1. Organometallic Catalysts
Saber Askari
Presenter :
Dr.Mirzaaghayan
Advisor :
May 2012

2. 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

3. 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 .

4. 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

5. 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

6. History

7. Fundamental Reaction

8. Mechanistic Concept

9. Homogeneous Catalysis
Wilkinson`s Catalyst : Olefin Hydrogenation
Hydroformylation
Monsanto Acetic acid Process
Wacker Process
Heterogeneous Catalysis
Ziegler-Natta Catalysts

10. Homogeneous Catalysis
Homogenous catalysts are used when selectivity is critical and product-catalyst separation problems can be solved.

11. Advantages : Disadvantages : 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 :
far more difficult for achieving product/catalyst separations

12. 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

13. 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.

14. Bennett , IC , 1977 , 16 , 665

15. 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

16. 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

17. 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

18. Wilkinson’s catalyst selectivity
Hydrogenation is stereoselective:
Rh preferentially binds to the least sterically hindered face of the olefin:

19. Wilkinson’s catalyst selectivity
Cis-disubstituted C=C react faster than trans-disubstituted C=C:
Schneider, JOC 1973, 38, 951

20. Cationic catalysts
Cationic catalysts are the most active homogeneous hydrogenation catalysts developed so far:

22. Halpern’s mechanism of hydrogenation for cationic Rh catalysts with bidentate phosphines
Steps:
alkene addition,
(2) H2 addition,
(3) migratory insertion,
(4) reductive elimination of the alkane, regeneration of the catalyst.
Halpern, Science 1982, 217, 401.

23. 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.

24. Synthesis of derivative of L-dihydroxyphenylalanine

25. 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

26. Intermediates in Noyori’s transfer hydrogenation
Knowles, JACS 1975, 97, 2567.

27. 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 1-hexene with a TOF = 120,000 hr-1 at 1 atm H2, 25ºC!! This is one of the most active hydrogenation catalysts known.

28. Catalytically active species
With bidentate ligands, olefin coordination can precede oxidative addition of H2 (S = methanol, ethanol, acetone).
Halpern, JACS 1977, 99, 8055

29. 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.

30. Heck , JACS,1961,83,4023

31. Cobalt Phosphine modified catalyst

32. Cobalt Phosphine catalyst Mechanism

33. Monsanto Acetic acid Process
1960 basf 1966 monsanto

35. CATIVA Process

36. CATIVA Process

37. 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.

39. Heterogeneous Catalysis
Heterogeneous catalysts dominate chemical and petrochemical industry: ~ 95% of all chemical processes use heterogenous catalysts.

40. 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.

41. The German chemist Karl Ziegler ( ) 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 ( ), 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.

42. There are typically three parts to most polymerizations:

44. Thanks for your attention