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

Abstract

Reduction of nitrite anions (NO₂^-) to nitric oxide (NO), nitrous oxide (N₂O) and ultimately dinitrogen (N₂) 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 (NO₂^2-), 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 (N₂O) and nitrite (NO₂^-). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO₂^- to NO.