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Organometallic Catalysts. Saber Askari. Presenter :. Dr.Mirzaaghayan. Advisor :. May 2012. Contents :. The basis for catalysis. Catalytic Cycle. History. Mechanistic Concept. Homogeneous Catalysis. Wilkinson’s Catalyst. Asymmetric hydrogenation. Hydroformylation.

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organometallic catalysts

Organometallic Catalysts

Saber Askari

Presenter :

Dr.Mirzaaghayan

Advisor :

May 2012

contents
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
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
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
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

homogeneous catalysis
Homogeneous Catalysis

Wilkinson`s Catalyst : Olefin Hydrogenation

Hydroformylation

Monsanto Acetic acid Process

Wacker Process

Heterogeneous Catalysis

Ziegler-Natta Catalysts

slide10

Homogeneous Catalysis

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

slide11

Relatively high specificity

  • Advantages :
  • 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
slide12

Most catalytic process can be built up from a small number of different types of step

  • Catalytic steps in homogeneous reactions

– 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
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.

olefin hydrogenation using wilkinson s catalyst
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

slide16

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

slide17

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
slide18

Wilkinson’s catalyst selectivity

Hydrogenation is stereoselective:

Rh preferentially binds to the least sterically hindered face of the olefin:

slide19

Wilkinson’s catalyst selectivity

Cis-disubstituted C=C react faster than trans-disubstituted C=C:

Schneider, JOC 1973, 38, 951

cationic catalysts
Cationic catalysts

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

slide22

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.

asymmetric hydrogenation
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.

slide25

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

slide27

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.

catalytically active species
Catalytically active species

With bidentate ligands, olefin coordination can precede oxidative addition of H2 (S = methanol, ethanol, acetone).

Halpern, JACS 1977, 99, 8055

hydroformylation
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.

monsanto acetic acid process
Monsanto Acetic acid Process

1960 basf 1966 monsanto

wacker 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 Catalysis

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

ziegler natta catalysis for the polymerization of olefins
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.

slide41

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.

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