Oxidatively Induced 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 di-iron edge of its FeMo cofactor (FeMoco). Here, we describe that oxidation of a pyrazolato-based dinickel (II) dihydride complex KL (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 (μ1, 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 (ṽ̃NN = 1894 cm-1). 3N2 is also formed upon 1e- oxidation of KLNi2I (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 and DFT calculations confirm that N2 binding by the LNi2 platform occurs only 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 reduction 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 inspired by the FeMoco function, and for the development of synthetic platforms that avoid strongly reducing conditions for N2 activation.