Oxygen atom transfer promoted nitrate to nitric oxide transformation: a step-wise reduction of nitrate → nitrite → nitric oxide.

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO ) to nitrite (NO ) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO from NO , which further produces nitric oxide (NO) either in acid-induced NO reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl (V ion source) induced step-wise reduction of NO in a Co-nitrato complex, [(12-TMC)Co(NO )] (2,{Co-NO }), to a Co-nitrosyl complex, [(12-TMC)Co(NO)] (4,{CoNO}), bearing an -tetramethylated cyclam (TMC) ligand. The VCl inspired reduction of NO to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, , OAT-1 = NO → NO (r) and OAT-2 = NO → NO (r). In these OAT reactions, VCl functions as an O-atom abstracting species, and the reaction of 2 with VCl produces a Co-nitrosyl ({CoNO}) with V-Oxo ({V[double bond, length as m-dash]O}) species, a proposed Co-nitrito (3, {Co-NO }) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl, which showed the generation of 4 with V-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl as a limiting reagent, as the second-order rate constant of OAT-2 ( ) is found to be ∼1420 times faster than that of the OAT-1 ( ) reaction. Binding constant ( ) calculations also support our proposition of NO to NO transformation in two successive OAT reactions, as is higher than , hence the reaction moves in the forward direction (OAT-1). However, is comparable to , and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using N-labeled-NO and NO revealed that the N-atom in the {CoNO} is derived from NO ligand. This work highlights the first-ever report of VCl induced step-wise NO reduction (NRs activity) followed by the OAT induced NO reduction and then the generation of Co-nitrosyl species {CoNO}.