Activation of Strong π-Acids at [Fe₄S₄]⁺ Clusters Enabled by a Noncanonical Electronic Structure

Although Fe-S clusters are privileged metallocofactors for the reduction of N₂, CO, and other π-acidic substrates, their constituent metal ions─high-spin Fe²⁺ and Fe³⁺─are typically not amenable to binding and activating strong π-acids. Here, we demonstrate that [Fe₄S₄]⁺ clusters can overcome this limitation by adopting a noncanonical electronic structure. Specifically, we report the synthesis and characterization of a series of 3:1 site-differentiated [Fe₄S₄]⁺ clusters in which the unique Fe site is bound by one of 10 electronically variable arylisocyanide ligands. Rather than being continuously tuned as a function of the arylisocyanides' electronic properties (e.g., as quantified by linear free energy relationships), the structures of the clusters are divided into two groups: (i) those with moderately π-acidic isocyanides, which adopt a "typical" structure characterized by standard bond metrics and geometric distortions from tetrahedral symmetry, and (ii) those with more strongly π-acidic isocyanides, which adopt a "contracted" structure with an unusually symmetric geometry and a compressed cluster core. Computational studies revealed that although the "typical" structure has a canonical electronic structure, the "contracted" structure has a noncanonical arrangement of spin density, with a full complement of π-backbonding electrons and more substantial Fe-Fe delocalization. These features of the "contracted" structure enable substantial C≡N bond weakening of the strongest π-acceptors in the series. More generally, the experimental characterization of the "contracted" electronic isomer suggests that other noncanonical electronic structures of Fe-S clusters remain to be discovered.