Nitrogen Subvalency in a Four-Member Redox Series of Iron-Sulfur Cluster Imido Complexes

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.