Acid-induced Conversion of Nitrite to Nitric Oxide at the Copper(ii) Center: a New Catalytic Pathway

Acid-induced reduction of nitrites (NO2-) to nitric oxide (NO) at Cu/Fe centers is one of the key steps in the nitrogen cycle and serves as an essential path to NO generation. In this study, we report the acid-catalysed conversion of NO2- to NO at the Cu-II centers in Cu-II-nitrito complexes, [(Me2BPMEN)Cu-II(NO2-)](+) (1) and [(H2BPMEN)Cu-II(NO2-)](+) (2). Both the Cu-II-NO2- complexes showed the formation of NO(g) along with H2O2 when reacted with one equivalent acid (H+) via the N-O bond homolysis of the presumed Cu-II-nitrous acid ([Cu-ONOH](2+)) intermediate. However, the H2O2 amount decreased with time or an increase in H+ and completely disappeared when H+ was more than about two equivalents accompanied by the generation of H2O. We detected the released NO(g) by using headspace gas chromatography/mass spectrometry; moreover, the NO(g) evolution was confirmed by the formation of a significant amount of {CoNO}(8), [(12-TMC)Co(NO)](2+) up to (90 +/- 5%) in the above reactions. Mechanistic investigations using N-15-labeled-(NO2-)-N-15 and O-18-labeled-(ONO-)-O-16-N-14-O-18 revealed that the N-atom in NO is derived from the (ONO-)-O-18 ligand, which was further confirmed by the detection of (NO)-N-15 and (NO)-O-18 gas in headspace gas chromatography/mass spectrometry. We also monitored and characterized the formation of H2O2 (one equivalent of H+) and H2O (two equivalents of H+) and the results describe the rationale behind biological NO2- reduction reactions generating NO along with H2O. We observed more than 90% recovery of (1) after 10 catalytic cycles of NO(g) generation.