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

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 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 indicates 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.