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

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 ^- (r₁) and OAT-2 = NO₂ ^- → NO (r₂). 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[double bond, length as m-dash]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 ∼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 K _b(Co is higher than K _b(Co, hence the reaction moves in the forward direction (OAT-1). However, K _b(Co is comparable to K _b{CoNO} , and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using ¹⁵N-labeled-¹⁵NO₃ ^- and ¹⁵NO₂ ^- 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.