Periodic Trends in M–Fe Interactions across an Isostructural and Isoelectronic Series of [MFe₃S₄] Clusters (M = V, Cr, Mo, W)

The Mo nitrogenase is more efficient for N2 reduction than the alternative nitrogenases for reasons that are not fully understood. A set of related hypotheses center on the Mo ion facilitating electron delocalization, which could aid in substrate activation and/or provide enhanced stability during turnover, among other possibilities. To understand how the incorporation of Mo or other ‘heterometals’ influences the electronic structures of Fe-S clusters, we prepared and characterized an isostructural and isoelectronic series of [MFe3S4] clusters (M = V, Cr, Mo, W) that includes the first example of a [CrFe3S4] cluster. Comparison of the clusters’ structural, spectroscopic, and computed properties reveals that the Mo- and W-containing clusters have greater M–Fe delocalization than those containing only 3d metals. Although the [CrFe3S4] cluster adopts the same ground spin state as the other clusters (S = 3/2), the smaller Cr atom engages in substantially attenuatedthough not fully obliteratedM–Fe delocalization. Moreover, electrochemical and Mössbauer spectroscopic studies reveal that the clusters with more substantial M–Fe delocalization have more electron-rich Fe sites. Such trends, if they apply to the nitrogenase cofactors, could account for the greater efficiency of the Mo nitrogenase in biological nitrogen fixation.