Transition metal organometallic compounds & Catalysis

French Chemist L. C. Cadet 1760 As2Me4 dicacodyl

Which one is organometallic? Ni(CO)4 or NaCN ?

Metal-carbon bond: a few from many?

O O
CH3 C C
R2 R2 Ph3P PPh3
R3P CO P Cl P
Rh
Re Mo Mo
PR3 P Cl P Ph3P Cl
R2 C C R2
CO
O O

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Organometallic Compound: Looking closer

Ligand Name Bonding Type

Molecular Hydrogen: H2 H
M
H
Hydride H- M-H

Phosphine: PR3 M-PR3

Carbonyl: C≡O M C O

Alkyl , Aryl M-CR M-Ph

Alkene M
H2C CH2
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 Catalysis : Basics

Catalyst Catalyzed rxn

A+B C proceeding through
an intermediate

Heterogeneous Ea

Ea
Homogeneous catalyzed
∆G

A catalyst lowers the Reactants

∆G
activation barrier for a Products
transformation, by introducing
a new reaction pathway.
Reaction Coordinate
It does not change the thermodynamics!!

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 Catalysis : Why

Synthesis of chemicals… pharmaceutical, agricultural

Catalytic converter … environmental

Biological system – efficient catalyst

Homogeneous and heterogeneous

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 How to select a good catalyst?

Activity: related to rate of reaction (also called turnover)

efficient catalyst: good activity

Selectivity: Byproducts should be minimized

Lifetime: It is costly to replace the catalyst frequently

Cost: The acceptable cost depends upon the catalyst

lifetime, product value lifetime and product value

Poisoning: decomposition of catalyst

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 Good Catalysts

Selectivity TON TOF

Chemo - Regio - Stereo- Enantio-

Structure of the Catalyst: Selectivity 8

 Coordination compounds in catalysis
Nobel Prizes

2005 Chauvin, Grubbs, Schrock

2001 Knowles, Noyori, Sharpless
1973 Wilkinson
1963 Ziegler, Natta
1918 Haber
1909 Ostwald

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Hydrogenation of Unsaturated Hydrocarbons

H H
-CH=CH- + H2 → -CH-CH-

NOBEL : 2001

The most common catalyst

⇓
Wilkinson’s Catalyst, [RhCl(PPh3)3]

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 Wilkinson’s Catalyst (WC)

Ph3P PPh3

Rh

Cl PPh3

Chlorotris(triphenylphosphine)rhodium(I)
square planar d8 configuration
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 Geoffrey Wilkinson
• Born July 14, 1921, Yorkshire, England
• Ph.D from Cal Berkeley studying with Glenn Seaborg
• 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.

Organometallic compounds prepared by

Wilkinson in display at Harvard Univ.

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 Synthesis of WC

Ph3P PPh3
EtOH
RhCl3 + 3 H2O + >4 PPh3 Rh + Ph3PO
78 oC
Cl PPh3

Commercially available

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 WC in alkene Hydrogenation: Catalytic Steps
(1) Oxidative addition
H
Ph3P PPh3 Ph3P PPh3
Rh + H2 Rh
Ph3P Cl Ph3P H
Rh+1 Cl
Rh3+
(2) Ligand Dissociation

H
H
Ph3P PPh3 Ph3P
Rh Rh H + PPh3
Ph3P H Ph3P
Cl Cl
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 WC in alkene Hydrogenation: Catalytic Steps
(3) Ligand Association

H H CH
Ph3P H H2
Ph3P CH2 Ph3P Rh
Rh H + Rh CH2
PPh3
Ph3P CH2 Ph3P Cl H
Cl
H2CCl CH2
(4) Migration/Insertion

H H CH2
Ph3P H CH
H 2
Ph3P Rh CH2
Cl Rh
CH2
PPh Ph3P
3
Ph3P H Rh H
H2CCl CH2 Ph3P
Cl 17
 WC in alkene Hydrogenation: Catalytic Steps

(5) Ligand association

H CH2 H CH2
CH2 CH2
Ph3P Ph3P PPh3
Rh H + PPh3 Rh
Ph3P Ph3P H
Cl Cl

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 WC in alkene Hydrogenation: Catalytic Steps

(6) Reductive elimination

H CH2
CH2
Ph3P PPh3 Ph3P PPh3
Rh Rh
Ph3P H Ph3P + CH3 CH3
Cl
Cl

(note: regeneration of the catalyst)

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WC
IN
A
C
T
I
O
N
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 WC in alkene Hydrogenation: Additional Notes

Rate of the reaction decreases as the alkyl substitution increases

Highly sensitive to the nature of the phosphine ligand
Analogous complexes with alkylphosphine ligands are inactive
Highly selective for C=C over C=O

Applications

* Laboratory scale organic synthesis

* Production of fine chemicals

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 Alkene Hydrogenation & Chirality & Nobel

Chiral phosphine ligands have been developed to synthesize optically

active products.
Synthesis of L-DOPA (Used in the treatment of Parkinson’s diseases)
Synthetic route was developed by Knowles & co-workers at Monsanto

Dr. William S. Knowles received Nobel prize in chemistry 2001

along with other two scientists.

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 Alkene Hydrogenation, Chirality & Nobel

This reaction, developed by Knowles, Vineyard, and Sabacky, was used at

Monsanto as a commercial route to the Parkinson's drug L-DOPA.
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 Additional notes For interested students
(a) Ligand coordination and dissociation
Facile coordination of the reactant and facile loss of products.

Coordinatively unsaturated - 16-electron complexes

(b) Oxidative addition
-occurs when a complex behaves simultaneously as a Lewis base
and a Lewis acid
Metal must possess a non-bonding electron pair

Coordinatively unsaturated

Oxidation of metal by two units – Mn to Mn+2

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Oxidative addition…

X-Y X
+ X-Y LnMn+2
LnMn
LnMn Y

Ph3P H
Cl
Ph3P Cl Ir
Ir + H2
Ph3P H
Ph3P CO CO
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(c) Insertion or migration
Migration of alkyl and hydride ligands
R L R
L+ M CO M C
S O
LnM CR

O
S

L'
R L' LnM CR
LnM LnM CR
C O
O
O

R
C
LnM O
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H CH
2
M M CH2CH3
CH2

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 (d) Nucleophilic attack

R R 2+ H R +
C
L3Pd L3Pd C C OH
OH2
C R R
H R

-
O
L5M - -
CO + OH L5M C OH L5M H
+
CO2
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(e) Reductive elimination

Involves decrease in the oxidation and coordination number

Ph3P Cl
Cl Rh
Ph3P Me Ph3P CO
Rh
Ph3P COR +
CO RCOMe

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