Reductive Nitric Oxide Coupling at a Dinickel Core: Isolation of a Key cis-Hyponitrite Intermediate en route to N2 O Formation

Abstract

Reductive coupling of nitric oxide (NO) to give N₂ O 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-[N₂ O₂ ]^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 [N₂ O₂ ]^2- unit, which allows to access an isomer that is prone to N₂ O release. Protonation of 2 is now found to trigger rapid N₂ O 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 N₂ O formation at bimetallic sites.

Keywords: dinuclear complexes; hyponitrite complexes; nickel; nitric oxide; small molecule activation.