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Published March 4, 2019 | Accepted Version + Supplemental Material
Journal Article Open

Electronic Structures of an [Fe(NNR_2)]^(+/0/–) Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation

Abstract

The intermediacy of metal–NNH_2 complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N_2) fixation. In this context, we have shown that triphosphine-supported Fe(N_2) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH_2) 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_2) 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_2" moiety. Specifically, although two closed-shell configurations of the "NNR_2" 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_2]^•–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_2 fixation activity.

Additional Information

© 2019 American Chemical Society. Received: January 14, 2019; Published: February 14, 2019. This work was supported by the Resnick Sustainability Institute at Caltech (a graduate fellowship to N.B.T.), as well as the NIH (Grant GM 070757). The EPR facility at the California Institute of Technology is supported by the NSF via its MRI program (Grant NSF-1531940) and the Dow Next Generation Educator Fund. The authors declare no competing financial interest.

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Accepted Version - nihms-1037227.pdf

Supplemental Material - ic9b00133_si_001.pdf

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