Chem 652 Spring 2013
Anionic Ligands
Prof. Donald Watson!
Read Hartwig Chapters 3-4
Hydrocarbyl Ligands
Most common types:
LnM
Me Me
Me
Me
benzyl
neopentyl
LnM
LnM
phenyl
LnM
LnM
LnM
methyl
SiMe3
vinyl
methyltrimethylsilyl
LnM
alkynyl
What do they have in common?
All lack β-hydrides.
Strength of Alkyl Metal Bonds
•  M-alkyl BDE’s range over ~28-70 kcal/mol.
•  First row metals weakest – subject to thermally homolysis.
•  Note: M-H stronger than M-R.
Synthesis of Metal Alkyls
Via transmetallation:
Cp2Zr
Cl
O
ZrCp2
Cl
Me2
N
Cl
Pd
Cl
N
Me2
Me3Al
MeLi
Cp2Zr
Me
Cl
Me2
N
Me
Pd
Me
N
Me2
+ (Me2Al)2O
Hard Nucleophiles with CO Ligands
MeLi
Co(PMe3)3(CO)Cl
Co(PMe3)3(CO)Me
-
O
Re(CO)5Br
MeLi
(CO)4Re
Me
Br
•  CO ligands can interfere with transmetallation approach.
Via Oxidative Addition
Anionic Complexes:
+ I–
Cp
[CpFe(CO)2]– + EtI
Fe
OC
Et
CO
Cp*
[Cp*Ir(PMe3)H]– •Li+ + C5H11OTf
Me
Ir
Me3P
H
+ LiOTf
Neutral Complexes:
(Me3P)4Co–Me + MeBr
Me
(Me3P)3Co Me
Br
Alkene Insertion
Hydrometallation (hydrozircanation):
Cp2Zr
H
Cl
+
Me
Cp2Zr
Me
Cl
Nucleophilic Addition of Alkene
O
R R
P
Cl
Pd
P
R R
+
R
R R
P
Cl
Pd
P
R R
+
N
H
R
O
R R
P
Cl
Pd
P
R R
N
•  Note: Electrophilic metal coordinates alkene and activates it
towards nucleophilic attack.
R
Fluoroalkyl Ligands
Electrophilic (Oxidative Addition):
Ir(PPh3)2(CO)(Cl)(I)CF3
Ir(PPh3)2(CO)Cl + CF3I
Nucleophilic:
Ar
R3P Pd Br
Rh
Me3SiCF3 + CsF
–Me3SiF
F
PR3
Me3SiRF
–Me3SiF
Ar
R3P Pd CF3
Rh
RF
PR3
•  Fluoroalkyl ligands are particularly strong and stable ligands.
Reactions of Metal Alkyls
α-Elimination:
TaCl5
1.5 Zn(CH2tBu)3
Ta(CH2
tBu)
3Cl2
2 LiCH2tBu
H3C CH3
CH3
t
( BuCH2)3Ta
However, not general:
TaCl5
+
1.5 Zn(CH3)2
Ta(CH3)3Cl2
2 CH3Li
Ta(CH3)5
Reactions of Metal Alkyls
Beta-Hydride Elimination:
R3P
I Pd
R3P
H
R3P
Ph
Ph
I Pd
R3P
H
Ph
+
Ph
Reactions of Metal Alkyls
2° Metal Alkyls Often Rearrange to 1°
H3C
+
Cp2Zr(H)Cl
Cp2Zr
CH3
Cl
CH3
Cl
Cp2Zr
CH3
CH3
PR3
S
R2N
Pd
R2N
CH3
S
Pd
S
H3C
Cp
OC
Ph3P
Fe
PR3
S
K = 10
CH3
Cp
CH3
OC
Ph3P
CH3
Fe
CH3
•  Driven by steric effects.
Common mechanism:
M
H
C
R
H
H
M
R
H
CH2
H
R
M
H
M
CH2
C
R
H
H
Reactions of Metal Alkyls
•  Electronic effects can override:
Cp
Cp
OC
Ph3P
Fe
R2N
CN
PR3
S
Pd
S
NC
OC
Ph3P
CH3
Fe
CN
CH3
PR3
S
K < 0.01
R2N
Pd
S
CN
Reactions of Metal Alkyls
•  Agostic interactions can also drive equilibrium with open coordination sites:
N
Pd
N
CH3
CH3
–115 °C
N
Pd
H
H
N
Observed
Not observed
CH3
N
Pd
N
N
N
CH3
CH3
N
Pd
CH3
NCCH3
CH3
Pd
–66 °C
N
–65 °C
Keq = 0.29
NCCH3
N
Pd
N
CH3
CH3
Keq = 43
CH3
•  Agostic interaction with 2° C–H bond stronger than with 1° C–H bond.
Aryl–Metal and Vinyl-Metal Complexes
Trends:
•  Aryl-metal and vinyl-metal bonds stronger than alkyl-metal.
R
BDEs:
>
M
R
M C
R
•  β-Elimination rare from aryl-metal and vinyl-metal bonds.
M
H
M
H
+
BDE’s Methyl vs Phenyl
R–Mn(CO)5
R = Ph vs. Me
Ph > Me 4 kcal/mol
O
SiBut3
O
SiBut3
Ti
R
N
H
SiBut3
R = Ph vs. Me
Ph > Me 7.5 kcal/mol
H
B
•  Greatest difference in 3rd row.
N
But
N
N
N Rh R
C
H
R = Ph vs. Me
Ph > Me 16 kcal/mol
R = Ph vs. Me
Cp*Ir(PMe3)R2
Ph > Me 26 kcal/mol
(82 vs 56 kcal/mol)
More Evidence of Strong Aryl-Metal Bonds
H3C
Os
CH3
CH2Cl2, rt
CH3
4
CH3
4
H3C
B(OH)2
Br
Br
Os
X
H3C
Os
H3C
Pyridinium tribromide
Fe powder
X
Os
Pd(0), K2CO3, DMF
4
CH3
4
•  Can sometimes carry out transformation of aryl ligands with breaking M-Ar bond!
Synthesis of Aryl–Metal and Vinyl-Metal Complexes
Via transmetallation:
Ph2
Cl
P
Rh
N
P
Ph2
p-TolLi
RT
THF
Ph2
p-Tol
P
Rh
N
P
Ph2
Cl
Bu3Sn
Pt
+
PtCl2
ZrCl4 + 4 Li(PhC=CMe2)
Zr(CPh=CMe2)4
Role of CO Ligands and Transmetalating Agent
Cp
Cp
ON
OC
PhLi
Re
Cl
Re
Cl
O
Cp
ON
OC
ON
Ph
Cp
PhCu
Re
Cl
ON
OC
Re
Ph
•  CO ligands can also be attached by aryl and vinyl nucleophiles.
•  Nature of the reagent effects outcome. Soft copper complexes
favor transmetallation.
Synthesis of Aryl–Metal and Vinyl-Metal Complexes
Via Oxidative Addition:
PPh3
Pd(PPh3)4
+
I
Pd
PhI
Ph
+
PPh3
Ph
Ph
Pt(PPh3)3
+
Br
Ph3P Pt PPh3
Br
2 PPh3
Synthesis of Aryl–Metal and Vinyl-Metal Complexes
Via Decarboxylation:
[Mn(CO)5]
O
O
+
Ph
Cl
Ph
– CO
Mn(CO)5
PhMn(CO)5
Via C–H Bond Activation:
hν
Cp*Ir(PMe3)H2
Cp*Ir(PMe3)(Ph)(H)
Via Alkyne Hydrometallation (Vinyl Only):
Cp2ZrHCl
+
CH3
CH3
Cp2Zr
Cl
Dynamics of Aryl-Metal Bonds
•  Some Ar–M bonds have low rotation barriers.
•  However, Ar–M bond can have slow rotation on NMR time scale.
•  Steric in nature.
J = 7.1 Hz
N Pd
H3C
H3C
C6Cl2F3
C6Cl2F3
=
Cl
F
Cl
F
C
Pd
N
F
F
F
J = 4.2 Hz Cl
•  Two orthro fluorides are diastereotopic.
Cl
F
Alkynyl Complexes
BDEs:
R
M
R
>
M
>
R
M C
R
Alkynyl Complex Synthesis
Salt Metastasis with Metal Acetylide:
N
Re
OC
OC
HC CR
BuLi, Et2O
N
N
Cl
Re
OC
CO
OC
N
C C R
n
CO
With Weak Base:
N
OC
OC
Re
N
N
H(C C)nR + AgOTf
Cl
OC
+ Et3N THF
CO
OC
acidic
H
M
M
R
R
Re
N
C C R
n
CO
Alkynyl Complex Synthesis
Transmetallation:
PR3
Cl M Cl
+
2 HC CR
CuI catalyst
base
PR3
Via
CuC CR
PR3
R C C M C C R
PR3
(M = Ni, Pd, Pt)
Metal Enolate Complexes
Types:
O
R
MLn
O
R
R
O-bound
O
MLn
C-bound
LnM
MLn
R
O
O
R
η3
MLn
bridged dimer
Examples:
CH3
H3C
O
H3C
Ta
H3C
O
O
OC W
OC
CO
CH3
CH3
CH3
OEt
OC Cr
OC
O
CH3
OMe
O vs. C Bound
L
L
L
Ru
O
CH3
L
CH3
L
L
L = PMe3
Et3P
Ph
L
CH3
CH3
PEt3
Ph
OC Rh O
C6D6
Et3P
Ph2
P
Pd
P
Ph2
O
Ru
CO, 25 °C
O
Rh
O
L
PEt3
Me
CH3
PPh3
Pd
vs.
Ph3P
O
Me
•  Early metals favor Obound (oxophilicity).
•  Late metals can be Obound or C-bound or in
equilibrium.
•  Steric crowding favors
O-bound.
•  Trans ligand influences
O- vs. C- bound.
Synthesis of Metal Enolates
Via Salt Metathesis:
t-Bu
Ph2
P
Pd
Br
P
Ph2
t-Bu
Ph2
P
Pd
P
Ph2
O
OK
CH3
Ph
PhCH3, 25 °C
CH3
Via Addition to Unsaturated Carbonyls:
H
Ph3P Rh PPh
3
Ph3P
CO
O
PhCH3
+
CH3
H C
PPh3 3
OC Rh
H
O
PPh3
CH3
PPh3
+
OC Rh
H
CH3
O
PPh3
CH3
Synthesis of Metal Enolates
Via Oxidative Addition:
Pd(PPh3)4
+
Cl
H3C
PhCH3
O
CH3
25 °C
O
Ph3P
Pd
PPh3
Cl
Via Alkylation of Nucleophilic Metals:
OC
OC
+
W
CO
O
O
Cl
OEt
OC
OC
W
OEt
CO
π-Allyl Complexes
Most common types:
R2
MLn
R1
R2
MLn
R1
an η1-allyl complex
an η3- or π-allyl complex
Examples:
Pd
Ni
Ni(II)
(η3-C
3H5)3Cr
Cr(III)
(η3-C
3H5)4Zr
Zr(IV)
Cl
Cl
Pd
Br
Et3P
Pt
PEt3
Pd(II)
•  η3 most common mode for early, middle and late TM complexes.
The MO’s of Allyl
Allyl Anion
Allyl Cation
η3 Allyl Bonding
C3V
pz
py
px
s
z
y
x
xz
yz
z2
Co
xy
x2-y2
OC
CO
CO
Closer Look At [Pd(allyl)Cl]2
Pd
Cl
Cl
Pd
•  C-C-C bond angle:119.8°
•  Pd-C distances: 2.132 Å, 2.108 Å, 2.121 Å
•  C-C bond distances:1.357 Å, 1.395 Å
•  Metal bound to allyl face.
•  Center carbon lies above Pd-Cl axis.
NMR of Allyl Groups
For static η3 allyl:
1H
typically
4-6.5 ppm
H
HH
"syn"
1H typically 2-5 ppm
MLn
HH
"anti"
sheilded
1H typically 1-3 ppm
Allyl Often Dynamic!
Hc
Hc
Hb
Hb
Hb
Hc
Hb
Hb
Ha Ha M
Ha M Ha
Hc
Ha
Ha
M
Hb
Hb
Ha
Ha M Hb
•  Note η3 to η1 isomerization allows for metal to access both faces of allyl.
•  Important in π-allyl substitution reactions.
BF4
BF4
H
Ph2P
N
Pd
HA
HB
CH3
CH2
HC
HD
Dissociation
to form a
monodentate
ligand
H
Ph2P
Pd
HA
HB
H
CH3 Rotation H3C
N
CH2
HC
HD
BF4
BF4
N
CH2
HA
PPh2
Pd
HB
HC
HD
Coordination to
regenerate a
bidentate ligand
H
H3C
N
CH2
HA
PPh2
Pd
HB
•  Dynamic behavior also seen by rearrangement of other ligands.
HC
HD
Synthesis of Metal Allyl Complexes
•  Via Nucleophilic Displacement:
MgBr
2
+
NiBr2
Ni
•  Via Oxidative Addition:
Cl
+
Ni(CO)4
PhH
1/2
Cl
Ni
Ni
Cl
+
4 CO
Synthesis of Metal Allyl Complexes
•  Via Insertions Into 1,3-Dienes:
RCo(CO)4
Co(CO)4
+
Co(CO)3
– CO
R
R
R = H, alkyl, R(O)C
•  Via Nucleophilic or Electrophilic Attack of 1,3-Diene Complex:
R
+
R
Mo(CO)2
Cp
Mo(CO)2
Cp
H
+
Fe(CO)3
DX
D
Fe(CO)3
X
η3 Benzyl Complexes
L
Cl Pd
L
H
D
NaBPh4
LiCl
H
D
L
Pd
L
BPh4
Benzylic ligands can also have η3 character.
Cyclopentadienyl Ligands
•  Cp ligands approximately occupy three facially oriented
coordination sites.
•  Bond Strengths:
Cp2Fe
(η5-Cp)MCl
Cp·
3
+
Cp·
·FeCp
+
·MCl3
ΔH = 79 kcal/mol
ΔH = 79 kcal/mol for Ti
ΔH = 100 kcal/mol for Zr
ΔH = 101 kcal/mol for Hf
MO’s of Cp
η5 Cp in Bonding
C2V
py
px
pz
s
z
y
x
dyz
dxz
dx2-y2
Co
dxy
dz2
OC
CO
Synthesis of Metal Cp Complexes
•  Accessing Cp Anions:
retro
[4+2]
H
2
H
base
CpM
or Na
"cracking"
CpH
Cp-dimer
•  Via Nucleophilic Attack:
FeCl2
+
NaCp
Cp2Fe
+
2 NaCl
Synthesis of Metal Cp Complexes
•  With Basic Ligands:
Zr(NMe2)4
+
CpH
Cp2Zr(NMe2)2
•  Electron-rich Metals:
H
H
+ Co2(CO)8
Co
OC
CO
Modified Cp Ligands Modulate Reactivity
Larger Ligands:
H3C
Annulated Cp’s:
CH3
CH3
H3C
CH3
Cp*
indenyl
fluorenyl
Chiral Cp’s:
H3C
H3C
CH3
M
CH3
CH3
H3C
M
H3C
CH3
M
CH3
Cp vs. Cp*
Cp*:
•
•
•
•
More steric protection; leads to kinetic stability.
Better solubility.
Better crystallinity.
More electron donation:
•  IR: CpM-CO vs Cp*M-CO shift ~50 cm-1
•  E° Cp2M0/+ vs. Cp*2M0/+ ~ 0.5 V
•  Drawback: synthesis. (Bergman and Bercaw, Org. Synth.
1987, 65, 42)
Cp Complex Geometry
•  Four Classes of Cp Complexes:
M
M
"sandwich"
compound
or
"metallocene"
=
M
L
L
L
"bent
metallocene"
M
L
L
L
"half
sandwich"
Metallocene Geometry
•  Relative Orientation of Cp’s in Metallocenes:
•
•
•
•
M
M
eclipsed
D5h
staggered
D5d
In gas phase: eclipsed slightly lower than staggered.
Barrier to rotation usually low (< 1 kcal/mol).
Ferrocene eclipsed in solid state.
Cp*2Fe is staggered (Me-Me repulsion).
Metallocenes with More than 18 e–
C-C = 1.41Å
Fe-C = 2.04Å
3.32Å
Co-C = 2.10Å
Ni-C = 2.18Å
Fe
Co
Ni
18 e–
D5h
19e–
D5d
20e–
D5d
Metallocene Redox Properties
Fe
+ e–
Fe
Co
+ e–
Co
E° = 0.41V
E° = -0.91V
Half-Sandwich Complexes
Ru
Ni
NO
Ph3P
Cl
PPh3
Re
OC
CO
CO
Cl
Cl Ta
Cl
Cl
Bent Metallocenes
Mo CO
Zr
Cl
Ta
Cl
Cp2Mo(CO)
Mo(II), d4, 18 e-
Cp2ZrCl2
Zr(IV), d0, 16 e-
Bonding Picture
empty “dx2” orbital
H
H
H
Cp2TaH3
d0, 18 e-
Ta(V),
X-ray structure of
Cp2Zr(Me)(THF)+
Note: THF 90 ° to Me-Zr-O plane
nO → dx2
Ansa Metallocenes
M
EBI complex
ethylenebis(indynyl)
M
EBTHI complex
ethylenebis(tetrahydroindynyl)
•  Cp Rings are linked with a “ansa” bridge.
•  Ansa ligands can force bent metallocene geometry.
•  Highly important in Z.N. polymerization.
Chiral Ansa Metallocenes
•  Structure is chiral.
•  Can be resolved.
M
•  Brintzinger/Jordan
Synthesis of
resolved
(EBTHI)2TiCl2.
Ph CH
3
Cl N
THF
Zr
THF
N
Cl
Ph CH3
X
N Zr
H3C
Ph
CH3
N
Ph
Li
THF
X
+
Li
4 HCl
X
Cl
Zr
Cl
Reactions of Metallocenes
•  Electrophilic Attack: Friedel Crafts, Formylation, etc.
H
E+
Fe
Fe E
Fe
– E+
– H+
E
E
Fe
•  Cp Rings Stabilize Cations at α carbon, important in chiral ligand synthesis.
PR2
OAc
Fe
Me
PPh2
HPR2
Fe
-HOAc
Me
PPh2
Fe
•  Deprotonation of Metallocenes
Fe
BuLi
Li
Fe
Li
Me
PPh2
Ring Slippage
L
Rh(CO)2
Rh(L)(CO)
-CO
L
Rh(CO)2
-CO
Rh(L)(CO)
Rh(CO)2
16 e–
slow,
associative
substitution
substitution
is 108 X faster
Metal Hydrides
Examples of terminal metal hydrides:
Cp2ZrH2
HMn(CO)5
Cl Pt
Ph3P
H5Re(PMePH2)3
PPh3
H
Examples of bridged hydrides:
CO
OC CO
OC
CO
CO
Cr
Cr
OC
H
OC CO
OC
–
H
Co
Co
H
H
(µ-H)3
up to 4 possible
Note:
[(CO)5Cr–H]–
Cr(CO)5
16 e–
18e–
3-center-2-e– bond
Note: This structure was initally misassigned as:
Cp*Co=CoCp*
paramagnetic; M-H hard to locate with X-ray
required neutron diffaction
Theopold and Casey, ACIE, 1992, 1341.
Properties of Metal Hydride
•
•
•
•
H covalent radius = 0.32 Å
M–H ~ 1.5-1.7 Å
X-ray underestimates M–H (by ~ 0.1 Å)
Even simple hydrides can effect structural details:
97°
PPh3
CO
CO
OC Mn CO
OC
H
83°
Rh
Ph3P
PPh3
PPh
3
H
tetrahedral
•  M-H BDE’s 60-75 kcal/mol.
Spectral Properties
•  1H NMR: Typically ∂ 0 to -40 ppm!
•  However bridging or other unusual structure can perturbe this:
[HCo6(CO)15]– ∂ = 23.2 ppm
•  IR: Typically ~ 1500-2200 cm–1
M–H Synthesis
•  Via Oxidative Addition to H2:
IrCl(PPh3)3 + H2
25 °C
1atm
H2IrCl(PPh3)3
reduction of complex open coordination site:
W(PMe3)3Cl4
H2, Na/Hg
–78 °C, THF
+ PMe3
WH4(PMe3)4
M–H Synthesis
•  Via Protonation:
Mn(CO)5
–
H+
HMn(CO)5
counter-ion can precoordinate with cationic complexes.
Ir
H
HCl
PMePh2
Ir
H2O
PMePh2
Cl
Cl–
PMePh2
PMePh2
H+
(COD)Ir(PMePh2)2Cl
can form cationic complexes
Os(CO)3(PPh3)2 +
HClO4
[Os(CO)3(PPh3)2H][ClO4]
M–H Synthesis
•  Reduction with main group hydrides:
WCl6
+
NaBH4
+
Cp2WH2
NaCp
•  Via β-hydride elimination:
Cp2ZrCl2
+
Me3CMgCl
Cp2ZrHCl
•  Reduction using alcohols:
K2IrCl6
CH3CD2OH/H2O
PPh3
H
D
H3C C O Ir(III)
D
H3C
IrDCl2(PPh3)3
O
C
+
D
D Ir(III)
Metal Hydride Reactivity
•  Acid-Base:
Despite bond polarization, some M-H “hydrides” are acidic.
pKa δ+ δM H
HCo(CO)4 0 H2Fe(CO)4 4.0 HMn(CO)5 7.1 Metal Hydride Reactivity
•  Insertions:
Cp2Zr
H
+
Me
Cl
Cp2Zr
Me
Cl
Note: this the microscopic reverse of β-hydride elimination!
•  H-Atom Transfer:
Me
HMn(CO)5
+
Me
Me
Mn2(CO)10 +
Mechanisms often occur via single electron pathways.
Metal Amido Complexes
t-Bu
Zr[N(CH3)2]4
Cp2Zr[N(CH3)2]2
OC
(DPPF)Pd
NRR'
Ph3P
R = R' = p-Tol
R = H, R' = Ph
Early metals:
Late metals:
•  Stronger π-bonding
•  More ionic bonding
•  Hard-hard match
•  Stronger bonds
•  π-Repulsion common
•  Less ionic bonding
•  Hard-soft mismatch
•  Weaker bonds
Ir
PPh3
NMePh
Metal Amido Bond Strengths
•  Early Metal Amidos:
M–N BDE’s for M[N(CH2CH3)2]4
M
Ti
M–N BDE (kcal/mol)
91
Zr
90
Hf
95
•  Late Metal Amidos:
Not well tabulated: ~ M-OR
Synthesis of Late Metal Amidos
•  Via Metathesis:
OC
Ph3P
Ir
PPh3
Cl
t-Bu
(DPPF)Pd
LiNMePh
Ph3P
Ir
PPh3
NMePh
t-Bu
HNRR'
– t-BuOH
O-t-Bu
OC
(DPPF)Pd
NRR'
R = R' = p-Tol
R = H, R' = Ph
Synthesis of Late Metal Amidos
•  Via N-H Activation:
PCy3
F
F
H
N
Pt
F
F
PCy3
H
(PCy3)2Pt + H2NC6F5
F
PtBu2
Ir
PtBu2 R
NH3
PtBu2
Ir H
NH2
t
P Bu2
Reactivity of Late Metal Amidos
•  β-elimination:
OC
Ph3P
Ir
PPh3
NPh
OC
Ph3P
Ir
PPh3
H
NPh
+
Ph
Ph
•  Reductive elimination:
Ph2
P
Fe
t-Bu
t-Bu
PPh3
R
Pd
P
Ph2
+ (dppe)2Pd +
N
R'
NRR'
•  Insertion:
CO
Cy2P Ru
PCy2
NH2
Cy2P Ru
PCy2
NH2
O
(PPh3)nPd
Synthesis of Early Metal Amidos
Metathesis:
MCln + n LiNR2
MCln + excess HNR2
M(NR2)n + n LiCl
M(NR2)n + HNR2·HCl
Early Metal Amidos
•  Very Reactive towards protic acids:
Zr[N(CH3)2]4
+
benzene
reflux, 2 h
Cp–H
Cp2Zr[N(CH3)2]2
+
2 HN(CH3)2
54%
Ti[N(CH3)2]4 + H–OtBu
Ti(OtBu)4
+
HN(CH3)2
Driven by Ti-X bond strength.
•  α-Elimination to give metal imidos:
Cp*
Cl
Cl
Ta
Cl
Cl
Cp*
ArHNLi
Cl
ArHN
Ta
Et3N
Cl
Cl
-Et3NHCl ArN
Me
Cp*
Ta
Cl
Cl
Ar =
Me
Porphyrin and Corrin Complexes
•  Important in bioinorganic chemistry and some aspects of
synthetic chemistry.
N
H
N
N
Me
Me
N
N
H
N
core ring system
of a porphyrin
Et
Me
Me
Me
H
N
Me
Me
Me
Me Me
Me Me
core ring system
of a corrin
Et
Et
N
Me
N
Et
Et
Me Me
Me
LiN
Et
Et
Et
N Cl N
FeCl3
Fe
NLi
N
Et
Et
Et
Et
N
N
Et
Et
Et
Et
Bissulfonamides
SO2Ar
NH
+
NH
SO2Ar
Ti(O-i-Pr)4
O
Ar S O
N
Ti
N
Ar S O
O
O-i-Pr
+
O-i-Pr
•  Important applications in asymmetric synthesis.
HO-i-Pr
Polypyrazolylborates
•  Developed by Trofimenko (Dupont and UD)
R
R
N
NH
3
R
+
R
NaBH4
R
•  a trispyrazolylborate (Tp)
•  6e–, anionic ligand
•  Often used in place of Cp
R
B
N
N
R
Na
N
N
H
N
N
Tp'
R
R
[(C2H4)2RhCl]2
+
KTp
DMF, rt
R
R
H
B RN
N
N N
N
R
N
R
Rh
Diketiminate Ligands (nacnac)
R1
R3
O
R2
R2
R2
2 NH2R4
O
R1
R4
R3
N
HN
NaOMe
MeOH
R4
R4
R3
N
iPr
iPr
Me
Me
iPr
N
N
N
+
Li
Me
iPr
R1
iPr
[Me3Pt(OTf)]4
N
Me
iPr
•  Anionic, 4 e– ligands.
•  Tunable steric and electronic properties.
iPr
Me Me
Pt
Me
iPr
N 4
R
Na
Alkoxide Ligands
Me
Me
Cp*2Zr(OH)Ph
Me
Me
Ir OPh
Ph3P
Me
N
O
Ni
Me
N
Me
Early metals:
Late metals:
•  Stronger π-bonding
•  More ionic bonding
•  Hard-hard match
•  Stronger bonds
•  π-Repulsion common
•  Less ionic bonding
•  Hard-soft mismatch
•  Weaker bonds
Synthesis of Early Metal Amidos
Metathesis:
LnM–X
NaOR
LnM–OR
LnM–X
HOR
base
LnM–R' or LnM–NR'2
or LnM–OR'
Driven by strong M–O bonds!
Reactivity of early metal alkoxides is limited. Most often serve as ancilllary ligands.
Late Metal Alkoxides BDE’s
M–O Bond
M–O BDE (kcal/mol)
Co–O in Co(CO)4(OH)
55
Rh–O in
octaethylporphyrin
rhodium alkoxides
~ 50–55
Late TM M–O ~ M–C bond strength.
Synthesis of Late Metal Alkoxides
•  Via Metathesis:
Me
Me
Me
Me
Me
Ir
Ph3P
Me
Cl
Me
AgOAc
Me
Me
Me
Me
Me
Ir OAc
Ph3P
Me
Toluene
Me
KOPh
Me
Me
Me
Ir OPh
Ph3P
Me
THF
•  Via Protonolysis:
N
+
Ni
N
N
Me
Me
rt
HO
O
+
Ni
N
Me
•  Via O–H Activation:
Pt(PCy3)2
+
HOPh
PCy3
H Pt OPh
PCy3
CH4
Reactions of Late Metal Alkoxides
•  β-elimination:
H
Et3P
Et3P
Ir
H
OCH3
Et3P
PEt3
Et3P
Cl
Ir
Cl
H
+
(CH2O)n
PEt3
•  Reductive elimination:
Ph2
P
Fe
t-Bu
Pd
P
Ph2
Ph3P
ArO
Rh
t-Bu
L
PPh3
PPh3
Δ
O
H
ArO
Ir
O
PPh2
PPh3
PPh3
Ph3P
-HOAr Ph3P
Ir
P
Ph2
Reactions of Late Metal Alkoxides
•  Insertion:
Et3P
Rh
Et3P
O
R
R'
PEt3
20 °C
(PEt3)4RhH
+
O
R
R'
Β-Diketonate Ligands (acac)
CH3
CH3
O
CH3
O
Mn
O
Fe
O
3
3
O
Ni
O
CH3
CH3
O
Co
O
CH3
CH3
Cu
O
CH3
3
O
CH3
2
•  Anionic, 4 e– ligands.
•  Tunable steric and electronic properties.
CH3
2
Halide Ligands
F
Cl
Br
I
Ionic radius (Å)
1.36
1.81
1.95
2.16
Cone angle (°)
92
102
105
107
•  Dramatic size difference between halides.
Halide Ligands
Dramatic Electronic
Differences too:
•  Fluoride is most
electronegative.
•  Would suggest iodide is
best ligand.
Not true!
Why?
F
E.N.
4.0
Cl
3.0
Br
2.8
I
2.5
π-Donor Ability
•  Fluoride is much stronger π-donor!
•  Increases M–X bond strength.
Ph3P
Ph3P
CF3SO3H
Os
X
BDE’s:
Ph3P
Ph3P
Os
X
H
CF3SO3
– ΔHM
(kcal/mol)
I
14.1
Br
16.3
Cl
19.7
F
37.3
Cp*2ZrCl2
Zr–Clave 115 kcal/mol
Cp*2ZrI2
Zr–Iave 80.4 kcal/mol
Effect of π-Donation Depends on D-Orbitals
•  π-donation into empty orbital increases bond strength.
•  π-donation into filled orbital decreases bond strength.
Xc
OC
OC
Ir
Xc
Xt
Xt
Relative Binding Affinities
Xt = Cl > Br > I
Xc = I > Br > Cl
Halides as Bridging Ligands
NH2
Prn3P
X
X
Pd
X
Pd
X
PPrn3
H3C
K
K: X = Cl > X = I
Prn3P
X
X
Pd
NH2Ar
•  Larger halides for stronger bridging interactions.
Hydrogen Bonding in Halogen Ligands
H
NH
N
L
H
Ir
X
L H
L = PPh3
X
IrX–HN(kcal/mol)
I
<1.3
Br
1.8
Cl
2.1
F
5.2
•  Smaller halogens form stronger hydrogen bonds.
Reactions of Metal Halides
•  Most often halogens are ancillary ligands or leaving groups.
•  Nature of halogen can effect reactivity.
H
OC
R3P
Ir
PR3
X
+ H2
(16 e-)
OC
R3P
Ir
𝑣CO (cm-1)
F
Cl
Br
I
1957 1965 1966 1967 H
PR3
X
H
H
R3P
Ir
PR3
CO
(18 e-)
X
ΔG (kcal/mol)
> -­‐10 -­‐14 -­‐17 -­‐19 •  Fluoro complex most electron rich… would be expected to favor O.A.
•  However, π-donation stabilizes SM more than product.
Reactions of Metal Halides
k1
L
X
Ru
X PR
3
R
k2
L
-PR3
Ru
+PR3
X
L
+
OMe
R
X
k-2
X
k-1/k2
Cl
1.25
I
330
X
R
X
–
k-1
Ru
OMe
OMe
Better π-donation of Cl’s slows back reaction in first equilibrium.
Not Always Ancillary Ligands
CF3
Cy
Cy
P Pd
MeO
i-Pr
CF3
Me
Br
AgF, CH2Cl2
i-Pr
i-Pr
MeO
74%
Cy
Cy
P Pd
MeO
i-Pr
Me
CF3
F
90 °C, tol
added ArBr
i-Pr
i-Pr
MeO
isolated, fully characterized
Me
F
45%
(15% no ArBr)
Reading on Your Own
•  Metal-Nitrosyl Complexes
•  Metal-Boryl Complexes
•  Metal-Phosphido Complexes
•  Metal-Thiolate Complexes
•  Metal-Silyl Complexes
•  All covered in Hartwig Chapter 4.