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

Gas Therapy: Generating, Delivery, and Biomedical Applications

Oxygen (O2), nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S), and hydrogen (H2) with direct effects, and carbon dioxide (CO2) with complementary effects on the condition of various diseases are known as therapeutic gases. The targeted delivery and in situ generation of these therapeutic gases with controllable release at the site of disease has attracted attention to avoid the risk of gas poisoning and improve their performance in treating various diseases such as cancer therapy, cardiovascular therapy, bone tissue engineering, and wound healing. Stimuli-responsive gas-generating sources and delivery systems based on biomaterials that enable on-demand and controllable release are promising approaches for precise gas therapy. This work highlights current advances in the design and development of new approaches and systems to generate and deliver therapeutic gases at the site of disease with on-demand release behavior. The performance of the delivered gases in various biomedical applications is then discussed.

Nitrate Upcycling Mediated by Organonickel Catalysis

Nitrogen oxides (NOx) are major environmental pollutants and to neutralize this long-term environmental threat, new catalytic methods are needed. Although there are biological denitrification processes involving four different enzymatic reactions to convert nitrate (NO3 -) into dinitrogen (N2), it is unfortunately difficult to apply in industry due to the complexity of the processes. In particular, nitrate is difficult to functionalize because of its chemical stability. Thus, there is no organometallic catalysis to convert nitrate into useful chemicals. Herein, we present a nickel pincer complex that is effective as a bifunctional catalyst to stepwise deoxygenate NO3 - by carbonylation and further through C-N coupling. By using this nickel catalysis, nitrate salts can be selectively transformed into various oximes (>20 substrates) with excellent conversion (>90 %). Here, we demonstrate for the first time that the highly inert nitrate ion can be functionalized to produce useful chemicals by a new organonickel catalysis. Our results show that the NOx conversion and utilization (NCU) technology is a successful pathway for environmental restoration coupled with value-added chemical generation.

Recent advances on the development of NO-releasing molecules (NORMs) for biomedical applications

Nitric oxide (NO) is an important biological messenger as well as a signaling molecule that participates in a broad range of physiological events and therapeutic applications in biological systems. However, due to its very short half-life in physiological conditions, its therapeutic applications are restricted. Efforts have been made to develop an enormous number of NO-releasing molecules (NORMs) and motifs for NO delivery to the target tissues. These NORMs involve organic nitrate, nitrite, nitro compounds, transition metal nitrosyls, and several nanomaterials. The controlled release of NO from these NORMs to the specific site requires several external stimuli like light, sound, pH, heat, enzyme, etc. Herein, we have provided a comprehensive review of the biochemistry of nitric oxide, recent advancements in NO-releasing materials with the appropriate stimuli of NO release, and their biomedical applications in cancer and other disease control.

Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand

The coupled NO-vibrational peaks [IR νNO 1775 s, 1716 vs, 1668 vs cm-1 (THF)] between two adjacent [Fe(NO)2] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)2(μ-ON2Me)Fe(NO)2] (1). Electronic interplay between [Fe(NO)2] units and [ON2Me]- ligand in DNIC 1 rationalizes that "hard" O-phenoxide moiety polarizes iron center(s) of [Fe(NO)2] unit(s) to enforce a "constrained" π-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2- electron configuration (Stotal = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)2]10 unit, while preserving the {Fe(NO)2}9 core in DNICs {Fe(NO)2}9-[·ON2Me]2- [K-18-crown-6-ether)][(ON2Me)Fe(NO)2] (2) and {Fe(NO)2}9-[·ON2Me] [(ON2Me)Fe(NO)2][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)2}9-[·ON2Me]2-] DNIC 3 ↔ [{Fe(NO)2}9-[ON2Me]-] ↔ [{Fe(NO)2}9-[·ON2Me]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(μ-ONMe)2Fe2(NO)4] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)2] (5). In parallel with this finding, the electronic structures of [{Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2-] DNIC 1, [{Fe(NO)2}9-[·ON2Me]2-] DNIC 2, [{Fe(NO)2}9-[·ON2Me]] DNIC 3, [{Fe(NO)2}9-[ONMe]-]2 DNIC 4, and [{Fe(NO)2}9-[·ONMe]2-] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively.

T-Shaped Palladium and Platinum {MNO}10 Nitrosyl Complexes

The synthesis and characterization of a homologous series of T-shaped {MNO}10 nitrosyl complexes of the form [M(PR3)2(NO)]+ (M = Pd, Pt; R = tBu, Ad) are reported. These diamagnetic nitrosyls are obtained from monovalent or zerovalent precursors by treatment with NO and NO+, respectively, and are notable for distinctly bent M-NO angles of ∼123° in the solid state. Adoption of this coordination mode in solution is also supported by the analysis of isotopically enriched samples by 15N NMR spectroscopy. Effective oxidation states of M0/NO+ are calculated, and metal-nitrosyl bonding has been interrogated using DFT-based energy decomposition analysis techniques. While a linear nitrosyl coordination mode was found to be electronically preferred, the M-NO and P-M-P angles are inversely correlated to the extent that binding in this manner is prevented by steric repulsion between the bulky ancillary phosphine ligands.

Photolytic C-Diazeniumdiolate Disassembly in the β-Diketiminate Complexes [MeLM(O2N2CPh3)] (M = Fe, Co, Cu)

The reaction of [K(18-crown-6)][O2N2CPh3] with [MeLCo(μ-Br)2Li(OEt2)] (MeL = {(2,6-iPr2C6H3)NC(Me)}2CH) generates the trityl diazeniumdiolate complex, [MeLCo(O2N2CPh3)] (1), in moderate yield. Similar metathesis reactions result in the formation of the Fe and Cu analogues, [MeLM(O2N2CPh3)] (Fe, 2; Cu, 3), which can also be isolated in moderate yields. Complexes 1-3 were characterized by ultraviolet-visible (UV-vis) spectroscopy, and their solid-state structures were determined by X-ray crystallography. These complexes were further characterized via 1H NMR spectroscopy (in the case of 1 and 2) or EPR spectroscopy (in the case of 3). Irradiation of complexes 1 and 2 with 371 nm light generates the known dinitrosyl complexes, [MeLM(NO)2] (M = Co, 4; Fe, 5), along with Ph3CH and 9-phenylfluorene. We propose that 4 and 5 are formed via the putative hyponitrite intermediates, [MeLM(κ2-O,O-ONNO)], which are formed by photoinduced homolysis of the C-N bond of the [O2N2CPh3] ligand. In contrast, irradiation of complex 3 with 371 nm light, in the presence of 1 equiv of PPh3, led to the formation of the Cu(I) complexes, [MeLCu(PPh3)], [(ArNCMeC(NO)CMeNAr)Cu(PPh3)] (6), and [(ArNCMeC(NO)CMeNAr)Cu]2 (7), of which the latter two are products of γ-nitrosation of the β-diketiminiate ligand. Also formed in this transformation are Ph3CN(H)OCPh3, Ph3PO, and N2O, along with trace amounts of NO.

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).

Nickel-Catalyzed NO Group Transfer Coupled with NOx Conversion

Nitrogen oxide (NO_x) conversion is an important process for balancing the global nitrogen cycle. Distinct from the biological NO_x transformation, we have devised a synthetic approach to this issue by utilizing a bifunctional metal catalyst for producing value-added products from NO_x. Here, we present a novel catalysis based on a Ni pincer system, effectively converting Ni-NO_x to Ni-NO via deoxygenation with CO(g). This is followed by transfer of the in situ generated nitroso group to organic substrates, which favorably occurs at the flattened Ni(I)-NO site via its nucleophilic reaction. Successful catalytic production of oximes from benzyl halides using NaNO₂ is presented with a turnover number of >200 under mild conditions. In a key step of the catalysis, a nickel(I)-^•NO species effectively activates alkyl halides, which is carefully evaluated by both experimental and theoretical methods. Our nickel catalyst effectively fulfills a dual purpose, namely, deoxygenating NO_x anions and catalyzing C-N coupling.

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.

Photoinduced linkage isomers in a model ruthenium nitrosyl complex: Identification and assignment of vibrational modes

Photoinduced NO-linkage isomers were investigated in the solid state of labelled trans-[Ru(^14/15NO)(py₄)F](ClO₄)₂ complex by combined IR-spectroscopy and DFT calculations. Based on the experimental data and the DFT calculations of this isotopically labelled ^14/15NO nitrosyl compound, we present a complete assignment of the vibrational bands of three nitrosyl linkage isomers in a range from 4000 to 200 cm^-1. The calculated IR-spectra match well with the experimental data allowing reliable assignment of the vibrational bands. The structural change from the Ru-NO (GS) to the Ru-ON (MS1) and Ru-η²-(NO) (MS2) linkage configuration leads to the downshift of the ν(NO) and ν(Ru-(NO)) bands, and a corresponding increase of the energy of the ν(Ru-F) band. The shift of the bands corresponds to the change of the Ru-(NO) and Ru-F bond lengths: increase of the Ru-(NO) bond length leads to the decrease of the energy of the ν(Ru-(NO)) band; decrease of the Ru-F bond length leads to the increase of the energy of the ν(Ru-F) band. These observations can be extrapolated to the family of related nitrosyl complexes and therefore be used for the qualitative prediction of the Ru-(NO) and Ru-L_trans-to-NO bond lengths of different linkage isomers in the framework of one complex. While the formation of linkage isomers is a reversible process, long-time irradiation sometimes induces irreversible reactions such as the release of NO. Here, we show that the photolysis of trans-[Ru(^14/15NO)(py₄)F](ClO₄)₂ in KBr pellets may lead to the release of nitrous oxide N₂O, conceivably through the formation of a {Ru-(κ²-ONNO)} intermediate.

NO Reduction to N2O Triggered by a Dinuclear Dinitrosyl Iron Complex via the Associated Pathways of Hyponitrite Formation and NO Disproportionation

In spite of the comprehensive study of the metal-mediated conversion of NO to N2O disclosing the conceivable processes/mechanism in biological and biomimetic studies, in this study, the synthesis cycles and mechanism of NO reduction to N2O triggered by the electronically localized dinuclear {Fe(NO)2}10-{Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(NO)2(μ-bdmap)Fe(NO)2(THF)] (1) (bdmap = 1,3- bis(dimethylamino)-2-propanolate) were investigated in detail. Reductive conversion of NO to N2O triggered by complex 1 in the presence of exogenous ·NO occurs via the simultaneous formation of hyponitrite-bound {[Fe2(NO)4(μ-bdmap)]2(κ4-N2O2)} (2) and [NO2]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO2)] (3) (NO disproportionation yielding N2O and complex 3). EPR/IR spectra, single-crystal X-ray diffraction, and the electrochemical study uncover the reversible redox transformation of {Fe(NO)2}9-{Fe(NO)2}9 [Fe2(NO)4(μ-bdmap)(μ-OC4H8)]+ (7) ↔ {Fe(NO)2}10-{Fe(NO)2}9 1 ↔ {Fe(NO)2}10-{Fe(NO)2}10 [Fe(NO)2(μ-bdmap)Fe(NO)2]- (6) and characterize the formation of complex 1. Also, the synthesis study and DFT computation feature the detailed mechanism of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 DNIC 1 reducing NO to N2O via the associated hyponitrite-formation and NO-disproportionation pathways. Presumably, the THF-bound {Fe(NO)2}9 unit of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 complex 1 served as an electron buffering reservoir for accommodating electron redistribution, and the {Fe(NO)2}10 unit of complex 1 acted as an electron-transfer channel to drive exogeneous ·NO coordination to yield proposed relay intermediate κ2-N,O-[NO]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO)] (A) for NO reduction to N2O.

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.

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.

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.

Quantification of Ni-N-O Bond Angles and NO Activation by X-ray Emission Spectroscopy

A series of β-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

Electronic effect of a perfluorinated β-diketiminate ligand on the bonding nature of copper carbonyl complexes

Two copper complexes 17Fnac2Cu(C6H6) (3) and 17Fnac2CuCO (4) containing the monoanionic, perfluorinated β-diketiminate 17Fnac2- ligand (1) (17Fnac2 = FC[C(CF3)N(C6F5)]2) were synthesized and characterized by IR and NMR spectroscopy (1H, 13C, 19F), cyclovaltammometry (CV), elemental analysis and single crystal X-ray diffraction. The perfluorinated 17Fnac2- ligand marginally reduces the π-back-bonding capacity of the copper centre to the carbonyl group in 4 when compared with the corresponding 16Fnac2- substituted complexes but substantially when compared with the fluorine free substituted derivatives. Quantum chemical calculations gave deeper insight into the bonding situation of this carbonyl complex, while CV studies were performed to determine the oxidation potential of 3 in solution. Based on these data, the influence of the degree of fluorination in different β-diketimine ligands on the electronic nature of the corresponding copper complexes is discussed.

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.

Coordinating Ability of Anions, Solvents, Amino Acids, and Gases towards Alkaline and Alkaline-Earth Elements, Transition Metals, and Lanthanides

After briefly reviewing the applications of the coordination ability indices proposed earlier for anions and solvents toward transition metals and lanthanides, a new analysis of crystal structures is applied now to a much larger number of coordinating species: anions (including those that are present in ionic solvents), solvents, amino acids, gases, and a sample of neutral ligands. The coordinating ability towards s-block elements is now also considered. The effect of several factors on the coordinating ability will be discussed: (a) the charge of an anion, (b) the chelating nature of anions and solvents, (c) the degree of protonation of oxo-anions, carboxylates and amino carboxylates, and (d) the substitution of hydrogen atoms by methyl groups in NH₃ , ethylenediamine, benzene, ethylene, pyridine and aldehydes. Hit parades of solvents and anions most commonly used in the areas of transition metal, s-block and lanthanide chemistry are deduced from the statistics of their presence in crystal structures.

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.

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.

One metal is enough: a nickel complex reduces nitrate anions to nitrogen gas

A stepwise reduction sequence from nitrate to dinitrogen gas at a single nickel center was discovered. A PNP nickel scaffold (PNP- = N[2-P i Pr2-4-Me-C6H3]2) emerged as a universal platform for the deoxygenation of NO x substrates. In these reactions carbon monoxide acts as the oxygen acceptor and forms CO2 to provide the necessary chemical driving force. Whereas the first two oxygens are removed from the Ni-nitrate and Ni-nitrite complexes with CO, the deoxygenation of NO requires a disproportionation reaction with another NO molecule to form NO2 and N2O. The final deoxygenation of nitrous oxide is accomplished by the Ni-NO complex and generates N2 and Ni-NO2 in a relatively slow, but clean reaction. This sequence of reactions is the first example of the complete denitrification of nitrate at a single metal-site and suggests a new paradigm of connecting CO and NO x as an effective reaction pair for NO x removal.