Homogeneous catalysis AB Catalyst C Heterogeneous Homogeneous A
Homogeneous catalysis A+B Catalyst C Heterogeneous Homogeneous A catalyst lower the activation barrier for a transformation, by introducing a new reaction pathway. It does not change thermodynamics!!
Important catalyst properties * Activity: A reasonable rate of reaction is needed Turnover frequency (N) N = /[Q] Large turnover frequency – efficient catalyst * Selectivity: Byproducts should be minimized * Lifetime: It is costly to replace the catalyst frequently * Cost: The acceptable cost depends upon the catalyst lifetime and product value
Transition metal organometallic compounds Metal-carbon bond 18 en rule Organic compounds – Octet rule Organometallic – 18 electron rule * 18 valance electron – inert gas configuration Oxidation state method Neutral atom method
Ligand Name Bonding Type Molecular Hydrogen: H 2 Hydride H- M-H Formal Charge Electrons donated 0 2 -1 2 Halide X- M-X -1 2 Amine, phosphine, arsine: NR 3, PR 3, As. R 3 M-NR 3 M-PR 3 0 2 -1 2 Carbonyl: C O Alkyl , Aryl Alkene M-CR M-Ph
16 electron complexes Group 9 and 10 Exception to 16/18 electron rule V(CO)6 17 electrons W(CH 3) 12 electrons 9+1+2+2+2 = 16
Catalytic steps (a) Ligand coordination and dissociation Facile coordination of the reactant and facile loss of products. Coordinatively unsaturated - 16 -electron complexes (b) Oxidative addition Non-bonding electron pair in the metal Coordinatively unsaturated Oxidation of metal by two units – Mn to Mn+2
Oxidative addition…
(c) Insertion or migration Migration of alkyl and hydride ligands
(d) Nucleophilic attack
(e) Reductive elimination Involves decrease in the oxidation and coordination number
Wilkinson’s Catalyst Tris(triphenylphosphine)rhodium(I) chloride square planar 16 electron species d 8 configuration
Geoffrey Wilkinson • Born July 14, 1921, Todmorden, Yorkshire, England • Received Ph. D from Cal Berkeley studying with Glenn Seaborg • 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.
Synthesis commercially available – Sigma Aldrich, Acros, etc.
(1) Oxidative addition 16 en Rh+1 18 en Rh 3+ (2) Ligand Dissociation
(3) Ligand Association (4) Insertion
(5) Ligand association
(6) Reductive elimination
WC IN A C T I O N
Highly sensitive to the nature of the phosphine ligand Analogous complexes with alkylphosphine ligands are inactive Chiral phosphine ligands have been developed to synthesize optically active products.
Applications * Laboratory scale organic synthesis * Production of fine chemicals * Synthesis of L-DOPA Used for the treatment of Parkinson’s diseases Synthetic route was developed by Knowles and co-workers at Monsanto Dr. William S. Knowles received Nobel prize in chemistry 2001 along with other two scientists.
This reaction, developed by Knowles, Vineyard, and Sabacky, was used at Monsanto as a commercial route to the Parkinson's drug LDOPA.
Metal Carbonyl Complexes CO as a ligand strong donor, strong π-acceptor strong trans effect small steric effect CO is an inert molecule that becomes activated by complexation to metals Homoleptic Heteroleptic
Group Formula Valence en 6 Cr(CO)6 7 Mn 2(CO)10 7+10+1 = 18 8 Fe(CO)5 8+10 = 18 9 Co 2(CO)8 9+8+1= 18 10 Ni(CO)4 10+8 = 18 6 +12 = 18 Structure
O C M orbital serves as a very weak donor to a metal atom
Metal should be in low oxidation state ie en rich
Synthesis Direct combination 30 o. C and 1 atm CO Reductive carbonylation Al. Cl 3 and Benzene 250 o. C, 350 atm CO
Substituted carbonyls Oxidative Addition Concomitant oxidation of the metal and addition of new ligand(s) Fe(CO)5 + Br 2 Fe 0 Fe 2+ Fe(CO)4 Br 2 + CO
Reductive Carbonylation Metal halide and CO
Properties Reduction to form metal carbonylates Substitution
Oxidation Oxidative cleavage of M-M bond Protonation …more negative the anion, higher its Bronsted basicity … use in the synthesis of different organometallic
Application Hydroformylation
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