Transcript MS PowerPoint - Catalysis Eprints database

Homogeneous Catalysis
HMC-2- 2010
Dr. K.R.Krishnamurthy
National Centre for Catalysis Research
Indian Institute of Technology, Madras
Chennai-600036
Homogeneous Catalysis- 2
Hydrogenation & Isomerization
Hydrogenation of olefins-Wilkinson’s catalyst
Isomerization of olefins
Activation of Hydrogen by Transition Metal Catalysts
Three routes are possible
Oxidative addition
Most common method for activation of H2
Need the d2 or higher electronic configuration
Need the empty co-ordination site to bind H2
A change in oxidation state of metal ion occurs
Hydrogenolysis
Only way the early transition metals with d0 configuration
can activate H2
Lanthanides and actinides typically use hydrogenolysis
Needs empty co-ordination site to bind H2
Needs anionic ligands (e.g., alkyl, halide) that can be
protonated off
No change in oxidation state of Metal occurs
Heterolytic cleavage
Similar to hydrogenolysis except an external base added, which will
transfer back the proton to complete the catalytic cycle
Ru+2 is the most common metal that uses heterolytic cleavage
as a mechanism
No change in oxidation state of the metal occurs
Hydrogenation catalysts
Widely used for hydrogenation of several
functional groups
Addition of H2 to multiple bonds / Selective
hydrogenation
R
+
H2
C
C
C
C
C
O
C
N
C
N
N
O
N
N
N
N
Functional groups for
hydrogenation
R
Wilkinson’s Catalyst (discovered in 1964)
RhCl(PPh3)3 first highly active homogeneous catalyst
Synthesis:
The most studied of all rhodium (I) phosphine species.
It has two allotropes, the bright red and orange forms
The first compound discovered to allow catalytic hydrogenation of alkenes and
other unsaturated substances in homogeneous solutions at room temperature
and pressure.
It’s discovery stimulated an enormous development in the synthesis of
complexes with metals and tertiary phosphine ligands.
The yardstick against which all other homogeneous catalysts are measured
Geoffrey Wilkinson
Born July 14, 1921, Todmorden, Yorkshire, England
Died Sept. 26, 1996, London
Received Ph.D from Cal Berkeley studying with
Glenn Seaborg
In 1955, appointed to the chair of Inorganic
Chemistry at Imperial College in the University of
London, at which time was the only established chair
in the subject in the United Kingdom
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“
Hydrogenation ofMechanism
olefins- Catalytic cycle
(Ph3P)3Rh1Cl
[16 e]
Four main steps
dissociation -Ph3P
RCH2CH3
H2
(Ph3P)2Rh1Cl
[14 e]
reductive
elimination
oxidative
addition
(III) H
(Ph3P)2ClRh
[16 e] (Ph3P)2ClRh
CH2CH2R
coordination
migratory
insertion
(Ph3P)2ClRh(III)
H
H
RCH
Oxidative addition
Olefin coordination
Migratory insertion
Reductive elimination
CH2
[18 e]
(III)
H
[16 e]
H
RCH
CH2
Addition of H2 in cis fashion
Oxidative addition-Promoted by
substitution of more electron rich
phosphines on Rh complex
Electron withdrawing groups
promote reductive elimination to
ethane
Oxidative addition of H2 is the
RDS
Hydrogenation- Reaction sequence
L- -P(Ph)3 ; S-Solvent
Hydrogenation-Ligand effect
Hydrogenation of cyclohexene on Wilkinson’s catalyst
Ligand
(4-ClC6H4)3P
(Ph)3P
(4-CH3C6H4)3P
(4-CH3OC6H4)3P
Relative activity
1.7
41
86
100
Rh complexes for hydrogenation
The catalyst is compatible with a variety of functional groups - ketones, esters,
carboxylic acids, nitriles, nitro groups & ethers
The metal hydride intermediate is primarily covalent in character.
It is observed that cationic complexes, which are more Electrophilic and favors
the alkene coordination are more active for hydrogenation.
Nobel Prize in 2005 with Prof.Grubbs
Ir 1,5 Cyclooctadiene, tris cyclohexyl phosphine pyridinium
Selective hydrogenation
Selectivity:
Selectively hydrogenate the most reactive multiple bond
Steric and electronic effects play an important role
e.g.,
Metal-Hydride/ Heterolytic Cleavage routes
Ru has a strong tendency to perform a heterolytic activation of H2
that occurs either via hydrogenolysis or heterolytic cleavage
Complexation of H2 with metal decrease H-H sigma-bond character
making more acidic or easier to deprotonate with a base
Both mechanisms give the same net result
Mechanism
Catalyst
Hydrogenolysis
+ H2
Olefin coordination
Hydride migration
No change in oxidation state of Ru+2 in catalytic cycle
Homogeneous Isomerization
But-1-ene
But-2-ene
Key steps
Migratory insertion
β- Elimination
Homogeneous Isomerization