NO2– Activation and Reduction to NO by a Nonheme Fe(NO2)2 Complex

Influence of diimine bidentate ligand in the nitrosyl and nitro terpyridine ruthenium complex on the HSA/DNA interaction and antiviral activity

Nitric oxide (NO) acts in different physiological processes, such as blood pressure control, antiparasitic activities, neurotransmission, and antitumor action. Among the exogenous NO donors, ruthenium nitrosyl/nitro complexes are potential candidates for prodrugs, due to their physicochemical properties, such as thermal and physiological pH stability. In this work, we proposed the synthesis and physical characterization of the new nitro terpyridine ruthenium (II) complexes of the type [RuII(L)(NO2)(tpy)]PF6 where tpy = 2,2':6',2″-terpyridine; L = 3,4-diaminobenzoic acid (bdq) or o-phenylenediamine (bd) and evaluation of influence of diimine bidentate ligand NH.NHq-R (R = H or COOH) in the HSA/DNA interaction as well as antiviral activity. The interactions between HSA and new nitro complexes [RuII(L)(NO2)(tpy)]+ were evaluated. The Ka values for the HSA-[RuII(bdq)(NO2)(tpy)]+ is 10 times bigger than HSA-[RuII(bd)(NO2)(tpy)]+. The sites of interaction between HSA and the complexes via synchronous fluorescence suppression indicate that the [RuII(bdq)(NO2)(tpy)]+ is found close to the Trp-241 residue, while the [RuII(bd)(NO2)(tpy)]+ complex is close to Tyr residues. The interaction with fish sperm fs-DNA using direct spectrophotometric titration (Kb) and ethidium bromide replacement (KSV and Kapp) showed weak interaction in the system fs-DNA-[RuII(bdq)(NO)(tpy)]+. Furthermore, fs-DNA-[RuII(bd)(NO2)(tpy)]+ and fs-DNA-[RuII(bd)(NO)(tpy)]3+ system showed higher intercalation constant. Circular dichroism spectra for fs-DNA-[RuII(bd)(NO2)(tpy)]+ and fs-DNA-[RuII(bd)(NO)(tpy)]3+, suggest semi-intercalative accompanied by major groove binding interaction modes. The [RuII(bd)(NO2)(tpy)]+ and [RuII(bd)(NO)(tpy)]3+ inhibit replication of Zika and Chikungunya viruses based in the nitric oxide release under S-nitrosylation reaction with cysteine viral.

Reduction of nitrite to nitric oxide and generation of reactive chalcogen species by mononuclear Fe(II) and Zn(II) complexes of thiolate and selenolate

Comparative reactivity of a series of new Zn(II) and Fe(II) compounds, [(Py2ald)M(ER)] (E = S, R = Ph: M = Zn, 1aZn; M = Fe, 1aFe; E = S, R = 2,6-Me2-C6H3: M = Zn, 1bZn; M = Fe, 1bFe; E = Se, R = Ph: M = Zn, 2Zn; M = Fe, 2Fe), and [(Py2ald)M]22+ (M = Zn, 5Zn; M = Fe, 5Fe) is presented. Compound 1aZn could react with nitrite (NO2-) to produce [(Py2ald)Zn(ONO)] (3Zn), which, upon treatment with thiols and PhSeH (proton source), could regenerate either 1aZn/5Zn and 2Zn respectively, along with the production of nitric oxide (NO) where the yield of NO increases in the order tBuSH ≪ PhCH2SH < PhSH < PhSeH. In contrast to this, 1aFe, 2Fe and 5Fe could affect the direct reduction of NO2- in the absence of protons to generate NO and [{(Py2ald)(ONO)Fe}2-μ2-O] (8Fe). Moreover, 8Fe could regenerate 5Fe and 1aFe/2Fe upon treatment with 4 and 6 equiv. of PhEH (E = S/Se), respectively, along with the generation of NO. Finally, a comparative study of the mononuclear Zn(II) and Fe(II) compounds for the transfer of the coordinated thiolate/selenolate and the generation and transfer of reactive sulfur/selenium species (RES-, E = Se, S) to a series of organic substrates has been provided.

Modeling Reactivity of Nitrite and Nitrous Acid at a Phenolate Bridged Dizinc(II) Site: Insights into NO Signaling at Zinc

Although nitrite-to-NO transformation at various transition metals including Fe and Cu are relatively well explored, examples of such a reaction at the redox-inactive zinc(II) site are limited. The present report aims to gain insights into the reactivity of nitrite anions, nitrous acid (HONO), and organonitrite (RONO) at a dizinc(II) site. A phenolate-bridged dizinc(II)-aqua complex [LH ZnII (OH2 )]2 (ClO4 )2 (1H -Aq, where LH =tridentate N,N,O-donor monoanionic ligand) is illustrated to react with t BuONO to provide a metastable arene-nitrosonium charge-transfer complex 2H . UV-vis, FTIR, multinuclear NMR, and elemental analyses suggests the presence of a 2 : 1 arene-nitrosonium moiety. Furthermore, the reactivity of a structurally characterized zinc(II)-nitrite complex [LH ZnII (ONO)]2 (1H -ONO) with a proton-source demonstrates HONO reactivity at the dizinc(II) site. Reactivity of both RONO (R=alkyl/H) at the phenolate-bridged dizinc(II) site provides NO+ charge-transfer complex 2H . Subsequently, the reactions of 2H with exogenous reductants (such as ferrocene, thiol, phenol, and catechol) have been illustrated to generate NO. In addition, NO yielding reactivity of [LH ZnII (ONO)]2 (1H -ONO) in the presence of the above-mentioned reductants have been compared with the reactions of complex 2H . Thus, this report sheds light on the transformations of NO2 - /RONO (R=alkyl/H) to NO/NO+ at the redox-inactive zinc(II) coordination motif.

NO Generation from Nitrite at Zinc(II): Role of Thiol Persulfidation in the Presence of Sulfane Sulfur

Nitrite-to-NO transformation is of prime importance due to its relevance in mammalian physiology. Although such a one-electron reductive transformation at various redox-active metal sites (e.g., Cu and Fe) has been illustrated previously, the reaction at the [ZnII] site in the presence of a sacrificial reductant like thiol has been reported to be sluggish and poorly understood. Reactivity of [(Bn3Tren)ZnII-ONO](ClO4) (1), a nitrite-bound model of the tripodal active site of carbonic anhydrase (CA), toward various organic probes, such as 4-tert-butylbenzylthiol (tBuBnSH), 2,4-di-tert-butylphenol (2,4-DTBP), and 1-fluoro-2,4-dinitrobenzene (F-DNB), reveals that the ONO-moiety in the [ZnII]-nitrite coordination motif of complex 1 acts as a mild electrophile. tBuBnSH reacts mildly with nitrite at a [ZnII] site to provide S-nitrosothiol tBuBnSNO prior to the release of NO in 10% yield, whereas the phenolic substrate 2,4-DTBP does not yield the analogous O-nitrite compound (ArONO). The presence of sulfane sulfur (S0) species such as elemental sulfur (S8) and organic polysulfides (tBuBnSnBntBu) during the reaction of tBuBnSH and [ZnII]-nitrite (1) assists the nitrite-to-NO conversion to provide NO yields of 65% (for S8) and 76% (for tBuBnSnBntBu). High-resolution mass spectrometry (HRMS) analyses on the reaction of [ZnII]-nitrite (1), tBuBnSH, and S8 depict the formation of zinc(II)-persulfide species [(Bn3Tren)ZnII-Sn-BntBu]+ (where n = 2, 3, 4, 5, and 6). Trapping of the persulfide species (tBuBnSS-) with 1-fluoro-2,4-dinitrobenzene (F-DNB) confirms its intermediacy. The significantly higher nucleophilicity of persulfide species (relative to thiol/thiolate) is proposed to facilitate the reaction with the mildly electrophilic [ZnII]-nitrite (1) complex. Complementary analyses, including multinuclear NMR, electrospray ionization-MS, UV-vis, and trapping of reactive S-species, provide mechanistic insights into the sulfane sulfur-assisted reactions between thiol and nitrite at the tripodal [ZnII]-site. These findings suggest the critical influential roles of various reactive sulfur species, such as sulfane sulfur and persulfides, in the nitrite-to-NO conversion.

Sequential Deoxygenation of CO2 and NO2- via Redox-Control of a Pyridinediimine Ligand with a Hemilabile Phosphine

The deoxygenation of environmental pollutants CO2 and NO2- to form value-added products is reported. CO2 reduction with subsequent CO release and NO2- conversion to NO are achieved via the starting complex Fe(PPhPDI)Cl2 (1). 1 contains the redox-active pyridinediimine (PDI) ligand with a hemilabile phosphine located in the secondary coordination sphere. 1 was reduced with SmI2 under a CO2 atmosphere to form the direduced monocarbonyl Fe(PPhPDI)(CO) (2). Subsequent CO release was achieved via oxidation of 2 using the NOx- source, NO2-. The resulting [Fe(PPhPDI)(NO)]+ (3) mononitrosyl iron complex (MNIC) is formed as the exclusive reduction product due to the hemilabile phosphine. 3 was investigated computationally to be characterized as {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand.

Acid-induced nitrite reduction of nonheme iron(ii)-nitrite: mimicking biological Fe-NiR reactions

Nitrite reductase (NiR) catalyzes nitrite (NO₂ ^-) to nitric oxide (NO) transformation in the presence of an acid (H⁺ ions/pH) and serves as a critical step in NO biosynthesis. In addition to the NiR enzyme, NO synthases (NOSs) participate in NO production. The chemistry involved in the catalytic reduction of NO₂ ^-, in the presence of H⁺, generates NO with a H₂O molecule utilizing two H⁺ + one electron from cytochromes and is believed to be affected by the pH. Here, to understand the effect of H⁺ ions on NO₂ ^- reduction, we report the acid-induced NO₂ ^- reduction chemistry of a nonheme Fe^II-nitrito complex, [(12TMC)Fe^II(NO₂ ^-)]⁺ (Fe^II-NO₂ ^-, 2), with variable amounts of H⁺. Fe^II-NO₂ ^- upon reaction with one-equiv. of acid (H⁺) generates [(12TMC)Fe(NO)]²⁺, {FeNO}⁷ (3) with H₂O₂ rather than H₂O. However, the amount of H₂O₂ decreases with increasing equivalents of H⁺ and entirely disappears when H⁺ reaches ≅ two-equiv. and shows H₂O formation. Furthermore, we have spectroscopically characterized and followed the formation of H₂O₂ (H⁺ = one-equiv.) and H₂O (H⁺ ≅ two-equiv.) and explained why bio-driven NiR reactions end with NO and H₂O. Mechanistic investigations, using ¹⁵N-labeled-¹⁵NO₂ ^- and ²H-labeled-CF₃SO₃D (D⁺ source), revealed that the N atom in the {Fe^14/15NO}⁷ is derived from the NO₂ ^- ligand and the H atom in H₂O or H₂O₂ is derived from the H⁺ source, respectively.

Lewis acid-assisted reduction of nitrite to nitric and nitrous oxides via the elusive nitrite radical dianion

Reduction of nitrite anions (NO2-) to nitric oxide (NO), nitrous oxide (N2O) and ultimately dinitrogen (N2) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (-0.74 V versus NHE). Detailed characterization confirms the formation of the borane-capped radical nitrite dianion (NO22-), which features a N(II) oxidation state. Protonation of the nitrite dianion results in the facile loss of nitric oxide (NO), whereas its reaction with NO results in disproportionation to nitrous oxide (N2O) and nitrite (NO2-). This system connects three redox levels in the global nitrogen cycle and provides fundamental insights into the conversion of NO2- to NO.

NO Generation from the Cross-Talks between Ene-diol Antioxidants and Nitrite at Metal Sites

The one-electron reduction of nitrite (NO₂^-) to nitric oxide (NO) and ene-diol oxidation are two important biochemical transformations. Employing mononuclear cobalt-nitrite complexes with Co^III and Co^II oxidation states, [(Bz₃Tren)Co^III(nitrite)₂](ClO₄) (1) and [(Bz₃Tren)Co^II(nitrite)](ClO₄) (2), this report illustrates NO release coupled to stepwise oxidation of ene-diol antioxidants such as l-ascorbic acid (AH₂) and catechol. Analysis of the AH₂ end-product reveals that the reaction with complex 1 affords dehydroascorbic acid. Intriguingly, a controlled oxidation of AH₂ with complex 2 results in a [Co^II]-bound ascorbyl radical-anion (8). Finally, NO release with the concomitant generation of metal-bound 3,5-di-tert-butyl-semiquinone radical anion from the reactions of 3,5-di-tert-butyl-catechol and [(Bz₃Tren)M^II(nitrite)](ClO₄) (2, M = Co; 4, M = Zn) provides mechanistic insights into the cross-talk between nitrite and ene-diols at the metal sites.

Secondary Coordination Sphere Influences the Formation of Fe(III)-O or Fe(III)-OH in Nitrite Reduction: A Synthetic and Computational Study

The reduction of nitrite (NO₂^-) to generate nitric oxide (NO) is a significant area of research due to their roles in the global nitrogen cycle. Here, we describe various modifications of the tris(5-cyclohexyliminopyrrol-2-ylmethyl)amine H₃[N(pi^R)₃] ligand where the steric bulk and acidity of the secondary coordination sphere were explored in the non-heme iron system for nitrite reduction. The cyclohexyl and 2,4,6-trimethylphenyl variants of the ligand were used to probe the mechanism of nitrite reduction. While previously stoichiometric addition of nitrite to the iron(II)-species generated an iron(III)-oxo complex, changing the secondary coordination sphere to mesityl resulted in an iron(III)-hydroxo complex. Subsequent addition of an electron and two protons led to the release of water and regeneration of the starting iron(II) catalyst. This sequence mirrored the proposed mechanism of nitrite reduction in biological systems, where the distal histidine residue shuttles protons to the active site. Computational studies aimed at interrogating the dissimilar behavior of the cyclohexyl and mesityl ligand systems resulting in Fe(III)-oxo and Fe(III)-hydroxo complexes, respectively, shed light on the key role of H-bonds involving the secondary coordination sphere in the relative stability of these species.

Magnetic Responsive Release of Nitric Oxide from an MOF-Derived Fe3O4@PLGA Microsphere for the Treatment of Bacteria-Infected Cutaneous Wound

Nitric oxide (NO) is an essential endogenous signaling molecule regulating multifaceted physiological functions in the (cardio)vascular, neuronal, and immune systems. Due to the short half-life and location-/concentration-dependent physiological function of NO, translational application of NO as a novel therapeutic approach, however, awaits a strategy for spatiotemporal control on the delivery of NO. Inspired by the magnetic hyperthermia and magneto-triggered drug release featured by Fe₃O₄ conjugates, in this study, we aim to develop a magnetic responsive NO-release material (MagNORM) featuring dual NO-release phases, namely, burst and steady release, for the selective activation of NO-related physiology and treatment of bacteria-infected cutaneous wound. After conjugation of NO-delivery [Fe(μ-S-thioglycerol)(NO)₂]₂ with a metal-organic framework (MOF)-derived porous Fe₃O₄@C, encapsulation of obtained conjugates within the thermo-responsive poly(lactic-co-glycolic acid) (PLGA) microsphere completes the assembly of MagNORM. Through continuous/pulsatile/no application of the alternating magnetic field (AMF) to MagNORM, moreover, burst/intermittent/slow release of NO from MagNORM demonstrates the AMF as an ON/OFF switch for temporal control on the delivery of NO. Under continuous application of the AMF, in particular, burst release of NO from MagNORM triggers an effective anti-bacterial activity against both Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli). In addition to the magneto-triggered bactericidal effect of MagNORM against E. coli-infected cutaneous wound in mice, of importance, steady release of NO from MagNORM without the AMF promotes the subsequent collagen formation and wound healing in mice.

Exacerbated Protein Oxidation and Tyrosine Nitration through Nitrite-Enhanced Fenton Chemistry

Nitrite is a common additive used during meat curing to prevent microbial contamination and retain an attractive red color in the product. However, the effects of nitrite on Fenton reactions catalyzed by free iron in meat products are not well understood, although such processes can induce protein oxidation and nitration, affecting the nutritional and aesthetic quality of meat products. This contribution reveals the mechanism through which nitrite affects Fenton reactions that generate reactive nitrogen and oxygen species by increasing the availability of Fe³⁺, facilitating its reduction and stabilizing Fe²⁺, and accelerating Fe³⁺/Fe²⁺ cycling, leading to exacerbated oxidative and nitrosative stress on proteins, with implications not only for meat processing but also in many biological and environmental processes due to the ubiquitous presence of iron, hydrogen peroxide, and nitrite in nature.

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.

Mechanism of O-Atom Transfer from Nitrite: Nitric Oxide Release at Copper(II)

Nitric oxide (NO) is a key signaling molecule in health and disease. While nitrite acts as a reservoir of NO activity, mechanisms for NO release require further understanding. A series of electronically varied β-diketiminatocopper(II) nitrite complexes [Cu^II](κ²-O₂N) react with a range of electronically tuned triarylphosphines PAr^Z₃ that release NO with the formation of O═PAr^Z₃. Second-order rate constants are largest for electron-poor copper(II) nitrite and electron-rich phosphine pairs. Computational analysis reveals a transition-state structure energetically matched with experimentally determined activation barriers. The production of NO follows a pathway that involves nitrite isomerization at Cu^II from κ²-O₂N to κ¹-NO₂ followed by O-atom transfer (OAT) to form O═PAr^Z₃ and [Cu^I]-NO that releases NO upon PAr^Z₃ binding at Cu^I to form [Cu^I]-PAr^Z₃. These findings illustrate important mechanistic considerations involved in NO formation from nitrite via OAT.

The dual roles of a V(III) centre for substrate binding and oxygen atom abstraction; nitrite reduction mediated by a V(III) complex

A V(iii) complex bearing a tris(thiolato)phoshine derivative mediates the reduction of nitrite without the assistance of external protons or oxophilic substrates. The metal site plays dual roles for nitrite binding and deoxygenation. The reaction is monitored by spectroscopy combined with isotopic labeling experiments. The formed product, a {VNO}4 species, is isolated and characterized.

A reagent for heteroatom borylation, including iron mediated reductive deoxygenation of nitrate yielding a dinitrosyl complex

4,4'-Bipyridyl is shown to be a catalyst for transfer of pinacolboryl groups from (Bpin)₂ to nitrogen heterocycles and to Me₃SiN₃. Using stoichiometric (Bpin)₂(pyrazine) or (Bpin)₂(bipyridine) in an analogous manner, an aromatic nitro group is deoxygenated and subsequently borylated, and four-fold deoxygenation of (DIM)Fe(NO₃)₂(MeCN) to yield the dinitrosyl complex (DIM)Fe(NO)₂ is facile. The co-product O(Bpin)₂ is the quantitative fate of the removed oxo groups. With borylation of both nitrogen heterocycles and doubly deoxygenating two nitrates coordinated to a single metal center, broad spectrum methodology is demonstrated.

Finding a new pathway for acid-induced nitrite reduction reaction: formation of nitric oxide with hydrogen peroxide

Here, we report a new pathway for nitrite reduction chemistry, formation of cobalt-nitrosyl ({Co^II-NO}⁸) with H₂O₂ in the reaction of a Co^II-nitrito complex with a one-fold acid (H⁺) via the formation of a Co^II-nitrous acid intermediate ({Co^II-ONOH}). Mechanistic investigations using ¹⁵N-labeled-¹⁵NO₂^- revealed that the N-atom in the {Co^II-NO}⁸ complex is derived from the nitrito ligand, and H₂O₂ came from the homolysis of the ON-OH moiety. Spectral evidence supporting the formation of the Co^II-ONOH intermediate and the generation of H₂O₂ is also presented.

Crystal-facet-dependent denitrosylation: modulation of NO release from S-nitrosothiols by Cu2O polymorphs

Nitric oxide (NO), a gaseous small molecule generated by the nitric oxide synthase (NOS) enzymes, plays key roles in signal transduction. The thiol groups present in many proteins and small molecules undergo nitrosylation to form the corresponding S-nitrosothiols. The release of NO from S-nitrosothiols is a key strategy to maintain the NO levels in biological systems. However, the controlled release of NO from the nitrosylated compounds at physiological pH remains a challenge. In this paper, we describe the synthesis and NO releasing ability of Cu2O nanomaterials and provide the first experimental evidence that the nanocrystals having different crystal facets within the same crystal system exhibit different activities toward S-nitrosothiols. We used various imaging techniques and time-dependent spectroscopic measurements to understand the nature of catalytically active species involved in the surface reactions. The denitrosylation reactions by Cu2O can be carried out multiple times without affecting the catalytic activity.

Computational analysis of non-heme iron-oxo formation by direct NO release in nitrite reduction

A direct NO-releasing reaction of nitrite catalyzed by [N(afa^Cy)₃Fe(OTf)]⁺ (afa (azafulvene-amine); OTf (trifluoromethanesulfonate); Cy (cyclohexyl)) was investigated using density functional theory (DFT) with D3 dispersion correction. The complex featured a secondary coordination sphere that facilitated the formation of the iron-oxo product [N(afa^Cy)₃FeO]⁺ with three (Fe)OH-N hydrogen bonds. As a high-spin iron(ii), the O-binding initial intermediate Fe(O)-nitrito was thermodynamically favorable in the S = 2 state. The cleavage of the (Fe)O-NO bond was performed by a β-electron shift to produce Fe(iii)-O by electron rearrangement in the S = 5/2 state. The different electron configurations are responsible for the structural properties, the valence of iron in the complexes, and the pathways of the reactions. Moreover, the two different H-bonds, (Fe)OH-N and (Fe)O-HN (by O-protonation), in the product complexes played a role in determining the reaction channels by impacting the N-H bond rotation. Thus, an exothermic sequence of conversions Fe(ii) → Fe(iii)-O → Fe(iii)-OH → Fe(iii)-O was established for the targeted product formation. This process provided a clue to build two key intermediates, iron-oxo and iron-hydroxo, in a variety of biological and synthetic systems. The results of this study are in agreement with experimental observations and describe the roles of H-bonding in nitrite reduction catalyzed by the non-heme iron complex.

Cobalt Metallopeptide Electrocatalyst for the Selective Reduction of Nitrite to Ammonium

A cobalt-tripeptide complex (CoGGH) is developed as an electrocatalyst for the selective six-electron, eight-proton reduction of nitrite to ammonium in aqueous buffer near neutral pH. The onset potential for nitrite reduction occurs at -0.65 V vs Ag/AgCl (1 M KCl). Controlled potential electrolysis at -0.90 V generates ammonium with a faradaic efficiency of 90 ± 3% and a turnover number of 3550 ± 420 over 5.5 h. CoGGH also catalyzes the reduction of the proposed intermediates nitric oxide and hydroxylamine to ammonium. These results reveal that a simple metallopeptide is an active functional mimic of the complex enzymes cytochrome c nitrite reductase and siroheme-containing nitrite reductase.

A Nonheme Sulfur-Ligated {FeNO}6 Complex and Comparison with Redox-Interconvertible {FeNO}7 and {FeNO}8 Analogues

A nonheme {FeNO}⁶ complex, [Fe(NO)(N3PyS)]²⁺ , was synthesized by reversible, one-electron oxidation of an {FeNO}⁷ analogue. This complex completes the first known series of sulfur-ligated {FeNO}^6-8 complexes. All three {FeNO}^6-8 complexes are readily interconverted by one-electron oxidation/reduction. A comparison of spectroscopic data (UV/Vis, NMR, IR, Mössbauer, X-ray absorption) provides a complete picture of the electronic and structural changes that occur upon {FeNO}⁶ -{FeNO}⁸ interconversion. Dissociation of NO from the new {FeNO}⁶ complex is shown to be controlled by solvent, temperature, and photolysis, which is rare for a sulfur-ligated {FeNO}⁶ species.

A mononuclear nonheme {FeNO}6 complex: synthesis and structural and spectroscopic characterization

While the synthesis and characterization of {FeNO}^7,8,9 complexes have been well documented in heme and nonheme iron models, {FeNO}⁶ complexes have been less clearly understood. Herein, we report the synthesis and structural and spectroscopic characterization of mononuclear nonheme {FeNO}⁶ and iron(iii)-nitrito complexes bearing a tetraamido macrocyclic ligand (TAML), such as [(TAML)Fe^III(NO)]^- and [(TAML)Fe^III(NO₂)]^2-, respectively. First, direct addition of NO_(g) to [Fe^III(TAML)]^- results in the formation of [(TAML)Fe^III(NO)]^-, which is sensitive to moisture and air. The spectroscopic data of [(TAML)Fe^III(NO)]^-, such as ¹H nuclear magnetic resonance and X-ray absorption spectroscopies, combined with computational study suggest the neutral nature of nitric oxide with a diamagnetic Fe center (S = 0). We also provide alternative pathways for the generation of [(TAML)Fe^III(NO)]^-, such as the iron-nitrite reduction triggered by protonation in the presence of ferrocene, which acts as an electron donor, and the photochemical iron-nitrite reduction. To the best of our knowledge, the present study reports the first photochemical nitrite reduction in nonheme iron models.

Converting between the oxides of nitrogen using metal-ligand coordination complexes

The oxides of nitrogen (chiefly NO, NO3(-), NO2(-) and N2O) are key components of the natural nitrogen cycle and are intermediates in a range of processes of enormous biological, environmental and industrial importance. Nature has evolved numerous enzymes which handle the conversion of these oxides to/from other small nitrogen-containing species and there also exist a number of heterogeneous catalysts that can mediate similar reactions. In the chemical space between these two extremes exist metal-ligand coordination complexes that are easier to interrogate than heterogeneous systems and simpler in structure than enzymes. In this Tutorial Review, we will examine catalysts for the inter-conversions of the various nitrogen oxides that are based on such complexes, looking in particular at more recent examples that take inspiration from the natural systems.

Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}(7) Complex with Dioxygen

The nonheme iron complex, [Fe(NO)(N3PyS)]BF4, is a rare example of an {FeNO}(7) species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin S = 1/2 ground state is populated at 0-150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 150 K. These results explain the observation of two N-O vibrational modes at 1737 and 1649 cm(-1) in CD3CN for [Fe(NO)(N3PyS)]BF4 at room temperature. This {FeNO}(7) complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron(III)-nitro complex, [Fe(III)(NO2)(N3PyS)](+), which was characterized by EPR, FTIR, UV-vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O2 is incorporated in the Fe(III)-NO2 product. The O2 reactivity of [Fe(NO)(N3PyS)]BF4 in methanol is dramatically different from CH3CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both Fe(III)-superoxo and Fe(III)-peroxynitrite intermediates and takes into account the experimental observations. The stability of the Fe(III)-nitrite complex is limited, and decay of [Fe(III)(NO2)(N3PyS)](+) leads to {FeNO}(7) species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO}(7) complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented.

cis and trans influences on [Pt(SRF)(triphos)]+ complexes (SRF = polyfluorobenzothiolate)

Platinum(ii) derivatives of the tridentate ligand Ph₂PCH₂CH₂P(Ph)CH₂CH₂PPh₂ (triphos), including [Pt(SR_F)(triphos)](CF₃SO₃) R_F = C₆F₄CF₃-4 1, C₆F₅2, C₆HF₄-4 3, C₆H₄CF₃-4 4, C₆H₃F₂-2,4 5, C₆H₄F-2 6, C₆H₄F-4 7 and C₆H₅8, have been prepared and characterised.