Organometallic Chemistry Introduction,, hapticity, p y, 18 e- rule Typical ‘organometallic’ organometallic ligands e.g. Carbonyl, Cyclopentadienyl, Alk l andd Alkene Alkyl Alk etc. t Oxidative Addition, Reductive Elimination g y Insertion reactions and Migratory Catalysis, Homogeneous Catalysis: Olefin Catalysis Hydrogenation (Wilkinson catalyst)
Catalysis : Basics Catalyst
A + B C
Catalyzed rxn proceeding through an intermediate
Heterogeneous \ Heterogeneous g \ Homogeneous g Ea
A catalyst lowers the activation barrier for a transformation, by introducing
Ea catalyzed
G Reactants
G G Products
a new reaction pathway. It does not change the thermodynamics !!
Reaction Coordinate
Transition metal organometallic compound
Transition metal organometallic compounds & Catalysis
1760 F 1760, French h Ch Chemist, it L L. C C. C Cadet, d t M Me2As-OAsMe A OA M 2 Cacodyl-Oxide C d l O id On Wed, Mar 23, 2011 at 7:39 PM, rajesh p wrote
French Chemist
L. C. Cadet
1760 As2Me4 dicacodyl
1760 In an attempt to make invisible ink, Cadet deGassicourt heats arsenic containing cobalt minerals
Which one is organometallic?
Ni(CO)4 or NaCN ?
with sodium acetate and obtains a foul smelling, fuming liquid. Thi li This liquid id contains t i M Me2As-O-AsMe A O A M 2 ("Cacodyloxide") ("C d l id ") which hi h iis th the fi firstt organometallic t lli compound ever made.
Metal-carbon bond: a few from many?
"Cadet's fuming liquid" can be reproducibly obtained by dry distillation of arsenic (As2O3) with potassium acetate.
O
"Cadet's fuming liquid" should be treated with utmost respect as it is very poisonous, which is characteristic of all arsenic compounds. p Cacodyl y compounds p Me2As-X were further investigated g by Bunsen who obtained "cacodyl" Me2As-AsMe2 and cacodyl cyanide Me2As-CN. The isolation of these compounds led to the concept of functional groups and substituents
CH3 R3P
Re
("radicals" in the language of the day). day)
R2 P
CO
CO
P R2
PR3
Cacodyloxide and its derivatives ignite very easily. In one of his experiments, Bunsen found that
O
C Mo
C Cl
Mo
Cl C
C
O
O
Ph3P
R2 P
P R2
Rh
Ph3P
PPh3 Cl
one drop of cacodyl oxide produced a flame several feet high and covering everything with a blank layer of arsenic. Bunsen stopped his research on cacodyl compounds after an explosion in 1843 in which he lost his right eye.
Catalysis : Basics
Organometallic Compound: Looking closer Ligand Name
Bonding Type H
Molecular Hydrogen: H2
M H
H d id HHydride
MH M-H
A+B
Catalyst
C
Heterogeneous \ Homogeneous the activation barrier
Carbonyl: CO Alkyl , Aryl
by introducing
M-PR3
Alkene
M-Ph
M H2C
Ea catalyzed
G Reactants
G G Products
a new reaction pathway.
M C O
M-CR
Ea
A catalyst lowers for a transformation,
Phosphine: PR3
Catalyzed rxn proceeding through an intermediate
CH2
It does not change the thermodynamics !!
Reaction Coordinate
How to select a good catalyst?
Catalysis : Why Synthesis of chemicals… pharmaceutical, agricultural Catalytic converter … environmental
Activity:
related to rate of reaction (also called turnover)
Efficient:
good activity
Biological g system y – efficient catalyst y
Selectivity: Byproducts should be minimized
Homogeneous and heterogeneous
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
Coordination compounds p in catalysis y Nobel Prizes
Hydrogenation y g of Unsaturated Hydrocarbons y
H 2010 Heck, Suzuki, Neigishi 2005 Chauvin, Grubbs, Schrock 2001 Knowles, Knowles Noyori, Noyori Sharpless 1973 Wilkinson 1963 Ziegler, Natta 1918 Haber
H
-CH=CH- + H2 -CH-CHNOBEL : 2001 The most common catal catalyst st Wilkinson’s Catalyst, [RhCl(PPh3)3]
Geoffrey Wilkinson
Wilkinson’s Catalyst y ((WC))
• Born July 14, 1921, Yorkshire, England
PPh3
Ph3P
• Ph.D from Cal Berkeley studying with Glenn Seaborg • First published WC in 1965 in the Journal 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.
Rh PPh3
Cl
Organometallic compounds prepared by Wilkinson in display at Harvard Univ.
Chlorotris(triphenylphosphine)rhodium(I) d8 configuration g
square q planar p
WC in alkene Hydrogenation: Catalytic Steps
SYNTHESIS OF WC
(1) Oxidative addition
PPh3
Ph3P
H
EtOH
RhCl3
+
3 H2O
+
+
Rh
>4 PPh3
Ph3PO
Ph3P
o
78 C Cl
Commercially available
PPh3
Rh
Ph3P
PPh3 Cl
Rh+1
Ph3P + H2
Rh
Ph3P
PPh3 H
Cl Rh3+
WC in alkene Hydrogenation: Catalytic Steps
WC in alkene Hydrogenation: Catalytic Steps (3) Alkene Binding
(2) Ligand Dissociation
H
H Ph3P
Ph3P
PPh3 3
Rh
Ph3P
H
H + PPh3
Rh
Ph3P
H
Ph 3 P
Rh
Ph 3 P
WC in alkene Hydrogenation: Catalytic Steps
P h 3P
Cl
P h 3P
H
H
P PChH 2
Rh
3
H H 2 CC l C H 2
Ph 3 P
Rh
Cl
CH 2
Rh
H
PPh 3
H 2 CCl CH 2
(5) Ligand association
H
CHH 2
Rh
CH 2
Ph 3 P
CH H2
WC in alkene Hydrogenation: Catalytic Steps
(4) Migration/Insertion P h 3P
CH 2
Cl
Cl
Cl
H +
H
H
Ph 3 P
P h 3P
CH2 CH2 Rh
P h 3P Cl
H CH2
H CH2
CH2
CH2
Ph3P H
Rh
Ph3P Cl
H + PPh3
Ph3P
Rh
Ph3P
PPh3
H Cl
WC in alkene Hydrogenation: Catalytic Steps (6) Reductive elimination
IN
H CH2 Ph3P
CH2 Rh
Ph3P
PPh3
H Cl
WC A
Ph3P
Rh
Ph3P
PPh3 Cl
+ CH3 CH3
(note: regeneration of the catalyst)
C T I O N
WC in alkene Hydrogenation: Additional Notes Highly selective for C=C over C=O Rate of the reaction decreases as the alkyl substitution increases Highly sensitive to the nature of the phosphine ligand A l Analogous complexes l with ith alkylphosphine lk l h hi li ligands d are iinactive ti Applications * Laboratory y scale organic g synthesis y * 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 g with other two scientists. along
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.
Additional notes For interested students
Oxidative addition… addition
(a) Ligand coordination and dissociation Facile coordination of the reactant and facile loss of products.
X-Y
Coordinatively unsaturated - 16 16-electron electron complexes
(b) Oxidative addition -occurs when a complex behaves simultaneously as a Lewis base and a Lewis acid M t l mustt possess a non-bonding Metal b di electron l t pair i
LnMn
+ X-Y
Ph3P
Cl Ir
Coordinatively Coo d ve y unsaturated u s u ed Oxidation of metal by two units – Mn to Mn+2
Ph3P
CO
LnMn
+ H2
X LnMn+2 Y
Ph3P H Cl Ir Ph3P H CO
(c) Insertion or migration
H CH 2 M CH2
Migration of alkyl and hydride ligands
L+
R
L
M CO
M C
R
M CH2CH3
O
S LnM
CR O
S
L' R
L' LnM
LnM
LnM
CR
C
CR O
O
O
R C LnM
O
(e) Reductive elimination
(d) Nucleophilic attack R
C
Involves decrease in the oxidation and coordination number
2+ R 2
H R
L3Pd
L3Pd
OH2
C C OH R
C H
R
Ph3P
R O
L5 M
+
CO + OH
-
L5M
C OH
L5M H + CO2
-
Ph3P
Cl Rh
Ph3P
Me
COR CO
Ph3P
Cl Rh CO
+
RCOMe
Catalysis : Basics
Organometallic Compound: Looking closer Ligand Name Molecular Hydrogen: H2
Bonding Type H
Catalyst A + B --------------- --------------- C
Heterogeneous \ Homogeneous
A catalyst lowers the activation barrier for a transformation, by introducing a new reaction ti pathway. th It d does nott change h th the th thermodynamics d i !!
M H
Hydride HPhosphine: PR3 Carbonyl: CO
M C O
Alk l , Aryl Alkyl A l
M CR M-CR
IN A C T I O N
Ea catalyzed
G Reactants
G
M M-Ph Ph
Products
M H2C
CH2
Nobel in Catalysis 2010
Ea
M-PR3
Alkene
WC
Catalyzed rxn proceeding through an intermediate
M-H M H
Reaction Coordinate
Heck, Suzuki, Neigishi
2005
Chauvin, Grubbs, Schrock
2001
Knowles, Noyori, Sharpless
1973
Wilkinson
1963
Ziegler, Natta
1918
Haber
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