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

03 June 2025, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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.

Keywords

nickel complexes
dinuclear complexes
hydride complexes
dinitrogen complexes
electronic structure

Supplementary materials

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Description
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Title
Supporting Information for "H2-Eliminating Reductive N2 Binding: a Dinickel-Bridging Bent N2 Radical Anion and its Redox-Triggered N2 Release"
Description
Synthetic procedures and materials; NMR, IR, Raman, EPR and ESI mass spectra; additional (spectro)electrochemical data; detailed crystallographic information; details about DFT calcula-tions
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