Activation\nof Strong π–Acids at [Fe<sub>4</sub>S<sub>4</sub>]<sup>+</sup> Clusters Enabled by a Noncanonical\nElectronic Structure

Although Fe–S clusters are\nprivileged metallocofactors for\nthe reduction of N<sub>2</sub>, CO, and other π-acidic substrates,\ntheir constituent metal ionshigh-spin Fe<sup>2+</sup> and\nFe<sup>3+</sup>are typically not amenable to binding and activating\nstrong π-acids. Here, we demonstrate that [Fe<sub>4</sub>S<sub>4</sub>]<sup>+</sup> clusters can overcome this limitation by adopting\na noncanonical electronic structure. Specifically, we report the synthesis\nand characterization of a series of 3:1 site-differentiated [Fe<sub>4</sub>S<sub>4</sub>]<sup>+</sup> clusters in which the unique Fe\nsite is bound by one of 10 electronically variable arylisocyanide\nligands. Rather than being continuously tuned as a function of the\narylisocyanides’ electronic properties (e.g., as quantified\nby linear free energy relationships), the structures of the clusters\nare divided into two groups: (i) those with moderately π-acidic\nisocyanides, which adopt a “typical” structure characterized\nby standard bond metrics and geometric distortions from tetrahedral\nsymmetry, and (ii) those with more strongly π-acidic isocyanides,\nwhich adopt a “contracted” structure with an unusually\nsymmetric geometry and a compressed cluster core. Computational studies\nrevealed that although the “typical” structure has a\ncanonical electronic structure, the “contracted” structure\nhas a noncanonical arrangement of spin density, with a full complement\nof π-backbonding electrons and more substantial Fe–Fe\ndelocalization. These features of the “contracted” structure\nenable substantial CN bond weakening of the strongest π-acceptors\nin the series. More generally, the experimental characterization of\nthe “contracted” electronic isomer suggests that other\nnoncanonical electronic structures of Fe–S clusters remain\nto be discovered.