Gauging Iron-Sulfur Cubane Reactivity from Covalency: Trends with Oxidation State

We investigated the room-temperature metal and ligand K-edge X-ray absorption (XAS) spectra of a complete redox series of cubane-type iron-sulfur clusters. The Fe K-edge position provides a qualitative, but convenient alternative to the traditional spectro¬scopic descriptors used to identify oxidation states in these systems, which we demonstrate by providing a calibration curve. Furthermore, high energy resolution fluorescence detected XAS (HERFD-XAS) at the S K-edge was used to measure Fe-S bond covalencies and record their variation with the average valence of the Fe atoms. While the Fe-S(thiolate) covalency evolves linearly, gaining 11% per bond and hole, the Fe-S(3) covalency evolves asystematically, reflecting changes in the magnetic exchange mechanism. A strong discontinuity manifested for superoxidation to the all-ferric state, distinguishing its electronic structure and its potential (bio)chemical role from its redox congeners. We highlight the functional implications of these trends on the reactivity of iron-sulfur cubanes.