Transition metal organometallic compounds & Catalysis
Metal-carbon bond: a few from many?
Which one is organometallic? Ni(CO)4 or NaCN ?
French Chemist L. C. Cadet 1760 As2Me4 dicacodyl
ReR3P CO
PR3
CH3
CO
MoPR2
R2P Cl
C
C
MoCl
R2P
PR2
C
C
O O
OO
Ligand Name Bonding Type
Molecular Hydrogen: H2
Hydride H- M-H
Phosphine: PR3 M-PR3
Carbonyl: CO
Alkyl , Aryl M-CR M-Ph
Alkene
MH
H
CM O
CH2H2C
M
Organometallic Compound: Looking closer
18 en rule
Organic compounds – Octet rule
Organometallic – 18 electron rule
* 18 valance electron – inert gas configuration
1920 British Chemist Sidgwick
To determine the electron count for a metal complex:
Determine the oxidation state of the transition metal center(s) and
the metal centers resulting d-electron count. To do this one must:
a) note any overall charge on the metal complex
b) know the charges of the ligands bound to the metal
center
c) know the number of electrons being donated to the metal
center from each ligand
2) Add up the electron counts for the metal center and ligands
Counting the number of electronsCounting the number of electrons
Ligand Name Bonding Type Formal Charge
Electrons donated
Molecular Hydrogen: H2
0 2
Hydride H- M-H -1 2
Halide X- M-X -1 2
Amine, phosphine, arsine: NR3, PR3, AsR3
M-NR3 M-PR3 0 2
Carbonyl: CO 0 2
Alkyl , Aryl M-CR M-Ph -1 2
Alkene -1 2
MH
H
CM O
CH2H2C
M
Counting the number of electronsCounting the number of electrons
Fe
OC
OCCO
CO
CO
Fe is 4s23d6 = 8e
each Co is neutral so Fe0
each CO donates 2 e = 10e
8e + 10e = 18ecoordinately saturated
RhPh3P Cl
Ph3P PPh3 Rh is s1d8 = 9e
since Cl is -1, Rh is +1 (the complex is neutral)
4 ligands x 2e each = 8e9e - 1e + 8e = 16e
therefore coordinately unsaturated
Catalysis
Reaction Coordinate
G
GReactants
Products
Ea
E acatalyzed
Catalyzed rxn proceeding through
an interm ediateA + B CA + B C
Catalyst
HeterogeneousHeterogeneous HomogeneousHomogeneous
A catalyst lowers the activation barrier for a transformation, by introducing a new reaction pathway.
It does not change the thermodynamics!!It does not change the thermodynamics!!
Synthesis of chemicals… pharmaceutical, agricultural
Catalytic converter … environmental
Biological system – efficient catalyst
Catalysis : Why?
Organometallic compounds, metals etc.
How to select an efficient catalyst?
Activity: related to rate of reaction (also called turnover)
efficient catalyst: good activity
Turnover frequency (N) N = N = /[Q]/[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, product value lifetime and product value
Poisoning: decomposition of catalyst, adsorption of reactant/product
Coordination compounds in catalysis Nobel Prizes
2005 Yves Chauvin,Robert H. Grubbs
and Richard R. Schrock.
2001 KNOWLES, NOYORI, SHARPLESS
1973 WILKINSON
1963 ZIEGLER, NATTA
1918 HABER
1909 OSTWALD
Hydrogenation of Unsaturated Hydrocarbons
The most common catalyst
Wilkinson’s Catalyst, [RhCl(PPh3)3]
-CH=CH- + H2 -CH-CH-
H H
NOBEL : 2001
Wilkinson’s Catalyst (WC)
Rh
Ph3P PPh3
PPh3ClChlorotris(triphenylphosphine)rhodium(I)
square planar dsquare planar d88 configuration configuration
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.
Synthesis of WC
Rh
Ph3P PPh3
PPh3Cl
+EtOH
78 oC
+ Ph3PORhCl3 3 H2O + >4 PPh3
Commercially available
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
Metal must possess a non-bonding electron pair
Coordinatively unsaturated
Oxidation of metal by two units – Mn to Mn+2
-occurs when a complex behaves simultaneously as a Lewis base and a Lewis acid
IrCO
Ph3P
Ph3P
Cl+ H2 Ir
CO
Ph3P
Ph3P
ClH
H
Oxidative addition…
LnMn + X-YLnMn
X-Y
LnMn+2
X
Y
(c) Insertion or migrationMigration of alkyl and hydride ligands
L + M CO
R
M C
L
O
R
M
H CH2
CH2
M CH2CH3
LnM
R
CO
LnM
S
CR
O
LnM CR
O
S
LnM
RC
O
LnM
L'
CR
O
L'
(d) Nucleophilic attack(d) Nucleophilic attack
C
C
R R
H R
L3Pd OH2C
R
H
C OH
R
R
2+
L3Pd
+
L5M CO + OH- L5M C
O
OH L5M H
-
-
+CO2
(d) Reductive elimination
Involves decrease in the oxidation and coordination number
Rh
COPh3P
Ph3P Me
COR
Cl RhCOPh3P
Ph3P Cl
RCOMe
+
Hydrogenation of Unsaturated Hydrocarbons
-CH=CH- + H2 -CH-CH-
H H
G0 = -101 kJ/mol
RhPh3P Cl
Ph3P PPh3
+ H2 RhPh3P
Cl
Ph3P PPh3
H
HRh+1
Rh3+
RhPh3P
Cl
Ph3P PPh3
H
HRh
Ph3PCl
Ph3PH
H + PPh3
(1) Oxidative addition(1) Oxidative addition
(2) Ligand Dissociation(2) Ligand Dissociation
WC in alkene Hydrogenation: Catalytic Steps
(3) Ligand Association(3) Ligand Association
(4) Migration/Insertion(4) Migration/Insertion
RhPh3P
Cl
Ph3PH
HCH2
CH2Rh
Ph3PCl
Ph3PH
H
CH2
CH2+Rh
Cl PPh3
Ph3PH
H
CH2H2C
RhPh3P
Cl
Ph3PH
H
CH2
CH2
RhPh3P
Cl
Ph3PCH2
H
CH2H
Rh
Cl PPh3
Ph3PH
H
CH2H2C
WC in alkene Hydrogenation: Catalytic Steps
+ PPh3RhPh3P
Cl
Ph3PCH2
H
CH2H
RhPh3P
Cl
Ph3PCH2
H
CH2H
PPh3
(5) Ligand association(5) Ligand association
WC in alkene Hydrogenation: Catalytic Steps
RhPh3P Cl
Ph3P PPh3RhPh3P
Cl
Ph3PCH2
H
CH2H
PPh3
+ CH3 CH3
(note: regeneration of the catalyst)
(6) Reductive elimination(6) Reductive elimination
WC in alkene Hydrogenation: Catalytic Steps
WC
IN
A
C
T
I
O
N
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
WC in alkene Hydrogenation: Additional Notes
* Laboratory scale organic synthesis* Production of fine chemicals
Applications
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 Dr. William S. Knowles received Nobel prize in chemistry 2001 along with other two scientists.along with other two scientists.
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.
Alkene Hydrogenation, Chirality & Nobel
Non-superimposable mirror image
Enantiomeric excess = (moles of major enantiomer - moles of other enantiomer / Total moles of both enantiomers) 100
phenylanisylmethylphosphine (PAMP)
Dimeric product is DiPAMP
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
Metal must possess a non-bonding electron pair
Coordinatively unsaturated
Oxidation of metal by two units – Mn to Mn+2
-occurs when a complex behaves simultaneously as a Lewis base and a Lewis acid
IrCO
Ph3P
Ph3P
Cl+ H2 Ir
CO
Ph3P
Ph3P
ClH
H
Oxidative addition…
LnMn + X-YLnMn
X-Y
LnMn+2
X
Y
(c) Insertion or migrationMigration of alkyl and hydride ligands
L + M CO
R
M C
L
O
R
LnM
R
CO
LnM
S
CR
O
LnM CR
O
S
LnM
RC
O
LnM
L'
CR
O
L'
M
H CH2
CH2
M CH2CH3
(d) Nucleophilic attack(d) Nucleophilic attack
C
C
R R
H R
L3Pd OH2C
R
H
C OH
R
R
2+
L3Pd
+
L5M CO + OH- L5M C
O
OH L5M H
-
-
+CO2
(e) Reductive elimination
Involves decrease in the oxidation and coordination number
Rh
COPh3P
Ph3P Me
COR
Cl RhCOPh3P
Ph3P Cl
RCOMe
+
