Molybdenum doping enhances backbonding at neighboring iron centers in iron-sulfur clusters

Nitrogenases employ Fe-S co-factors to reduce the weakly π-acidic ligand N₂ to NH₃. The Mo nitrogenase is more efficient than the V and Fe-only nitrogenases, but how the Mo ion enhances N₂ reduction is not fully understood. In this study, we undertake a comparative structural and spectroscopic analysis of synthetic [MoFe₃S₄] and [Fe₄S₄] clusters that ultimately reveals how the presence of Mo impacts the ability of Fe-S clusters to interact with π-acids. Specifically, we find that compared with the Fe centers in the [Fe₄S₄] clusters, those in the [MoFe₃S₄] clusters are more adept at backbonding to their weakly π-acidic N-heterocyclic carbene ligands. Such differences can be attributed to enhanced metal-sulfide mixing and Mo-Fe delocalization in the [MoFe₃S₄] clusters, whereby the diffuse Mo 4d orbitals more efficiently transfer electron density from the sulfides to the Fe ions and, ultimately, to Fe-bound π-acids.