Acid-induced conversion of nitrite to nitric oxide at the copper(ii) center: a new catalytic pathway

Abstract

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 ¹⁵N-labeled-¹⁵NO₂⁻ and ¹⁸O-labeled-¹⁶O¹⁴N¹⁸O⁻ revealed that the N-atom in NO is derived from the ¹⁸ONO⁻ ligand, which was further confirmed by the detection of ¹⁵NO and N¹⁸O 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.