Applications of organometallic compounds

Use of Organometallic Compounds as Catalysts in Chemistry and Industries

1. Catalysis by ORGANOMETALLIC COMPOUNDS Paper II – Organometallics & Main Group Chemistry Jaiswal Priyanka M.Sc. II (Inorganic) Semester IV Mithibai College (2015-16)

2. Catalysis by Organometallic Compounds A catalyst is a substance that increases the rate of a reaction but it is not itself consumed. Catalysis plays a vital role in the production of fuels, commodity chemicals, fine chemicals and pharmaceuticals as well as providing the means for experimental safeguards all over the world. More than 60% of all chemical products and 90% of all chemical processes are based on catalysis. Most catalysts used in industrial and research laboratories are inorganic (often organometallic) compounds .

3. Catalysis (contd)….. A whole new technology appeared based on organometallic catalysis in olefin polymerization . Nobel prizes for chemistry have been awarded to Zieglar and Natta (1963), Fischer and Wilkinson (1973) for their discoveries in Organometallic chemistry and homogeneous catalysis. More recently, in 2005, Chauvin, Schrock, and Grubbs were awarded Nobel Prize for developing organometallic catalysts for olefin methathesis. Catalysis can be of two types Heterogeneous and Homogeneous.

4. Types of Catalysts Homogeneous : Catalyst and Reactant in the same phase e.g .) Organometallic Compounds Heterogeneous : Catalyst and Reactant in different phases e.g.) Metal Surface Cativa Process Catalytic decomposition of formic acid on noble metals

5. Homogenous versus Heterogenous Catalysis Homogenous (Monophase) Heterogenous (Biphase/Multiphase) Form Soluble metal complexes Metals, usually supported or oxide Temperature Low (<250 O C) High (250-500 O C) Activity (of metal content) High Variable Selectivity High Variable Conditions of Reaction Mild Harsh Lifetime of Catalysts Variable Long Sensitivity of Deactivation Low High Problems due to Diffusion None Difficult to solve Recycling of Catalysts Usually difficult Can easily be done Mechanism Realistic models do exist Not obvious

6. Heterogeneous Catalysis Desorption of Products (STEP 4) There is a re-arrangement of electrons and the products are then released from the active sites Adsorption of Reactants (STEP 1) Incoming species lands on an active site and forms bonds with the catalyst. It may use some of the bonding electrons in the molecules thus weakening them and making a subsequent reaction easier. Reaction of adsorbed intermediates (STEPS 2 and 3) Adsorbed gases may be held on the surface in just the right orientation for a reaction to occur. This increases the chances of favourable collisions taking place. Adsorptive property of a catalyst = Catalytic activity

7. Homogeneous Catalysis Catalytic steps in homogeneous reactions Association / dissociation of a ligand (requires labile complexes) Catalysis steps often requires facile coordination of reactants to metal ions and equally facile loss of products. Both processes must occur with low Activation Energy. For this purpose, highly labile complexes are needed as they are co-ordinatively unsaturated (having an open coordination site or being weakly coordinated ) e.g. Square-planar 16-electron complexes are co-ordinatively unsaturated. ML4 complexes of Pd(II), Pt(II) and Rh(I ) - [RhCl(PPh 3 ) 3 ] – hydrogenation catalyst Insertion and elimination reactions The migration of alkyl and hydride ligands to unsaturated ligands (Migratory insertion )

8. Nucleophilic attack on a coordinated ligand The coordination of ligands (CO, alkenes) to metals in positive oxidation states results in the activation of coordinated C atoms towards attack by nucleophile . e.g . Stereochemical evidence indicates that the reaction occurs by direct attack on the most highly substituted C atom of the coordinated olefin. Oxidative addition / reductive elimination Oxidative addition of a molecule AX to a complex brings about dissociation of the A – X bond and coordination of the two fragments. Reductive elimination is the reverse of oxidative addition and often follows it in a catalytic cycle

9. 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.

10. Hydrogenation mechanism Steps: ( 1 ) H 2 addition, ( 2 ) alkene addition, ( 3 ) migratory insertion, ( 4 ) reductive elimination of the alkane, regeneration of the catalyst

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

12. Asymmetric hydrogenation A variety of bidentate chiral diphosphines have been synthesized and used to make amino acids by hydrogenation of enamides: One area where homogeneous catalysis rules is asymmetric hydrogenation. This involves the use of chiral catalyst and a prochiral alkene substrate that generates a chiral carbon center in hydrogenation.

13. Synthesis of derivative of L-dihydroxyphenylalanine

14. 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 Noyori received the 2001 chemistry Nobel prize for the development of asymmetric hydrogenation catalysis.

15. Intermediates in Noyori’s transfer hydrogenation

16. Lanthanide Hydrogenation Catalysts Tobin Marks reported the extraordinary activity of ( Cp 2 LuH) 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 H 2 , 25ºC !! This is one of the most active hydrogenation catalysts known.

17. 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.

18. 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.

19. There are typically three parts to most polymerizations:

20. References Organometallic Chemistry and Catalysis, Didier Astruc Organometallic Chemistry , R.C. Mehrotra Inorganic Chemistry: Principles of Structure and Reactivity, James E. Huheey, Ellen A. Keiter, Richard L. Keiter, Okhil K. Medhi Reaction Mechanisms of Inorganic and Organometallic Systems, Robert B. Jordan; Professor of Chemistry, University of Alberta http ://www.chem.iitb.ac.in/~rmv/ch102/ic6.pdf