Imido complexes of high-spin 3d metals can feature N subvalency, wherein configurations with hole character at N contribute significantly to the ground state electronic structure. Reported herein are the synthesis and characterization of a series of Fe–S cluster terminal imido complexes isolated in four charge states, [(IMes)3Fe4S4(NAr)]n (IMes, 1,3-dimesitylimidazol-2-ylidene; Ar = 2,4,6-tri-tert-butylphenyl; n = 1–, 0, 1+, 2+), in which the weak, trigonal ligand field of the sulfides enforces high-spin configurations at the Fe sites. It is shown that whereas the ground states of the anionic and neutral members of the series feature dominant imide ([NAr]2–) character, the mono- and dicationic clusters have significant imidyl ([NAr]•–) and/or triplet nitrene (3NAr) character. This was demonstrated experimentally using variable-temperature 1H NMR spectroscopy to reveal the spin topology of the [Fe(NAr)] unit across the series. Additional structural, spectroscopic, and computational analysis revealed the extent to which electron density changes occur at the imido-bound Fe versus the peripheral Fe sites. These results establish the preference for high-spin Fe-imido sites in Fe–S clusters and the possibility for such sites to have multiconfigurational electronic structures with imide, imidyl, and/or nitrene character.

We herein present a comprehensive analysis of the oxidation-state-dependence of the 57Fe partial vibrational density of states (PVDOS), measured using nuclear resonance vibrational spectroscopy (NRVS)...

Elucidating the nature of the metallocofactors in nitrogenase enzymes, and preparing synthetic analogues of these clusters, is a classic target for bioinorganic chemists. Now the transformation of [Fe...