Conversion of Nitric Oxide into Nitrous Oxide as Triggered by the Polarization of Coordinated NO by Hydrogen Bonding

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.

Organometallic chemistry confined within a porphyrin-like framework

The world of modified porphyrins changed forever when an N-confused porphyrin (NCP), a porphyrin isomer, was first published in 1994. The replacement of one inner nitrogen with a carbon atom revolutionised the chemistry that one is able to perform within the coordination cavity. One could explore new pathways in the organometallic chemistry of porphyrins by forcing a carbon fragment from the ring or an inner substituent to sit close to an inserted metal ion. Since the NCP discovery, a series of modifications became available to tune the coordination properties of the cavity, introducing a fascinating realm of carbaporphyrins. The review surveys all possible carbatetraphyrins(1.1.1.1) and their spectacular coordination and organometallic chemistry.

Reductive Coupling of Nitric Oxide by Cu(I): Stepwise Formation of Mono- and Dinitrosyl Species En Route to a Cupric Hyponitrite Intermediate

Transition-metal-mediated reductive coupling of nitric oxide (NO(g)) to nitrous oxide (N2O(g)) has significance across the fields of industrial chemistry, biochemistry, medicine, and environmental health. Herein, we elucidate a density functional theory (DFT)-supplemented mechanism of NO(g) reductive coupling at a copper-ion center, [(tmpa)CuI(MeCN)]+ (1) {tmpa = tris(2-pyridylmethyl)amine}. At -110 °C in EtOH (<-90 °C in MeOH), exposing 1 to NO(g) leads to a new binuclear hyponitrite intermediate [{(tmpa)CuII}2(μ-N2O22-)]2+ (2), exhibiting temperature-dependent irreversible isomerization to the previously characterized κ2-O,O'-trans-[(tmpa)2Cu2II(μ-N2O22-)]2+ (OOXray) complex. Complementary stopped-flow kinetic analysis of the reaction in MeOH reveals an initial mononitrosyl species [(tmpa)Cu(NO)]+ (1-(NO)) that binds a second NO molecule, forming a dinitrosyl species [(tmpa)CuII(NO)2] (1-(NO)2). The decay of 1-(NO)2 requires an available starting complex 1 to form a dicopper-dinitrosyl species hypothesized to be [{(tmpa)Cu}2(μ-NO)2]2+ (D) bearing a diamond-core motif, en route to the formation of hyponitrite intermediate 2. In contrast, exposing 1 to NO(g) in 2-MeTHF/THF (v/v 4:1) at <-80 °C leads to the newly observed transient metastable dinitrosyl species [(tmpa)CuII(NO)2] (1-(NO)2) prior to its disproportionation-mediated transformation to the nitrite product [(tmpa)CuII(NO2)]+. Our study furnishes a near-complete profile of NO(g) activation at a reduced Cu site with tripodal tetradentate ligation in two distinctly different solvents, aided by detailed spectroscopic characterization of metastable intermediates, including resonance Raman characterization of the new dinitrosyl and hyponitrite species detected.

NO Coupling at Copper to cis-Hyponitrite: N2O Formation via Protonation and H-Atom Transfer

Copper nitrite reductases (CuNIRs) convert NO2- to NO as well as NO to N2O under high NO flux at a mononuclear type 2 Cu center. While model complexes illustrate N-N coupling from NO that results in symmetric trans-hyponitrite [CuII]-ONNO-[CuII] complexes, we report NO assembly at a single Cu site in the presence of an external reductant Cp*2M (M = Co, Fe) to give the first copper cis-hyponitrites [Cp*2M]{[CuII](κ2-O2N2)[CuI]}. Importantly, the κ1-N-bound [CuI] fragment may be easily removed by the addition of mild Lewis bases such as CNAr or pyridine to form the spectroscopically similar anion {[CuII](κ2-O2N2)}-. The addition of electrophiles such as H+ to these anionic copper(II) cis-hyponitrites leads to N2O generation with the formation of the dicopper(II)-bis-μ-hydroxide [CuII]2(μ-OH)2. One-electron oxidation of the {[CuII](κ2-O2N2)}- core turns on H-atom transfer reactivity, enabling the oxidation of 9,10-dihydroanthracene to anthracene with concomitant formation of N2O and [CuII]2(μ-OH)2. These studies illustrate both the reductive coupling of NO at a single copper center and a way to harness the strong oxidizing power of nitric oxide via the neutral cis-hyponitrite [Cu](κ2-O2N2).

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.

Influence of the primary and secondary coordination spheres on nitric oxide adsorption and reactivity in cobalt(ii)-triazolate frameworks

Nitric oxide (NO) is an important signaling molecule in biological systems, and as such, the ability of porous materials to reversibly adsorb NO is of interest for potential medical applications. Although certain metal-organic frameworks are known to bind NO reversibly at coordinatively unsaturated metal sites, the influence of the metal coordination environment on NO adsorption has not been studied in detail. Here, we examine NO adsorption in the frameworks Co2Cl2(bbta) (H2bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) and Co2(OH)2(bbta) using gas adsorption, infrared spectroscopy, powder X-ray diffraction, and magnetometry. At room temperature, NO adsorbs reversibly in Co2Cl2(bbta) without electron transfer, with low temperature data supporting spin-crossover of the NO-bound cobalt(ii) centers of the material. In contrast, adsorption of low pressures of NO in Co2(OH)2(bbta) is accompanied by charge transfer from the cobalt(ii) centers to form a cobalt(iii)-NO- adduct, as supported by diffraction and infrared spectroscopy data. At higher pressures of NO, characterization data indicate additional uptake of the gas and disproportionation of the bound NO to form a cobalt(iii)-nitro (NO2 -) species and N2O gas, a transformation that appears to be facilitated by secondary sphere hydrogen bonding interactions between the bound NO2 - and framework hydroxo groups. These results provide a rare example of reductive NO binding in a cobalt-based metal-organic framework, and they demonstrate that NO uptake can be tuned by changing the primary and secondary coordination environment of the framework metal centers.

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 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 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> 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 (k₂^/) is found to be ∼1420 times faster than that of the OAT-1 (k₂) reaction. Binding constant (K_b) calculations also support our proposition of NO₃⁻ to NO transformation in two successive OAT reactions, as K_{b(Co^II–NO2⁻)} is higher than K_{b(Co^II–NO3⁻)}, hence the reaction moves in the forward direction (OAT-1). However, K_{b(Co^II–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.

Single metal-induced reduction of NO₃⁻ → {NO₂⁻} → NO via oxygen atom transfer reaction.

Reduction of NO by diiron complexes in relation to flavodiiron nitric oxide reductases

Reduction of nitric oxide (NO) to nitrous oxide (N₂O) is associated with immense biological and health implications. Flavodiiron nitric oxide reductases (FNORs) are diiron containing enzymes that catalyze the two electron reduction of NO to N₂O and help certain pathogenic bacteria to survive under "nitrosative stress" in anaerobic growth conditions. Consequently, invading bacteria can proliferate inside the body of mammals by bypassing the immune defense mechanism involving NO and may thus lead to harmful infections. Various mechanisms, namely the direct reduction, semireduction, superreduction and hyponitrite mechanisms, have been proposed over time for catalytic NO reduction by FNORs. Model studies in relation to the diiron active site of FNORs have immensely helped to replicate the minimal structure-reactivity relationship and to understand the mechanism of NO reduction. A brief overview of the FNOR activity and the proposed reaction mechanisms followed by a systematic description and detailed analysis of the model studies is presented, which describes the development in the area of NO reduction by diiron complexes and its implications. A great deal of successful modeling chemistry as well as the shortcomings related to the synthesis and reactivity studies is discussed in detail. Finally, future prospects in this particular area of research are proposed, which in due course may bring more clarity in the understanding of this important redox reaction.

Theoretical study of hydrogen bonding interactions in substituted nitroxide radicals

Interaction energy (E_int) of hydrogen bonded complexes of nitroxide radicals can be assessed in terms of the deepest minimum of molecular electrostatic potential (V_min).

Copper(I) Complex Mediated Nitric Oxide Reductive Coupling: Ligand Hydrogen Bonding Derived Proton Transfer Promotes N2O(g) Release

A cuprous chelate bearing a secondary sphere hydrogen bonding functionality, [(PV-tmpa)Cu^I]⁺, transforms ^•NO_(g) to N₂O_(g) in high-yields in methanol. Ligand derived proton transfer facilitates N-O bond cleavage of a putative hyponitrite intermediate releasing N₂O_(g), underscoring the crucial balance between H-bonding capabilities and acidities in (bio)chemical ^•NO_(g) coupling systems.

Ruthenium N-Confused Porphyrins: Selective Reactivity for Ambident 2-Heteroatom-Substituted Pyridines Serving as Axial Ligands

Three types of ruthenium(II) N-confused porphyrin (NCP) complexes bearing an axial 2-thiopyridine, 2-pyridone, and 2-iminopyridine moiety at the inner carbon atom, respectively, were synthesized. The unique reactivity of the 2-substituted pyridine derivatives (2-X-pyridine; X=SH, OH, NH₂ ) toward the inner carbon atom of the NCP allows the formation of two types of coordinated products (i. e., pyridine donor versus 2-heteroatom donors), as inferred from single-crystal X-ray structures. The selective reactivity was investigated by using density functional theory (DFT) calculations. Finally, the catalytic activity of these ruthenium complexes was demonstrated through the styrene oxidation reactions. As a result, the ruthenium(II) NCP complex bearing a 2-thiopyridine moiety, together with aqueous H₂ O₂ as an oxidant showed the highest selectivity for benzaldehyde (benzaldehyde/styrene oxide=20 : 1).

Copper(I)/NO(g) Reductive Coupling Producing a trans-Hyponitrite Bridged Dicopper(II) Complex: Redox Reversal Giving Copper(I)/NO(g) Disproportionation

A copper complex, [CuI(tmpa)(MeCN)]+, effectively reductively couples NO(g) at RT in methanol (MeOH), giving a structurally characterized hyponitrito-dicopper(II) adduct. Hydrogen-bonding from MeOH is critical for the hyponitrite complex formation and stabilization. This complex exhibits the reverse redox process in aprotic solvents, giving CuI + NO(g), leading to CuI-mediated NO(g)-disproportionation. The relationship of this chemistry to biological iron and/or copper mediated NO(g) reductive coupling to give N2O(g) is discussed.