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Transcript
Transition metal organometallic compounds & Catalysis
French Chemist
L. C. Cadet
1760
Which one is organometallic?
As2Me4 dicacodyl
Ni(CO)4 or NaCN ?
Metal-carbon bond: a few from many?
CH3
R3P
Re
CO
CO
PR3
R2
P
P
R2
O
O
C
C
Mo
Cl
Mo
Cl
C
C
O
O
R2
P
P
R2
Ph3P
Ph3P
Rh
PPh3
Cl
9e - 1e + 8e = 16e
therefore coordinately
Organometallic Compound: Looking closer
Ligand Name
Bonding Type
H
Molecular Hydrogen: H2
M
H
Hydride H-
Phosphine: PR3
Carbonyl: CO
Alkyl , Aryl
M-H
M-PR3
M C O
M-CR M-Ph
Alkene
M
H2C
CH2
18 en rule
1920
British Chemist
Sidgwick
Organic compounds – Octet rule
Organometallic – 18 electron rule
* 18 valance electron – inert gas configuration
Counting the number of electrons
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 electrons
Ligand Name
Molecular Hydrogen: H2
Bonding Type
H
M
Formal
Charge
Electrons
donated
0
2
-1
2
H
Hydride 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
Alkene
M
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
d3
21
d4
22
d5
d6
d7
d8
28
d 10s 1
24
25
Sc Ti
V
Cr
Mn Fe Co Ni Cu
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
39
40
41
42
43
44
45
46
47
Y
Zr
Nb Mo Tc Ru Rh Pd Ag
Yttrium
Zirconium
Niobium
Molybdenum Technetium Ruthenum
Rhodium
Palladium
Silver
57
72
73
74
77
78
79
76
27
d 10
23
75
26
d9
29
La Hf Ta W
Re Os Ir
Pt Au
Lanthanum
Rhenium
Platinum
Hafnium
Tantalum
Early Transition
Metals
16e and sub-16e
configurations
are common
Coordination
geometries
higher than 6
Tungsten
Osmium
Iridium
Gold
Middle Transition
Metals
Late Transition
Metals
18e
configurations
are common
16e and sub-16e
configurations
are common
Coordination
geometries
of 6 are common
Coordination
geometries
of 5 or lower
Catalysis
A+B
Catalyst
Catalyzed rxn
proceeding through
an intermediate
C
Ea
A catalyst lowers the
G
activation barrier for a
transformation, by introducing
a new reaction pathway.
Ea
c a ta ly ze d
Re a c ta nts
G
Produc ts
Re a c tion Coordina te
It does not change the thermodynamics!!
Heterogeneous
Homogeneous
Catalysis : Why?
Synthesis of chemicals… pharmaceutical, agricultural
Catalytic converter … environmental
Biological system – efficient catalyst
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 = /[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
H
H
-CH=CH- + H2  -CH-CHNOBEL : 2001
The most common catalyst

Wilkinson’s Catalyst, [RhCl(PPh3)3]
Wilkinson’s Catalyst (WC)
PPh3
Ph3P
Rh
Cl
PPh3
Chlorotris(triphenylphosphine)rhodium(I)
square planar
d8 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
PPh3
Ph3P
RhCl3
+
3 H2O
+
EtOH
+
Rh
>4 PPh3
78 oC
Cl
Commercially available
PPh3
Ph3PO
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
-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
Oxidative addition…
X-Y
+ X-Y
LnMn
Ph3P
Cl
Ir
Ph3P
CO
LnMn
+ H2
X
LnMn+2
Y
H
Ph3P
Cl
Ir
Ph3P H 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
S
LnM
S
CR
O
L'
R
L'
LnM
LnM
C
CR
O
O
R
C
LnM
O
LnM
CR
O
(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
-
(d) Reductive elimination
Involves decrease in the oxidation and coordination number
Ph3P
Ph3P
Cl
Rh
Ph3P
Me
COR
CO
Ph3P
Cl
Rh
+
CO
RCOMe
Hydrogenation of Unsaturated Hydrocarbons
H
H
-CH=CH- + H2  -CH-CHG0 = -101 kJ/mol
WC in alkene Hydrogenation: Catalytic Steps
(1) Oxidative addition
H
Ph3P
Rh
Ph3P
PPh3
Cl
Ph3P
+ H2
Ph3P
Rh3+
(2) Ligand Dissociation
H
Rh
Ph3P
PPh3
H
Cl
H
Cl
Rh+1
Ph3P
Rh
PPh3
H
Ph3P
Rh
Ph3P
Cl
H + PPh3
WC in alkene Hydrogenation: Catalytic Steps
(3) Ligand Association
H
Ph3P
Rh
H +
Ph3P
CH2
CH2
Cl
Ph3P
Ph3P
Ph3P Cl
H
CH
H2
H
Rh
CH2
Rh
H
PPh3
H2CCl CH2
(4) Migration/Insertion
Ph3P
Ph3P
H
H
Rh
Cl Rh
Ph3P
H
CH
H 2
CH2
PPh
3
H
H2CCl CH2
Ph3P
CH2
CH2
Rh
Ph3P
Cl
H
WC in alkene Hydrogenation: Catalytic Steps
(5) Ligand association
H CH2
H CH2
CH2
CH2
Ph3P
Rh
Ph3P
Cl
H + PPh3
Ph3P
Rh
Ph3P
PPh3
H
Cl
WC in alkene Hydrogenation: Catalytic Steps
(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
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
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.
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.
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
-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
Oxidative addition…
X-Y
+ X-Y
LnMn
Ph3P
Cl
Ir
Ph3P
CO
LnMn
+ H2
X
LnMn+2
Y
H
Ph3P
Cl
Ir
Ph3P H CO
(c) Insertion or migration
Migration of alkyl and hydride ligands
L+
R
L
M CO
M C
O
S
LnM
S
CR
O
L'
R
L'
LnM
LnM
C
CR
O
O
R
C
LnM
R
O
LnM
CR
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