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 (NO3-) to nitrite (NO2-) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO2- from NO3-, which further produces nitric oxide (NO) either in acid-induced NO2- reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl3 (V3+ ion source) induced step-wise reduction of NO3- in a Co-II-nitrato complex, [(12-TMC)Co-II(NO3-)](+) (2,{Co-II-NO3-}), to a Co-III-nitrosyl complex, [(12-TMC)Co-III(NO)](2+) (4,{CoNO}(8)), bearing an N-tetramethylated cyclam (TMC) ligand. The VCl3 inspired reduction of NO3- to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, i.e., OAT-1 = NO3- -> NO2- (r(1)) and OAT-2 = NO2- -> NO (r(2)). In these OAT reactions, VCl3 functions as an O-atom abstracting species, and the reaction of 2 with VCl3 produces a Co-III-nitrosyl ({CoNO}(8)) with V-V-Oxo ({V-V=O}(3+)) species, via a proposed Co-II-nitrito (3, {Co-II-NO2-}) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl3, which showed the generation of 4 with V-V-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl3 as a limiting reagent, as the second-order rate constant of OAT-2 (k(2)') is found to be similar to 1420 times faster than that of the OAT-1 (k(2)) reaction. Binding constant (K-b) calculations also support our proposition of NO3- to NO transformation in two successive OAT reactions, as Kb(CoII-NO2-) is higher than Kb(CoII-NO3-), hence the reaction moves in the forward direction (OAT-1). However, Kb(CoII-NO2-) is comparable to K-b{CoNO}8, and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using N-15-labeled-(NO3-)-N-15 and (NO2-)-N-15 revealed that the N-atom in the {CoNO}(8) is derived from NO3- ligand. This work highlights the first-ever report of VCl3 induced step-wise NO3- reduction (NRs activity) followed by the OAT induced NO2- reduction and then the generation of Co-nitrosyl species {CoNO}(8).