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 N 2 to NH 3 . The Mo nitrogenase is more efficient than the V and Fe-only nitrogenases, but how the Mo ion enhances N 2 reduction is not fully understood. In this study, we undertake a comparative structural and spectroscopic analysis of synthetic [MoFe 3 S 4 ] and [Fe 4 S 4 ] 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 [Fe 4 S 4 ] clusters, those in the [MoFe 3 S 4 ] 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 [MoFe 3 S 4 ] 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.