Redox Non-Innocent Behavior of a Terminal Iridium Hydrazido(2-) Triple Bond

Electronic Structures of an [Fe(NNR2)]+/0/- Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation

The intermediacy of metal-NNH₂ complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N₂) fixation. In this context, we have shown that triphosphine-supported Fe(N₂) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH₂) 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(NNMe₂) 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 "NNMe₂" moiety. Specifically, although two closed-shell configurations of the "NNR₂" 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 [NNR₂]^•- 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 N₂ fixation activity.

A Highly-Reduced Cobalt Terminal Carbyne: Divergent Metal- and α-Carbon-Centered Reactivity

Reported here is the isolation of a dianionic cobalt terminal carbyne derived from chemical reduction of an encumbering isocyanide ligand. Crystallographic, spectroscopic and computational data reveal that this carbyne possesses a low-valent cobalt center with an extensively filled d-orbital manifold. This electronic character renders the cobalt center the primary site of nucleophilicity upon reaction with protic substrates and silyl electrophiles. However, reactions with internal alkynes result in [2+2] cycloaddition with the carbyne carbon to form a new C-C bond.