Electronic Structures of an [Fe(NNR)] Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation.

The intermediacy of metal-NNH complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N) fixation. In this context, we have shown that triphosphine-supported Fe(N) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH) complexes over an array of charge and oxidation states. Upon exposure to further H/e equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N-H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the "NNMe" moiety. Specifically, although two closed-shell configurations of the "NNR" ligand have been commonly considered in the literature-isodiazene and hydrazido(2-)-we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR] ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N fixation activity.