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Transition metal organometallic compounds & Catalysis French Chemist L. C. Cadet 1760 Which one is organometallic? As2Me4 dicacodyl Ni(CO)4 or NaCN ? Metal-carbon bond: a few from many? CH3 R3P Re CO CO PR3 R2 P P R2 O O C C Mo Cl Mo Cl C C O O R2 P P R2 Ph3P Ph3P Rh PPh3 Cl 9e - 1e + 8e = 16e therefore coordinately Organometallic Compound: Looking closer Ligand Name Bonding Type H Molecular Hydrogen: H2 M H Hydride H- Phosphine: PR3 Carbonyl: CO Alkyl , Aryl M-H M-PR3 M C O M-CR M-Ph Alkene M H2C CH2 18 en rule 1920 British Chemist Sidgwick Organic compounds – Octet rule Organometallic – 18 electron rule * 18 valance electron – inert gas configuration Counting the number of electrons To determine the electron count for a metal complex: Determine the oxidation state of the transition metal center(s) and the metal centers resulting d-electron count. To do this one must: a) note any overall charge on the metal complex b) know the charges of the ligands bound to the metal center c) know the number of electrons being donated to the metal center from each ligand 2) Add up the electron counts for the metal center and ligands Counting the number of electrons Ligand Name Molecular Hydrogen: H2 Bonding Type H M Formal Charge Electrons donated 0 2 -1 2 H Hydride H- M-H Halide X- M-X -1 2 Amine, phosphine, arsine: NR3, PR3, AsR3 M-NR3 M-PR3 0 2 0 2 -1 2 -1 2 Carbonyl: CO Alkyl , Aryl M C O M-CR M-Ph Alkene M H2C CH2 CO OC Fe OC Fe is 4s23d6 = 8e CO each Co is neutral so Fe0 CO each CO donates 2 e = 10e 8e + 10e = 18e coordinately saturated Ph3P Ph3P Rh PPh3 Rh is s1d8 = 9e Cl since Cl is -1, Rh is +1 (the complex is neutral) 9e - 1e + 8e = 16e 4 ligands x 2e each = 8e therefore coordinately unsaturated d3 21 d4 22 d5 d6 d7 d8 28 d 10s 1 24 25 Sc Ti V Cr Mn Fe Co Ni Cu Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper 39 40 41 42 43 44 45 46 47 Y Zr Nb Mo Tc Ru Rh Pd Ag Yttrium Zirconium Niobium Molybdenum Technetium Ruthenum Rhodium Palladium Silver 57 72 73 74 77 78 79 76 27 d 10 23 75 26 d9 29 La Hf Ta W Re Os Ir Pt Au Lanthanum Rhenium Platinum Hafnium Tantalum Early Transition Metals 16e and sub-16e configurations are common Coordination geometries higher than 6 Tungsten Osmium Iridium Gold Middle Transition Metals Late Transition Metals 18e configurations are common 16e and sub-16e configurations are common Coordination geometries of 6 are common Coordination geometries of 5 or lower Catalysis A+B Catalyst Catalyzed rxn proceeding through an intermediate C Ea A catalyst lowers the G activation barrier for a transformation, by introducing a new reaction pathway. Ea c a ta ly ze d Re a c ta nts G Produc ts Re a c tion Coordina te It does not change the thermodynamics!! Heterogeneous Homogeneous Catalysis : Why? Synthesis of chemicals… pharmaceutical, agricultural Catalytic converter … environmental Biological system – efficient catalyst Organometallic compounds, metals etc. How to select an efficient catalyst? Activity: related to rate of reaction (also called turnover) efficient catalyst: good activity 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, product value lifetime and product value Poisoning: decomposition of catalyst, adsorption of reactant/product Coordination compounds in catalysis Nobel Prizes 2005 Yves Chauvin,Robert H. Grubbs and Richard R. Schrock. 2001 KNOWLES, NOYORI, SHARPLESS 1973 WILKINSON 1963 ZIEGLER, NATTA 1918 HABER 1909 OSTWALD Hydrogenation of Unsaturated Hydrocarbons H H -CH=CH- + H2 -CH-CHNOBEL : 2001 The most common catalyst Wilkinson’s Catalyst, [RhCl(PPh3)3] Wilkinson’s Catalyst (WC) PPh3 Ph3P Rh Cl PPh3 Chlorotris(triphenylphosphine)rhodium(I) square planar d8 configuration Geoffrey Wilkinson • Born July 14, 1921, Yorkshire, England • 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. Organometallic compounds prepared by Wilkinson in display at Harvard Univ. Synthesis of WC PPh3 Ph3P RhCl3 + 3 H2O + EtOH + Rh >4 PPh3 78 oC Cl Commercially available PPh3 Ph3PO 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 -occurs when a complex behaves simultaneously as a Lewis base and a Lewis acid Metal must possess a non-bonding electron pair Coordinatively unsaturated Oxidation of metal by two units – Mn to Mn+2 Oxidative addition… X-Y + X-Y LnMn Ph3P Cl Ir Ph3P CO LnMn + H2 X LnMn+2 Y H Ph3P Cl Ir Ph3P H CO (c) Insertion or migration Migration of alkyl and hydride ligands L+ R L M CO M C R O H CH 2 M CH2 M CH2CH3 S LnM S CR O L' R L' LnM LnM C CR O O R C LnM O LnM CR O (d) Nucleophilic attack R C R 2+ L3Pd H R L3Pd OH2 C C OH R R C H R O L5M + - CO + OH L5M C OH L5M H + CO2 - (d) Reductive elimination Involves decrease in the oxidation and coordination number Ph3P Ph3P Cl Rh Ph3P Me COR CO Ph3P Cl Rh + CO RCOMe Hydrogenation of Unsaturated Hydrocarbons H H -CH=CH- + H2 -CH-CHG0 = -101 kJ/mol WC in alkene Hydrogenation: Catalytic Steps (1) Oxidative addition H Ph3P Rh Ph3P PPh3 Cl Ph3P + H2 Ph3P Rh3+ (2) Ligand Dissociation H Rh Ph3P PPh3 H Cl H Cl Rh+1 Ph3P Rh PPh3 H Ph3P Rh Ph3P Cl H + PPh3 WC in alkene Hydrogenation: Catalytic Steps (3) Ligand Association H Ph3P Rh H + Ph3P CH2 CH2 Cl Ph3P Ph3P Ph3P Cl H CH H2 H Rh CH2 Rh H PPh3 H2CCl CH2 (4) Migration/Insertion Ph3P Ph3P H H Rh Cl Rh Ph3P H CH H 2 CH2 PPh 3 H H2CCl CH2 Ph3P CH2 CH2 Rh Ph3P Cl H WC in alkene Hydrogenation: Catalytic Steps (5) Ligand association H CH2 H CH2 CH2 CH2 Ph3P Rh Ph3P Cl H + PPh3 Ph3P Rh Ph3P PPh3 H Cl WC in alkene Hydrogenation: Catalytic Steps (6) Reductive elimination H CH2 Ph3P CH2 Rh Ph3P PPh3 H Cl Ph3P Ph3P Rh PPh3 Cl + CH3 CH3 (note: regeneration of the catalyst) WC IN A C T I O N WC in alkene Hydrogenation: Additional Notes Rate of the reaction decreases as the alkyl substitution increases Highly sensitive to the nature of the phosphine ligand Analogous complexes with alkylphosphine ligands are inactive Highly selective for C=C over C=O Applications * Laboratory scale organic synthesis * Production of fine chemicals Alkene Hydrogenation & Chirality & Nobel Chiral phosphine ligands have been developed to synthesize optically active products. Synthesis of L-DOPA (Used in the treatment of Parkinson’s diseases) Synthetic route was developed by Knowles & co-workers at Monsanto Dr. William S. Knowles received Nobel prize in chemistry 2001 along with other two scientists. Alkene Hydrogenation, Chirality & Nobel This reaction, developed by Knowles, Vineyard, and Sabacky, was used at Monsanto as a commercial route to the Parkinson's drug L-DOPA. Non-superimposable mirror image Enantiomeric excess = (moles of major enantiomer - moles of other enantiomer / Total moles of both enantiomers) 100 phenylanisylmethylphosphine (PAMP) Dimeric product is DiPAMP Additional notes For interested students (a) Ligand coordination and dissociation Facile coordination of the reactant and facile loss of products. Coordinatively unsaturated - 16-electron complexes (b) Oxidative addition -occurs when a complex behaves simultaneously as a Lewis base and a Lewis acid Metal must possess a non-bonding electron pair Coordinatively unsaturated Oxidation of metal by two units – Mn to Mn+2 Oxidative addition… X-Y + X-Y LnMn Ph3P Cl Ir Ph3P CO LnMn + H2 X LnMn+2 Y H Ph3P Cl Ir Ph3P H CO (c) Insertion or migration Migration of alkyl and hydride ligands L+ R L M CO M C O S LnM S CR O L' R L' LnM LnM C CR O O R C LnM R O LnM CR O H CH 2 M CH2 M CH2CH3 (d) Nucleophilic attack R C R 2+ L3Pd H R L3Pd OH2 C C OH R R C H R O L5M + - CO + OH L5M C OH L5M H + CO2 - (e) Reductive elimination Involves decrease in the oxidation and coordination number Ph3P Ph3P Cl Rh Ph3P Me COR CO Ph3P Cl Rh + CO RCOMe
