The Influence of Heterometals on Dinitrogen Binding at Synthetic Iron-Sulfur Clusters

Of the three nitrogenase isozymes (Mo, V, and Fe-only), the Mo nitrogenase is the most efficient at catalyzing N₂ reduction. To better understand the chemical basis for this difference in reactivity, we herein study how the identity of M in an isostructural series of synthetic [MFe₃S₄] clusters (M = V, Cr, Fe, Mo, or W) impacts the ability of neighboring Fe sites to bind N₂. We find that only for M = Mo or W does reduction of the chloride-bound clusters result in formation of an N₂ complex; those clusters with M = V, Cr, or Fe instead undergo bimolecular reactivity, with no evidence for N₂ binding. We argue that the divergent N₂ chemistry between clusters with 4d or 5d "heterometals" and those comprised only of 3d ions can be ascribed to electronic structure differences, and that such differences may underlie the higher efficiency of the Mo nitrogenase.