H2-Eliminating Reductive N2 Binding: a Dinickel-Bridging Bent N2 Radical Anion and its Redox-Triggered N2 Release

Nitrogenase accumulates reducing equivalents in hydrides and couples H2 elimination to the reductive binding of N2 at a diiron edge of its FeMo cofactor (FeMoco). Here, we describe that oxidation of a pyrazolato-based dinickel(II) dihydride complex K[L(Ni-H)2] (1K), either electrochemically or chemically using H+ or ferrocenium, triggers H2 elimination and binding of N2 in a constrained and extremely bent bridging mode in [LNi2(1,2-N2)] (3N2). Spectroscopic and computational evidence indicate that the electronic structure of 3N2 is best described as NiII–(N2•–)–NiII, with a rare 1e‒ reduced and significantly activated N2 substrate (n(NN) = 1894 cm–1). 3N2 is also formed upon 1e‒ oxidation of K[LNiI2] (2K) under N2. This is an unusual and counterintuitive scenario where the oxidation of a dinickel(II) dihydride, or of a dinickel(I) complex, induces the reductive activation of N2. Detailed (spectro)electrochemical studies confirm that N2 binding by the {LNi2} platform only occurs in the regime of the mixed valent NiIINiI species, while both oxidation and reduction induce the release of N2 from 3N2; the latter represents a redox-induced electron transfer (RIET) process where metal re-duction leads to N2•– oxidation due to intramolecular back electron transfer. These findings offer new perspectives for understanding the multi-e‒/H+ scenarios of N2 fixation via hydride intermediates akin to the FeMoco function, and for the development of synthetic platforms that avoid strongly reducing conditions for N2 activation.