Chemistry in Transition: Research in Group IV and V Organometallics, 1995-2000
An Honors Thesis (HONRS 499)
by
Katherine A. Rees
-
Thesis Advisor: Dr. Robert Morris
Ball State University
Muncie, IN
May 2001
Graduation: May 5, 2001
-
SpC o II
Th eSIS
L/)
--
::.l4gCj
• l-tfAbstract
'-, ,,\I
• ,K '"f"+
This compilation of abridged research in the area of Group IV and V transition metal
organometallic chemistry is generated in the pursuit of creating a body of works as reference
material for continuing faculty-student research here at Ball State University. Brief synopses of
the most pertinent aspects of the collected research articles, including necessary reactions and
structures, related to my specific area of research as an undergraduate student fellow were
designed as sources of information and suggestion for further research. Each section is
consequently divided into individual or paired metal subject matter to provide an efficient
reference for the research that has been propagated over the past five years in the study of Group
IV and V transition metal organometallic chemistry, 1995-2000.
Acknowledgements
Sincere gratitude is due to Dr. Robert Morris, my thesis advisor and wonderful faculty
mentor over the past year, for his patience and encouragement in the sometimes difficult process
--
;;;'001
of completing this thesis. Without his support and advice during those first information-gathering
sessions, then the arduous organization of thought, and again throughout the writing process, this
thesis would have undoubtedly been a lost cause. Thank you, Dr. Morris, for being a friend and
counselor as well as editor and taskmaster.
-
I.
Introduction
Chemical reactions are the underlying foundation for many industrial synthetic processes.
. complexes 0 f th e Group IV transItion
. . meta Is (22T 1,' 40Z r, 72Hf) contammg
. . one or
OrganometallIc
two cyclopentadienyl (Cp) or indenylligands are versatile and powerful reagents which have now
been developed and manipulated for close to thirty years for use in catalytic and stoichiometric
reactions. These compounds and their variations are very effective as stereoselective olefin
polymerization catalysts and as catalysts or reagents for a wide range of synthetic organic
reactions, including olefin hydrogenation, hydrooligomerization, cyclopolymerization,
epoxidation and isomerization, olefin-pyridine coupling, imine and enamine hydrogenation,
Diels-Alder reactions, allylic alcohol and amine synthesis, dehydrogenative phenylsilane
oligomerization, enantioselective carbomagnesiation reactions of alkenes, and hydrosilation of
ketones. 7,lI,12,18,19,20,36,43 Thus, new preparative methods for, and chemically and confonnationally
-
altered derivatives of, Group IV organometallic complexes have significant impact on many areas
of chemical synthesis and, consequently, industrial applications as well.
The most common preparation for coordination of a Cp or indenyl ligand and their many
variations requires the use of salts containing the ligand anion, such as LiL, NaL, MgL2' et cetera,
which then participate in chloride displacement reactions of MCl x or MClxLn compounds
(M=Group IV metal). 12 Disadvantages of these procedures include the need to protect the
moisture, air, and sometimes light-sensitive ligand anion and metal halide reactants, and the metal
complex products, which involves dry box techniques and working in an argon/nitrogen
environment. 36 Metal halide by-products also pose difficulties with their removal, as well as the
tendency for the metal complexed products to fonn dimers and polymers, which resist
crystallization into monomeric fonn. 36 Most efforts to produce monomeric complexes employ
two bulky cyclopentadienyl (Cp) or pentamethylcyclopentadienyl (Cp*) ligands attached to the
8
metal center. Amine elimination reactions have been introduced as a recourse for use, providing
a more efficient alternative to the more conventional reactions between ligand salts and metal
chlorides.9, 12,4 1,46
-
More modem syntheses focus on the necessity of producing the active, chiral, rae
. Iatmg
. or reso I'
.mactIve,
..
metallocenes and methods f,or ISO
vmg th em tirom th elr
meso f,orms. 161824
' ,
The C2 symmetric environment of the rae Cp-metallocenes mediates a significant number of
stereoselective C-C andlor C-H bond-forming processes, which are integral to many
polymerization reactions. 43 Researchers hope that controlling the enantioselectivity in the
precursor catalyst will transfer that selectivity to the polymerized molecule.
Reduced flexibility and enhanced availability of the metal center for the approaching
monomer molecule are considered to be the best organometallic catalyst characteristics for
stereoregular polymerization. Most ligand systems are chelating, containing either one or two Cp
or indenyl type groups (half-sandwich or sandwich complexes).46 Chelating half-sandwich
metallocenes are especially important in "constrained geometry" type industrial catalysts. 39,46
Most chelating ligands lead to either non-chiral or chiral complexes of C2symmetry; C 2
symmetrical complexes offer the advantage of limiting the directions of approach of a substrate to
-
a metal, but their synthesis requires purification of the chiral, active rae-diastereomers from the
inactive meso-diastereomers. 46 Attaching ligands at an unsymmetrical site has occasionally led to
C2 symmetric metallocenes, but they are often conformationally too mobile to provide welldefined geometry.21 Chiral complexes incorporating two appropriately substituted Cp or indenyl
ligands are being applied increasingly in enantioselective synthesis and stereoselective
polymerization. Much recent work in the area of Group IV organometallic research has focused
on defining the relationship between the ligand structure of the metallocenes and their catalytic
activity.
Emphasis has thus been given to increasing catalyst activities and selectivities as well as
the molecular mass of the polymers formed by fine-tuning the metallocene properties using
specific ligand substitution patterns. 35 Various functional groups are theorized to affect
metallocene activity, since it is well-known that changes in the electron density at the metal
center have a pronounced influence on the polymerization and catalytic behavior. The
development of new ligand systems for Group IV transition metal chemistry is a major direction
ofresearch. 32 ,46 By well-defining the metal environment, researchers aim at controlling the
tacticity in olefin polymerization and other catalyzed transformations. The synthesis of
2
-
complexes containing functional groups with atoms such as nitrogen, sulfur, and boron are under
study to examine the effect of the interaction between the metal centers and the heteroatoms'
differing electronic contributions. 26 .28 ,32,34 Bimetallic complexes are also under investigation due
to the theory that molecules containing two closely adjacent metal atoms of differing kind or
environment may permit cooperative effects resulting in unique catalytic applications.
Group V transition metals
16
eV, 41Ta, 73Nb) and their associated chemistry in
3
complexation with similar cyclopentandienyl and indenylligands as used in Group IV transition
metal chemistry are relatively unexplored areas of research. Only within the past few years have
serious forays been made into methods of preparation of these compounds, which have been
notoriously difficult, frequently requiring unusually high pressures and extensive reaction time as
well as precautions to maintain the integrity of the thermally and optically unstable precursors
and products. 4,6 Ongoing research is gradually building upon initial preparative methods to yield
more substantial means of producing chemically viable compounds. While not much is known
-
about their possible uses, Group V metallocenes, like Group IV complexes, are believed to hold
promise for industrial applications as their potential for diverse development is investigated.
II. The Group IV Transition Metals
The Group IV transition elements: titaniume 2Ti), zirconium(40Zr), and hafnium(72Hf),
have become a prominent component in the research of olefin polymerization catalysts and
related reaction catalysts since their initial discovery some 30 years ago. Due to their unique
electronic properties and coordination characteristics as early d block transition elements, these
metals exhibit strong behavior as ligand-metal complexed precursor catalysts whose properties
can be manipulated to exert individual control over the many reactions they influence. There is a
pronounced valence orbital energy dependence for these metals which can be observed from their
behavior as sterically-affected bent metallocene complexes of the order of reactivity 40Zr> 22Ti >
72Hf.35 This translates into a dominating electronic control of catalytic behavior related directly to
the steric and electronic demand of ligand systems on the metal center. 35 In conjunction with the
need for chiral isomerization, researchers aim at producing ligand systems and syntheses that
optimize the desired qualities of catalytic behavior.
3
-
III-A. Zirconium,4oZr
Zirconium possesses the largest volume of research, due in part to its individual
characteristics that result from its particular electronic configuration. The zirconium metal atom
occupies a seemingly unique position among the Group IV transition elements; its larger 4do
outer orbitals produce an ansa, bent conformation in conjunction with the steric demands of the
ligand system that optimizes the chirality of the metallocene complex in most cases. The smaller
titanium (3d) and much larger hafnium (4f 45d) atoms are less reactive and possess a more limited
utility in much of the research presented here.
In 1995, Luttikhedde, et al., hypothesized that the most stereos elective catalysts are
bridged ansa-metallocenes containing two indenyl, tetrahydroindenyl, or otherwise substituted
cyclopentadienylligands that are linked by a bridge between the 1 and l' positions. His research
produced the ansa-[ 1,1' -bis(inden-l, I-diyl)-l-silacyclopentane)]ZrCh. 29
-
The reaction of I-silacyclopentylbis(l-indenyl) with 2 equivalents ofBuLi gives the
doubly deprotonated dianion (CH2)4Si(C9H6)t, which reacts with ZrCl 4 to make the ansametallocene dichlorides rae- and meso- (CH2)4Si(C9H6)2ZrCh(1,2) [rae and meso-lsilocyclopentylbis( l-indenyl)ZrCh] and hydrogenation of 1 yielded rae-lsilacyclopentylbis(4,5,6,7 -tetrahydro-l-indenyl)ZrCh.
-4
-
The planar C5 rings adopt C2 symmetry in the rac isomer. The meso isomer contains 2
inequivalent chloride atoms, of which one is sterically hindered by benzo units at the top and
bottom while the other is essentially unencumbered. Changes in the Cp-metal-Cp angle (smaller
than CHr Si-CH3 bridge) cause steric strain in the complex, which increases the stereorigidity.
In that same year, Diamond, et al., investigated the possibilities inherent in the "new"
synthesis of Group IV metallocenes via amine elimination. The 1,2-bis(3-indenyl)ethane system
was introduced in reaction to Zr(NMezk This synthesis yielded pure racemic (EBI)Zr(NMe2h
(EBI=1,2-ethylenebis(l-indenyl), and subsequent treatment with two equivalents of Me2-NH-HCI
in a one pot synthesis yielded rac-(EBI)ZrClz. 12
a)
2
;
3
-
b)/
rac-4
rac-1
Previously, ansa/ rac (EBI)ZrC}z was synthesized by the reaction of ZrCI4(THF)2 and (EBI)Liz
with low variable yields of 30-50%, or (EBI)Kz in 20% yields with a general raclmeso ratio of
2/1. The rac isomer was not always separable from the achiral meso isomer. This particular
synthesis (metal dialkylamide amine elimination) provided racemic, ansa zirconocenes in high
yield:
Zr(NMeZ)4 + 1,2-bis(3-indenyl)ethane (N2/toluene, 100°C, 17 hours) -- >
(EBI)Zr(NMeZ)2, 90% yield, raclmeso 13: 1.
Recrystallization from toluene afforded pure rac-(EBI)Zr(NMe2)Z in 68% yield without
discernible photosensitive properties; x-ray crystallography confirmed a monomeric state. The
-
amine elimination, however, was found to be reversible and catalyzed by Me2NH to the rac form,
which is the thermally stable product.
5
-
In 1996, Christopher, et aI., performed the amine elimination on (SBI)H2(Me2Si( 1indenyl)2) in synthesis with Zr(NMe2)4 to yield rac-(SBI)Zr(NMe2)2 in 65% yield via a
mono(indenyl) intermediate n5-C9H6SiMe2C9H7)Zr(NMe2)3, which reacted with a second
equivalent of the ligand to a binuclear complex {(Il-n5: n5-Me2Si(1-indenyl)z) {Zr(NMe 2)3}z.9
Catalyzation to raclmeso by Me2NH was also observed. The removal of the dimethylamine was
found to control the equilibrium yield of raclmeso.
e
. G
(@L·
I
'"
& &
15 ~
..
1/2 Me2SiCI2
Et~
Me-Si-Me
Me-Si-Me
mes0-3
rac-3
·20OC--> RT
(1 )
Ratio:
hexanes
reflux. 8 h
-
..
3
·~4
2
rae· ..
Ratio:
mesc-..
14
1·
Rac-(SBJ)Zr(NMe2)2 was then converted to rac-(SBJ)ZrClz in reaction with Me3SiCl.
Working under the knowledge that variations in metallocene complexes allow for
exertion of microstructural control, Plenio and Burth studied a synthesis in 1996 to produce a new
metallocene complex with one nitrogen (N) atom directly bonded to the Cp or indenyl ring
-
system, which would influence its catalytic olefin polymerization behavior. They introduced the
reaction of2-indanone or 3,4-diphenylcyclopent-2-anone with pyrrolidene and N,N'-
6
-
dimethylethylene diamine, which led to 2-aminoindenes and l-amino-3,4diphenylcyclopentadienes whose lithium salts could be reacted with ZrCl 4 to generate the
corresponding aminozirconocene dichlorides (which were also moisture sensitive).34
00=0
fr
-!.....
2....
©O-{)
1
~T-«
I
Ph
I
2
h
~ »-{J
3
Ph
Scheme 1. (a) pyrrolidine. MeOH. 15 min; (b) NoN' -dimethylethylenediamine. benzene:. 48 h ~f1ux; (c) pyrrolidine. benzene. 48 h reflux.
4
5
6
Scheme 2. Aminozirconocene:-dichlorides 4. 5 and 6 we~ prepared
by reaction of the ~spective deprotonaled aminoindenes 0.2) and
aminocyclopentadiene:s (3) with ZrCI •.
The structure of di-(2-N-pyrrolidene-n 5-indenyl)ZrClz existed exclusively as the racemic isomer;
no meso form appeared to necessitate resolution. The indenyl C-N bond length range was of
multiple bond character, which would indicate that the Zr-cationic complex should be relatively
stable with an electron-rich metal center.
7
-
In that same year, Lefeber, et al., presented a reaction for the reductive synthesis of rac(EBTHI)ZrCh (EBTHI= 1,2-ethylene-l, l' -bis(n 5-tetrahydroindenyl) with Mg in Me3SiC2SiMe3,
which gave the first zirconocene-alkyne complex containing no additional ligands: rac(EBTHI)Zr(n2-Me3SiC2Me3).27
Scheme 1
Mg, MIIlSlCzSlMIil
THF
•
-MgC12
rile
rae-1 M" TI
rae-2 M-Zr
-
Scheme 3
tac-3
tac-4
L .. py
L - (S)-{-}-nlcotlne
tlIC-2
j
tac-5
2 PhN-CHPh
,t,!e,SlCzSIMe,
The use of optically pure EBTHI in conjunction with the alkyne ligand was believed to possibly
retain the unique catalytic ability in subsequent products_
-.
Also in 1996, Bell, et al., focused on the symmetry aspects of the metallocene complex,
producing an alternative route to the ethylmagnesiation of terminal alkenes. At that time, the
8
-
compounds were usually catalyzed by (R,R)-ethylene-1 ,2-bis(n 5-4,5,6,7 -tetrahydro-1-indenyl)Zr1,1' -binapth-2,2' -diolate, but only with low turnovers and enantioexcess. Bell and her colleagues
created instead a novel C j symmetric ZrClz complex: cpcp'zrCh(Cp=C 5H5,Cp'=1-neomenthyl4,5,6,7-tetrahydroindenyl), which is more highly enantiose1ective, cheaper, catalytically more
active and recoverable. 3
H>-
~4
~ H
6
i)
7.
CJ~CJ
2&
-
•
Lt'+
Scheme 1
The C j symmetric complex induced asymmetry in the carbomagnesiation reaction, culminating in
the design and synthesis of novel chiral complexes and useful enantio inductions. The most
widely synthesized complex at the time was the C 2-symmetric ethylenebis(tetrahydroindenyl)ZrCh. A C j symmetric chiral zirconocene was designed and synthesized in
this research, featuring induced planar chirality in one Cp ligand which also proved to provide the
control elements (a "roof and wall") needed for chiral induction.
Fig. 1
Fig. 2
In 1997, Voskoboykinov, et al., reported on organotin reagents used to produce rae and
meso-2,2'-propylidene-bis(1-indenyl)ZrCh in high yield, as well as an improved synthesis of
2,2'-bis(1-indenyl)propane. 42 Previously, a nucleophilic addition of indeny I lithium to the 2,2'dimethyl-fulvene derivative of indene in THF was utilized for the synthesis.
-
I
iJddJ'
, / = ' .\
\
'\
9
(1)
Me.C=O
NaOH. 15-crov.n-:;
THF
•
(2)
In this synthesis, acetone and two equivalents of distilled indene, an excess ofNaOH powder, and
a catalytic amount of crown ether was used in situ to produce the fulvene ligand 2,2' -bis(1indenyl)propane.
Tin substituted cyclopentadienylligands normally involved an exchange reaction of
alkali-metal derivatives of cyclopentadienyl compounds with triorganotin halides, but the product
was plagued by halide contaminations and low yield due to side redox reactions. Here, an
alternative reaction of the corresponding cyclopentadienyl compound with organotin ami des of
the order R3SnNR'2 (R, R'=alkyl) was used. 2.2'-bis(indenyl)propane and two equivalents of
R3SnNEt2 (R=Et, nBu) in THF proceeded to diethylamine and 2,2' -bis[(3-trialkyltin)inden-I-yl]
propane in quantitative yield. A tin-substituted hydrocarbon could be isolated after vacuum
evaporation.
(3)
The catalytic ability of transition metallocenes is known to be sensitive to modifications of both
substrate structure and reaction conditions. Ansa-complexes of zirconium usually involved
exchange reactions of ZrCl 4 with alkali metal derivatives of corresponding cyclopentadienyl
ligands involving bridging fragments. A chelating ligand may coordinate two zirconium atoms,
however, forming a metal containing polymers and oligomers under appropriate conditions and
-
thus decreasing the yield of overall ansa-metallocenes. Trialkyltin derivatives of various
cyclopentadienylligands are convenient selective precursors for transition element 1t complexes
10
-
of early transition metals. Here, organotin substituted cyclopentadienylligands are reacted with
ZrCl4 in toluene to produce equimolar rae and meso-2,2'-propylidene-bis(1-indenyl)ZrClz in 92%
yield.
~
. R,SnCI
2. ru<-
I. me."",
(-+ )
k5'~,
IIn~
:
l
~.
CI.z~ lIn
-
CI
cr
-
n
(5)
Isolation of the rae isomer resulted from the low solubility of the compound in organic solvents,
-
but LiCI and polymeric products have similar solubilities. Researchers noted that Y2 equivalents
of the organotin formed polymeric and oligomeric intermediates with bridging ligands and
chlorides.
In 1997, Ziniuk, et al., also published work on new chelating (1-) ligands based on
indenyl groups and either a pendant pyridyllmethyl arm or pendant quinolyl/methyl arm which
were synthesized to react with tetrakis( dimethylamido)Zr by amine elimination to complexes of
C] symmetry.46 The indenylligand was found to coordinate to the n 5 position to block the top of
the zirconium metal center while the pyridine coordinated in a long N--Zr bond to block the back
of the metal atom.
3
-
2
4
Scheme :!. Synthesis of the 'constrained geometry' zirconium complexes 3 and 4.
-
These chelating half-metallocenes especially were and are expected to play an important role in
constrained geometry catalysts. The advantage of Cz symmetry is the limiting approach effected
to the metal center; however, such syntheses generally require isolation and purification of the
rae isomer from the meso form. Complexes using C, symmetry, though, have thus far been
limited in usage.
In that same year, 32Vogel, et aI., also reacted tetrakis(dimethylamido)Zr with
dimethylsilylene- and isopropylene- bridged bis(indenyl)ligands to yield the corresponding ansabis(dimethylamido) zirconocenes containing n5-coordinated 1t aromatic ligand systems.
4
'
Zr[N(CHJ)zl. + (CHJhE(lndH)1
3
• 2 NH(CK.I.
",5
1," 2,5
E
-.
SI
rac-1,rac-2
C
Scheme 1.
The amine elimination involved the ready binding of the first indenyl group, while the second one
proceeded much more slowly, during which careful control of the experiment must be maintained
to select the desired products. Here, two new chelating ligands bind through an indenyl arm and
aromatic amine to the metal center. The indenyl and pyridyl or quinolyl moieties were rigid,
constraining the geometry at the metal center.
In 1999, Baker and Wallace published a report on the reaction of axially chiral (M)-1-(2'methyl-3' -indenyl)-napthalene-2-methanol with Zr(NEtZ)4, which enantiospecifically provided
the planar chiral complex (P)-bis( diethylamido )-[ 1-(2' -methyl-I' -indenyl)naphthalene-2methoxo ]Zr(Etz}z.2 These planar chiral Cp metal complexes were known to be more successful
stereoselective catalysts, but were usually obtained in enantiomerically pure form through
resolution procedures. The most common approach to avoid resolution used cyclopentadienyl
-.
ligands containing other stereo genic elements which rendered the ligand faces diastereotopic;
12
-
however, the stereo selectivity for the planar chiral element during metallations was not
guaranteed and separation of diastereomer products might still be required.
Baker and Wallace instead created a ligand design by which a second metal coordination
site was constrained to one face of an asymmetrically substituted Cp ring through hindered
rotation about a bond between Cp and the naphthalene. The enantiomeric synthesis of planar
chiral bidentate indenyl-alkoxide complexes of zirconium using an axially chiral indene ligand
proceeded by metallation through amine elimination with one equivalent of Zr(NEt2)4.
i. ii. iii
5 R.SPh
4
iv
-
[
6 R. S(O)Ph
viii
3
7
Scheme 1 Reagents and conditions: i. SOCh (15 equiv.), 25 ·C. 20 h; ii,
(lR)-menthol (2 equiv.), pyridine (2 equiv.). CH~Ch, 25 ·C, 30 min; iii,
PhSNa (1.2 equiv.). DMF, 60 ·C. 20 h; iv, OXONE" (3 equiv.), NaHC03
(5 equiv.), acetone-MeCN-water (20: 10: I). 5 ·C, 20 h; v, 2-methylindenyllithium (1.3 equiv.), THF. 0 ·C, 40 min; vi, NEt3-toluene (I: I).
reflux, 24 h; vii, LAH (5 equiv.), diethyl ether. 25 ·C, I h; viii, Zr(NEt2)4 (I
equiv.). toluene. reflux. 16 h.
In the year 2000, Fischer, et al., documented the reaction ofCp2ZrCb with two
equivalents ofEtMgCI in THF to effect the formation of the binuclear complex [Cp2Zr(Et)2](Ilethene), where the bridging ethene could be considered to be the dianion C 2H/- asymmetrically
coordinated to two neighboring Cp2Zr(Et) centers. 14 [(Me3SiCp)2]zrCh and two equivalents of
-
EtMgCl formed [(Me3Si-Cp)2Zr(Et)2h (Il-ethene); Cp2ZrCh and Et2Mg(dioxane) formed
[CpZr(ethane)(THF)]; [(indenyl)2Zr(ethane)(THF)] and [Cp2Zr( ethene)-(pyridine)] were
13
-
synthesized in good yield, then reacted with norbornene to form a metallocycle. The complexes
were subsequently involved in CP2ZrX2 catalyzed carbomagnesiation reactions of norbornene
with EtMgX.
Three isolated intermediates of catalytic carbomagnesiation and related complexes were
characterized through this research: the binuclear ethene ethyl complex, which was believed to be
a reservoir for formation of Cp2Zr( ethene)(THF) as well as two closely related mononuclear
ethene complexes and the metallocycle under catalytic carbomagnesiation conditions using
norbornene as a substrate. This reaction was a highly regioselective and stereoselective synthesis
of functionally substituted complexes of in situ generated Cp2ZrEt2 reactivity.
Scheme 1. Proposed Catalytic Cycle of the
Carbomagnesation Reactionl.l2,-l~
CP2ZlE~
.j
-
c~·.,.....-1
"
1'2L'~
A
"
R
~~R
EI
I
~yR
CIMg~R
~~
EtMgCl
In the same year, Zhang, et al., also generated a high yield synthesis ofrac-SiMe2bridged bis(indenyl) zirconocenes that exploited conformational properties of a simple chelating
diamide ligand to control diastereoism. 45 The chelated propylene-diamide complex
Zr{PhN(CH 2hNPh}Ch(THF)2 was created through the reaction ofZrCl 4 and 2 equivalents of
Li2[PhN(CH2)3-NPh] in toluene and one equivalent of the dianion in THF/Et20. ZrCl4 and
-
Li4{PhN(CH2)3-NPh} were also reacted in THFlEt20. The resulting complex had approximate C2
symmetry and a distorted octahedral geometry.
14
Scheme 1
2 Uz(PhN(CH.bNPh)
toluene, - 4 UCI
...;.:..;:..::.;.c"-----''--_.~
Uz(PhN(CH,)3NPh)
TliF I ElzO
-2UCI
J
Zr(PhN(CHV3NPhl.
~4
/
THF/ElzO
CI
THI\I
N
Ph"
~
• """TliF
. I 'N"~Ph
I
R"
CI
R4
e H
- 2 LiCllUz(SBI')(E1zO)
+
R'
H H
b Me H H
e Me benzo
d Me Ph H
2a-d
SBI'
SBI
MSBI
MBSBI
MPSBI
Me3 SiCI
or
-
HCI
llIo-(SBI')ZrCI.
4e-d
3a-d
-
R'
R'
~~~Ph
l"f""'N"
~
I
Ph/
~
R4
R'
la)
(b)
lei)
Ie)
Figure 3. Schematic drawings of the molecular structures of rae- and
meso-(MBSBI)Zr(NMePhh (a, b) and rae- and meso-(MBSBI)Zr(PbCH,.
CH2NPh) (c, d) based on X-ray crystallographic analyses (R4 and RS =
benzo).
15
-
The meso isomer was sterically disfavored due to the twist conformation of the product. The
zirconocene-diamide could be used in situ in reaction with Me3SiCI to convert the complex to
rac-( SBI' )ZrCI2•
Corradi, et al., also reported in 2000 an introduction of a reduction scheme of
CpRZrCI 3(dme) (CpR=Cp, C5H4SiMe3, C 5H4Me, Indenyl) with sodium (Na) amalgam in the
presence of isoprene. JO
Scheme 1
M8aSi~SiMe3
----..
I
-/'-. """"'H
/ CSF5
R1
R2
R1 =R2 =H,Me
R1 =H,R2 =Me
.A
-
c
B
Scheme 2
~R
Ie '>
0-f':'.:·:/·"
-Ol" ",CobB
B(CefslJ
-.....:....~:--.
2
., CpR =C5H5
b, CpR =C5H4 SiMea
c, CpR = C5H4 Me
warm IoRT
d, CpR = Ind
in vacuo
~R
I
~Zr-CsF5
(C.F5l:zB
~
~
1 atm
Me
16
::t
H..
b
Me
-:/
la-d
warmloRT
-
The addition of allylmagnesium chloride gave diene complexes CpRZr(n 3-allyl)(n 4-isoprene)
conveniently performed in a one pot synthesis. The further addition ofB(C6Fs)3 produced a
thermally unstable zwitteronic complex which decomposed as the reaction mixture was warmed
to room temperature, acquiring C-H activation and a propene elimination as well as the migration
ofa B-C 6 Fs group to the metal center. The resulting compounds were fluxional, with the rotation
of the Zr-C 6F S influenced by the steric demand of the Cp ligand.
-
17
-
III-B. Zirconium, 40Zr, and Hafnium, 72Hf
The bulk of research on hafnium metallocene complexes appears in conjunction with
discussions on similar syntheses for zirconium complexes. For the purposes of organization,
these abstracts are separated into their own section.
In 1995, Shaw, et al., introduced a monoindenyl species appropriate for both zirconium
were prepared in two straightforward syntheses.
ZrC1 4 +
37
("1\
~SnBu3
H
toluene
-C1SnBu,
-----+.
[(
~
)
]
11--C
9 H 7 ZrC1 3 n
(2)
(1)
(2)
(1 )
-
~
[IT'I~ -C~-ilJCl)Jn
1.1 )
or
.
JlS
DME
toluene
CI .....
•
CI"'-
I .... cl
~~ OCH,
HJCO~
(T'I~ -C~-IHfCI:(j.I-C1 l12
'll
~
(3)
.
=Zr(3,. H(·4)
As noted, the complexes were prepared using 1-trimethylsilylindene and I-tributylstannylindene
as indene transfer reagents. The hafnium dimer shown represented the first structurally
characterized Cp(substituted Cp) hafnium trichloride complex. These mono-Cp complexes were
determined to display different reactivities than the corresponding Cp2MCh complexes.
Rae isomers of ansa-metallocenes can be prepared from the racemic forms of dielementsubstituted bis(Cp) ligands, and in 1997, Nifant'ev and Ivchenko produced the distannylated
-
bis(Cp) and bis(indenyl) ligands in reaction with ZrCl 4 and HfCl 4 to give the corresponding ansametallocenes in high yield to confirm this. 31
18
-
Chart 1
Z=Me,C (1)
Z=Me,Si (2)
i
8
Z
C-"art 2
M8JS~Z~SnMe3
MC~
Z=Me,C (3)
Z=Me,Si (4)
Z=CH,CH, (5)
~"I
X
X
Scheme 2
~snMe3
Scheme 1
z
".y
L,ZrHal
~'H
- Me3SnH~1
"---/
.
R'~~R'
u+
u+
*.
Scheme 3
zra,~
Oi.a,
6h, reflux
6-anti100% anti-form
at least 98% rac-.
fr(;{6)
~F-
Zra,
~
toluene
4Oh, reflux
7-anti-
100% anti-form
8
rac-/meso-= 4: 1
During that year, Jany, et a!., also presented a synthesis for preparation of
enantiomerically pure ethylene-bridged ansa -zirconocene and -hafnocene complexes bearing
different Cp fragments. The ring opening of (R)-epoxystyrene with fluorenyllithium was
determined to proceed enantiospecifically to optically pure 2-(9-fluorenyl)-l-(S)-phenylethanol
and 2-(9-fluorenyl)-2-(S)-phenylethanol in nearly quantitative yield. The trifluoromethane
sulfonate derivatives could be treated with fluoreneyl and indenyllithium salts, resulting in the
19
ZIL,
-
formation of dilithio salts which could then react with ZrCl4 and HfCl 4 to form enantiomerically
pure complexes in up to 63% yield?4
Ph
FlU~H
j
I. (Flu)Li.
2.H+
Flu~H
+
Ph
2b
2.
Ph
~°
~ ~Hl
Ib
+ HOC-~H-Ph
2.HCI
3
pyridine
II I
C-CH-Ph
•
of
J
2b
0 NH)Cl
Flu'
Ph
Ph
(CpR)Li.1
Flu~' ","-OSO,CF) ----+
Flu~ .'\...CpR
5
6.,b
a: CpR-Inc!
b: CpR = Flu
Scheme 2
-
Ph
Flu~lnd
CF~ Zl .-eH,
Ph ....
I. BuLi
/
Ind
2. z,cl,
7a
,
CH,
IOa.b
7b
Ht'J'tO.
1
ow~ ~
_C~+~CI
Sa
l
CH,li.
Cy.
H.Flu rJ
. . ( " Zl r-
Ib
'" ,CH,
H.lnd
lla.b
C,H,NO••
NE~
a: R,R-lsomer
b: R,S-Isomer
J3
Scheme 3
I. BuLi
2. Mel,
-
~
c: M=Zr
d:M=Hf
~
8e,d
-
In 1998, Thiyagarajan and Jordan presented the reaction of Li2SBI(SBI=Me2Si( 1indenyl)2) with two equivalents of AlMe2CI in Et20, followed by THF to produce
{AlMe2(THF)(indenyl)}z-SiMe2 in 111 racimeso, from which the rae isomer could be
recrystallized in toluene. 38 The rae isomer existed as the 1,3 isomer in its solid state; in solution,
toluene-dB, rae existed as a 211 mixture of 1,3 and 1,1 Al-indenyl isomers which interconverted
rapidly at room temperature. There was found to be a slow isomerization to 111 racimeso and
slow, partial disproportionation by ligand redistribution.
+
,"
Me
Me
/N,
2 Me2Al, /AIM82
Me
-
Scheme 1
~
_~IM"(THF)
(THF)M8zA1 _ P
Me2Si
~
1 ,&
[1.5J-AI
....~
;:;r......."
~
(THF)Me,AI P
~
.
~
(THF)MezAI p
[1.5]·AI
~
~
MezSI
(THF)Me;e
.
_lb
..0-1.
n
1~ [1.5J-H
[1.5]·H
[. .~21'. .""'"l
~
(THF)MezAl p
~
. ox1:
~
(THF)MezAI p
(THF)MezAI
~
11
[1.5]·H
MezSl
~
.H
?!JAlMe{fHF)
..AIM..(THF)
1 b_
~
[1.5J-H
[1.5J-Si
~
~
(THF)MeaAl p
~
-it)
H
1,&
1~
[1.5].AI
~
....
[1.5]-Si
[1.5J-H
~
....~
~
(THF)Me,AI P
1~ [1.5J-H
(THF)MezAI P
1~
"-~e
(THF)MezAI
.... ~
1,&
1,&
Table 1. Syntheai. 01 Aluminum _Indeuyl Compounds
compel
iaoJaled yield (...)
{AIMeiTHF).indenyl)},,8iM... (1)
[{ 1.AJMeo(U-dioune)u.2.Me-l.indenyl}oSiMesJ. (lIa)
(l.AlMes<EtoO).2-M.....5-benz..1.indenyl}OSiM... (3)
(1.AlMes<EtoO).2.Me+Ph-1-indenyl}oSiMe.. (4a)
(1.AIMei'fHF').2.Me+Ph-1-indenyl},,8iM.. (4b)
l,2-{3-AlMeiTHF).l·indenyl}..CoH. (5)
41
70
6
62
54
54
21
1.211
rue
211
2/3
rue
lingle iaomer"
{AIMez(Lj-lnd}zSiMez
-
..
+
M(NMeV4
AI reagent
2.
3
4.
(8)
MIII8lIocene
NMR yield (%)
90
7 M. HI, RZ" R4= RS. H
70
1011
MzZr, ~=Me, R4 .R5=H
90
314
M.Zr, R2=Me, R4& RS .. benzo
10 M .. Zr, ~.Me, R4,.ph,R'.H
75
9110
75
314 .
8
8
-
fJId_
8 MzZr, RZ=~=R'=H
....1.
The ligand reacts with Zr(NMe2)4 and Hf(NMe2)4 under mild conditions to (SBI)Zr(NMe2)2 with
a racimeso of 4.5/1. The amine elimination was less useful for hafnium as well as titanium; the
order of reactivity for the Group IV metals being generally Zr>Hf>Ti in the amine elimination
syntheses, and even more defined with crowded metal amides or crowded or weakly acidic Cp
ligands.
In 1998, Nifant'ev, et aI., again published a report of bridged ligands that were
incorporated into an attempt to prepare zirconocenes from distanny1 derivatives of the Cp ligand.
The bridged ligand 4,4-di(Cp)-N-methylpiperidine and 4,4-di(indenyl)-N-methylpiperidine were
prepared by condensation of cyclopentadiene or indene with N-methylpiperidine. The dilithium
salts of these reagents were reacted to form the corresponding ansa-zirconocenes, which appeared
to have no direct contact between the Zr and N atoms. 32 The lone pair of nitrogen is known to
-
interact with the acidic Zr center, effecting chemical and catalytic activity of the metal atom,
which usually leads to the (undesired) deactivation of the metal catalyst. The interaction of the
22
-
two atoms can be prevented by simply putting distance between the two. The current complex
was prepared from Cp (or indenes) and N-methylpiperidine in a two phase KOHIDME, which
was a similar reaction to simple ketones. Product decomposition was a threat during distillation,
but no interaction between the Zr and N appeared. A bis(trimethylstannyl) derivative could be
introduced as a cyclopentadienylating agent in situ to obtain the best results in the synthesis .
•
R"
R',N
~
C~art
1
n
ZrCI,
1
An
3
Scheme 1
-
£
Me
,0· ..-00:1
SchemeS
4,25'16
-
KOH
OME
4
Me
2Et,&9
ether
*
Et,Sri
5,43'16
5b
ZrCll,
toluene
SnEt,
1: 1 rac/meso
Scheme 2
3,42'16
23
8,22%
-
In 1999, Chirik, et al., published research on zirconium dihydride complexes, which are
important catalysts and intermediates in olefin hydrogenation and polymerization reactions;
[(RnCp)2ZrCh]x was synthesized, having substituted Cp ligands by hydrogenation of the
corresponding dimethyl compounds. 8 The most sterically crowded complexes: (Cp*(n 5_
CsHMe4)ZrH2, (Cp*=n5-CsMes)), Cp*{n5-CsH3-1,3-(CMe3MZrH2 and {n 5 -C sH3-1,35
(CMe3)2hZrH2 were monomeric. The less crowded ([Cp* {n -CSH4(CMe3)} ZrH212,
[Cp*(THI)ZrH2h (THI=n 5-tetrahydroindenyl) and [{ n 5-CsH3-1 ,3-(CHMe2)2}ZrH2 and (n 5 _
CsHMe4)2ZrH: existed in eqUilibrium monomeric and dimeric forms in benzene solution.
~
'Zr.-CI
~
2CH3Li
~"""CI.
&!P-CH3
I
I
R"
-
~.....cH3
~CH3
I
R"
(1)
~CH3
Et20
R"
Hz(l aim)
25"C •
benzene
or
cyclohexane
~H
~H
...........•
•...........
dimer
(2)
I
R,.
.Scheme 2
trans
cis
Scheme 3
The hydride could also be replaced by deuterium, incorporated on the Cp rings of monomeric
dihydride compounds, in an equilibrium H2 < -- >D 2.
24
-
In the same year, Rogers, et al., produced a 6,6-dimethylfulvene complex with
M(CH2Ph)4 (Zr, Hf) in benzene which reacted cleanly to give [n5-C5H4(CMe2CH2Ph)]M(CH2Ph)3
without byproducts. 36 Titanium complexes were not observed.
~._/Me
~Me
v-'Me
•
-
v-"Me
+
~/Ph
~C(Ph}2CH2Ph
~Ph
I
Ph~/Hf"""'CH Ph
+
Hf(CH~h)4
PhHzC
4
(2)
z
,
3
()~
.,.......
Scheme 1
G>=<
~
Me
Zr(N~)4
~#
Me
~
I
MezN
_Zr.......
/
MezN
Me
(4)
NMez
•
+
o
@ - f 3Nu
~=:=?//
,Me
\, ,/ .J
CH3
Nu'
•
(XH3
0::,...
CH 3
T@--<
Me
RJi-R
A three-legged piano stool geometry was believed to be assumed by the complex, with n2-bound
-
benzyl ligands. A second equivalent of ligand produced no reaction, while the bulkier 6,6diphenylfulvenl;: reacts only with the more Lewis acidic Hf(CH 2Ph)4 to yield [n 5 _
25
CH3
-..
C5H4(CPh2CH2Ph)]Hf(CHz-Phk Tetraamido M(NMe2)4 and 6,6-dimethylfulvene produced
dimethyl amino and [n5-C5H4(CmeCH2)]M(NMe2)3Zr due to deprotonation of the fulvene methyl
group and coordination of the resulting 2-propenyl-Cp fragment. The reaction of6,6dimethylfulvene and [n5-CsH4(CMeCH2)]M(NMe2)3Zr yielded [n5-C5H4(CMeCH2)hZr(NMe2h.
An excess of8,8-dimethylbenzofulvene was used in reaction with M(NMe2)4 to produce [n 5_
C5H4(CMeCH2)hZr(NMe2)2 exclusively with one coordinated indene. The M(CH2Ph)4 complex
gave products through a nucleophilic attack of a benzyl ligand onto the exocyclic carbon of the
coordinated fulvene.
In the year 2000, 4lWarren, et al., experimented with the symmetry ofmetallocene
ligands, using a symmetrically bridged indenyl or tetrahydroindenylligand to work with the
precursors ofM(NMe2)2Ch(dme) (Zr,Hf) in reaction with dilithium derivatives ofsilyl-bridged
bis( l-indenyl) ligands bearing pendant 4-pentanyl groups, Lh-(3,3' -5R2-SBI) in toluene to make
-
bis(dimethylamido)metallocenes in 6-711 racimeso. 43 In an excess ofTMS-CI in CH 2Ch and
catalytic HCI, rae-[3,3' -5R2-SBI]ZrCh appeared as a product, free from the meso isomer, in 40%
yield. Similarly, Lh[SBI]OEt2 and Lh[EBI]OEt2Ied to selective formation of the rae isomer in
as high as 2011 ratio for [SBI]Zr(NMe2h
Scheme 2"
8
rac/meso
-
~
.
2 Li+
-.iii62%
~ .....-:
<:)
2SiMc2
Li 2[3,3'-~R2"SBII
2
om 1 equiv of BuLiII'HF, then ROTs/O 'C; (ll) BuLilE140,
then 0.5 equiv of Mtl2SiCb/-78 'C to room temperature; (iii)
2 equiv of BuLilpentane.
-
Scheme 3
M(NMe,l. + 2 TMS·CI
EtO
-f.Fm;-
Li,[3,3'-'R,-SBI) + M(NMe,l,CI,(dmel
[3,3'-'R,-SBljZr(NMe,J,
_"'._ ....h ...... C
M(NMe,l,C1,(dme)
3 M:Zr
• M"Hf
toluene.
-3S·C·RT
4TMS-CI
S mol % HCI,
CH,CI,
[3Y"R,-SBI)M(NMe,),
5 M:: Zr
, M = Hf
rQc/rMSO
C••
TIIC-[3,3'-'R,-SBI)ZrCI,
7
611
-
Scheme4G
initial
M~N~'
c2
'NMc,
I
racJmtso
0) M=Zr
10 M=Hf
~20/1
1611
Y
)I(NMc1i1Cl1(dmc)
~
~<NM_
NMc2
11 M=Zr
Il M=Hf
5/1
811
Sodium(Na) amalgam reduction ofrac-[3,3,-5Rr SBI]ZrCb resulted in tail to tail coupling of the
pendant olefin:;; to a,a' -disubstituted zirconocyclopentane. The reduction of HfCl4 led to the
symmetric fonnation of a,a' -disubstituted and a,I3' -substituted hafnacyclopentane in 1: 1 ratio.
SchemeS
-
13
Scheme 6"
xsf.~g
t
M=Zr
~
Me i
2
17
M".... CI
7 M=Zr
"CI8M=Hf
xsNalHg
THF
I M=Hf
16
+
~
VP
~
MC 2
CH,
18
14
15
27
III-C. Zirconium, 40Zr, and Titanium, 22Ti
Zirconium and titanium, of the 3d and 4d Group IV transition metals, also often appear
together in resc!arch syntheses that take advantage of their particular properties. Here, research
existing for thc:se two elements is presented separately from other divisions.
In 1996, Halterman, et aI., prepared a synthesis of2,2'-bis[(4,7-dimethyl-inden-l-yIJ1,1' -binapthyl and 2,2' -bis[(4,5,6, 7-tetrahydroinden-l-yl)methyIJ-l, l' -binapthyl-titanium andzirconium dichlorides from 2,2' -bis(bromomethyl)-l, l' -binapthalene. 19
,6)
3121
lal9! M=Ti. lbI'IM=Zr
6112)
51111
~: 0",""
~u
..."
....,.,·-..-'c......-'"
LI
~
~~
I
I
1) n-BulJ .
_
A
I
2)0<:0
3)W
21
I
~
8
12
~
~
(5)-H-7
::".
Scheme I.
Li
:::
-
~
(5)-<+,
-(l)n'BuLi ~""
(2J~CI).[OI·
I:
"",
..."
Li
1)
n-Buli
2)0~
3)W~
M
(RH+)-S
(R)-H-12
.a
Ka
'<:
_ (2) ZrCI.
(5)-(+HO M=Ti 24<;<
II M=Zr 21 ..
1)
LI
..." LI
..
n-BuLi
2'0../'("\
3)H~
18
~'
'<:
,,,&
28
12
..." U I
..."
I
A
(R)-(+)-12
Scheme 2.
~
~I
(RH+)-7
1) n-BuLI
..
2)O-o:J<
3)W
a?
~II
I
,
a
Scheme .l. Fonnal",n of binaphlhy) bridgoo ligand.,
-
2,2' -bis(bromomethyl)-I, l' -binapthalene was alkylated with the lithium salt of 4,7dimethylindene. The alkyl substituted at C4 and/or C7 of indenyl in bis(indenyl) metal
complexes containing achiral bridges.
In 1997, Halterman and Ramsey reported the coupling of enantiomerically enriched 2,2'dilithium-l,1 ' -binapthyl with various annulated cyc1opentenones or 2-indanone as a facile route
to the preparation of a series of annulated bis(Cp) or bis(indenes) bridged at a symmetric Cp
position. 16
0
1) LOA
0
NCsH"
13
2)Br~
8
~
OCH3
3) oxalic acid
4) 5% KOH
a
1) LDA
NCsH11
15
-
14
0
[)
~
2)Br~
OC~
3) oxalic acid
4) 5% KOH
16
r
TM~
\CI
Z·
~
o
\
A TiCl~ \J ~
;=;"
~
18
-=:
1)LDA
-----~
2) CIPO(OEt)2
3) LDA
KC iCO)8
O
C H~ N •
$
\1
0
\4 quant
lb
~
Scheme 3.
19
20
Scheme ~. Preparation of annuJated cydopentenone,.
~
~
I
I
_I_ln_.au_l.J-t-I_~ ~£.,Ct
e
~I
I
(RH+1-5
L«)
21 Mel,or Met.
31 HCI. air
M= Ti. ZI
II n-BuU.
21Ta,
-
~tBc,
.."
"CI
3IHCL .....
or
21
ZIG"
(RH+I-22 M = Ti
(RH+I-ZI M = ZI
~
l)n_Bu~I"'~"'}{·>··~'
~
lln_BuU ..
21ra,
~?
~CI
~
31 HCL, air
(RH+1-7
-
(3' 'CI
~
I
or
21
ZIG"
CI
(RH+I-34 M = Ti
m~
I
.M,
0:
.~
31[0)
•
M = ri. ZI
...,;
Scheme 4. MelJllation of binaphthyl bridged ligands.
29
-
The six or seven membered annulated bis(tetrahydroindene) or bis(hexahydrozulene) ligands
could readily be converted to titanium/zirconium dichloride complexes. Only a single Czsymmetric isomer of the metallocene could form; the faces of the Cp ligand were homotopic,
while the chiral bridge forced a chiral conformation.
~
CsHsli 1)0=0:)<
~ ~
31
or
----~.
CH3i-il 2)p-TsOH
XI
__ ~
~
CeCI3
1) n-BuU
..
2) TCl3
3) Hel. air
R =Me 30
Scheme 5. Preparation of bis(letrahydropentalenyl)titanium dichl<>-
rides.
-
•
A bis(indenyl) and five-membered annulated bis(tetrahydropentalene) ligand could not be
metalated in this synthesis. Unbridged dimethyl and diphenyl substituted tetrahydropentalenes,
however, were prepared and converted to TiCh complexes.
30
-
In that same year, Halterman, et al., also published the synthesis of a "new" C 2hsymmetric, doubly-bridged 1,2' -(ethane-l ,2-diyl)-2, l' -( ethane-l ,2-diyl)bis(indenyl) ligand from
1,5-dimethyl-l ,5-cyc100ctadiene via a selenium catalyzyed oxidation reaction and a Nazarov 1t
cyc1ization reaction. 18 Only one bridged C2-symmetric diastereomer formed in metalation
because of the geometry of the doubly-bridged ligand system, preventing the formation of the
meso isomer. There was no evidence of oligomeric metallocene products in the synthesis of
Ti/ZrCh complexes with the ligand. A related bis(indene) containing bicyc10[3.3.1] nonane
moiety was synthesized from Meerwein's diketone but couldn't be metalated.
Scheme 1. Syntheail of Doubly Bridled
BiI(indenyl)metaJ Compleltea 10 and 11
Q Q PhMgBrO~
Rl
20H
5.0
---...
_
PPA
.,-
1.
l-BuOOH
2. Swem
CHzC,:!
(m 25%
(14%)
C H0
120 "C
(54%)
. . , . . Ai
...
4
1.Wmethyl
_
{73%
two st""-)
iIIomer)
~OH
~
o/):b
~- ---;;
0
o
LiAlH.
§
HO
7
•
1.n-BuU
2.T~
--
p-rSOHQocoI
I
3. [0)
(42%)
~ A
or
--..
II
.M
I"
1
1. Zr(NMez.).
2.
HNM~-HCI
"""
M • TI, 10 (63%)
Zt.11 (67%)
In 1998, Halterman, et al., again reported the creation of "new" metallocenes bridged at
the 7,7' position of indenyI ligands, including a methylene bridge [dl-bis(4,6-dimethylinden-7yl)methylidene jTiCh and [dl-bis( 4,6-dimethylinden-7 -yl)methylidene ]zrCh prepared from mxylene. [dl-l ,2-bis(4-methylinden-7-yl)ethylidene] and [dl-l ,2-bis(4-isopropylinden-7yl)ethylidene] --ZrCh were likewise prepared fromp-cymene; and [dl-l,2-bis(4-isopropylinden-7yl)ethylidene]TiC}z was prepared fromp-xylene. The complexes exhibited C) symmetry in the
-
solid state and C2 symmetry in solution as well as a very open "architecture", which was believed
to contribute to a more reactive metal center. 20
31
-
~~,
C!fo
'Ti ....0
"0
Tr'
V.
'CI
12
1
~
~R
C1
M··..•
... 51
(56\'CI
Sll
4"
*
3"
1
"
~'
'CI
's~CI R~
I
M·····
'~CI
C1
R
37a
t>
14
;.'CI
6"
Figure 1.
D
0
dA
B
VERY OPEN
!1:
,~ ~
A
CI.oSED
Figul''' 2.
6:
7:
C
OPEN
~
01 Bia(indenyl) Lig .. I Schem.. 1. S yntheaiaand
11h
an... 11a
o
,:'
y
.
Q9
R
R
tBuOK. nBuLi
....
2)12
1aR=Me
1b R= iF'r
R
I
1'"
....
I
I) 3..:hloropropionyl·
chloride/AlCl]
2)H2SO•• 11
l1a R.= Me
SaR=Me
8bR=iPr
~
=
iPr
and 0Ibcr isomers
1'"
MeOH
....
Metala!ion
til
1'"
I .... I
lOaR .. Me
lOb R iPr
and other isomers
=
R
PhH
lib R
NaBH. H O w
R R
OH
-
9aR=Me
9bR=iPr
and other isomers
TsOH
of Bis(ind
Scheme 2. Metalation
and
llb
enyl) Ligands lla
R
W
Conditions
M
l)nBuLi
2)ZrCl.
Zr
I) Zr(NMe,).
2) Me1NH1CJ
l)nBuU
2) TiCI)
3)[0)
UaR=Me
UbR=iPr
and other isomers
32
. . .0
flUlfU.O
Yield
llb R-iPr
ca. SO:SO
cso..
Zr
128 R .. Me
llb R= iPr
ca. 100:0
ca. 100:0
63..
61 ..
Ti
13 R=iPr
ca.lOO:O
14'1&
-
In that same year, Halterman, et al., also published a report on the palladium-catalyzed
coupling of a 1,2-diiodobenzene with (indenyl)zinc complex obtained from indene, 4methylindene, 4,7-dimethylindene, and hydrindancene to prepare 1,2-bis(1-indenyl)benzenes in
moderate yield. 17
~
~
'CI+~CI
~--
1.2%~
I
THF. 95 "C. 36 h
I
l.vzncbBr~Br
-~
-
8
~ A'
V
1.2% PcI(PPI\J4
THF.95"C.24h
P
,.,.
.....
I
--,
or
~
~
\"''0",
1) Zr(NMe21,,; 2) HNMerHCI
'M .•
v.;'CI
dlM.Zr:6a
M .TI: 7a
1.2%~
THF. 95 "C. 24 h
-
1) n-BuU; 2) ZrCt.
8
r?
or
~
~
I A
1) n-BuU; 2) TlC1~ 3) HCI
_
Scheme I. Palladium-catalyzcd coupling of indene and dihalobcnzenes.
~CI
meso-
M.Zr: 6b
M.TI: 7b
Scheme 2. Metalation of 1,2-bis(l-indenyl)bcnzene.
New phenyl bridged ansa-bis(indene) titanium and zirconium dichlorides from these ligands
provided a good yield by addition ofTiCh or ZrCl4 to their lithium salts or Zr(NMe2)4 to neutral
ligands. In both cases, the zirconium tetraamide metalation produced very high dl selectivity. NBuLilZrCl4 metalation of phenyl-bridged unsubstituted indene gave 3/2 ratio of dl(rac)- to mesowhile placing substituents at 4,7- and 5,6- positions led to 1011 selectivity dllmeso. The nBuLi/TiCb metalation proceeded to a 111 up to 411 ratio selectivity dllmeso.
33
-
IV. Titanium, 22Ti
Titanium, the smallest of the Group IV transition metals, is a popular subject for many
different syntheses and experimentation in Cp/indenyl derivative ligand chemistry. Owing to its
own unique properties as a result of its position among the early transition metals, titanium may
often be successfully used in chemistry for which lower Group IV metals are not suitable.
In 1995, Urabe, et al., produced a bis(indenyl) titanium complex of the character (n 5 _
indenylMn3-alkyl-l-methylallyl) generated from (n 5-indenyl)2TiCh, 2-alkylbutadiene, and i-Pr
MgCI, which reacted with CO2 or RCHO (R=Ph, Et, i-Pr, or t-Bu) to give optically active p,yunsaturated carboxylic acid or threo-homoallyl alcohols with high enantiomeric purities. 40
-
C!l2TIC~
(1)
--
(¢. @),~
:+2
/"..,
R'
R'
~R2
7
R3R4C.O
TICPz
HO
(1 )
(4)
R3
~
R'
(2)
(5)
(3)
(6)
c» ToCIz
0
(7)
R
~..~
.
':'PrMgCl
R
~
I
Tl{lnd·)2
(7)
or (8)
(9.) R. CaHIsr
(9b) MezC-CH(CH2l2"
(10)
~/
/'
C~H
(11.) R. CIl"I,II(11 b) Me~H(CH2l2"
R~ ~.....
HO). R'
thrBO
34
J... . .
+
HO'·..lR'
erythro
(12)-(15)
(2)
(8)
-
Introduction of chiral elements to the ligands enabled asymmetric delivery of the allyl group, but
to increase the optical purity of the products, a more rigid and sterically demanding n5-indenyl
group was used to increase efficiency.
In the same year, Willoughby, et al., reported the synthesis of a new titanocene
compound containing both a Cp and an indenylligand. The preparation occurred in a one pot
synthesis from indene and pentamethylene fulvene. A tetrahydroindenyl analog was also
prepared for reaction as the saturated derivative of the indenyl group.44 The catalytic
hydrogenation of2-phenylpyrroline was conducted with each of the two complexes to find that
the tetrahydroindenyl analog complex catalyzed the reaction at a rate five times faster than the
Cp/indenyl complex. This was attributed to the difference in electron density at the titanium
metal center, and it was projected that reactivity was affected by the steric environment of the Cp
ligand.
-
- - - - - [~l- 2.1.n-BUU~"""
(JO
~ I
/J
1.n-BuU
2.0=0
_
I
CI
TI····
'CI
TlCI,
3. PbCI 2
~
2a
"""
1-U
80 pslg
18 % Ov....11 YIeld
Hz/Pt02~
.CI
'CI
------
T.. ·•
2b
82% Yield
Scheme I. Preparalion or complexes 2. and 2b.
35
-
In 1996, Chin and Buchwald reported a new preparative method to produce optically pure rae and
meso -ethylem~-1 ,2-bis(n 5:4,5,6, 7-tetrahydro-l-indenyl) titanium derivatives that effectively
doubled the available yield without the need for chromatographic separation or the use of sodium
metal. 7
Scheme 1
~
;n
1. Na, 0.5 equiv
(R~binap/lthol ~
-----------------.~
CI~.
x~'o,x
~
Chromatography over alumina
(rac~l
(R,R.RH
35·37%yieICI
(R~BlNOL
Scheme 2
(rac~1
0.6 (R)-2,2'-bonaphth-l. 1'ClIOI
1.0 p-amonobenzoic acid
excess NEt3
toluene reflUX
,.-
"'.R.RJ-' .
u
(S.S>E.TH,.~L~
\:
I
NHd-z
(5,5)-3
(5.5)-3
(R,R,R~2
34% yield
optically pure
j
Cone. HCII
dichloromethane
Scheme 3
oN
4
5 mol % catalyst
10 mol % n-BuLi
7.5 mol % PhSiH 3
80psig H 2 ,
65 ·C, THF
oR
I
H
h
5
(5,5)-1
(5).598'''
(R,R.R}-2
(R}.599.....
catalyst =
36
-
In 1996, Amor and Okuda reported the titanium complex Ti(n5 : nl-C9H6SiMe2NCMe3)X2
(X=CI, Me, CH2SiMe3, CH 2Ph), containing the tert-amido-functionalized indenylligand
C9H6SiMe2NCMe3.] Titanium complexes with one amido functionalized Cp ligand were the new
single site polymerization catalysts, with the ability to promote efficient polymerization of ethene
with l-alkenes (i.e. l-octene) to produce new types of elastomeric polyolefins. Here, the complex
was synthesize:d by the reaction of the dilithium derivative Lh(C9H6SiMe2NCMe3) with
TiCb(THF)3 followed by oxidation or alkylation of the dichloro derivative.
~~5;
c(
LINHCM83
•
Me~~
l
M83~H
2 BuLl
•
[~~]
(LI+)z M83C)-
R = H (a), SIMe, (b)
Aa>!TH'"
2.} PbCI2
Me~
--
M83~nv'
CI
1a
Me1~
~Fb
7
~
I
Me_
M83~~~'
Me~nvHzPh
CHzPh
R·= lie, 2a
R·= CHzS11le3; 3a
4a
Unexpectedly, the reaction of C9H6(SiMe3)(SiMe2Cl) with TiCl 4 did not produce Ti(n5 C9H6SiMe2Cl)Ch. The attempt to prepare linked amido-indenylligands and titanium complexes
limited stability in the mono(indenyl) titanium complex, but this was expected to be improved by
the introduction of the linked amido functionality.
Also in 1996, Halterman, et aI., presented the development of a complex containing a C2-
-
symmetric 8,10-dimethyl-tricyclo[5.2.2.02,6]-2,5-undecadiene(DMeBCOCpH) ligand bridged by
silicon, methyl~me, and ethylene. 21 Dimethyl Si-bridged bis(BCOCp) ligands were accessible but
37
-
couldn't be converted to the corresponding titanium, zirconium, or (niobium) complexes. The
8,1 O-dimethyl.4-( I-methylethylidene)tricyc10(5 .2.2.02,6)-2,5-undecadiene(DMeBCOCpdimethylfulvene) ligand was prepared but unreactive toward Cp anions.
Chart 2
'R~"l::
RR
Chart 1
'R
.&
'R .
~ I
'R
G, p
~
0) 'cl
R
Chart 3
Here, the presence of achiral bridges between the
-
chiral Cp ligands were used to install the chiral
-((~
~~
~,
a·site
environment around the metal center.
102
~,Il-Bridged 11
a,a·Bridged 12
In 1997, Huttenloch, et at., reported chiral biphenyl bridged metallocene complexes ofa
general formula: biph(3,4-R2C5H2)2MCh (biph= 1,1' -biphenyldiyl), which were synthesized and
characterized. 23 Dimethyl-substituted titanium and zirconocenes were prepared with optimized
yields and resolution. The bis(tetrahydroindenyl) complexes (R,R=(CH 2)4) were prepared by an
alternate route and characterized as were the phenyl-substituted derivatives.
~~'
M: TI nte.l.
Zr nte-z.
38
-
~ l'+2
Q
\
0
litO
>-.
'"
R
ether
•
-78'C 10 r I
R
R
R
•
7
• b 160'C
2KH
c 180'C
decahn
R
190 'C
R
a
R
•
R = Me. (CH 71. Ph
•
c
b
Scheme I.
~
QkMe
RR
1) TiCI,(THFl,. THF
2)6NHCI ...r
R=
Me.
I
•
I
-
O~~Me
CI
~~~CI
HO
(i)-15a
R
(CH,)•. Ph
ether. S h. 2S"C
abc
·2CH.
Scheme 2.
x::
-
I
"
OH~
..,0
OAe
To
- \7;( "~O
I
Ph
TH'H
o Ok
(S.RR)-14a
1'1
Scheme 5.
R
I Jd>~o
~
-
'A
~
1) 0 5 ell (R)-IINClI1t1101
2)
'0
R
~.
THF. -SO'C.
12". r.l
•
»:~<
R
R
." .
(R.R)-11a.b
Scheme 6.
'
k"Me
~
:
I
-
Ph
H'H
o Ok
+ 2
'/
0.5 eq. (R)-bInaPhtIloI
(")/Zr ~Me --e-IJ'le(--.-2S-·-C--"·
u
-2 CH.
(RR}-17.
rac-11a
Scheme 7.
39
(R.R.R)-14a
-
In the same year, Luttikhedde, et al., also published the reaction of indenyl-potassium
with Y2 an equivalent of chloromethylpivalate to yield bis(3-indenyl)methane. A double
deprotonation of the ligand complex with two equivalents of butyl lithium (BuLi) followed by
reaction with TiCI4~2THF and subsequent hydrogenation yields rac-methylenebis(4,5,6,7tetrahydro-l-indenyl)TiCh in moderately low yield. 30 Analysis of the molecular structure
revealed the presence of a very small acute angle at the methylene bridge carbon, resulting in a
reduced Cp-Ti-Cp angle.
2 IndI< + (CH ,)3CCOOCH 2C1
THF.O"C
•
Scheme I.
-
2
:I
Scheme 2.
In 1998, Lensink completed a reaction of TiCNMe2)4 with a bidentate ligand,
CSH4CH2CH2KHS02C6H4CH3, yielding a titanium sulfonamido cyclopentadiene complex Ti(n 5 :
cr-CSH4CH2CH2NHS02C6H4CH3)(NMe2)2?8 Upon reaction with 2 equivalents of Me3SiCI, a
"new" metallo(:ene dichloride complex Ti(n 5 : cr-C5H4CH2CH2NHS02C~4CH3)Ch was
generated.
40
-
(a)
~)
¢
(b)
N ........... "'NR
I
I
O=S=O
¢
+ Ti(NR2).
-HNR2
1
O~~~O
¢
2
O=S=O
¢
-HNR 2
7
3 (R= Me)
5 (R=Et)
&M.
~ .-/
+ Ti(NMe,).
"NMe,
O=S=O
¢
-2 HNMe 2
2
(e)
~.NR,
(R2NhTi'N~
HN
I
O=S=O
6
~NMe2
~/ "NMe2
O=S=O
d
~/
+ 2 Me3 SiCI
O=S=O
..
¢
CI
CI
¢
- 2 Me,SiNMe,
3
4
Scheme L
-
This synthesis of a chiral, enantiomerically pure sulfonamide Cp/indene bidenate ligand was
hypothesized to engender the formation of very stable, strongly bound Group IV complexes,
which would subsequently contain a metal-sulfonamide bond less susceptible than other bonds to
aminolysis or alcoholysis.
In 1999, Palodonken, et al., derived new ansa titanocenes from 1,2-bis(2-indenyl)ethane,
prepared from a titanium-mediated reductive coupling of2-(hydroxymethyl)indenes, which
provided a convenient method for substrate dimerization as well. 33
I-indenyl
aI-:toa
bRei. 5
" .
[co-]
"8
-91 .....,
05· I:: ~ cO
I
I'
1
b
R.f.6
2-indenyl
-
R.f.8
~h"~~~
~" .', ~ I
,,:;~
2
d
pres."t
study
~
~
h
~Br
~Br
~~
~ I
41
4
la. n-BuLi
lb. TiC1 3o(th0 3
Ie. aq. HCI. air
2. H 2• Pt0 2
R-...2........,.-,....
RI
~
6-1CrY'" , - I ~,
Ph
v-vi
.&
OH
8
9
vii-viii
OTBS
A
OH
7
Alkyl substitution of the indene ring at the C3 position improved the regioselectivity of the
reductive coupling to prepare ethylenebis(2-indenyl) ansa ligands in low to moderate yield (2962%).
In 2000, Lancaster, et al., presented research for a (boryICp) titanium complex,
-
(CpB)TiCb, (CpB=n5-C5H4B(C6Fs)2), which reacts with LiCsHs (LiCp), LiCsH4SiMe3 (LiCp') and
LiC 9H 7 (Lilnd) to give the corresponding titanocene complexes: (CpB)CpTiCb, (CpB)Cp'TiCh,
and (CpB)IndTiCI/6 The first complex (CpB)TiCb exhibits piano stool geometry with an
uncoordinated trigonal planar boryl group. The reacted complexes possess -B(C6FSh substituents
which act as intramolecular Lewis acids by coordinating to chloride ligands with formation of BCI-Ti bridges, characterized by relatively short B-CI and elongated Ti-CI bonds.
3
6
4
42
-
-
The compounds are fluxional, with the -B(C 6F5)2 group switching rapidly from one chloride to
another. Recrystallization of the first reacted TiCb complex in the presence of trace amounts of
moisture afforded (CpB)CpTi(~-OH)CI with rigid B-O-Ti chelate coordination.
Also in 2000, Halterman, et al., published a report on the synthesis ofnonracemic
bis(indene) (+)-(1R,2R,4R,5R)-1,4-bis(3-indenyl)-2,5-diisopropylcyclohexane in 60% yield from
the addition of indeny I lithium to the corresponding bis(methanesulfonate)ester of2,4diisopropyl-l ,4-cyclohexanediol. 22 The synthesis consisted of deprotonation of the bis(indene)
with n-BuLi followed by metallation with TiCh and an oxidative work-up (HCI, air, chloroform),
which gave a single stereoisomeric 2,5-diisopropylcyclohexane-1 ,4-diyl-bridged
bis(indenyl)TiCb in 80% yield. Catalytic hydrogenation of the bis(indenyl)TiCh gave 2,5diisopropylcyclohexane-l ,4-diyl-bridged bis(tetrahydroindenyl)TiCh in 76% yield.
-
( tR.2R.4R.5R)-J
43
-
The nonracemilc mixtures of chiral bis(indenyl)TiCb and bis(tetrahydroindenyl)TiCb complexes
were also examined as catalysts in the pinacol coupling of benzaldehyde in the presence of
manganese metal. In this first comparison for enantioselectivities for corresponding complexes,
the selectivity for dihydrobenzoin was found to be 0% for bis(indenyl) and 32% for
bis(tetrahydroindenyl).
V. The Group V Transition Metals
The Group V transition metals: Vanadium, 23 V , Niobium, 41Nb, and Tantalum, 73Ta,
occupy a relatively unexplored area ofCp and indenylligand chemistry due to their generally
demanding syntheses, which require extremely high pressures and long reaction times, and fragile
nature in the presence of light and air. Knowledge of their specific properties and reaction
proclivities is still limited; however, recent years have provided heightened research on their
-
syntheses as these metals have attracted more attention to their possible roles in a olefin
polymerization and ethylene/CO copolymerization as well as generating interest in other possible
catalyst behaviors.
In 1997, Klaus, et al., expanded his work on transition metal complexes having pentalene
or pentalene derivatives. Formerly, his research found compounds containing late transition
metals: Ni, Co, Fe, and Ru, formed multinuclear complexes in which bicyclic ligands assume
planar or nearly planar geometry and act as bridges between the metal atoms. With his most
recent report, complexes with all 8 atoms coordinated to a single metal atom were achieved for
the first time with pentalene. 25 Vanadium containing starting materials (vanadocene
monohalides) of the form [Cp2VX] (Cp=C5H5, X=CI, Br) and derivatives which react with two
mole equivalents of allyllithium. The allyllithium reacts to replace a Br and Cp ligand to form a
16e- vanadium complex in 80-90% yield. Dilithium compounds can also be used to form 18ediamagnetic complexes.
44
(CPlVXI
~
7.
I
v.
~
·Ux,·UCp
Sa
[00]
-¥
(Cp 'CpVXI 7b
I
v.
~
<s?O
I
~
·UX.·UCp
Sa
8b
IInd,VXI
7.
V.
·!.iX.· UInd
Be
{OO>-]
-
[CPlVX]
~
7.
•
. ux,· UCp
Sb
I
v.
~
ScI
In 1997, Bitterwolf, et al., reported the reaction ofNa[M2().1-CIMCO)s], where M=Nb,Ta,
with NaCsH4R (COCH3 , C02CH3, C02C2H s, COC 6H s, COCH2C 6H s, LiC sH4C(CH3)CH2 , or
NaC9H7 to giv~: the corresponding (n 5_ C SH4R)M(CO)4 or (n 5_ C 9H 7)M(CO)4 compounds in good
yield. 6
45
-
In most cases, these complexes have been prepared by reductive carbonylation in
extremely high pressure conditions of the corresponding (n 5_ CsH 4R)MCI4 • This research was an
extension of previous theory and work by Calderazzo and Pamploni, resulting in the preparation
air stable as solids, but underwent ready oxidation in solution.
In 1998, Djakovitch and Hermann produced the reaction of [Nb(NMe2)3 (=N-2,4-
iPr2CJI3)] with an Cp-indenyl isopropylidene ligand to form the new niobium complex [Nb(n 5_
CSH4R)(NMe2)2 (=N-2,4-iPr2C6H3), R=CMe 2C9H 7] in high yield. 13 The new ansa-niobocene
[Nb(NMe2) (=N-2,4-iPr2C6H3)(n5:nJ_{CsH4}C(CH3h{C9H6} was obtained at higher temperatures
by an intramolecular deprotonation by concomitant coordination of the indenyl group.
-
Toluene. lOO"C. 2h
88%
3
1
Scheme I.
Too....... lOO"C. 2h
1
3
Toluene. llO"C. 7h
n'lHroml
TMSCI. Toluene
-
4
2O"C. 2b
93%
Scheme 2.
s
4
Scheme 3.
46
In 1998, Bitterwolf, et al., again presented research on improved preparative routes for
Cp and substituted Cp(CO)4 derivatives ofNb and Ta complexes to produce five compounds:
(vinylCp)Nb(CO)4' (acetylCp)Nb(CO)4' (Cp)Ta(CO)4' (indenyl)Ta(CO)4' and
(benzoylCp)Ta(COk 4 Photolysis of both light-sensitive (Cp)M(CO)4 and (indenyl)M(CO)4
metal complexes in Nujol glass matrices was found to result in the reversible loss of one to three
CO ligands as a function of the wavelength of light.
Sc:bemcIa
Cu(I)C1
RmTcmp
Ihr
(j-R
Na
[M(CO»f
-.
RmTcmp
IS miD
Scheme I. Synthesis of ring-substituted derivatives using [M(CO)6J - .
MCl, +
~TMS
"\;;;;;;/ - R
+ 4CO
Scheme II. High yield synthesis of
CsHs)MCI •.
('1~-CsH5)M(CO)4
from (,'-
In 2000, Bitterwolf, et al., reported on kinetic studies for CO substitution reactions of (n 5_
5
CsHs)M(CO)4 and (n _ C9H 7)M(CO)4 with phosphines and phosphites. Niobium derivatives were
-
found to undergo ligand exchange at higher rates than tantalum, and indenyl derivatives
exchanged fast€:r than Cp derivatives. s
47
Scheme 1. Dissociative mechanism for ligand exhanae, M oz Nb, Ta.
«p@ ~ o.po> ka
OC-1\CO
C C
o
0
Ic~
«f#'>O
'oc7i'
OC I\CO -OC7\C
0
L
L
C
o
Scheme 2. AJsociative mcdlanism involving indenyl effect. Dissocia·
tive pathway is minor and is not illustrated.
The previous belief that Cp ligands reacted by an associative mechanism was also
refuted; dissociative or dissociative interchange mechanisms were found to proceed for Cp
ligands, while indenylligands were predicted to react by an associative mechanism. Rate and
thermodynamic data also suggest that the "indenyl effect" was operative within Group V metals.
The referred to "effect" was a term coined by Basola, and used to describe the rate enhancement
-
found for indenyl compounds, which is attributed to indene's ability to undergo n5 to n 3
haptotropic rearrangement to facilitate ligand association. The overall reactivity order for the
Group V was also found to generally follow V>Nb>Ta rather than the supposed triad rule,
V<Nb>Ta.
48
-
References
lAmor, Francisco and Jun Okuda. "Linked Amido-indenyl Complexes of Titanium." Journal 0/
Organometallic Chemistry 1996, 520, 245-248.
2Baker, Robert W. and Brian J. Wallace. "Enantiospecific Synthesis of a Planar Chiral Bidentate
Indenyl-alkoxide Complex of Zirconium Using an Axially Chiral Indene Ligand." Chem
Communications 1999.
3Bell, Louise, et al. "Catalytic Asymmetric Carbomagnesiation of Un activated Alkenes."
Tetrahedron Letters 1996, 37(39), 7139-7142.
4B itterwolf, Thomas E., et al. "Improved Synthesis of (n 5 -CpR)M(CO)4 Compounds and the
Nujol Matrix Photochemistry of (n 5-C9H7)M(CO)4' where M= Nb, Ta." Journal 0/
Organometallic Chemistry 1998,557, 77-92.
5Bitterwolf, Thomas E., et al. "Kinetics and Mechanism of the Ligand Substitution of (n 5 _
C9H 7)M(CO)4, M=Nb or Ta ... " Journal o/Organometallic Chemistry 2000,168-173.
6Bitterwolf, Thomas E., et al. "Ring Functionalized Cyc10pentadienyl Derivatives ofNb and Ta."
Journal o/Organometallic Chemistry 1997, 545-546, 27-33.
7Chirik, Paul J., et al. "Preparation and Characterization of Monomeric and Dimeric Group IV
Metallocene Dihydrides Having Alkyl-Substituted Cyc10pentadienyl Ligands."
Organometallics 1999, 18, 1873-1881.
sChin, Bain and Stephen Buchwald. "An Improved Procedure for the Resolution of (rac)Ethylenebis(tetrahydroindenyl) Titanium Derivatives." Journalo/Organometallic
Chemistry 1996, 61, 5650-5651.
9Christopher, Joseph N., et al. "Synthesis, Structure, and Reactivity of rac-Me2Si (indenyl)2
Zirconium(NMe2)2." Organometallics 1996, 15,4038-4044.
IOCorradi, Marco M., et al. "Synthesis, Fluxionality, and Propene Insertion Reactions of
Zirconium Boryldiene Complexes with Sterically Undemanding Cp Ligands."
Organometallics 2000, 19, 1150-1159.
llDiamond, Gary M. and Richard G. Jordan. "Synthesis of Group IV Metal rac-(EBI)M(NR2)2
Complexes by Amine Elimination, Scope and Limitations." Organometallics 1996, 15,
4030-4037.
l2Diamond, Gary M., et al. "Efficient Synthesis of rac-(Ethylenebis(indenyl) Zirconium
Dichloride Complexes via Amine Elimination." Organometallics 1995, 14,5-7.
13 Djakovitch,
Laurent and Wolfgang Hermann. "Half-sandwich and ansa-Niobicenes: Synthesis
and Reactivity." Journal 0/ Organometallic Chemistry 1998, 562, 71-78.
14Pischer, Reinald, et al. "Reactive Intermediate of the Catalytic Carbomagnesation Reaction:
Isolation and Structures of [CpZrEth(u-ethene) ... " Organometallics 2000, 19(13),
2532-2540.
15Gaede, Petra Escarpa, et al. "Transition Metal Complexes of 2-cyclopentadienyl-2-indenylpropane." Journal o/Organometallic Chemistry 1998, 559, 107-121.
16Halterman, Ronald L. and Timothy H. Ramsey. "Preparation of 1,1 '-binapthyl-2,2'-diyl
Bridged ansa-bis(annulated cyclopentadienyl) Titanium and -Zirconium Dichloride
Complexes." Journal o/Organometallic Chemistry 1997, 530, 225-234.
17Halterman, Ronald L., et al. "Synthesis and Structure of [1,2-Bis(1-indenyl) benzene] Titanium
and Zirconium Dichlorides." Journal 0/ Organometallic Chemistry 1998, 568,41-57.
18Halterman, Ronald L., et al. "Synthesis and Structure of C 2-symmetric Doubly-bridged bis(indenyl) Titanium and -Zirconium Dichlorides." Organometallics 1997, 16,3333-3339.
19Halterman, Ronald L., et al. "Synthesis of ansa-2,2'-bis[(4,7-dimethyl-inden-l-yl)methyl]-I,I'binapthyl and nsa-2,2' -bis[(4,5,6,7 -tetrahydroindenl-yl)methyl]-I, l' -binapthyl Titanium
and Zirconium Dichlorides." Journal o/Organometallic Chemistry 1996, 520, 163-170.
2oHalterman, Ronald L., et al. "Synthesis ofC7, C7'-Ethylene and C7, CT-Methylene Bridged
C 2-symmetric bis(indenyl) Zirconium and -Titanium Dichlorides." Organometallics
1998,17(18),3900-3907.
21Halterman, Ronald L., et al. "Synthesis, Structure Determination, and Reactivity of C 2symmt~tric,
Ethylene-bridged ansa-bis(diMeBCOCp)TiCh." Organometallics 1996, 15,
3957-3967.
22Haltermen, Ronald L., et al. "Preparation of [2,5-diisopropylhexane-l ,4-bis(indenyl)]TiCh and
[2,5-diisopropylhexane-l ,4-bis(tetrahydroindenyl)]TiClz." Organometallics 2000, 19,
3824-3829.
23Huttenloch, Monika E., et al. "Ansa-metallocene derivatives XXXIX ... Syntheses, Crystal
Structure, and Enantioseparations." Journal o/Organometallic Chemistry 1997, 541,
219-232.
24Jany, Gerhard, et al. "Synthesis of Enatiomerically Pure Ethylene-bridged ansa-Ziconocene
and-Hafnocene Complexes ... " Organometallics 1997, 16,544-550.
25Jonas, Klaus, et al. "Novel Mononuclear Vanadium Complexes Having Pentalene Ligands n8 _
bonded to a Single Metal Atom." Angew. Chem. Int. Ed. Engl. 1997,36(16), 1712-1714.
26Lancaster, Simon J., et al. "Synthesis, Reactivity, and Structural Studies on
-
Borylc:yclopentadienyl Complexes with Titanium." Organometallics 2000, 19, 15991608.
-
27Lefeber, Claudia, et al. "Rac-[1,2-ethylene-l,1 '-bis(n 5 -tetrahydroindenyl)] [n
5
_
bis(trimethylsilyl)acetylene] Zirconium ... " Organometallics 1996, 15, 3486-3490.
28Lensink, Comelis. "Synthesis and Structure of Sulfonamidocyc1opentadiene Titanium
Compll~xes."
Journal o/Organometallic Chemistry 1998, 553, 387-392.
29 Luttikhedde, Hendrik, et al. "Synthesis and Characterization of ansa-[ 1, I-bis(inden-l, I-dyl)-Isilacyc1opentane] Zirconium Dichlorides." Journal o/Organometallic Chemistry 1995,
486, 193-198.
30Luttikhedde, Hendrik, et al. "Synthesis and Molecular Structure ofrac-methylenebis-(4,5,6,7tetrahydro-l-indenyl)TiCh." Journal o/Organometallic Chemistry 1997,547, 129-132.
31Nifant'ev, Ilya E.and Pavel V. Ivechenko." Synthesis of Zirconium and Hafnium ansaMetallocenes vis Transmetallation of Dielement-substituted Bis( cyc1opentadienyl) and
Bis(indenyl) Ligands." Organometallics 1997, 16, 713-715.
32Nifant'ev, Ilya, et al. "Preparation of the ansa-Zirconocenes with an N-methylpiperidine
Bridge." Organometallics 1998,17,4734-4738.
33Palandoken, Hasan, et al. "Reductive Dehydroxy Coupling of2-(hydroxymethyl)indenes to
-
Prepare Ethano-bridged Bis(2-indenyl) ansa-Titanocenes." Journalo/Organometallic
Chemistry 1999, 579, 338-347.
34Plenio, Herbt:rt and Dirk Burth. "Aminozirconocenes: A New Class of Zirconocenes with a
Nitrogen Directly Bonded to an n5-cyc1opentadienyl(indenyl) Ligand." Journal 0/
Organometallic Chemistry 1996, 519, 269-272.
35Psiorz, Christian, et al. "Synthesis and Application of Ring-Substituted Analogues of the [4- (ncyc10pentadienylidene)-4,5,6, 7-tetrahydro-n 5-indenyl] ZrCh ansa-Metallocene System."
Chem Ber. 1995, 128,357-364.
36Rogers, Jonathan S., et al. "Fulvene to Cyc10pentadienyl Conversion with Homoleptic
Complexes of Zirconium and Hafnium." Organometallics 1999, 18,3976-3980.
37Shaw, Scott L., et al. "Monoindenyl Halides of Zirconium and Hafnium." Journal 0/
Organometallic Chemistry 1995, 489, C4-C6.
38Thiyagarajan, B. and Richard F. Jordan. "Aluminium ansa-Indenyl Compounds ... "
Organometallics 1999, 18,5347-5359.
39Thom, Matthew G., et al. "Coordination Modes of2-(indenyl)pheroxide Ligation at Early DBlock Metal Centers." Chem Communications 1999, 2543-2544.
-
4°Urabe, Hirokazu, et al. "Generation ofChiral (n5-indenyl)2 (n 3-allyl) Titanium Complex and Its
Highly Enantioselective Reaction with Carbonyl Compounds." Tetrahedron Letters
1995, 36(31), 5595-5598.
-
41 Voge l, Alexander, et al. "Ansa-bis(indenyl-)dimethylamido Zirconocenes." Journal of
Organometallic Chemistry 1997, 527, 297-300.
42Voskoboynikov, Alexander Z., et al. "Synthesis and X-ray Crystal Structures ofrac and meso2, 2'-propylidene-bis(1-indenyl) Zirconium Dichlorides." Journal of Organometallic
Chemistry 1997, 530, 75-82.
43Warren, Timothy H., et al. "1,3-Doubly bridged Group IV Metallocenes by Intramolecular
Reductive Coupling to Pendant Olefins." Organometallics 2000, 19, 127-134.
44Willoughby, Christopher A., et al. "Preparation and X-ray Structure of a novel Chiral
Methylene-bridged Titanocene Complex." Journal of Organometallic Chemistry 1995,
497, 11-15.
45Zhang, Xingwang, et al. "General Synthesis of Racemic Me2Si-bridged bis(indenyl)
Zirconocene Complexes." Journal of the American Chemical Society 2000, 122,80938094.
46Ziniuk, Zeev, et al. "New Chelating pyridyl-indenyl and quinolyl-indenyl Ligands Leading to
C 1 Symmetric Complexes of Zirconium via Amine Elimination." Journal of
--
Organometallic Chemistry 1997, 545-546,441-446.