Oxidatively Induced Reductive N₂ Binding: A Dinickel-Bridging Bent N₂ Radical Anion and Its Redox-Triggered N₂...

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 K[L(Ni-H)₂] (1^K), 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(μ_1,2-N₂)] (3^N2). Spectroscopic and computational evidence indicate that the electronic structure of 3^N2 is best described as Ni^II-(N₂^•-)-Ni^II, with a rare 1e^- reduced and significantly activated N₂ substrate (ṽ_̃NN = 1894 cm^-1). 3^N2 is also formed upon 1e^- oxidation of K[LNi₂^I] (2^K) 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 Ni^IINi^I species, while both oxidation and reduction induce the release of N₂ from 3^N2; 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.