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Homogeneous catalysis
A+B
Catalyst
Catalyzed rxn
proceeding through
an intermediate
C
Heterogeneous
Homogeneous
Ea
G
Ea
c a ta ly ze d
A catalyst lower the activation Re a c ta nts
G
barrier for a transformation,
Produc ts
by
introducing a new reaction
Re a c tion Coordina te
pathway.
It does not change the thermodynamics!!
Important catalyst properties
* Activity: A reasonable rate of reaction is needed
Turnover frequency (N)
N = /[Q]
Large turnover frequency – efficient catalyst
* Selectivity: Byproducts should be minimized
* Lifetime: It is costly to replace the catalyst frequently
* Cost: The acceptable cost depends upon the catalyst
lifetime
and product value lifetime and product value
Transition metal organometallic compounds
Metal-carbon bond
18 en rule
Organic compounds – Octet rule
Organometallic – 18 electron rule
* 18 valance electron – inert gas configuration
Oxidation state method
Neutral atom method
Ligand Name
Molecular Hydrogen:
H2
Hydride H-
Bonding Type
H
M
Formal
Charge
Electrons
donated
0
2
-1
2
H
M-H
Halide X-
M-X
-1
2
Amine, phosphine,
arsine: NR3, PR3, AsR3
M-NR3 M-PR3
0
2
0
2
-1
2
-1
2
Carbonyl: CO
Alkyl , Aryl
M C O
M-CR M-Ph
M
Alkene
H2C
CH2
CO
OC
Fe
OC
Fe is 4s23d6 = 8e
CO
each Co is neutral so Fe0
CO
each CO donates 2 e = 10e
8e + 10e = 18e
coordinately saturated
Ph3P
Ph3P
Rh
PPh3
Rh is s1d8 = 9e
Cl
since Cl is -1, Rh is +1 (the complex is neutral)
9e - 1e + 8e = 16e
4 ligands x 2e each = 8e
therefore coordinately unsaturated
Ph3P
16 electron complexes
Ir
OC
Group 9 and 10
Exception to 16/18 electron rule
V(CO)6
17 electrons
W(CH3)
12 electrons
Cl
PPh3
9+1+2+2+2 = 16
Catalytic steps
(a) Ligand coordination and dissociation
Facile coordination of the reactant and facile loss of products.
Coordinatively unsaturated - 16-electron complexes
(b) Oxidative addition
Non-bonding electron pair in the metal
Coordinatively unsaturated
Oxidation of metal by two units – Mn to Mn+2
Oxidative addition…
Ph3P
Cl
Ir
Ph3P
CO
+ RI
R
Ph3P
Cl
Ir
Ph3P I CO
(c) Insertion or migration
Migration of alkyl and hydride ligands
L+
R
L
M CO
M C
R
O
H CH
2
M
CH2
M CH2CH3
(d) Nucleophilic attack
R
C
R 2+
L3Pd
H R
L3Pd
OH2
C C OH
R R
C
H
R
O
L5M
+
-
CO + OH
L5M
C OH
L5M H
+
CO2
-
(e) Reductive elimination
Involves decrease in the oxidation and coordination number
Ph3P
Ph3P
Cl
Rh
Ph3P
Me
COR
CO
Ph3P
Cl
Rh
+
CO
RCOMe
Wilkinson’s Catalyst
PPh3
Ph3P
Rh
Cl
PPh3
Tris(triphenylphosphine)rhodium(I) chloride
square planar
16 electron species
d8 configuration
Geoffrey Wilkinson
• Born July 14, 1921,
Yorkshire, England
Todmorden,
•Received Ph.D from Cal
studying with Glenn Seaborg
Berkeley
•First published compound in 1965 in
Journal of the Chemical Society Chemical Communications
•Nobel Prize in Chemistry 1973 (shared
with Ernst Otto Fischer)
for their
pioneering
work,
performed
independently, on the chemistry of the
organometallic, so called sandwich
compounds.
Synthesis
PPh3
Ph3P
RhCl3
+
3 H2O
+
EtOH
+
Rh
>4 PPh3
78 oC
Cl
PPh3
commercially available – Sigma Aldrich, Acros, etc.
Ph3PO
(1) Oxidative addition
H
Ph3P
Rh
Ph3P
PPh3
Cl
Ph3P
+ H2
Rh
Ph3P
PPh3
H
Cl
16 en Rh+1
18 en Rh3+
(2) Ligand Dissociation
H
Ph3P
Rh
Ph3P
PPh3
H
Cl
H
Ph3P
Rh
Ph3P
Cl
H + PPh3
(3) Ligand Association
H
Ph3P
Rh
Ph3P
H +
CH2
CH2
Cl
CH2
H
Ph3P
CH2
Rh
Ph3P
H
Cl
(4) Insertion
H
Ph3P
CH2
CH2
Rh
Ph3P
H
Cl
H CH2
Ph3P
CH2
Rh
Ph3P
Cl
H
(5) Ligand association
H CH2
H CH2
CH2
CH2
Ph3P
Rh
Ph3P
Cl
H + PPh3
Ph3P
Rh
Ph3P
PPh3
H
Cl
(6) Reductive elimination
H CH2
Ph3P
CH2
Rh
Ph3P
PPh3
H
Cl
Ph3P
Ph3P
Rh
PPh3
Cl
+ CH3 CH3
(note: regeneration of the catalyst)
WC
IN
A
C
T
I
O
N
Highly sensitive to the nature of the phosphine ligand
Analogous complexes with alkylphosphine ligands are inactive
Chiral phosphine ligands have been developed to synthesize
optically active products.
Applications
* Laboratory scale organic synthesis
* Production of fine chemicals
* Synthesis of L-DOPA
Used for the treatment of Parkinson’s diseases
Synthetic route was developed by Knowles and co-workers at Monsanto
Dr. William S. Knowles received Nobel prize in chemistry 2001
along with other two scientists.
This reaction, developed by Knowles, Vineyard, and Sabacky, was
used at Monsanto as a commercial route to the Parkinson's drug LDOPA.
Metal Carbonyl Complexes
M-CO
CO as a ligand
strong s donor, strong π-acceptor
strong trans effect
small steric effect
CO is an inert molecule that becomes activated by
complexation to metals
Homoleptic
Heteroleptic
Group
Formula
Valence en
Structure
CO CO
6
Cr(CO)6
OC Cr
CO
OC CO
6 +12 = 18
7
Mn2(CO)10
7+10+1 = 18
8
Fe(CO)5
8+10 = 18
OC
CO COCO
OC Mn
Mn CO
OC CO OC CO
CO
OC Fe CO
OC CO
CO OC
OC
9
Co2(CO)8
9+8+1= 18
CO
Co CO
Co
OC
C
O
C
O
CO
Ni
OC
10
Ni(CO)4
10+8 = 18
CO
CO
O
M
C
s orbital serves as a very weak donor to a metal atom
O C
M
CO-M sigma bond
O
C
M
M to CO pi backbonding
O C
M
CO to M pi bonding
(rare)
Metal should be in low oxidation state ie en rich
Synthesis
Direct combination
Ni(s) + 4CO(g)
Ni(CO)4(l)
30oC and 1 atm CO
Reductive carbonylation
CrCl3(s) + Al(s) + 6CO(g)
AlCl3(soln) + Cr(CO)6 (soln)
AlCl3 and Benzene
Re2O7(s) + 17 CO(g)
250oC, 350 atm CO
Re2(CO)10(s) + 7CO2(g)
Substituted carbonyls
Oxidative Addition
Concomitant oxidation of the metal and addition of new ligand(s)
Fe(CO)5 + Br2
Fe0
Fe2+
Fe(CO)4Br2 + CO
Reductive Carbonylation
Metal halide and CO
RhCl3.3H2O
CO/EtOH
100
o
Rh2(CO)4Cl2
Properties
Reduction to form metal carbonylates
Fe(CO)5
Na, THF
2-
[Fe(CO)4] + CO
Substitution
Cr(CO)6 + Solv
Cr(CO)5(solv) +PR3
Cr(CO)5(solv) + CO
Cr(CO)5PR3 + solv
[Cr(CO)5Solv]
Cr(CO)5(PR3)
Cr(CO)6(PR3)]#
[Cr(CO)5(PR3)]
O v
C
l
o
+S
Cr(CO)6
R3
+P
-CO
Oxidation
(CO)5Mn-Mn(CO)5 + Br2
2Mn(CO)5Br
Oxidative cleavage of M-M bond
Protonation
Mn(CO)5-(aq) + H+(aq)
HMn(CO)5(s)
…more negative the anion, higher its Bronsted basicity
… use in the synthesis of different organometallic
[Mn(CO)5]- + CH3I
[Mn(CH3)(CO)5] + I-
Application
Hydroformylation
H2
Co2(CO)8
2HCo(CO)4
CH2=CHR
CO
HCo(CO)3
HCo(CO)4
H(CO)3Co
CH2
CHR
RCH2CH2CHO
(CO)3Co C-CH2CH2R
(CO)4CoCH2CH2R
(CO)3CoCH2CH2R
CO