Coordination and homologation of CO at Al(I): mechanism and chain growth, branching, isomerization and reduction

Homologation of carbon monoxide is central to the heterogeneous Fischer Tropsch Process for the production of hydrocarbon fuels. C–C bond formation has been modelled by homogeneous systems, with [CnOn]2- fragments (n = 2-6) formed by two-electron reduction being commonly encountered. Here we show that four- or six-electron reduction of CO can be accomplished by the use of anionic aluminium(I) ('aluminyl') compounds, to give both topologically linear and branched C4/C6 chains. We show that the mechanism for homologation relies on the highly electron-rich nature of the aluminyl reagent, and on an unusual mode of interaction of the CO molecule, which behaves primarily as a Z-type ligand in initial adduct formation. The formation of [C6O6]4- from [C4O4]4- shows a solution-phase CO homologation process that brings about chain branching via complete C-O bond cleavage, while comparison of the linear [C4O4]4- system with the [C4O4]6- congener formed under more reducing conditions, models the net conversion of C–O bonds to C–C bonds in the presence of additional reductant.