In biological systems, nitrite reductase enzymes (NIRs) are responsible for reduction of nitrite (NO2-) to nitric oxide (NO). These NIRs have mostly Cu- or Fe-containing active sites, surrounded by amine-containing ligands. Therefore, mononuclear Cu complexes with N-donor ligands are highly relevant in the development of NIR model systems and in the mechanistic investigation of the nitrite reduction reaction. Herein, we report on a terpyridine-based CuII complex with square planar geometry for H+-assisted electrochemical reduction of NO2-. Through electrochemical measurements, spectroscopic characterization and isotope-labelling experiments we propose a mechanistic reaction pathway involving an unstable HNO2 state. The CuI intermediate, formed electrochemically, was isolated and its molecular structure was deduced, showing linkage isomerism of the nitrite ligand. Moreover, qualitative and quantitative product analysis by GC-MS shows N2O formed as a side product along with the main product NO. Furthermore, by obtaining single crystals and conducting structural analysis we were able to determine the structural arrangement and redox state of the complex after electrochemical treatment.

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 (H+ ions). However, NO synthases (NOSs) generate NO via L-arginine oxidation 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 CuII-nitrito-CoII complex, [(LN8H)CuII-NO2 --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.

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.

Acid-catalyzed conversion of nitrite to nitric oxide at the copper( ii ) center: a new catalytic pathway.

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.

Herein, we have synthesized three different types of BiOBr nanomaterials (nanosheets, nanoplates and nanoflowers) by simply varying the pH of the solvent medium. Results suggest that the nanosheets contain predominantly (001) plane as the major exposed crystal facets. On the other hand, nanoflowers possess mainly (110) plane as the major exposed facet. However, nanoplates show both (001) & (110) crystal facets predominantly. Detailed morphological and elemental studies have been carried out to investigate the variation of positively charged oxygen vacancies depending on the nature of the predominantly exposed crystal facets. Furthermore, the relative extent of oxygen vacancies was further correlated with the overall surface area and the porosity of the as synthesized nanomaterials. Finally, all these three different types of nanomaterials with varying predominantly exposed crystal facets have been utilized for the catalytic reduction of para-nitrophenol to para-aminophenol as a model system through mild reducing agent NaBH4. A probable mechanism has been proposed to explain the facet dependent catalytic activities of different BiOBr nanomaterials. Finally, Fe (III)/ Fe (II) ions were specifically incorporated in BiOBr nanomaterials. The incorporation of Fe (III)/Fe (II) ions has been further correlated with the detail structural, elemental and optoelectronic properties. Depending on the exposed crystal facets, a huge enhancement has been observed for overall catalytic efficiency upon incorporation of Fe (III)/ Fe (II) ions. A suitable mechanism has been proposed.

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.

Here, we report the nitric oxide dioxygenation (NOD) reactions of Ni-III-peroxo, [(12-TMC)Ni-III(O-2(2-))](+) (Ni-III-O-2(2-), 1), and Co-III-peroxo, [(12-TMC)Co-III(O-2(2-))](+) (Co-III-O-2(2-), 2), complexes bearing N-tetramethylated cyclam (TMC) ligands. The reaction of complex 1 with nitric oxide (NO) generates Ni-II-nitrato complex [(12-TMC)Ni-II(NO3-)](+) (Ni-II-NO3-, 3). However, in contrast to the NOD reaction of 1, compound 2 produces Co-II-nitrito complex, [(12-TMC)Co-II(NO2-)](+) (Co-II-NO2-, 4), with O-2 gas formation. Results suggested the formation of a presumptive Ni-peroxynitrite ([Ni-PN](+), [Ni-ONOO-](+)) intermediate, in the NOD reaction of 1, prior to the generation of Ni-II-NO3- (3). Although the end product of the reaction of 2 with NO is different from that of 1, the mechanistic study explored by using 2,4-di-tert-butylphenol (2,4-DTBP) suggested that the reaction of 2 with NO also occurs via a presumed Co-peroxynitrite ([Co-PN](+), [Co-ONOO-](+)) intermediate. Furthermore, exploring these NOD reactions and tracking the oxygen atom using O-18-labeled hydrogen peroxide ((H2O2)-O-18) revealed that two oxygen atoms of 3 ((NiII-18O2NO-)-O-16) and one oxygen atom of 4 ((CoII-18ONO-)-O-16) are derived from the peroxo (O-2(2-)) moieties of 1 and 2, respectively. Furthermore, we have explored the M-peroxo regeneration from NOD products (3 and 4), and for the first time, we have observed the formation of M-III-O-2(2-) complexes, 1 and 2, upon treatment with H2O2 and triethylamine (TEA), respectively. This work is the first-ever report which showed the exact NO dioxygenase (NOD) enzyme catalytic cycle. Furthermore, we have also explored the NO activation of 3 and 4, and surprisingly, we observed the formation of Co-nitrosyl (5) from 4 upon reaction with NO; in contrast, 3 was unreactive towards NO. Evidence supporting the formation of [Ni-PN](+) and [Co-PN](+) intermediates is also presented.