(borylamide) complexes discussed in
this chapter shows that the coordination environment of bis(borylamide) based systems are
somewhat more crowded. Furthermore, it has been generally found in ansa-metallocenesystems
that Cp-substituents which lay directly above and/or below the center of the coordination wedge
serve to shut down propylene insertion, but not ethylene insertion, presumably due to unfavorable
steric interactions between the Cp- and olefin substituents. 3 1 The bis(borylamide) ligands
discussed herein place their organoboryl substituent directly over and under the center of the
coordination wedge due to N-B it-bonding, as illustrated in the structure of (CyBen)ZrEt 2 (5a).
Initially such a orientation was deemed desirable, to sterically protect potentially very reactive alkyl
groups from bimolecular interactions with other metal centers. In the context of Ziegler-Natta
catalysis, however, such an orientation may serve to impede migration of the metal-bound alkyl
group to a coordinated olefin.
156
Conclusions
A family of easily accessible, thermally stable zirconium and hafnium 3-H containing
dialkyl complexes based on the [(Cy 2BNCH 2 CH 2 NBCy 2 )]2- ligand have been prepared which
cleanly form cations in chlorobenzene through reaction with [Ph 3 C][B(C 6 F5 )4 ]. These species
interact with arenes, preferentially binding toluene in chlorobenzene solution which confers added
thermal stability to the P-H containing alkyl cations. In all cases, the metal-arene interaction is less
labile in the hafnium derivatives relative to the corresponding zirconium species, consistent with
the greater metal-ligand bond strengths for third row vs. second row metals. Although 3-H
containing cations may be observed and characterized in chlorobenzene, the polymerization of 1hexene proved irreproducible in this solvent and suggests that the bis(borylamide) alkyl cations are
not responsible for the polymerization activity. Polymerization carried out in dichloromethane led
to regioirregular poly- 1-hexene of low polydispersity, but the active species could not be identified
in this solvent.
157
Experimental
General Procedures. All experiments were performed under nitrogen in a Vacuum
Atmospheres drybox or under argon using standard Schlenk techniques. All solvents were
purified by standard techniques while deuterated NMR solvents were dried and stored over 4 A
molecular sieves before use.
Butyllithium and all Grignard reagents with the exception of Bu 2 CHMgBr were obtained
from Aldrich and titrated immediately before use. Iodine (99.999%) and ZrC14 (99.6%) were
obtained from Strem while HfC14 containing less than 0.1% Zr was obtained from Cerac.
Cyclohexene (99%), H2 BCI*SMe 2 and 5-nonanone were used as received from Aldrich.
Ethylenediamine containing 0.6% water was stored over 4-A molecular sieves before use.
[Ph 3 C][B(C 6 F5 )4] 10 was received as a gift from Exxon and was used as a yellow solid after
recrystallization from neat CH 2C12 followed by recrystallization from CH2C12/pentane to remove
any CH 2 C12 of crystallization. 19 LiCH 2 SiMe 3 32 and Ph 3 PBr23 3 were prepared according to
literature procedures.
1H, 13 C, 19 F,
and
1 1B
NMR spectra were recorded at 300, 75.4, 282, and 96.2 MHz.
respectively. Proton spectra were referenced internally by the residual solvent proton signal
relative to tetramethylsilane. Carbon spectra were referenced internally relative to the
the NMR solvent relative to tetramethylsilane.
13 C
signal of
Fluorine and boron spectra were referenced
externally to neat CFC13 and BF 3 etherate, respectively. IR spectra were recorded as Nujol mulls
between KBr plates on a Perkin-Elmer 1600 FT-IR spectrometer. Elemental analyses were
performed on a Perkin-Elmer PE2400 microanalyzer in our laboratories.
GPC analyses were carried out on a system equipped with two Alltech columns (Jordi-Gell
DVB mixed bed - 250 mm x 10 mm (i.d.)). The solvent was supplied at a flow rate of 1.0
mL/min. with a Knauer 64 HPLC pump. HPLC grade CH 2 Cl 2 was continuously dried and
distilled from CaH 2 . A Wyatt Technology mini Dawn light scattering detector coupled to a Knauer
differential-refractometer was employed.
In cases where polymer conversion was high.
differential refractive index increment, dn/dc, was determined assuming that all polymer that was
158
weighed for the run (ca. 10 mg accurate to ± 0.1 mg) eluted from the column. In cases where
polymer yield was low (< 25%), an average value of dn/dc (0.049) determined above was used to
compensate for the increased portion of the polymer weight due to metal and activator fragments.
Cy 2 BNHCH 2 CH 2 NHBCy 2 (H2(CyBen)) (1). A ethereal solution of Cy 2 B-C1 was
prepared by addition of H2 BC1'SMe 2 (25.00 g, 226.5 mmol) in ether (45 mL) to a stirring
solution of cyclohexene (40.92 g, 498.2 mmol) in ether (150 mL) maintained at 0 OC. The solution
became cloudy and was stirred at 0 'C for 1h. The hydroboration solution was then allowed to
warm to room temperature and was stirred overnight. An suspension of [LiNHCH2CH 2 NHLi]x
was prepared by adding butyllithium (95.1 mL, 238 mmol, 2.5 M in hexane) to a stirring solution
of ethylenediamine (7.15 g, 118.9 mmol) in THF (375 mL) at -78 'C. After the addition was halfcomplete, a flocculent white solid began to precipitate. After stirring 30 min at -78 'C. the
suspension was allowed to warm to room temperature and was stirred an additional 90 min. After
cooling the suspension of [LiNHCH 2 CH 2 NHLi]x once again to -78 "C, the clear solution of
Cy 2 B-Cl was added, consuming the white suspension to give a light yellow solution. The
solution was then warmed to room temperature and stirred overnight. The volatiles were removed
in vacuo, and the residue was extracted with pentane (ca. 400 mL). The extracts were filtered
through Celite and concentrated to ca. 100 mL which produced a large amount of crystalline solid.
After the filtrate stood at -30 OC for 3 h, the solid was collected on a large frit, washed with
pentane (-20 mL), and dried in vacuo to give 31.75 g (68%) of colorless crystals. An analytical
sample was doubly recrystallized from ether at -30 'C:
1H
NMR 8 (CDCl3 ) 3.872 (br t. 2, NH).
2.993 (pseudo t, 4, NCH 2 ), 1.7 - 0.9 (br resonances, 44, Cy);
13 C
NMR 6 44.88 (NCH 2 ).
29.66 (Cp), 28.95 (B-C), 28.72 (Cp), 28.36 (B-C), 28.06, 28.00 (Cy), 27.25, 27.12 (C8):
11 B
NMR 8 46.3; IR (Nujol/KBr) 3396 v(NH) cm - 1. Anal. Calcd for C26 H50 N2 B2 : C, 75.75; H.
12.21; N, 6.80. Found C, 76.01; H, 12.33; N, 6.83.
Li2(CyBen)*OEt 2 (2). tBuLi (22.0 mL, 36.08 mmol, 1.64 M in hexane) was slowly
added over 15 minutes with stirring to a chilled (-30 'C) solution of H2(CyBen) (7.26 g, 17.61
mmol) in pentane (200 mL) with a small amount ether (10 mL) added. Upon stirring the chilled
159
H2(CyBen) solution, H2(CyBen) began to crystallize, but after partial addition of the tBuLi, it
had redissolved. After 1/2 addition of the tBuLi, the product began to precipitate and became
more voluminous over 15 minutes. After the suspension was allowed to stir for 2 h at room
temperature, the solvent was concentrated to 1/2 its original volume and the flask was cooled to
-30 'C for 2 h. The white solid was collected by filtration, washed liberally with pentane, and
then dried to afford 8.30 g (95%) of the product.
1H
NMR (C6 D6 + 6 drops THF-d 8 ) 8 3.589
(br, 4, NCH 2 ), 3.252 (q, 4, OCH 2 CH 3 ), 2.0 - 1.2 (br resonances, 44, Cy),
OCH 2 CH 3 );
13 C
1.089 (t, 6.
NMR 8 65.86 (OCH 2 CH 3 ), 49.81 (NCH2 ), 33.92 (br, B-C), 32.40 (Cy),
31.60 (br, B-C), 30.71 (Cy), 49.48 (2 Cy), 28.42 (Cy), 28.30 (Cy),
15.49 (OCH 2 CH 3 ).
Low C and H analyses are consistent with gradual loss of ether in solid state.
(CyBen)Zr(CH 2 SiMe 3 )2 (3a). Zr(CH 2 SiMe 3 )2 C12 (ether)2 was first prepared in situ
as follows: under reduced lighting, a solution of LiCH 2 SiMe 3 (3.00 g, 31.9 mmol) in ether (95
mL) was added dropwise over 1 h to a vigorously stirring suspension of ZrC14 (3.71 g. 15.9
mmol) in ether (75 mL) at room temperature under reduced lighting. After stirring an additional 30
minutes, the reaction mixture was filtered through Celite and the filter cake was washed with ether
(2 x 20 mL) to give a clear, light yellow solution of Zr(CH 2 SiMe 3 )2 C12 (ether) 2 . This solution was
chilled to -30 oC and solid Li 2 (CyBen)*OEt2 (7.94 g, 15.9 mmol) was added with stirring. The
thick, flocculent suspension of Li2(CyBen)*OEt2 thinned over 30 minutes and the resulting mixture
was stirred overnight. After removal of the volatiles in vacuo, the residue was first titurated with
pentane (25 mL) and then was extracted with pentane (350 mL), filtering the resulting light yellow
solution through Celite. During concentration of the filtrate to ca. 25 mL, crystallization occurred
and after standing several hours at -30 oC the product was collected by filtration, washed with
pentane (3 x 5 mL), and dried in vacuo to yield 8.41 g (76%) of colorless crystals. An analytical
sample was obtained by recrystallization from ether at -30 oC. 1H NMR (C6D6 ) 6 3.766 (s, 4, NCH2 ), 2.05 - 1.20 (br m, 44, Cy), 0.590 (s, 4, Zr-CH2 ), 0.185 (s, 18, CH 2 SiMe3 );
13 C
NMR 8
55.50, 53.31, 32.78 (B-CH), 30.07 (B-CH), 29.84, 28.62, 28.53, 28.00, 27.60, 26.76 (Cy).
160
Anal. Calcd for C34 H 70 N2 B2 Si2 Zr: C, 60.42; H, 10.43; N, 4.15. Found C, 60.63; H, 10.68; N,
3.96.
(CyBen)Hf(CH 2 SiMe3) 2 (3b). Hf(CH 2 SiMe 3 )2 C12 (OEt 2 ) 2 was first prepared in situ
as follows: under reduced lighting, a solution of LiCH 2 SiMe 3 (2.70 g, 28.7 mmol) in ether (90
mL) was added dropwise over 1 h minutes to a vigorously stirring suspension of HfC14 (4.59 g.
14.3 mmol) in ether (75 mL) at room temperature. After stirring for an additional 30 minutes, the
reaction mixture was filtered through Celite and the filter cake was washed with ether (2 x 20 mL)
to give a clear, light yellow solution. This solution of Hf(CH 2 SiMe 3 )2 C12 (ether) 2 was chilled to
-30 'C and then solid Li 2 (CyBen)*OEt2 (7.00 g, 14.1 mmol) was added with stirring. The thick.
flocculent suspension of Li 2 (CyBen)*OEt 2 thinned over 30 minutes and the resulting mixture was
stirred overnight. The volatiles were removed in vacuo and the residue was first titurated with
pentane (25 mL) and then extracted with pentane (350 mL), filtering the resulting solution through
Celite. Upon concentrating the solution to ca. 30 mL crystallization occurred, and after standing
several hours at -30 'C, 7.47 g of the product was isolated by filtration. A second crop could be
obtained at -30 'C to give a total yield of 7.81 g (74%). An analytical sample was obtained by
recrystallization from ether at -30 'C: 1H NMR (C6 D6 ) 8 3.786 (s, 4, NCH 2 ), 2.05 - 1.20 (br m.
44, Cy), 0.186 (s, 18, CH 2 SiMe 3 ), 0.157 (s, 4, Hf-CH 2 );
13 C
NMR 8 64.42, 49.83, 32.91
(B-CH), 30.20 (B-CH), 29.54, 28.57, 28.08, 27.61, 26.84 (Cy), 3.95 (SiMe3). Anal. Calcd for
C 34 H 70 N2B 2 Si 2 Hf: C, 53.51; H, 9.24; N, 3.67. Found C, 53.36; H, 9.26; N, 3.53.
(CyBen)ZrI 2 (4a). Iodine crystals (3.35 g, 13.20 mmol) were first weighed out and
then added slowly to a rapidly stirring solution of (CyBen)Zr(CH 2 SiMe 3 )2 (4.46 g, 6.60 mmol) in
dichloromethane (200 mL) at room temperature until the color of iodine was no longer discharged
upon dissolution (0.06 g iodine remained). After stirring for a further 45 minutes, the light orange
solution was concentrated to ca. 20 mL and then allowed to crystallize at -30 'C overnight. The
crystals which had formed were collected on a frit and washed with pentane (3 x 10 mL). Two
further crops were obtained from the mother liquors combined with the pentane washings to give a
total of 4.42 g (88%) of product.
1H
NMR (C6 D6 ) 5 3.579 (s, 4, NCH 2 ), 2.35-2.15 (br, 4, Cy)
161
1.90-1.20 (br m, 40, Cy);
13 C
NMR 6 56.89 (NCH 2 ), 32.44 (B-CH), 30.28 (Cy), 30.00 (B-
CH), 28.16, 28.02, 27.35, 27.18, 26.29 (Cy). Anal. Calcd for C26 H4 8N 2B21 2 Zr: C, 41.35; H,
6.40; N, 3.71. Found C, 41.40; H, 6.81; N, 3.61.
(CyBen)HfI2 (4b). Iodine crystals (4.92 g, 19.4 mmol) were first weighed out and
then added slowly to a rapidly stirring solution of (CyBen)Hf(CH 2 SiMe 3 )2 (7.40 g, 9.70 mmol) in
dichloromethane (250 mL) at room temperature until the color of iodine was no longer discharged
upon dissolution (0.20 g iodine remained). After stirring an additional 30 minutes, the light
orange solution was concentrated to ca. 40 mL and then allowed to crystallize at -30 'C overnight.
The crystals which had formed were collected on a frit and washed with pentane (3 x 10 mL). An
additional crop was obtained from the mother liquors combined with the pentane washings to give
a total of 6.76 g (83%) of product. An alH NMR (C6 D6 ) 8 3.579 (s, 4, NCH 2 ), 2.35-2.15 (br.
4, Cy) 1.90-1.20 (br m, 40, Cy);
13 C
NMR 8 56.89 (NCH 2 ), 32.44 (B-CH), 30.28 (Cy).
30.00 (B-CH), 28.16, 28.02, 27.35, 27.18, 26.29 (Cy). Anal. Calcd for C26 H4 8 N2 B2 Hf: C.
37.06; H, 5.74; N, 3.33. Found C, 36.78; H, 5.65; N, 3.28.
(CyBen)Zr(CH 2 CH 3 )2 (5a).
A solution of CH 3 CH 2 MgCl (1.37 mL, 2.78 mmol.
2.03 M in ether) was added with stirring to a chilled (-30 "C) solution of (CyBen)ZrI2 (1.00 g.
1.32 mmol) in dichloromethane (35 mL). The solution immediately became cloudy and slowly
turned light yellow . After the reaction mixture was allowed to stand at -30 'C for 30 minutes. the
Mg salts were precipitated by the addition of 1,4-dioxane (0.28 g, 3.2 mmol) with stirring. The
volatiles were removed in vacuo, and the residue was triturated with pentane (10 mL). The
powdery residue was extracted with pentane, the extracts were filtered through Celite, and the
filtrate was concentrated to dryness. The resulting white solid was recrystallized from pentane at
-30 'C to afford 0.597 g (81%) of white crystals in two crops.
1H
NMR (C6 D6 ) 8 3.804 (s, 4,
NCH 2 ), 1.90 - 1.00 (br m, 44, Cy), 1.538 (t, 6, CH 2 CH 3 ), 0.965 (q, 4, Zr-CH 2 );
13 C
NMR 6
53.88 (NCH 2 ), 49.03 (Zr-CH 2 ), 32.38, 30.06 (B-CH), 29.77, 28.72, 28.59, 28.14, 28.59,
27.64, 26.87 (Cy), 9.95 (Zr-CH 2 CH 3 ). Anal. Calcd for C30 H58 N2 B 2 Zr: C, 64.39; H, 10.44; N,
5.01. Found C, 64.35; H, 10.44; N, 5.07.
162
(CyBen)Hf(CH 2 CH3)2 (5b). A solution of CH 3 CH 2 MgCl (0.886 mL, 1.87 mmol.
2.11 M in ether) was added with stirring to a chilled (-30 °C) solution of (CyBen)Hfl 2 (0.750 g,
0.890 mmol) in dichloromethane (15 mL). The solution immediately became cloudy. After the
reaction mixture was allowed to stand at -30 'C for 45 minutes, the Mg salts were precipitated by
the addition of 1,4-dioxane (0.160 g, 1.9 mmol) with stirring. The volatiles were removed in
vacuo, and the residue was triturated with pentane (5 mL). The powdery residue was extracted
with pentane (25 mL), the extracts were filtered through Celite, and the filtrate was concentrated to
dryness. The resulting white solid was recrystallized from pentane at -30 'C to afford 0.448 g
(78%) of colorless crystals in two crops.
1H
NMR (C6 D6 ) 8 3.778 (s, 4, NCH 2 ), 1.95 - 1.20 (br
m, 44, Cy), 1.540 (t, 6, CH 2 CH 3 ), 0.853 (q, 4, Zr-CH 2 );
13 C
NMR 8 61.71 (Hf-CH 2 ). 50.66
(NCH 2 ), 32.49, 30.35 (B-CH), 29.52, 28.67, 28.58, 28.18, 27.62, 26.91, 10.07 (HfCH 2 CH 3 ). Anal. Calcd for C30 H5 8 N2 B 2 Hf: C, 55.70; H, 9.03; N, 4.33. Found C, 55.74; H.
8.90; N, 4.33.
(CyBen)Zr(CH 2 CHMe 2 )2 (6a). A solution of Me 2 CHCH 2 MgCI (1.11 mL, 2.66
mmol, 2.39 M in ether) was added with stirring to a chilled (-30
oC)
solution of (CyBen)ZrI2
(0.980 g. 1.30 mmol) in dichloromethane (35 mL). The solution immediately became cloudy.
After the reaction mixture was allowed to stand at -30 'C for 45 minutes, the Mg salts were
precipitated by the addition of 1,4-dioxane (0.25 g, 2.8 mmol) with stirring. The volatiles were
removed in vacuo, and the residue was triturated with pentane (5 mL). The powdery residue was
extracted with pentane (25 mL), the extracts were filtered through Celite, and the filtrate was
concentrated to dryness. The resulting white solid was recrystallized from pentane at -30 OC to
afford 0.687 g (86%) of white crystals in three crops. 1H NMR (C6 D6 ) 8 3.777 (s, 4, NCH 2 ),
2.326 (m, 2, CHMe 2 ) 2.05-1.30 (br m, 44, Cy), 1.096 (d, 12, CHMe2 ), 1.004 (d, 4, Zr-CH 2 );
13 C NMR 6
76.60 (Zr-CH 2 ), 53.56 (NCH 2 ), 33.31, 30.07 (B-CH), 29.76, 29.62, 28.85.
28.65, 28.59, 27.93, 27.63, 26.89 (Cy and CHMe 2 ). Anal. Calcd for C34 H66 N2 B 2 Zr: C, 66.33:
H, 10.79; N, 4.55. Found C, 66.52; H, 10.85; N, 4.52.
163
(CyBen)Hf(CH 2 CHMe2)2 (6b). A solution of Me 2 CHCH 2 MgCl (0.861 mL,
1.87
mmol, 2.17 M in ether) was added with stirring to a chilled (-30 oC) solution of (CyBen)HfI 2
(0.750 g, 0.890 mmol) in dichloromethane (15 mL). The solution immediately became cloudy.
After the reaction mixture was allowed to stand at -30 oC for 45 minutes, the Mg salts were
precipitated by the addition of 1,4-dioxane (0.16 mL, 1.9 mmol) with stirring. The volatiles were
removed in vacuo, and the residue was triturated with pentane (5 mL). The powdery residue was
extracted with pentane (25 mL), the extracts were filtered through Celite, and the filtrate was
concentrated to dryness. The resulting white solid was recrystallized from pentane at -30 OC to
afford 0.535 g (85%) of white crystals in two crops.
1H
NMR (C6 D6 ) 6 3.778 (s, 4, NCH 2 ).
2.387 (m, 2, CHMe 2 ), 2.1-1.3 (br m, 44, Cy), 1.103 (d, 12, CHMe 2), 0.741 (d, 4, Hf-CH 2 ):
13C NMR 5 88.34 (Hf-CH 2 ), 50.22 (NCH 2 ), 33.30 (B-CH), 30.34 (B-CH), 29.97, 29.64.
29.51, 28.59, 28.00, 27.62, 26.94. Anal. Calcd for C3 4 H 66 N2 B 2 Hf: C, 58.09; H, 9.45; N,
3.99. Found C, 58.22; H, 9.83; N, 3.96.
(CyBen)Zr(CH2CH 2 Bu)
2
(7a). A solution of BuCH 2 CH 2 MgBr (0.506 mL. 1.05
mmol, 2.07 M in ether) was added with stirring to a chilled (-30 "C) solution of (CyBen)ZrI 2
(0.377 g, 0.499 mmol) in dichloromethane (15 mL). The solution remained clear while it was
allowed to stand at -30 'C for 45 minutes after which the Mg salts were precipitated by the addition
of 1,4-dioxane (0.10 mL, 1.2 mmol). After the volatiles were removed in vacuo, the oily residue
was triturated with pentane (5 mL) and then extracted with pentane (10 mL) and filtered through
Celite. The filtrate was concentrated to dryness, the residue extracted with pentane, and the filtrate
was concentrated to give the 0.297 g (98%) of the product as a slightly tan, analytically pure oil.
1H NMR (C6 D 6 ) 5 3.825 (s, 4, NCH 2 ), 1.9-1.2 (br m, 60, Cy + hexyl), 1.063 (m, 4, Zr-CH2 ),
0.910 (t, 6, CH 3 );
13 C
NMR 8 61.47 (Zr-CH 2 ), 53.77 (NCH 2 ), 35.75, 32.89 (BCH), 32.12.
30.15 (BCH), 29.85, 28.78, 28.60, 28.17, 27.63, 27.40, 26.91, 23.09, 14.33. Anal. Calcd for
C 38 H 74 N2 B 2 Zr: C, 67.94; H, 11.09; N, 4.17. Found C, 67.90; H, 11.40; N, 3.98.
(CyBen)Hf(CH 2 CH 2Bu) 2 (7b). A solution of BuCH 2 CH 2 MgBr (0.512 mL, 1.06
mmol, 2.07 M in ether) was added with stirring to a chilled (-30 °C) solution of (CyBen)Hfl2
164
(0.425 g, 0.504 mmol) in dichloromethane (10 mL). The solution remained clear while it was
allowed to stand at -30 'C for 45 minutes after which the Mg salts were precipitated by the addition
of 1,4-dioxane (0.10 mL, 1.2 mmol). After the volatiles were removed in vacuo, the oily residue
was triturated with pentane (5 mL) and then extracted with pentane (10 mL) and filtered through
Celite. The filtrate was concentrated to dryness, the residue was once again extracted with pentane
(10 mL), and the filtrate was concentrated to give the 0.334 g (87%) of the product as a slightly
tan, analytically pure oil.
1H
NMR (C6D 6) 8 3.811 (s, 4, NCH 2 ), 2.05-1.20 (br m, 60, Cy +
hexyl), 1.063 (m, 4, Zr-CH2 ), 0.912 (t, 6, CH 3 ), (m, 4, Hf-CH 2 );
13 C
NMR 8 73.57 (Hf-CH 2 ),
50.52 (NCH 2 ), 36.49, 32.91 (BCH), 32.18, 30.36 (BCH), 29.59, 28.69, 28.58, 28.19, 27.61.
27.54, 26.93, 23.11, 14.34. Anal. Calcd for C38 H74 N2 B 2 Hf: C, 60.13; H, 9.82; N, 3.69. Found
C, 60.06; H, 9.93; N, 3.42.
Bu 2 CHMgBr was prepared in three steps from 5-nonanone: (1) 5-nonanone (10.73 g.
78.79 mmol) was reduced with lithium aluminum hydride (1.50 g, 39.4 mmol) in ether 34 to give
10.81 g (99%) of 5-nonanol after workup with 2 N H2 SO4 , extraction with ether, and removal of
all volatiles by rotary evaporation under reduced pressure (water aspirator). (2) After degassing
and standing over 4 A molecular sieves for 12 h, 5-nonanol (5.25 g, 38.00 mmol) was placed in
dry, degassed DMF (50 mL) and chilled to 0 'C under argon. Solid Ph 3 PBr 2 (17.58 g. 41.65
mmol) was added 33 via solid addition tube to the solution over a period of 45 min, after which
time the solution was gently heated to 45 'C for 2 h. Hexane (300 mL) and water (200 mL) was
then added with stirring and the organic fraction was collected and wash with water (2 x 200 mL).
The hexane layer was dried with MgSO 4 , filtered, concentrated to ca. 5 mL, filtered through
activated alumina, and concentrated in vacuo to constant weight to give 4.70 g (61%) of 5bromononane as an oil. (3) After standing over 4 A molecular sieves for 12 h, 5-bromononane
(2.05 g, 10.17) was added slowly over 2 h to 0.60 g magnesium powder (activated by heating
followed by the addition of an 12 crystal after the addition of ether) in ether (10 mL). After
filtration through Celite, the Grignard solution was concentrated to <10 mL, transferred to a 10
165
mL volumetric flask and diluted to 10.0 mL. Titration with n-propanol indicated the yield of
Grignard formation was 45%.
(CyBen)Zr(CHBu 2 )2 (8). A solution of Bu 2 CHMgBr (3.24 mL, 1.46 mmol, 0.45 M
in ether) was added with stirring to a chilled (-30 'C) solution of (CyBen)ZrI 2 (0.500 g, 0.662
mmol) in dichloromethane (15 mL).
After standing at -30 'C for 5 h, the Mg salts were
precipitated by the addition of 1,4-dioxane (0.35 g, 4.0 mmol). After the volatiles were removed
in vacuo, the residue was extracted with pentane, filtered through Celite, and concentrated to
1H
dryness.
NMR analysis of this residue indicated that alkylation was not complete, showing
the presence of ca. 10% of an unsymmetric product assigned as (CyBen)Zr(CHBu 2 )I. This
residue was dissolved in ether (10 mL), chilled to -30 'C, and an additional 0.35 mL of the
Bu 2 CHMgBr solution was added. After standing a further 5 h, dioxane (35 tL) was added and
the mixture was concentrated to dryness. The residue was extracted with pentane (10 mL), the
extracts were filtered through Celite, and the filtrate was concentrated to dryness. Recrystallization
from pentane at -30 'C afforded 0.354 g (71%) of thin, colorless needles in 4 crops. 1H NMR
(C6 D6 ) 8 3.939 (s, 4, NCH 2 ), 2.2-1.2 (br m, 68, Cy and Bu), 0.999 (t, 12, CH 2 Me), 0.732 (br
q, 2, Zr-CHBu 2 );
33.08,
13 C
NMR 8 76.23 (Zr-CHBu 2 ), 53.50 (NCH 2 ), 34.61, 33.67 (B-CH).
30.94, 30.17 (B-CH), 29.56, 28.68, 27.96, 27.69, 27.02, 23.85 (CH 2 CH 3 ), 14.43
(CH 3 ).Anal. Calcd for C44 H 86 N 2B 2 Zr: C, 69.91; H, 11.46; N, 3.71. Found C, 70.15; H, 11.59:
N, 3.68.
Preparation and NMR Characterization of Alkyl Cations:
(6+).
Diisobutyl derivative 6 (0.027 mmol) was dissolved in chlorobenzene-d5 and
chilled to -30 'C. Upon addition of solid [Ph 3 C][B(C 6F5 )4 ] (0.025 g, 0.027 mmol) with stirring,
the intense yellow-orange color of [Ph 3 C] + was initially quenched. After complete addition, the
solution was light yellow, indicative of the presence of a slight excess of [Ph 3 C][B(C 6F 5 )4 ]. The
light yellow solution was rapidly transferred to an NMR tube and frozen at -78 'C before insertion
into a pre-cooled NMR probe at -30 'C.
1H
NMR data presented in Table 5.3;
-30 oC) 8 -137.97 (br s, 2, o-F), -167.83 (t, 1, p-F), -171.75 (br s, 2, in-F).
166
19 F
NMR (6a+,
(5+, 7+). 5 eq. toluene (14.2 gL, 0.133 mmol) was added to a chlorobenzene-d5 (0.70
mL) solution of dialkyl 5 or 7 (0.027 mmol) and the resulting solution was chilled to -30 OC. After
addition of solid [Ph 3 C][B(C 6 F5 )4 ] (0.050 g, 0.027 mmol) with stirring, the yellow-orange
solution was rapidly transferred to an NMR tube and frozen at -78 'C before insertion into a precooled NMR probe at -30 'C.
1H
NMR data presented in Table 5.3.
Polymerizations in Chlorobenzene (Table 5.4). 53 gmol (0.30 - 0.38 mg) of dialkyl 4 or 5
was weighed out, dissolved in 5.0 mL chlorobenzene, and the solution was chilled in the freezer
(-30 °C) for ca. 10 min.
The solution was briefly removed from the freezer when solid
[Ph 3 C][B(C 6F 5 )4 ] (0.050 g, 53 pimol) was added with stirring. After all the [Ph 3 C][B(C 6 F5 )4 1
had dissolved, the solution was returned to the freezer for 2 min prior to the addition of 1-hexene
(1.50 mL) with stirring. The vial was immediately returned to the freezer. In cases where
polymerization ensued, the solution became extremely viscous within 5-10 min. After standing at
-30 'C for 60 min, the mixture was quenched with 1.0 mL methanol. The viscous solution was
transferred to a pre-weighed flask, washing with dichloromethane, and much of the volatiles were
removed by rotary evacuation under reduced pressure (water aspirator). The polymer was then
dried at 60 mTorr and 110 'C for 8 - 12 h. Yields reported are not corrected for extra weight due to
the metal complex and activator fragments.
Polymerizations in Dichloromethane (Table 5.5). 53 gmol (0.30 - 0.38 mg) of dialkyl 4 or
5 was weighed out, dissolved in 5.0 mL dichloromethane, and the corresponding amount of
toluene was added (0, 28.4 mL (5 eq.), or 84.2 mL (15 eq.)). After standing ca. 10 min at -30 'C.
solid [Ph 3 C][B(C 6 F5 )4 ] (0.050 g, 53 imol) was added and the solution was allowed to stand at
room temperature, placed in an ice bath outside the box (0 °C) or returned to the freezer (-30 °C)
and allowed to stand 2 min. 1-Hexene (1.50 mL) was then injected by syringe and the solution
was stirred for 60 min (no stirring took place at -30 °C) at the appropriate temperature after which
time the solution was quenched with 1.0 mL methanol. After rotary evaporation of the volatiles
(water aspirator), the viscous oils were dried at room temperature at 100 mTorr to constant weight.
Yields reported are not corrected for extra weight due to the metal complex and activator.
167
Referenes
(1) Scollard, J. D.; McConville, D. H. J. Am. Chem. Soc. 1996, 118, 10008.
(2) Warren, T. H. Unpublished results.
(3) Fubstetter, H.; Kroll, R.; N6th, H. Chem. Ber. 1977, 110, 3829.
(4) N6th, H.; Prigge, H.; Rotsch, A.-R. Chem. Ber. 1986, 119, 1361.
(5) Brown, H. C.; Ravindran, N.; Kulkarni, S. U. J. Org. Chem. 1979, 44, 2417.
(6) Aoyagi, K.; Gantzel, P. K.; Kalai, K.; Tilley, T. D. Organometallics1996, 15, 923.
(7) The reaction of Li 2 (CyBen)*OEt2 with TiC14 in pentane gives a mixture of (CyBen)TiC12 (ca.
65 %), (CyBen)2Ti (ca. 25%), and H2(CyBen) from which (CyBen)TiC12 may be isolated in ca.
25% yield. Warren, T. H. Unpublished observations.
(8) Wengrovius, J. H.; Schrock, R. R. J. Organometal. Chem. 1981, 205, 319.
(9) Brand, H.; Capriotti, J. A.; Amold, J. Organometallics1994, 13, 4469.
(10) Chien, J. C. W.; Tsai, W. M.; Rausch, M. D. J. Am. Chem. Soc. 1991, 113, 8570.
(11) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem. 1988, 36, 1.
(12) See Chapter 4.
(13) van der Linden, A.; Schaverien, C. J.; Meijboom, N.; Ganter, C.; Orpen, A. G. J. Am.
Chem. Soc. 1995, 117, 3008.
(14) Babu, G. N.; Newmark, R. A.; Chien, J. C. Macromolecules 1994, 27, 3383.
(15) Asakura, T.; Demura, M.; Nishiyama, Y. Macromolecules 1991, 24, 2334.
(16) Coughlin, E. B.; Bercaw, J. E. J. Am. Chem. Soc. 1992, 114, 7606.
(17) Schrock, R. R.; Luo, S.; Lee, J. C.; Zanetti, N. C.; Davis, W. M. J. Am. Chem. Soc.
1996, 118, 3883.
(18) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1992, 434, Cl.
(19) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1995, 497, 55.
(20) Gillis, D. J.; Tudoret, M.-J.; Baird, M. C. J. Am. Chem. Soc. 1993, 115, 2543.
(21) Lancaster, S. J.; Robinson, O. B.; Bochmann, M.; Coles, S. J.; Hursthouse, M. B.
Organometallics1995, 14, 2456.
168
(22) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics1997, 16, 842.
(23) Canich, J. A.; Turner, H. W., PCT Int. Appl. WO 92/12162, 1991, Exxon.
(24) Andersen, R. A. Inorg. Chem. 1979, 18, 2928.
(25) Tinkler, S.; Deeth, R. J.; Duncalf, D. J.; McCamley, A. J. Chem. Soc., Dalton Trans.
1996, 2623.
(26) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg, H. Organometallics1996,
15, 2672.
(27) Horton, A. D.; de With, J. J. Chem. Soc., Dalton Trans. 1996, 1375.
(28) Scollard, J. D.; McConville, D. H.; Payne, N. C.; Vittal, J. J. Macromolecules 1996, 29,
5241.
(29) Baumann, R.; Davis, W. M.; Schrocl, R. R. J. Am. Chem. Soc. 1997, 119, 3830.
(30) Baumann, R.; Schrock, R. R. Manuscript in preparation.
(31) Brintzinger, H. H.; Fischer, D.; Mtillhaupt, R.; Rieger, B.; Waymouth, R. M. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1143.
(32) Tessier-Youngs, C.; Beachley, O. T. Inorg. Synth. 1986, 24, 95.
(33) Wiley, G. A.; Hershkowitz, R. L.; Rein, B. M.; Chung, B. C. J. Am. Chem. Soc. 1964.
64, 964.
(34) Noyce, D. S.; Denney, D. B. J. Am. Chem. Soc. 1950, 72, 5743.
169
ACKNOWLEDGMENTS
When applying to graduate school five years ago, I wanted to do my Ph.D. studies with
no advisor other than Professor Richard R. Schrock. If asked why back then, I would probably
rattle off something like "alkylidenes", "alkylidynes", "dinitrogen", "fundamentally new
compounds", and if that wasn't enough, I would have just have to say I had some sort of gut
feeling. It is now much easier to answer that question. I thank Dick Schrock not only for the
opportunity to explore ideas which were interesting and important to me, but especially for his
guidance which focused the efforts described in this thesis - it has been a tremendously challenging
and rewarding experience. I also thank him for not noticing the mess that has engulfed my desk
and its surroundings these last couple of months - I promise it will be cleaned up sometime very
soon.
The first impression I have of my undergraduate research advisor is a lasting one. After
apologizing for the smell of trimethylphosphine which permeated the laboratory, Professor
Gregory Girolami began explaining his research to me in a manner I understood and by which I
became genuinely excited. Not only am I truly grateful for the interest he took in my education
expressed through his mentoring, but the example he set through his teaching will remain with me
my entire career.
Before entering grad school, I had the good fortune to work for Al Casalnuovo and T.V.
RajanBabu at DuPont who shared their approaches to science with me and also on a more practical
level, who showed me how to do chemistry in a drybox. I particularly appreciate their confidence
in me. They emphasized that I shouldn't hesitate to try anything that I wanted, even though I was
working with HCN!
Upon arrival in the Schrock group, I immediately found an uncle that I didn't know I had.
Perhaps Moshe Kol recognized that I was quite impressionable and took advantage of me. I
soaked up his advice on life and chemistry, believing these words to have come from a wise (old)
man. Many discussions with Kit Cummins, Ivan Shih, and Howie Fox carried similar weight.
Although I might not have recognized their full import at the time, I now realize these
conversations were actually invaluable mentoring sessions.
Many thanks go to all those in the Schrock Group who not only have contributed to this
thesis by being helpful in many different ways, but who have made the lab a fun place to be: Anne
LaPointe, Scott "Scottie" Seidel, Steve Reid, Tom "wiggle-bug" Boyd, Gretchen Kappelmann.
Shifang "Shifangus" Luo, Deryn Fogg, Karen Totland, Myra O'Donohue, George Greco, John
Alexander, Eric Liang, Yann Schrodi, Michael Aizenberg, David Graf, and many others I have
overlapped with. A special thanks goes to Robert Baumann for many thoughtful and helpful
discussions concerning olefin polymerization and for going the extra mile in obtaining many of the
elemental analyses reported in Chapters 1 and 5.
Bill Davis deserves credit for collecting and solving all of the X-ray structures discussed in
this thesis. In addition to his efforts on the "agostic" and "gostic" neopentyl hydrogens. I
particularly appreciate his openness to the hundreds of crystallography questions that I bombarded
him with each time he did a structure for me. I also thank Steve Reid for working on the structure
of (BigBen)ZrMe2*pentane. (The molecule of pentane was worth one can of Coke from Steve.)
Since his arrival in the Spec. Lab this year, Jeff Simpson has been an invaluable resource and I
appreciate not only his advice on NMR experiments, but also his interest in my questions.
Outside of the laboratory, playing sports with many in the chemistry department and
particularly those in the Schrock Group has been an important part of my graduate career. Without
question, the event that has carried the most emotional weight is Chemistry Summer Volleyball.
After two losing but enjoyable seasons, a lot of hard work practicing led to an appearance in the
1995 Championship Match. Stung by a defeat, our efforts were redoubled in 1996 which
culminated in a 13-0 championship season. In all honesty, this was one of the most exciting and
rewarding teams I have ever been part of and I thank Flo, Steve, Sean, Shane, Carlos, and Jessie
for this privilege. I also thank Flo, Robert, Sean, Alex, Chris, and Big John for coming back to
play football in 1996 (after we were constantly annihilated and demoralized in 1995). as well as the
Swager Buam Rich, Scott, and David for a wonderful 5-0 season. I won't forget soon the
"immaculate reception" that Rich bobbled for 20 yards before Alex finally grabbed it from him.
170
Back within the confines of the fourth floor of Building 6, I thank Richard Tassoni for
starting what would become a parallel line of study for me in the Schrock Group. He taught me
my first (well, I already knew "Gesundheit") three German words: Stiuigkeiten, Waschmaschine,
sehr gut. The first was so that I could always have something to eat, the second so that I could
always have clean clothes, and the last so that I would be able to express my appreciation for these
items. "Sehr gut" describes the tutelage of Robert Baumann, Nadia Zanetti, and Klaus Wanniger.
I truly appreciate their incredible patience, speaking German with me until I understood what they
meant. Outside the group, I also thank Peter "Beta" Kuhn for his help with the important
Unterbleichfeldsaussprache. For reasons not related to space considerations, I unfortunately
cannot give individual credit for any particular phrases learned from these excellent teachers.
My primary German teacher, however, is the same guy who became my labmate in January
1994. Working with Florian Schattenmann was immediately a rewarding experience, from
discussing chemistry to sharing the same basic working style in the lab. After I heard him
speaking on the phone in German on day, I was curious as to a couple of words he said. So soon
started the German lessons: word lists, cases, conjugation. His patience is unbelievable. After all.
he has done a terrific job on two fronts - with his Ph.D. and as my personal German teacher. But
of all that has taken place in 6-427 between us, what I value most is the tremendous friendship that
has developed. I cherish it. As I thank Florian once again, I must too thank Fate who brought
together two Virgos stung by the sign of Capricorn.
John Gross and Dennis Hall have been a big part of life outside of the Schrock Group. I
appreciate John's friendship as well as all of his advice on among other things, the Greeks. I am
not sure if Dennis and I will collect all 50 goblets, chalices, and other drinking utensils by the end
of June, but I value the journey with him far more than those darned cups. Not to be forgotten is
Sherde's astrological black erotica on the beach and fried chicken at home, and exciting Weather
Channel commentary by Chris, my in-house meteorologist.
In so many ways over the last two years, Brigitte Neuner has been a tremendous influence
on me, both in and out of the lab. She is not only a constant springboard for chemistry ideas. but
her unfaltering belief in me has made tough times more bearable and each day helps drive me closer
toward my goals. One of the most personally rewarding results to come out of this thesis is that I
will be able to spend much more time with my pers6nliche Aussprachetrainerin daoben in Mtinster.
Lastly, these two pages would not be complete without recognizing my family. I
appreciate the interest my brothers Dan and Andy have always shown in what I have been doing
and their support on many different levels along the way. It would be impossible to thank my
parents for all of which they deserve credit. I can only begin by acknowledging their wonderful.
constant support to follow my dreams. It is to them that this thesis is dedicated.
171