Heterogeneous catalysis of C–C bond formation: black art or organometallic science?

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Abstract
Methylene polymerisation, which occurs during the heterogeneously catalysed hydrogenation of CO (Fischer–Tropsch synthesis) is analysed as an example of a transition metal-promoted C–C coupling reaction. A new catalytic cycle has been proposed, the alkenyl mechanism, which is based on simple stoichiometric organometallic reactions. Results from experiments using isotopically 13C2-labelled probe molecules indicate that surface vinyl/alkenyl species are directly involved in the Fischer–Tropsch polymerisation over rhodium and ruthenium surfaces. The data indicate that C–C couplings are favoured if one of the participants has a metal to sp2 carbon bond, and that vinyl (or alkenyl) is a better combining ligand than phenyl, which in turn is better than methyl (alkyl). C–C coupling reactions which occur during heterogeneous catalysis are well modelled by soluble organometallic metal complexes, where the C–C coupling reactions take place at one or two metal atoms. Model 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 heterogeneous catalysis conditions, those that take place on isolated and characterised metal complexes, and those that occur under ultra-clean conditions on single planes of metal crystals.