Transcript Organometallic Catalysts

Organometallic
Catalysts
Presenter :
Saber Askari
Advisor :
Dr.Mirzaaghayan
May 2012
Contents :
 The basis for catalysis
 Catalytic Cycle
 History
 Mechanistic Concept
 Homogeneous Catalysis
 Wilkinson’s Catalyst
 Asymmetric hydrogenation
 Hydroformylation
 Monsanto Acetic acid Process
 CATIVA Process
 Wacker Process
 Heterogeneous Catalysis
 Ziegler-Natta Catalyst
The basis for catalysis

A Catalyst is a substance which speed up the
rate of a reaction without itself being consumed.
A catalyst lowers the activation energy for a chemical reaction
The catalyzed reaction goes by a
multistep mechanism in which
the metal stabilizes intermediates
that are stable only when bound
to metal .
Importance of catalysis
 Many major industrial chemicals are prepared with the aid of
catalysts
 Many fine chemicals are also made with the aid of catalysts
– Reduce cost of production
– Lead to better selectivity and less waste
Catalytic Cycle
The catalytically active species must have a vacant coordination site
to allow the substrate to coordinate
The establishment of a reaction mechanism is always a difficult task.
It is even harder to definitively establish a catalytic cycle as all the
reactions are going on in parallel!
Late transition metals are
privileged catalysts (from
16e species easily)
In general , the total
electron count alternates
between 16 and 18
One of the catalytic steps in
the cycle is rate-determining
History
Fundamental Reaction
Mechanistic Concept
Homogeneous Catalysis
Wilkinson`s Catalyst : Olefin Hydrogenation
Hydroformylation
Monsanto Acetic acid Process
Wacker Process
Heterogeneous Catalysis
Ziegler-Natta Catalysts
Homogeneous
Catalysis
Homogenous catalysts are used when selectivity is
critical and product-catalyst separation problems can
be solved.

Advantages :
 Relatively high specificity
 Relatively low reaction temperatures
 Generally far more selective for a single product
 far more easily studied from chemical & mechanistic aspects
 far more active

Disadvantages :
o far more difficult for achieving product/catalyst separations

Catalytic steps in homogeneous reactions
 Most catalytic process can be built up from a small
number of different types of step
– Association / dissociation of a ligand
» requires labile complexes
– Insertion and elimination reactions
– Nucleophilic attack on a coordinated ligand
– Oxidation and reduction of a metal center
– Oxidative addition / reductive elimination
Wilkinson’s Catalyst:
RhCl(PPh3)3 was the first highly active homogeneous
hydrogenation catalyst and was discovered by Geoffrey
Wilkinson (Nobel prize winner for Ferrocene) in 1964.
Wilkinson’s Catalyst is a Rh(I) complex, Rh(PPh3)3Cl
containing three phosphine ligands and one chlorine.
As a result of the olefin insertion (hydrogen migration) we
obtain a Rh (III), 16e-, five coordinate species. A solvent
occupies the sixth coordination site to take it to a 18e- species.
Reductive elimination occurs to give the hydrogenated product
and the catalytically active species.
Bennett , IC , 1977 , 16 , 665
Olefin Hydrogenation using Wilkinson’s
Catalyst
The complex RhCl(PPh3)3 (also known as Wilkinson’s catalyst)
became the first highly active homogeneous hydrogenation
catalyst that compared in rates with heterogeneous
counterparts.
Wilkinson, J. Chem. Soc. (A) 1966, 1711

Hydrogenation mechanism
Steps:
(1) H2 addition,
(2) alkene addition,
(3) migratory insertion,
(4) reductive elimination of
the alkane, regeneration of
the catalyst
Halpern, Chem. Com. 1973, 629; J. Mol. Cat. 1976, 2, 65; Inorg. Chim. Acta. 1981, 50, 11

Wilkinson’s catalyst selectivity
The rate of hydrogenation depends on :
(a) presence of a functional group in the vicinity of the C=C bond
(b) degree of substitution of the C=C fragment

Wilkinson’s catalyst selectivity
Hydrogenation is stereoselective:
Rh preferentially binds to the least sterically hindered face of the olefin:

Wilkinson’s catalyst selectivity
Cis-disubstituted C=C react faster than trans-disubstituted C=C:
Schneider, JOC 1973, 38, 951
Cationic catalysts
Cationic catalysts are the most active homogeneous hydrogenation
catalysts developed so far:

Halpern’s mechanism of hydrogenation for
cationic Rh catalysts with bidentate phosphines
Halpern, Science 1982, 217, 401.
Steps:
(1) alkene addition,
(2) (2) H2 addition,
(3) migratory insertion,
(4) reductive elimination of
the alkane, regeneration of
the catalyst.
Asymmetric hydrogenation
A variety of bidentate chiral diphosphines have been synthesized and used
to make amino acids by hydrogenation of enamides:
Burk, Acc. Chem. Res 2000, 33, 363.
•
Synthesis of derivative of L-dihydroxyphenylalanine

Chiral hydrogenation catalysts
Catalysts similar to Wilkinson’s but using
chiral phosphine ligands have been used
for the asymmetric hydrogenation of small
molecules .
– Important in the fine chemicals
/pharmaceutical industry
Noles and Nyori received the 2001 chemistry Nobel prize for the
development of asymmetric hydrogenation catalysis

Intermediates in Noyori’s transfer hydrogenation
Knowles, JACS 1975, 97, 2567.

Lanthanide Hydrogenation Catalysts
Tobin Marks reported
the extraordinary
activity of (Cp*2LuH)2
for the hydrogenation of
alkenes and alkynes. The
monometallic complex
catalyzes the
hydrogenation of 1hexene with a TOF =
120,000 hr-1 at 1 atm
H2, 25ºC!! This is one of
the most active
hydrogenation catalysts
known.
Catalytically active species
With bidentate ligands, olefin coordination can precede oxidative addition of H2
(S = methanol, ethanol, acetone).
Halpern, JACS 1977, 99, 8055
Hydroformylation
The reaction of an alkene with carbon monoxide and hydrogen,
catalyzed by cobalt or rhodium salts to form an aldehyde is
called hydroformylation.
Hydroformylation was discovered by Otto Roelen in 1938.
Heck , JACS,1961,83,4023

Cobalt Phosphine modified catalyst
Cobalt Phosphine catalyst Mechanism
Monsanto Acetic acid Process
1960 basf 1966 monsanto
CATIVA Process
CATIVA Process
Wacker Process
This is one of the earliest industrial processes developed in
Germany for the conversion of ethylene into acetaldehyde.
Wacker process is more complex than the other catalytic processes
described above.
Heterogeneous
Catalysis
Heterogeneous catalysts dominate chemical and
petrochemical industry: ~ 95% of all chemical processes
use heterogenous catalysts.
Ziegler-Natta Catalysis for the
Polymerization of olefins
Polymers are large molecules with molecular weights in the
range of 104 to 106. These consist of small building units known
as monomers
For example polyethylene is made up of ethylene monomers
In all of these cases a single monomer is repeated several times
in the polymer chain. The number of repeating units determines
the molecular weight of the polymer.
The German chemist Karl Ziegler (1898-1973) discovered in
1953 that when TiCl3(s) and AlEt3 are combined together
they produced an extremely active heterogeneous catalyst for
the polymerization of ethylene at atmospheric pressure.
Giulio Natta (1903-1979), an Italian
chemist, extended the method to
other olefins like propylene and
developed variations of the Ziegler
catalyst based on his findings on the
mechanism of the polymerization
reaction.
The Ziegler-Natta catalyst family includes halides of titanium,
chromium, vanadium, and zirconium, typically activated by
alkyl aluminum compounds
Ziegler and Natta received the Nobel Prize in Chemistry for their
work in 1963.
There are typically three parts to most polymerizations:
Thanks for your attention