Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction

[Image: see text] Homologation of carbon monoxide is central to the heterogeneous Fischer–Tropsch process for the production of hydrocarbon fuels. C–C bond formation has been modeled by homogeneous systems, with [C(n)O(n)](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 aluminum(I) (“aluminyl”) compounds to give both topologically linear and branched C(4)/C(6) 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 [C(6)O(6)](4–) from [C(4)O(4)](4–) shows for the first time a solution-phase CO homologation process that brings about chain branching via complete C–O bond cleavage, while a comparison of the linear [C(4)O(4)](4–) system with the [C(4)O(4)](6–) congener formed under more reducing conditions models the net conversion of C–O bonds to C–C bonds in the presence of additional reductants.