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Flavodiiron nitric oxide reductases (FNORs), found in pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N(2)O) to detoxify NO released by the human immune system. Previously, we reported the first FNOR model system that mediates direct NO reduction (Dong et al., J. Am. Chem. Soc. 2018, 140, 13429–13440), but no intermediate of the reaction could be observed. Here we present a new set of model complexes that, depending on the ligand substitution, can either mediate direct NO reduction or stabilize a highly activated high-spin (hs) {FeNO}(7) complex, the first intermediate of the reaction. The precursors, [{Fe(II)(MPA-(RPhO)(2))}(2)] (1, R = H and 2, R = (t)Bu, Me), were prepared first and fully characterized. Complex 1 (without steric protection) directly reduces NO to N(2)O almost quantitatively, which constitutes only the second example for this reaction in model systems. Contrarily, reaction of sterically protected 2 with NO forms the stable mononitrosyl complex 3, which shows one of the lowest N-O stretching frequencies (1689 cm(−1)) observed so far for a mononuclear hs-{FeNO}(7) complex. This study confirms that an N-O stretch ≤1700 cm(−1) represents the appropriate level of activation of the FeNO unit to enable direct NO reduction. The higher activation level of these hs-{FeNO}(7) complexes compared to those formed in FNORs emphasizes the importance of hydrogen bonding residues in the active sites of FNORs to activate the bound NO ligands for direct N–N coupling and N(2)O formation. The implications of these results for FNORs are further discussed.

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.

In this paper, we report the preparation, spectroscopic and theoretical characterization, and reactivity studies of a Co(IV)-oxo complex bearing an N4-macrocyclic coligand, 12-TBC (12-TBC = 1,4,7,10-tetrabenzyl-1,4,7,10-tetraazacyclododecane). On the basis of the ligand and the structure of the Co(II) precursor, [CoII(12-TBC)(CF3SO3)2], one would assume that this species corresponds to a tetragonal Co(IV)-oxo complex, but the spectroscopic data do not support this notion. Co K-edge XAS data show that the treatment of the Co(II) precursor with iodosylbenzene (PhIO) as an oxidant at -40 °C in the presence of a proton source leads to a distinct shift in the Co K-edge, in agreement with the formation of a Co(IV) intermediate. The presence of the oxo group is further demonstrated by resonance Raman (rRaman) spectroscopy. Interestingly, the EPR data of this complex show a high degree of rhombicity, indicating structural distortion. This is further supported by the EXAFS data. Using DFT calculations, a structural model is developed for this complex with a ligand-protonated structure that features a Co═O···HN hydrogen bond and a four-coordinate Co center in a seesaw-shaped coordination geometry. Magnetic circular dichroism (MCD) spectroscopy further supports this finding. The hydrogen bond leads to an interesting polarization of the Co-oxo π-bonds, where one O(p) lone-pair is stabilized and leads to a regular Co(d) interaction, whereas the other π-bond shows an inverted ligand field. The reactivity of this complex in hydrogen atom and oxygen atom transfer reactions is discussed as well.

Non-heme high-spin (hs) {FeNO}8 complexes have been proposed as important intermediates towards N2 O formation in flavodiiron NO reductases (FNORs). Many hs-{FeNO}8 complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of non-heme iron nitrosyl complex that is stabilized by an unexpected spin-state change. Upon reduction of the hs-{FeNO}7 complex, [Fe(TPA)(NO)(OTf)](OTf) (1), the N-O stretching band vanishes, but no sign of DNIC or N2 O formation is observed. Instead, the dimer, [Fe2 (TPA)2 (NO)2 ](OTf)2 (2) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hs-{FeNO}8 intermediate, followed by a spin state change of the iron centers to low-spin (ls), and speculate that 2 models intermediates in hs-{FeNO}8 complexes that precede the disproportionation reaction.

The chemistry of nonheme iron centers with NO in different oxidation states has experienced a renaissance in the last 10 years because of recent findings of their involvement in N2O generation (in ...

The iron(II)-nitroxyl complex [Fe(NO)(L3)] (1) (with L3- = a hindered hydrotris(pyrazolyl)borate ligand), a high-spin (hs)-{FeNO}8 complex in the Enemark-Feltham notation, is surprisingly stable and is the first of its kind that could be structurally characterized. We further studied this compound using a variety of spectroscopic methods. These results indicate a hs iron(II) center with a bound 3NO- ligand where the spins are antiferromagnetic coupled ( St = 1). Vibrational data show that this complex has a very strong Fe-NO bond. DFT calculations support this result and link it to very strong π-donation from the 3NO- ligand to the iron(II) center. Furthermore, a very unusual equilibrium between the hs-{FeNO}8 complex and a dinitrosyl iron complex (DNIC) of {Fe(NO)2}9 type is observed. The O2 reactivity of the complex is finally reported.

Flavodiiron NO reductases (FNORs) are important enzymes in microbial pathogenesis, as they equip microbes with resistance to the human immune defense agent nitric oxide (NO). DFT calculations predict that a network of second coordination sphere (SCS) hydrogen bonds is critical for the key N-N coupling step in the NO reduction reaction catalyzed by FNORs. In this study, we report the synthesis of a model complex of FNORs with pendant hydrogen bond donors. For this purpose, the ligand H[BPMP] (= 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol) was modified with two amide groups in the SCS. Reaction of the precursor complex [Fe(2)(BPMP(NHCO(t)Bu)(2))(OAc)](OTf)(2) (1) (OTf(−) = triflate anion) with NO in the presence of base led to the surprising isolation of a diiron mononitrosyl complex, [Fe(2)(BPMP(NHCO(t)Bu)(NCO(t)Bu))(OAc)(NO)](OTf) (2) and a triiron decomposition product, [Fe(3)(BPMP(NHCO(t)Bu)(2)(OAc)(2)(μ-O)(2)(ONO)](OTf) (3), which were both structurally characterized. Complex 2 models the corresponding mononitrosyl adduct in FNORs. This result points towards a strategy that can be used to stabilize mononitrosyl diiron complexes, using the SCS.

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}(8−10) complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)](+/0/−). Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, despite the highly reduced nature of these complexes, they feature an NO(+) ligand throughout with strong Fe-NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)](−) complex, an example of a ls-{FeNO}(10) species, features a true reverse dative Fe→B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}(8) system, with two additional electrons “stored” on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}(9) complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-NO bond. These data are further contextualized by comparison with a related N(2) complex, [Fe(TPB)(N(2))](−), which is a key intermediate in Fe(TPB)-catalyzed N(2) fixation. Our present study finds that the Fe→B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

A phosphinite-based dinickel pincer complex, {2,3,5,6-(iPr2PO)4C6}Ni2Cl2, is synthesized by heating a mixture of 1,2,4,5-tetrahydroxybenzene, iPr2PCl, NiCl2, and Et3N at 150 °C in a microwave reactor or refluxing 1,2,4,5-(iPr2PO)4C6H2 with NiCl2 and Et3N at 175–250 °C in a flask. Substitution of chlorides by hydrides (from LiAlH4 or LiEt3BH) and methyl groups (from MeLi) generates {2,3,5,6-(iPr2PO)4C6}Ni2H2 and {2,3,5,6-(iPr2PO)4C6}Ni2Me2, respectively. Protonation of the methyl complex with HCO2H gives {2,3,5,6-(iPr2PO)4C6}Ni2(OCHO)2. Spectroscopic and crystallographic data of these Janus POCOP pincer complexes are compared to those of the analogous mononuclear complexes {2,6-(iPr2PO)2C6H3}NiX (X = Cl, H, Me, and OCHO). The hydride complex {2,3,5,6-(iPr2PO)4C6}Ni2H2 reacts with phenylacetylene and CO2 to give insertion products. It also catalyzes the reduction of CO2 with catecholborane (HBcat) to yield CH3OBcat and catBOBcat with a turnover frequency of 7.5 min–1. The stoichiometric and catalytic activi...

The nitrogen cycle is one of the most important biogeochemical cycles on Earth because nitrogen is an essential nutrient for all life forms. To supplement natural nitrogen fixation, farmers add large amounts of nitrogen-containing fertilizer to their soils such that nitrogen never becomes a limiting nutrient for plant growth. However, of the nitrogen added to fields — most of which is in the form of NH3 and NO3− — only 30–50% is taken up by plants, while the remainder is metabolized by soil microorganisms in processes with detrimental environmental impacts. The first of these processes, that is, nitrification, refers to the biological oxidation of NH3 to NO2− and NO3−, which have low retention in soil and pollute waterways, leading to downstream eutrophication and ultimately ‘dead zones’ (low oxygen zones) in coastal waters, for example, the Gulf of Mexico. In a second process, namely, denitrification, NO3− and NO2− undergo stepwise reduction to N2O and N2. Substantial amounts of the N2O produced in this process escape into the atmosphere, contributing to climate change and ozone destruction. Recent results suggest that nitrification also affords N2O. This Review describes the enzymes involved in NH3 oxidation and N2O production and degradation in the nitrogen cycle. We pay particular attention to the active site structures, the associated coordination chemistry that enables the chemical transformations and the reaction mechanisms. Nitrification and denitrification are responsible for the processing of ammonia fertilizer, ultimately leading to the generation of environmental pollutants that accumulate in waterways and the atmosphere. This Review describes the enzymes involved in these processes, which fascinate with their unusual active sites and the surprising reactions that they catalyse.

Non-heme high-spin (hs) {FeNO}(8) complexes have been proposed as important intermediates towards N(2)O formation in flavodiiron NO reductases (FNORs). Recent studies on diiron model complexes have further demonstrated this reactivity. At the same time, many hs-{FeNO}(8) complexes actually disproportionate by forming dinitrosyl iron complexes (DNICs). However, the mechanism of this reaction is not understood. In the process of investigating this reaction further, we isolated a new type of non-heme iron-nitrosyl complex that is stabilized by an unexpected spin state change. The hs-{FeNO}(7) complex, [Fe(TPA)(NO)(OTf)](OTf) (1), was synthesized first and characterized by different spectroscopic methods and X-ray crystallography. Upon reduction of 1 with 1 eq. of CoCp(2), the N-O stretching band of 1 disappears, but no sign of DNIC or N(2)O formation is observed. To our surprise, we isolated and structurally characterized a dimer, [Fe(2)(TPA)(2)(NO)(2)](OTf)(2) (2). We propose that this species is formed from dimerization of the initially formed hs-{FeNO}(8) intermediate, followed by a spin state change of the iron centers to low-spin (ls). This complex is further characterized by Mössbauer and vibrational spectroscopy. We speculate that the dinuclear structure of 2 models corresponding intermediates in hs-{FeNO}(8) complexes that precede the disproportionation reaction.

The iron(II)-nitroxyl complex [Fe(NO)(L3)] (1) (with L3

The intermediacy of metal-NNH(2) complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N(2)) fixation. In this context, we have shown that triphosphine-supported Fe(N(2)) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH(2)) complexes over an array of charge and oxidation states. Upon exposure to further H(+)/e(−) equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N−H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe(2)) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the “NNMe(2)” moiety. Specifically, although two closed-shell configurations of the “NNR(2)” ligand have been commonly considered in the literature isodiazene and hydrazido(2−) we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR(2)](•-) ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N(2) fixation activity.

Flavodiiron nitric oxide reductases (FNORs), a common enzyme family found in various types of pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N2O) as a protective detoxification mechanism. Utilization of FNORs in pathogenic bacteria helps them survive and proliferate in the human body, thus causing chronic infections. In this paper, we present a new diiron model complex, [Fe2((Py2PhO2)MP)(OPr)2](OTf), with bridging propionate ligands (OPr−) that is capable of directly reducing NO to N2O in quantitative yield without the need to (super)reduce the complex. We first prepared the diferric precursor and characterized it by UV–vis, IR, NMR and Mossbauer spectroscopies, cyclic voltammetry, and mass spectrometry. This complex can then conveniently be reduced to the diferrous complex using CoCp2. Even though this diferrous complex is highly reactive, we have successfully isolated and characterized this species using X-ray crystallography and various spectroscopic techniques. Most i...

A series of copper complexes bearing new 6-substituted tris(2-pyridylmethyl)amine ligands (LR) appended with NH(p-R-C6H4) groups (R = H, CF3, OMe) were prepared. These ligands are electronically tunable (ΔE1/2 = 160 mV) and CuI(LR)+ complexes react with oxygen to form hydrogen bonded (trans-1,2-peroxo)dicopper species.

The cobalt–nitrosyl complex [Co(NO)(L3)] is supported by a highly hindered tridentate nitrogen ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3−), and shows a linear Co–N–O unit. This complex was prepared by the reaction of the potassium salt of L3− with the cobalt–nitrosyl precursor [Co(NO)2(tmeda)](BPh4) (tmeda = N,N,N,′N′-tetramethylethylenediamine). The obtained cobalt–nitrosyl complex as well as the corresponding products from the reaction with dioxygen, [Co(η2-O2N)(L3)] and [Co(η2-O2NO)(L3)], were characterised by X-ray crystallography and a number of spectroscopic methods including IR/far-IR, UV-Vis, and NMR spectroscopy. We also performed MCD measurements and DFT calculations to further elucidate the electronic structure of [Co(NO)(L3)] and the optical properties of the complex. The MCD spectra reveal two NO-to-Co charge-transfer transitions with strong excited state displacements that give rise to vibrational progressions in the MCD spectra, indicative of a very covalent Co–NO bond. These results provide new insight into the properties of the Co–NO bond and the electron distribution in the complex, which is best described as [CoII(NO−)(L3)].

All 18 species of gibbons are considered threatened with extinction and listed on the IUCN Red List of Threatened Species. Because gibbons (Hylobatidae) are one of the most threatened primate families, a great need exists to determine and monitor their status effectively. To meet this need, we employed distance sampling methods to estimate the density and number of gibbon groups. We focused on southern yellow-cheeked crested gibbon in the Nam Cat Tien sector of Cat Tien National Park, Vietnam, from July to October, 2016. We used an auditory point count method at 48 listening posts to detect gibbon groups. We estimated our detection probability of calling groups of gibbons within 1,200 m of a listening post was 0.63 (95% CI: 0.54-0.74). In addition, we estimated the daily calling probability of a gibbon group to be 0.46 (95% CI: 0.33-0.59) and interpreted this as the probability that a group was available to be detected. We adjusted our group density and abundance estimates by both detection and availability probabilities. Ultimately we estimated 325 (95% CI: 232-455) gibbon groups in the Nam Cat Tien sector. Our results suggest that Cat Tien National Park contains one of the largest populations of southern yellow-cheeked crested gibbon in Vietnam. Our methods are one way of avoiding underestimation of gibbon group density and abundance by incorporating uncertainty in gibbon group availability and detection.