HETEROGENEOUS CATALYSIS OF C-C BOND FORMATION - BLACK ART OR ORGANOMETALLIC SCIENCE

Methylene polymerisation, which occurs during the heterogeneously cata lysed hydrogenation of CO (Fischer-Tropsch synthesis) is analysed as a n example of a transition metal-promoted C-C coupling reaction. A new catalytic cycle has been proposed, the alkenyl mechanism, which is bas ed on simple stoichiometric organometallic reactions. Results from exp eriments using isotopically C-13(2)-labelled probe molecules indicate that surface vinyl/alkenyl species are directly involved in the Fische r-Tropsch polymerisation over rhodium and ruthenium surfaces. The data indicate that C-C couplings are favoured if one of the participants h as a metal to sp(2) carbon bond, and that vinyl (or alkenyl) is a bett er combining ligand than phenyl, which in turn is better than methyl ( alkyl). C-C coupling reactions which occur during heterogeneous cataly sis are well modelled by soluble organometallic metal complexes, where the C-C coupling reactions take place at one or two metal atoms. Mode l studies further suggest that no step of the Fischer-Tropsch process requires the participation of cluster arrays. Bridges may now be built between the reactions which occur on metal surfaces under heterogeneo us catalysis conditions, those that take place on isolated and charact erised metal complexes, and those that occur under ultra-clean conditi ons on single planes of metal crystals.