Transcript Document

Chem 1140; Introduction to
Organometallic Chemistry
• General Mechanistic Principles
• Reactions with Wilkinson’s Catalyst
A. Organometallic Mechanisms
Oxidation State: The oxidation state of a metal is defined as the charge left
on the metal after all ligands have been removed in their natural, closed-shell
configuration. This is a formalism and not a physical property!
d-Electron Configuration: position in the periodic table minus oxidation state.
18-Electron Rule: In mononuclear, diamagnetic complexes, the total number
of electrons never exceeds 18 (noble gas configuration). The total number of
electrons is equal to the sum of d-electrons plus those contributed by the
ligands.
18 electrons = coordinatively saturated
< 18 electrons = coordinatively unsaturated.
Cl
Pd
Pd
for each Pd: Ox. state
, Cl
Cl
bridging by lone
pairs on Cl;
each Cl acts as a
2-electron, mono
negative ligands to
one of the Pd's,
and a 2-electron
neutral donor ligand
like PPh3 to the other
Pd(II)
d: 10 (4d 10 5s 0) - 2 = 8
electron count:
: 4e-
Cl
: 2e-
Cl
: 2e8e- + d 8 = 16eunsaturated
H
Ox. State: 2 Cp -, 1 H -, 1 Cl  Zr(IV)
Cl
d: 4 (4d 25s 2) - 4 = 0
Zr
electron count: 2 Cp -: 12
H-: 2
Cl - : 2
Zr : 0
16 e-, unsaturated
Bonding considerations
donation:
or
M
vacant
dsp hyb rid

backdonation:
M
filled d orb ital


for M-CO:
M
dspn acceptor
C
O
donor
Structure
• saturated (18 e-) complexes:
- tetracoordinate: Ni(CO)4, Pd(PPh3)4 are tetrahedral
- pentacoordinate: Fe(CO)5 is trigonal bipyramidal
- hexacoordinate: Cr(CO)6 is octahedral
z
• unsaturated complexes have high dx2-y2;
16e- prefers square planar
y
x
Basic reaction mechanisms
- ligand substitution: M-L + L’  M-L’ + L
can be associative, dissociative, or radical chain.
L
M
+ -
+ -
X
+
-
trans-effect: kinetic effect of a ligand on the role of substitution at
the position trans to itself in a square or octahedral complex (ground-state
weakening of bond).
L  M, repels negative charge to trans position.
Lc
Nu
Lc
Lt
X
+ Nu-
Pt
X
Lc
Pt
Lt
Lc
Lt
Pt
Lc
X
Lc
Lc
Pt
Lt
X
Nu
Lc
Lc
Lt
Nu
Nu
Pt
Lc
+ X-
- oxidative addition:
-L
[Ph3P]4Pd
-L
[Ph3P]3Pd
16 e-
[Ph3P]2Pd
Br
Ph
14 e-
strong -donor
L2Pd
H
(+II) Br
L2Pd
Br
H
Ph
agostic (2e-/3-center
bond) interactions
16 e-
16 e-
Ph
- reductive elimination: the major way in which transition metals are
used to make C,C- and C,H-bonds!
Me
Pd
PPh3
Ph3P
Pd
-L
Me
Ph3P
Pd
Me
Ph3P-Pd
Ph3P
Ph3P
Ph
cis!
Ph
12e-
14e?
Ph
+L
Ph
- migratory insertion:
R
Cp
Zr Cp
Cl
16e-
O
C
CO
R
Cp
Zr Cp
Cl
18e-
R
16e-
O
Cp
Zr Cp
Cl
- -elimination/hydrometalation:
-elimination
H
R
Cp
Zr Cp
Cl
R
H Cp
Zr Cp
Cl
hydrometalation
R
Cp
Cp
H Zr
Cl
- olefin metathesis:
Fischer carbene complex
OMe
, 80 °C
OMe
+
(OC) 5Cr
18e-
OEt
Ph
Ph
OEt
(OC) 5Cr
EtO
OMe
Ph
(OC) 5Cr
- transmetalation:
R-M + M’-X 
R-M’ + M-X
Summary of Mechanisms:
- ligand substitution
- oxidative addition/reductive elimination
- migratory insertion/-elimination (carbo-, hydrometalation)
- alkene metathesis
- transmetalation
Reactions with Wilkinson’s Catalyst
Alkene Hydrogenation
H2
cat. RhCl(PPh3)3
CO2Me
CO2Me
CO2Me
H2
96:4
cat. PtO2
49:26
Mechanism
PPh3
oxidative
addition
H Rh H
Cl
PPh3
H H
-PPh3
[RhCl(PPh3)2]
coordination
R
RhCl(PPh3)3
H
+PPh3
R
reductive
elimination
PPh3
R
PPh3
H
R
Cl Rh H
H
H
PPh3
Cl
Rh H
PPh3
migratory
insertion
Reductions
O
OH
OH
i-PrOH, KOH
i-PrOH, KOH
cat. RhCl(PPh3)3
cat. RhCl(PPh3)3
Where does the hydrogen come from????
Hydrosilylation
Et
or
Et
HSiMe2Ph
cat. RhCl(PPh3)3
Et
SiMe2Ph
Hydrocarbonylation
0.4 equiv
RhCl(PPh3)3
THPO
CHO
OTHP
CH2Cl2
40 oC, 20 h
H
THPO
O
H
OTHP
Decarbonylation
Et
Ph
O
1.0 equiv RhCl(PPh3)3
Et
Ph
H
93% retention
Hydroformylation
O
H2, CO
N
H
cat. RhCl(PPh3)3
O
N
H
other minor products
CHO
Markownikow Hydroboration
1. catecholborane
cat. RhCl(PPh3)3
Ph
2. H2O2, OH-
OH
Ph
primary alcohol
if the catalyst is
partially oxidized
Cycloisomerization
H
cat. RhCl(PPh3)3
TBDMSO
TBDMSO H