Acid-induced nitrite reduction of nonheme iron(ii)-nitrite: mimicking biological Fe-NiR reactions

Nitrite reductase (NiR) catalyzes nitrite (NO2-) to nitric oxide (NO) transformation in the presence of an acid (H+ ions/pH) and serves as a critical step in NO biosynthesis. In addition to the NiR enzyme, NO synthases (NOSs) participate in NO production. The chemistry involved in the catalytic reduction of NO2-, in the presence of H+, generates NO with a H2O molecule utilizing two H+ + one electron from cytochromes and is believed to be affected by the pH. Here, to understand the effect of H+ ions on NO2- reduction, we report the acid-induced NO2- reduction chemistry of a nonheme Fe-II-nitrito complex, [(12TMC)Fe-II(NO2-)](+) (Fe-II-NO2-, 2), with variable amounts of H+. Fe-II-NO2- upon reaction with one-equiv. of acid (H+) generates [(12TMC)Fe(NO)](2+), {FeNO}(7) (3) with H2O2 rather than H2O. However, the amount of H2O2 decreases with increasing equivalents of H+ and entirely disappears when H+ reaches approximately equal to two-equiv. and shows H2O formation. Furthermore, we have spectroscopically characterized and followed the formation of H2O2 (H+ = one-equiv.) and H2O (H+ approximately equal to two-equiv.) and explained why bio-driven NiR reactions end with NO and H2O. Mechanistic investigations, using N-15-labeled-(NO2-)-N-15 and H-2-labeled-CF3SO3D (D+ source), revealed that the N atom in the {(FeNO)-N-14/15}(7) is derived from the NO2- ligand and the H atom in H2O or H2O2 is derived from the H+ source, respectively.