Lewis acid-assisted reduction of nitrite to nitric and nitrous oxides via the elusive nitrite radical dianion.

Reduction of nitrite anions (NO ₂ ^- ) to nitric oxide (NO), nitrous oxide (N ₂ O) and ultimately dinitrogen (N ₂ ) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (-0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO ₂ ^2- ), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N ₂ O) and nitrite (NO ₂ ^- ). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO ₂ ^- to NO.
(© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)