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 .