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