Reactions of aluminium(i) with transition metal carbonyls: scope, mechanism and selectivity of CO homologation.

Over the past few decades, numerous model systems have been discovered that create carbon-carbon bonds from CO. These reactions are of potential relevance to the Fischer-Tropsch process, a technology that converts syngas (H2/CO) into mixtures of hydrocarbons. In this paper, a homogeneous model system that constructs carbon chains from CO is reported. The system exploits the cooperative effect of a transition metal complex and main group reductant. An entire reaction sequence from C1 → C2 → C3 → C4 has been synthetically verified. The scope of reactivity is broad and includes a variety of transition metals (M = Cr, Mo, W, Mn, Re, Co), including those found in industrial heterogeneous Fischer-Tropsch catalysts. Variation of the transition metal fragment impacts the relative rate of the steps of chain growth, allowing isolation and structural characterisation of a rare C2 intermediate. The selectivity of carbon chain growth is also impacted by this variable; two distinct isomers of the C3 carbon chain were observed to form in different ratios with different transition metal reagents. Based on a combination of experiments (isotope labelling studies, study of intermediates) and calculations (DFT, NBO, ETS-NOCV) we propose a complete mechanism for chain growth that involves defined reactivity at both transition metal and main group centres.