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

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 (NO22−), 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. The reduction of nitrite (NO2−) to nitric oxide (NO), relevant to the biogeochemical nitrogen cycle as well as radioactive waste, typically occurs at redox-active metal centres. Now, a Lewis acid-capped nitrite has been reduced to the nitrite dianion (NO22−), a nitrogen-centred radical that connects three redox levels in the global nitrogen cycle through NO2−, NO and N2O.