Methyl-Containing Iron–Sulfur Cluster with FeMoco Geometry

Biological production of NH₃ from N₂ in nitrogenases takes place at a complex octanuclear cluster displaying a bridging μ₆-carbide ligand. The carbide is derived from a methyl group of a radical SAM enzyme. To gain insight into the reactivity and electronic structure impact of alkyl ligands, we have accessed iron sulfur clusters with terminal and bridging methyl groups. Treatment of the tetranuclear halide bridged cluster [Tp*MoFe₃S₃Cl₄]^2- with excess methyl Grignard results in the formation of a high spin octanuclear MoFeS cluster with bridging CH₃ ligands. Spectroscopic characterization and computational analysis of this cluster, along with additional high spin MFeS (M = Mo or W) clusters with terminal alkyl ligands, suggest that MFeS clusters containing only Fe centers of oxidation states lower than Fe(III) achieve notably high spin states through ferromagnetic Fe-Fe interactions. Hydrogen and methane formation was demonstrated upon treatment with acid.