Disentangling Cooperative Electron Correlation and Structural Effects in Bio-inspired [Fe4S4]+ Stereo-valence Isomerism and π-Acid Activation

Fe-S clusters are widely-conserved metallocofactors that serve as the cornerstone to many of nature's most essential and kinetically challenging biocatalytic reactions, including nitrogen fixation and CO2 reduction, with their diverse functional and redox capabilities enabled by the facile perturbation and redistribution of cluster electron density in response to both short-range (e.g., ligation) and long-range (e.g., non-covalent) interactions. Despite recent advancements in the structural characterization of these systems, concurrent computational efforts continue to face significant challenges owing to their strongly correlated electronic structure. Here, we employ a computationally tractable, multi-reference methodology that captures both strong and dynamic correlation effects to simulate a series of electronically variable arylisocyanide-ligated [Fe4S4]+ clusters, inspired by an experimentally characterized structural series, to gain a more comprehensive understanding of Fe-S reactivity. We elucidate the effects of electron correlation on the electronic and physical structures of [Fe4S4]+ clusters, and their interplay, as well as their tunability via systematic ligand modification, which gives rise to experimentally observed stereo-valence isomerism.