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

Elucidation of the Electrocatalytic Nitrite Reduction Mechanism by Bio-Inspired Copper Complexes

Mononuclear copper complexes relevant to the active site of copper nitrite reductases (CuNiRs) are known to be catalytically active for the reduction of nitrite. Yet, their catalytic mechanism has thus far not been resolved. Here, we provide a complete description of the electrocatalytic nitrite reduction mechanism of a bio-inspired CuNiR catalyst Cu(tmpa) (tmpa = tris(2-pyridylmethyl)amine) in aqueous solution. Through a combination of electrochemical studies, reaction kinetics, and density functional theory (DFT) computations, we show that the protonation steps take place in a stepwise manner and are decoupled from electron transfer. The rate-determining step is a general acid-catalyzed protonation of a copper-ligated nitrous acid (HNO2) species. In view of the growing urge to convert nitrogen-containing compounds, this work provides principal reaction parameters for efficient electrochemical nitrite reduction. This contributes to the investigation and development of nitrite reduction catalysts, which is crucial to restore the biogeochemical nitrogen cycle.

A bioinspired cobalt catalyst based on a tripodal imidazole/pyridine platform capable of water reduction and oxidation

The prototypical drug carrier [Co^II(L¹)Cl]PF₆ (1), where L¹ is a tripodal amine bound to pyridine and methyl-imidazoles, had its electrocatalytic water splitting activity studied under different pH conditions. This species contains a high-spin 3d⁷ Co^II metal center, and is capable of generating both H₂ from water reduction and O₂ from water oxidation_. Turnover numbers reach 390 after 3 h for water reduction. Initial water oxidation activity is molecular, with TONs of 71 at pH 7 and 103 at pH 11.5. The results reveal that species 1 can undergo several redox transformations, including reduction to the 3d⁸ Co^I species that precedes a ^LS3d⁶ hydride for water reduction, as well as nominal Co^IVO and Co^III-OOH species required for water oxidation. Post-catalytic analyses confirm the molecular nature of reduction and support initial molecular activity for oxidation.

Reducing O2 sensitivity in electrochemical nitric oxide releasing catheters: An O2-tolerant copper(II)-ligand nitrite reduction catalyst and a glucose oxidase catheter coating

Electrocatalytic nitric oxide (NO) generation from nitrite (NO₂^-) within a single lumen of a dual-lumen catheter using Cu^II-ligand (Cu^II-L) mediators have been successful at demonstrating NO's potent antimicrobial and antithrombotic properties to reduce bacterial counts and mitigate clotting under low oxygen conditions (e.g., venous blood). Under more aerobic conditions, the O₂ sensitivity of the Cu(II)-ligand catalysts and the reaction of O₂ (highly soluble in the catheter material) with the NO diffusing through the outer walls of the catheters results in a large decreases in NO fluxes from the surfaces of the catheters, reducing the utility of this approach. Herein, we describe a new more O₂-tolerant Cu^II-L catalyst, [Cu(BEPA-EtSO₃)(OTf)], as well as a potentially useful immobilized glucose oxidase enzyme-coating approach that greatly reduces the NO reactivity with oxygen as the NO partitions and diffuses through the catheter material. Results from this work demonstrate that very effective NO fluxes (>1*10^-10 mol min^-1 cm^-2) from a single-lumen silicone rubber catheter can be achieved in the presence of up to 10% O₂ saturated solutions.

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.

Reductive NO Coupling at Dicopper Center via a [Cu2(NO)2]2+ Diamond-Core Intermediate

Treatment of a dicopper(I,I) complex with excess amounts of NO leads to the formation of a dicopper dinitrosyl [Cu₂(NO)₂]²⁺ complex capable of (i) releasing two equivalents of NO reversibly in 90% yield and (ii) reacting with another equivalent of NO to afford N₂O and dicopper nitrosyl oxo species [Cu₂(NO)(O)]²⁺. Resonance Raman characterization of the [Cu₂(NO)₂]²⁺ complex shows a ¹⁵N-sensitive N═O stretch at 1527.6 cm^-1 and two Cu-N stretches at 390.6 and 414.1 cm^-1, supporting a symmetric diamond-core structure with bis-μ-NO ligands. The conversion of [Cu₂(NO)₂]²⁺ to [Cu₂(NO)O]²⁺ occurs via a rate-limiting reaction with NO and bypasses the dicopper oxo intermediate, a mechanism distinct from that of diFe-mediated NO reduction to N₂O.

Direct Reduction of NO to N2O by a Mononuclear Nonheme Thiolate Ligated Iron(II) Complex via Formation of a Metastable {FeNO}7 Complex

Addition of NO to a nonheme dithiolate-ligated iron(II) complex, FeII(Me3TACN)(S2SiMe2) (1), results in the generation of N2O. Low-temperature spectroscopic studies reveal a metastable six-coordinate {FeNO}7 intermediate (S = 3/2) that was trapped at -135 °C and was characterized by low-temperature UV-vis, resonance Raman, EPR, Mössbauer, XAS, and DFT studies. Thermal decay of the {FeNO}7 species leads to the evolution of N2O, providing a rare example of a mononuclear thiolate-ligated {FeNO}7 that mediates NO reduction to N2O without the requirement of any exogenous electron or proton sources.

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).

Mechanistic study on reduction of nitric oxide to nitrous oxide using a dicopper complex

A density functional theory study was carried out to investigate the reduction mechanisms of NO to N₂O using a dicopper complex reported by Zhang and coworkers (J. Am. Chem. Soc., 2019, 141, 10159-10164). The reaction mechanism consists of three steps: N-N bond formation, isomerization of the resultant N₂O₂ moiety, and cleavage of the N-O bond.

Dynamic Change of Active Sites of Supported Vanadia Catalysts for Selective Catalytic Reduction of Nitrogen Oxides

Selective catalytic reduction of NOx by ammonia (NH₃-SCR) on V₂O₅/TiO₂ catalysts is a widely used commercial technology in power plants and diesel vehicles due to its high elimination efficiency for NOx removal. However, the mechanistic aspects of the NH₃-SCR reaction, especially the active sites on the V₂O₅/TiO₂ catalysts, are still a puzzle. Herein, using combined operando spectroscopy and density functional theory calculations, we found that the reactivity of the Lewis acid site was significantly overestimated due to its conversion to the Brønsted acid site. Such interconversion makes it challenging to measure the intrinsic reactivity of different acid sites accurately. In contrast, the abundant V-OH Brønsted acid sites govern the overall NOx reduction rate in realistic exhaust containing water vapor. Moreover, the vanadia species cycle between V⁵⁺═O and V⁴⁺-OH during NOx reduction, and the re-oxidation of V⁴⁺ species to form V⁵⁺ is the rate-determining step.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)-Platinum(II) Olefin Complexes as Catalysts

The nitrogen oxides NO₂, NO, and N₂O are among the most potent air pollutants of the 21^st century. A bimetallic Rh^I–Pt^II complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3–4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N₂O) to water and N₂ was recorded, making these Rh^I–Pt^II complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO₂, TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NO_x were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO₂ proceeds stepwise, to first give NO and H₂O, followed by the generation of N₂O and H₂O, which is then further converted to N₂ and H₂O. The nitrogen−nitrogen bond‐forming step takes place in the conversion from NO to N₂O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N₂O₂²⁻) complex, which was detected as an intermediate.

A Nonheme Mononuclear {FeNO}7 Complex that Produces N2 O in the Absence of an Exogenous Reductant

A new nonheme iron(II) complex, Fe^II (Me₃ TACN)((OSi^Ph2 )₂ O) (1), is reported. Reaction of 1 with NO_(g) gives a stable mononitrosyl complex Fe(NO)(Me₃ TACN)((OSi^Ph2 )₂ O) (2), which was characterized by Mössbauer (δ=0.52 mm s^-1 , |ΔE_Q |=0.80 mm s^-1 ), EPR (S=3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}⁷ complex with an S=3/2 spin ground state. The RR spectrum (λ_exc =458 nm) of 2 combined with isotopic labeling (¹⁵ N, ¹⁸ O) reveals ν(N-O)=1680 cm^-1 , which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm^-1 ). Complex 2 reacts rapidly with H₂ O in THF to produce the N-N coupled product N₂ O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N₂ O in the absence of an exogenous reductant.

Nickel-mediated N-N bond formation and N2O liberation via nitrogen oxyanion reduction

The syntheses of (DIM)Ni(NO₃)₂ and (DIM)Ni(NO₂)₂, where DIM is a 1,4-diazadiene bidentate donor, are reported to enable testing of bis boryl reduced N-heterocycles for their ability to carry out stepwise deoxygenation of coordinated nitrate and nitrite, forming O(Bpin)₂. Single deoxygenation of (DIM)Ni(NO₂)₂ yields the tetrahedral complex (DIM)Ni(NO)(ONO), with a linear nitrosyl and κ¹-ONO. Further deoxygenation of (DIM)Ni(NO)(ONO) results in the formation of dimeric [(DIM)Ni(NO)]₂, where the dimer is linked through a Ni–Ni bond. The lost reduced nitrogen byproduct is shown to be N₂O, indicating N–N bond formation in the course of the reaction. Isotopic labelling studies establish that the N–N bond of N₂O is formed in a bimetallic Ni₂ intermediate and that the two nitrogen atoms of (DIM)Ni(NO)(ONO) become symmetry equivalent prior to N–N bond formation. The [(DIM)Ni(NO)]₂ dimer is susceptible to oxidation by AgX (X = NO₃⁻, NO₂⁻, and OTf⁻) as well as nitric oxide, the latter of which undergoes nitric oxide disproportionation to yield N₂O and (DIM)Ni(NO)(ONO). We show that the first step in the deoxygenation of (DIM)Ni(NO)(ONO) to liberate N₂O is outer sphere electron transfer, providing insight into the organic reductants employed for deoxygenation. Lastly, we show that at elevated temperatures, deoxygenation is accompanied by loss of DIM to form either pyrazine or bipyridine bridged polymers, with retention of a BpinO⁻ bridging ligand.

Deoxygenation of nitrogen oxyanions coordinated to nickel using reduced borylated heterocycles leads to N–N bond formation and N₂O liberation. The nickel dimer product facilitates NO disproportionation, leading to a synthetic cycle.

DNA-Targeted Metallodrugs: An Untapped Source of Artificial Gene Editing Technology

DNA binding metal complexes are synonymous with anticancer drug discovery. Given the array of structural and chemical reactivity properties available through careful design, metal complexes have been directed to bind nucleic acid structures through covalent or noncovalent binding modes. Several recognition modes - including crosslinking, intercalation, and oxidation - are central to the clinical success of broad-spectrum anticancer metallodrugs. However, recent progress in nucleic acid click chemistry coupled with advancement in our understanding of metal complex-nucleic acid interactions has opened up new avenues in genetic engineering and targeted therapies. Several of these applications are enabled by the hybridisation of oligonucleotide or polyamine probes to discrete metal complexes, which facilitate site-specific reactivity at the nucleic acid interface under the guidance of the probe. This Review focuses on recent advancements in hybrid design and, by way of an introduction to this topic, we provide a detailed overview of nucleic acid structures and metal complex-nucleic acid interactions. Our aim is to provide readers with an insight on the rational design of metal complexes with DNA recognition properties and an understanding of how the sequence-specific targeting of these interactions can be achieved for gene engineering applications.

Spectroscopic Evidence of Hyponitrite Radical Intermediate in NO Disproportionation at a MOF-Supported Mononuclear Copper Site

Dianionic hyponitrite (N₂ O₂ ^2- ) is often proposed, based on model complexes, as the key intermediate in reductive coupling of nitric oxide to nitrous oxide at the bimetallic active sites of heme-copper oxidases and nitric oxide reductases. In this work, we examine the gas-solid reaction of nitric oxide with the metal-organic framework Cu^I -ZrTpmC* with a suite of in situ spectroscopies and density functional theory simulations, and identify an unusual chelating N₂ O₂ ^.- intermediate. These results highlight the advantage provided by site-isolation in metal-organic frameworks (MOFs) for studying important reaction intermediates, and provide a mechanistic scenario compatible with the proposed one-electron couple in these enzymes.

Polypyridyl Co complex-based water reduction catalysts: why replace a pyridine group with isoquinoline rather than quinoline?

The electronic effect of the substituent has been fully leveraged to improve the activity of molecular water reduction catalysts (WRCs). However, the steric effect of the substituents has received less attention. In this work, a steric hindrance effect was observed in a quinoline-involved polypyridyl Co complex-based water reduction catalyst (WRC), which impedes the formation of Co(iii)-H from Co(i), two pivotal intermediates for H₂ evolution, leading to significantly impaired electrocatalytic and photocatalytic activity with respect to its parent complex, [Co(TPA)Cl]Cl (TPA = tris(2-pyridinylmethyl)-amine). In sharp contrast, two isoquinoline-involved polypyridyl Co complexes exhibited significantly improved H₂ evolution efficiencies compared to [Co(TPA)Cl]Cl, benefitting mainly from the more basic and conjugated features of isoquinoline over pyridine. The dramatically different influences caused by the replacement of a pyridine group in the TPA ligand by quinoline and isoquinoline fully demonstrates the important roles of both the electronic and steric effects of a substituent. Our results may provide novel insights for designing more efficient WRCs.

Lewis Acid Coordination Redirects S-Nitrosothiol Signaling Output

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron-transfer, redox-innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C6 F5 )3 coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C6 F5 )3 coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer-sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans-[LA-O-N=N-O-LA]2- [LA=B(C6 F5 )3 ], which releases N2 O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C6 F5 )3 ] radical anion, which is susceptible to N-N coupling prior to loss of RSSR.

Biological and Bioinspired Inorganic N-N Bond-Forming Reactions

The metallobiochemistry underlying the formation of the inorganic N-N-bond-containing molecules nitrous oxide (N2O), dinitrogen (N2), and hydrazine (N2H4) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N-N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes as well as synthetic model complexes.

NO disproportionation over defective 1T′-MoS2 monolayers

NO disproportionation can be catalyzed by MoS₂ monolayers loaded with S vacancies. When the MoS₂ sheet is under 3% compressive strain, two NO molecules at a S vacancy can form a NO₂. The left N atom will react with a third NO to afford N₂O under 3% tensile strain.

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.

Nitric Oxide Generation On Demand for Biomedical Applications via Electrocatalytic Nitrite Reduction by Copper BMPA- and BEPA-Carboxylate Complexes

Intravascular (IV) catheters are essential devices in the hospital that are used to monitor a patient's blood and for administering drugs or nutrients. However, IV catheters are also prone to blood clotting at the point of insertion and infection by formation of robust bacterial biofilms on their surface. Nitric oxide (NO) is ideally suited to counteract both of these problems, due to its antimicrobial properties and its ability to inhibit platelet activation/aggregation. One way to equip catheters with NO releasing properties is by electrocatalytic nitrite reduction to NO by copper complexes in a multi-lumen configuration. In this work, we systematically investigate six closely related Cu(II) BMPA- and BEPA-carboxylate complexes (BMPA = bis-(2-methylpyridyl)amine); BEPA = bis-(2-ethylpyridyl)amine), using carboxylate groups of different chain lengths. The corresponding Cu(II) complexes were characterized using UV-Vis, EPR spectroscopy, and X-ray crystallography. Using detailed cyclic voltammetry (CV) and bulk electrocatalyic studies (with real-time NO quantification), in aqueous buffer, pH 7.4, we are able to derive clear reactivity relations between the ligand structures of the complexes, their Faradaic efficiencies for NO generation, their turnover frequencies (TOFs), and their redox potentials. Our results show that the complex [Cu(BEPA-Bu)](OAc) is the best catalyst with a high Faradaic efficiency over large nitrite concentration ranges and the expected best tolerance to oxygen levels. For this species, the more positive redox potential suppresses NO disproportionation, which is a major Achilles heel of the (faster) catalysts with the more negative reduction potentials.

Nitrosyl Linkage Isomers: NO Coupling to N2O at a Mononuclear Site

Linkage isomers of reduced metal-nitrosyl complexes serve as key species in nitric oxide (NO) reduction at monometallic sites to produce nitrous oxide (N₂O), a potent greenhouse gas. While factors leading to extremely rare side-on nitrosyls are unclear, we describe a pair of nickel-nitrosyl linkage isomers through controlled tuning of noncovalent interactions between the nitrosyl ligands and differently encapsulated potassium cations. Furthermore, these reduced metal-nitrosyl species with N-centered spin density undergo radical coupling with free NO and provide a N-N coupled cis-hyponitrite intermediate whose protonation triggers the release of N₂O. This report outlines a stepwise molecular mechanism of NO reduction to form N₂O at a mononuclear metal site that provides insight into the related biological reduction of NO to N₂O.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

HN2 O2 - as a Ligand in Mononuclear Hydrogenhyponitrite-κ2 -N,O Ruthenium Complexes with Bisphosphane Co-Ligands

The hyponitrite anion is a tentative intermediate in the reduction of nitric oxide (NO) to nitrous oxide (N₂ O) catalyzed by nitric-oxide reductase (NOR) in the process of bacterial denitrification. Owing to the considerable number of known coordination modes for the hyponitrito ligand, its actual bonding form in the enzymatic cycle is a point of current discussion. Here, we contribute to the hardly known ligand properties of a key intermediate, the monoprotonated hyponitrite anion. Three air- and water-stable ruthenium complexes with hydrogenhyponitrite as the ligand were synthesized by using commercially available bisphosphane co-ligands (1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,2-bis(diphenylphosphino)ethene (dppv)). The starting compounds [Ru(dppe)₂ (tos)]BF₄ (1) and [Ru(dppp)₂ (tos)]BF₄ (2) contained the bidentate coordinating tosylate anion (tos) as a particularly well-suited leaving group. To confirm the protonated and deprotonated species, X-ray diffraction, IR, UV/Vis spectroscopy (solution and solid state), solid-state NMR spectroscopy, and high-resolution mass spectroscopy were used. DFT calculations give insight into the bonding situation. We report on [Ru(dppe)₂ (HN₂ O₂ )]BF₄ (5), [Ru(dppp)₂ (HN₂ O₂ )]BF₄ (6), [Ru(dppv)₂ (HN₂ O₂ )]BF₄ (7), [Ru(dppp)₂ (HN₂ O₂ )]BF₄ ⋅Imi (9; Imi=imidazole) as the first mononuclear trans-hydrogenhyponitrite complexes. Isolated deprotonated analogs are [Ru(dppe)₂ (N₂ O₂ )]⋅HImi(BF₄ ) (8) and [Ru(dppv)₂ (N₂ O₂ )] ⋅HImi(BF₄ )⋅Imi (10).

NO˙ disproportionation by a {RhNO}9 pincer-type complex

NO˙ disproportionation by the pincer-type complex [Rh(PCP^tBu)(NO)]˙ (1˙) results in the formation of Rh(PCP^tBu)(NO)(NO₂) (2) with coordinated nitrite and quantitative release of N₂O.