Gauging Iron–Sulfur Cubane Reactivity from Covalency: Trends with Oxidation State
Liam Grunwald, Daniel F. Abbott, Victor Mougel
Abstract
High Resolution Image Download MS PowerPoint Slide We investigated 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 spectroscopic descriptors used to identify oxidation states in these systems, which we demonstrate by providing a calibration curve based on two analytic methods. 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 ± 0.4% per bond and hole, the Fe–S(μ 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 those of its redox congeners. We highlight the functional implications of these trends for the reactivity of iron–sulfur cubanes.