Reductive Nitric Oxide Coupling at a Dinickel Core: Isolation of a Key cis‐Hyponitrite Intermediate en route to N2O Formation

Emulated reaction trajectory: Two NO molecules are reductively coupled at a dinickel(II) dihydride complex that serves as a dinickel(I) synthon via substrate induced H₂ release. The resulting cis‐hyponitrite adopts an unusual N,O‐bridging binding mode in solid state, but facile rotational fluxionality within the bimetallic cleft allows to access an isomer that is prone to proton‐triggered N₂O formation.

Abstract

Reductive coupling of nitric oxide (NO) to give N₂O is an important reaction in the global nitrogen cycle. Here, a dinickel(II) dihydride complex 1 that releases H₂ upon substrate binding and serves as a masked dinickel(I) scaffold is shown to reductively couple two molecules of NO within the bimetallic cleft. The resulting hyponitrite complex 2 features an unprecedented cis‐[N₂O₂]²⁻ binding mode that has been computationally proposed as a key intermediate in flavodiiron nitric oxide reductases (FNORs). NMR and DFT evidence indicate facile rotational fluxionality of the [N₂O₂]²⁻ unit, which allows to access an isomer that is prone to N₂O release. Protonation of 2 is now found to trigger rapid N₂O evolution and formation of a hydroxido bridged complex, reminiscent of FNOR reactivity. This work provides fundamental insight into the biologically relevant reductive coupling of two NO molecules and the subsequent trajectory towards N₂O formation at bimetallic sites.

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