Acid-catalyzed Transformation of Nitrite to Nitric Oxide on Copper(II)-Cobalt(II) Centers in a Bimetallic Complex.

Nitrite (NO2-) serves as a pool of nitric oxide (NO) in biological systems under hypoxic conditions, and it is transformed to NO by nitrite reductase (NiR) enzyme in the presence of acid. However, NO synthases generate NO in normoxic conditions. Previously, acid-induced NO2- reduction chemistry was modeled on mono-metallic 3d-metals, generating metal-nitrosyls or NO(g) with H2O or H2O2 products. Herein, to understand the relative potency of a bimetallic system, we report the acid-induced reductive conversion of η2-bound NO2- to NO on CuII-CoII centers of a hetero-bimetallic CuII­nitrito-CoII complex, [(LN8H)CuII­NO2-­CoII]3+ (CuII-NO2--CoII, 2) bearing an octadentate N8-cryptand ligand (LN8H). The CuII-NO2--CoII generates [CuII(LN8H)CoII]4+ (1) upon reaction with one equiv. acid (HClO4, H+ ions source) with NO(g) via a presumed transient nitrousacid (ONOH) intermediate species. Likewise, this NO2- reduction was found to form H2O, which is believed to be from the decomposition of H2O2, an intermediate species. In addition, complex 2, in the presence of more than one equiv. H+ ions also showed the formation of NO(g) with H2O. Mechanistic investigations, using 15N-labeled-15NO2-, 18O-labeled-18O14N16O- and 2H-labeled-DClO4 (D+ source), revealed that the N-atom and O-atom in the 14/15NO and 14N18O gases are derived from NO2- ligand and H-atom in H2O derived from H+-source, respectively.

Mechanistic insights into nitric oxide oxygenation (NOO) reactions of {CrNO}5 and {CoNO}8.

Here, we report the nitric oxide oxygenation (NOO) reactions of two distinct metal nitrosyls {Co-nitrosyl (S = 0) vs. Cr-nitrosyl (S = 1/2)}. In this regard, we synthesized and characterized [(BPMEN)Co(NO)]2+ ({CoNO}8, 1) to compare its NOO reaction with that of [(BPMEN)Cr(NO)(Cl-)]+ ({CrNO}5, 2), having a similar ligand framework. Kinetic measurements showed that {CrNO}5 is thermally more stable than {CoNO}8. Complexes 1 and 2, upon reaction with the superoxide anion (O2˙-), generate [(BPMEN)CoII(NO2-)2] (CoII-NO2-, 3) and [(BPMEN)CrIII(NO2-)Cl-]+ (CrIII-NO2-, 4), respectively, with O2 evolution. Furthermore, analysis of these NOO reactions and tracking of the N-atom using 15N-labeled NO (15NO) revealed that the N-atoms of 3 (CoII-15NO2-) and 4 (CrIII-15NO2-) derive from the nitrosyl (15NO) moieties of 1 and 2, respectively. This work represents a comparative study of oxidation reactions of {CoNO}8vs. {CrNO}5, showing different rates of the NOO reactions due to different thermal stability. To complete the NOM cycle, we reacted 3 and 4 with NO, and surprisingly, only 3 generated {CoNO}8 species, while 4 was unreactive towards NO. Furthermore, the phenol ring nitration test, performed using 2,4-di-tert-butylphenol (2,4-DTBP), suggested the presence of a proposed peroxynitrite (PN) intermediate in the NOO reactions of 1 and 2.

Nitric Oxide Oxygenation Reactions of Cobalt-Peroxo and Cobalt-Nitrosyl Complexes.

Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of CoIII-peroxo (CoIII-O22-) and Co-nitrosyl ({CoNO}8) complexes bearing the same N4-donor ligand (HMTETA) framework. In this regard, we prepared and characterized two new [(HMTETA)CoIII(O22-)]+ (2, S = 2) and [(HMTETA)Co(NO)]2+ (3, S = 1) complexes from [(HMTETA)CoII(CH3CN)2]2+ (1). Both complexes (2 and 3) are characterized by different spectroscopic measurements, including their DFT-optimized structures. Complex 2 produces CoII-nitrato [(HMTETA)CoII(NO3-)]+ (CoII-NO3-, 4) complex in the presence of NO. In contrast, when 3 reacted with a superoxide (O2•-) anion, it generated CoII-nitrito [(HMTETA)CoII(NO2-)]+ (CoII-NO2-, 5) with O2 evolution. Experiments performed using 18/16O-labeled superoxide (18O2•-/16O2•-) showed that O2 originated from the O2•- anion. Both the NOO reactions are believed to proceed via a presumed peroxynitrite (PN) intermediate. Although we did not get direct spectral evidence for the proposed PN species, the mechanistic investigation using 2,4-di-tert-butylphenol indirectly suggests the formation of a PN intermediate. Furthermore, tracking the source of the N-atom in the above NOO reactions using 15N-labeled nitrogen (15NO) revealed N-atoms in 4 (CoII-15NO3-) and 5 (CoII-15NO2-) derived from the 15NO moiety.

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 CoII-nitrato complex, [(12-TMC)CoII(NO3 -)]+ (2,{CoII-NO3 -}), to a CoIII-nitrosyl complex, [(12-TMC)CoIII(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 CoIII-nitrosyl ({CoNO}8) with VV-Oxo ({VV[double bond, length as m-dash]O}3+) species, via a proposed CoII-nitrito (3, {CoII-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 VV-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(CoII-NO2 -) is higher than K b(CoII-NO3 -), hence the reaction moves in the forward direction (OAT-1). However, K b(CoII-NO2 -) is comparable to K b{CoNO}8 , 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.