Mechanism of N–N Bond Formation by Transition Metal–Nitrosyl Complexes: Modeling Flavodiiron Nitric Oxide Reductases

Nitric oxide (NO) has a number of important biological functions, including nerve signaling transduction, blood pressure control, and, at higher concentrations, immune defense. A number of pathogenic bacteria have developed methods of degrading this toxic molecule through the use of flavodiiron nitric oxide reductases (FNORs), which utilize a nonheme diiron active site to reduce NO → N₂O. The well-characterized diiron model complex [Fe₂(BPMP)­(OPr)­(NO)₂]²⁺ (BPMP⁻ = 2,6-bis­[(bis­(2- pyridylmethyl)­amino)­methyl]-4-methylphenolate), which mimics both the active site structure and reactivity of these enzymes, offers key insight into the mechanism of FNORs. Presently, we have used computational methods to elucidate a coherent reaction mechanism that shows how one and two-electron reduction of this complex induces N–N bond formation and N₂O generation, while the parent complex remains stable. The initial formation of a N–N bond to generate hyponitrite (N₂O₂^2–) follows a radical-type coupling mechanism, which requires strong Fe–NO π-interactions to be overcome to effectively oxidize the iron centers. Hyponitrite formation provides the largest activation barrier with Δ<i>G</i>^‡ = 7–8 kcal/mol (average of several functionals) in the two-electron, super-reduced mechanism. This is followed by the formation of a N₂O₂^2– complex with a novel binding mode for nonheme diiron systems, allowing for the facile release of N₂O with the assistance of a carboxylate shift. This provides sufficient thermodynamic driving force for the reaction to proceed via N₂O formation alone. Surprisingly, the one-electron “semireduced” mechanism is predicted to be competitive with the super-reduced mechanism. This is due to the asymmetry imparted by the BPMP^– ligand, allowing a one-electron reduction to overcome one of the primary Fe–NO π-interactions. Generally, mediation of N₂O formation by high-spin [{M-NO⁻}]₂ cores depends on the ease of oxidizing the M centers and breaking of the M–NO π-bonds to formally generate a “full” ³NO^– unit, allowing for the critical step of N–N bond formation to proceed (via a radical-type coupling mechanism)

Mechanism of N–N Bond Formation by Transition Metal–Nitrosyl Complexes: Modeling Flavodiiron Nitric Oxide Reductases

Nitric oxide (NO) has a number of important biological functions, including nerve signaling transduction, blood pressure control, and, at higher concentrations, immune defense. A number of pathogenic bacteria have developed methods of degrading this toxic molecule through the use of flavodiiron nitric oxide reductases (FNORs), which utilize a nonheme diiron active site to reduce NO → N₂O. The well-characterized diiron model complex [Fe₂(BPMP)­(OPr)­(NO)₂]²⁺ (BPMP⁻ = 2,6-bis­[(bis­(2- pyridylmethyl)­amino)­methyl]-4-methylphenolate), which mimics both the active site structure and reactivity of these enzymes, offers key insight into the mechanism of FNORs. Presently, we have used computational methods to elucidate a coherent reaction mechanism that shows how one and two-electron reduction of this complex induces N–N bond formation and N₂O generation, while the parent complex remains stable. The initial formation of a N–N bond to generate hyponitrite (N₂O₂^2–) follows a radical-type coupling mechanism, which requires strong Fe–NO π-interactions to be overcome to effectively oxidize the iron centers. Hyponitrite formation provides the largest activation barrier with Δ<i>G</i>^‡ = 7–8 kcal/mol (average of several functionals) in the two-electron, super-reduced mechanism. This is followed by the formation of a N₂O₂^2– complex with a novel binding mode for nonheme diiron systems, allowing for the facile release of N₂O with the assistance of a carboxylate shift. This provides sufficient thermodynamic driving force for the reaction to proceed via N₂O formation alone. Surprisingly, the one-electron “semireduced” mechanism is predicted to be competitive with the super-reduced mechanism. This is due to the asymmetry imparted by the BPMP^– ligand, allowing a one-electron reduction to overcome one of the primary Fe–NO π-interactions. Generally, mediation of N₂O formation by high-spin [{M-NO⁻}]₂ cores depends on the ease of oxidizing the M centers and breaking of the M–NO π-bonds to formally generate a “full” ³NO^– unit, allowing for the critical step of N–N bond formation to proceed (via a radical-type coupling mechanism)

A Smorgasbord of Carbon: Electrochemical Analysis of Cobalt–Bis(benzenedithiolate) Complex Adsorption and Electrocatalytic Activity on Diverse Graphitic Supports

Heterogeneous dihydrogen production manifolds comprised of bulk graphite, pencil graphite, graphite powder in Nafion films, graphene, and glassy carbon electrodes with adsorbed proton reduction catalyst TBA­[Co­(S₂C₆Cl₂H₂)₂] have been prepared and tested for their efficiency to generate dihydrogen in acidic aqueous media. The catalyst adsorbed on these inexpensive graphitic surfaces consistently displays similar electrocatalytic profiles compared to the same catalyst on highly ordered pyrolytic graphite (HOPG) supports, including high activity in moderately acidic aqueous solutions (pH < 4), moderate overpotentials (0.42 V vs platinum), and some of the highest reported initial turnover frequencies under electrolysis conditions (96 s^–1). The exceptions are glassy carbon and single-layer graphene surfaces, which only weakly adsorb the catalyst, with no sustained catalytic current upon acid addition. In particular, the improved stability and good activity observed for the catalyst adsorbed on graphite powder embedded in a Nafion film shows that this is a promising H₂ production system that can be assembled at minimal cost and effort

Reductive Transformations of a Pyrazolate-Based Bioinspired Diiron–Dinitrosyl Complex

Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to N₂O, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L^–, which serve as model systems for nonheme diiron active sites. Their <i>ferrous</i> nitrile precursors [L­{Fe­(R′CN)}₂­(μ-OOCR)]­(X)₂ (<b>1</b>) can be readily converted into the corresponding nitrosyl adducts ([L­{Fe­(NO)}₂­(μ-OOCR)]­(X)₂, <b>2</b>). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe­(NO)}⁷ (Enemark–Feltham notation) units. The nitrosyl adducts <b>2</b> can be (electro)­chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of N₂O. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of <b>2a</b> (R = Me, X = ClO₄). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L­{Fe­(NO)₂}₂]­(ClO₄), <b>3a′</b>) and a diacetato-bridged <i>ferrous</i> complex [LFe₂(μ-OAc)₂]­(ClO₄) (<b>3a″</b>). Especially <b>3a′</b> parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which N₂O release is much less efficient than for FNORs

Reductive Transformations of a Pyrazolate-Based Bioinspired Diiron–Dinitrosyl Complex

Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to N₂O, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L^–, which serve as model systems for nonheme diiron active sites. Their <i>ferrous</i> nitrile precursors [L­{Fe­(R′CN)}₂­(μ-OOCR)]­(X)₂ (<b>1</b>) can be readily converted into the corresponding nitrosyl adducts ([L­{Fe­(NO)}₂­(μ-OOCR)]­(X)₂, <b>2</b>). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe­(NO)}⁷ (Enemark–Feltham notation) units. The nitrosyl adducts <b>2</b> can be (electro)­chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of N₂O. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of <b>2a</b> (R = Me, X = ClO₄). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L­{Fe­(NO)₂}₂]­(ClO₄), <b>3a′</b>) and a diacetato-bridged <i>ferrous</i> complex [LFe₂(μ-OAc)₂]­(ClO₄) (<b>3a″</b>). Especially <b>3a′</b> parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which N₂O release is much less efficient than for FNORs

Reduction of Graphene Oxide Thin Films by Cobaltocene and Decamethylcobaltocene

Reduced graphene oxide (RGO) films have been prepared by immersion of graphene oxide (GO) films at room temperature in nonaqueous solutions containing simple, outer-sphere metallocene reductants. Specifically, solutions of cobaltocene, cobaltocene and trifluoroacetic acid (TFA), and decamethylcobaltocene each showed activity for the rapid reduction of GO films cast on a wide variety of substrates. Each reactant increased the conductivity of the films by several orders of magnitude, with RGO films prepared with either decamethylcobaltocene or cobaltocene and TFA possessing the highest conductivities (∼10⁴ S m^–1). X-ray photoelectron spectroscopy suggested that while all three reagents lowered the content of carbon–oxygen functionalities, solutions of cobaltocene and TFA were the most effective at reducing the material to sp² carbon. Separately, Raman spectra and atomic force micrographs indicated that RGO films prepared with decamethylcobaltocene consisted of the largest graphitic domains and lowest macroscopic roughness. Cumulatively, the data suggest that the outer-sphere reductants can affect the conversion to RGO but the reactivity and mechanism depend on the standard potential of the reductant and the availability of protons. This work both demonstrates a new way to prepare high-quality RGO films on a wide range of substrate materials without annealing and motivates future work to elucidate the chemistry of RGO synthesis through the tunability of outer-sphere reductants such as metallocenes

Stabilization of a Heme-HNO Model Complex Using a Bulky Bis-Picket Fence Porphyrin and Reactivity Studies with NO

Nitroxyl, HNO/NO–, the one-electron reduced form of NO, is suggested to take part in distinct signaling pathways in mammals and is also a key intermediate in various heme-catalyzed NOx interconversions in the nitrogen cycle. Cytochrome P450nor (Cyt P450nor) is a heme-containing enzyme that performs NO reduction to N2O in fungal denitrification. The reactive intermediate in this enzyme, termed “Intermediate I”, is proposed to be an Fe-NHO/Fe-NHOH type species, but it is difficult to study its electronic structure and exact protonation state due to its instability. Here, we utilize a bulky bis-picket fence porphyrin to obtain the first stable heme-HNO model complex, [Fe(3,5-Me-BAFP)(MI)(NHO)], as a model for Intermediate I, and more generally HNO adducts of heme proteins. Due to the steric hindrance of the bis-picket fence porphyrin, [Fe(3,5-Me-BAFP)(MI)(NHO)] is stable (τ1/2 = 56 min at −30 °C), can be isolated as a solid, and is available for thorough spectroscopic characterization. In particular, we were able to solve a conundrum in the literature and provide the first full vibrational characterization of a heme-HNO complex using IR and nuclear resonance vibrational spectroscopy (NRVS). Reactivity studies of [Fe(3,5-Me-BAFP)(MI)(NHO)] with NO gas show a 91 ± 10% yield for N2O formation, demonstrating that heme-HNO complexes are catalytically competent intermediates for NO reduction to N2O in Cyt P450nor. The implications of these results for the mechanism of Cyt P450nor are further discussed

Ligand Recruitment and Spin Transitions in the Solid-State Photochemistry of Fe^(III)TPPCl

We report evidence for the formation of long-lived photoproducts following excitation of iron­(III) tetraphenylporphyrin chloride (Fe^(III)TPPCl) in a 1:1 glass of toluene and CH₂Cl₂ at 77 K. The formation of these photoproducts is dependent on solvent environment and temperature, appearing only in the presence of toluene. No long-lived product is observed in neat CH₂Cl₂ solvent. A 2-photon absorption model is proposed to account for the power-dependent photoproduct populations. The products are formed in a mixture of spin states of the central iron­(III) metal atom. Metastable six-coordinate high-spin and low-spin complexes and a five-coordinate high-spin complex of iron­(III) tetraphenylporphyrin are assigned using structure-sensitive vibrations in the resonance Raman spectrum. These species appear in conjunction with resonantly enhanced toluene solvent vibrations, indicating that the Fe^(III) compound formed following photoexcitation recruits a toluene ligand from the surrounding environment. Low-temperature transient absorption (TA) measurements are used to explain the dependence of product formation on excitation frequency in this photochemical model. The six-coordinate photoproduct is initially formed in the high-spin Fe^(III) state, but population relaxes into both high-spin and low-spin state at 77 K. This is the first demonstration of coupling between the optical and magnetic properties of an iron-centered porphyrin molecule

Hydrotris(triazolyl)borate Complexes as Functional Models for Cu Nitrite Reductase: The Electronic Influence of Distal Nitrogens

Hydrotris­(triazolyl)­borate (Ttz) ligands form CuNO_<i>x</i> (<i>x</i> = 2, 3) complexes for structural and functional models of copper nitrite reductase. These complexes have distinct properties relative to complexes of hydrotris­(pyrazolyl)­borate (Tp) and neutral tridentate N-donor ligands. The electron paramagnetic resonance spectra of five-coordinate copper complexes show rare nitrogen superhyperfine couplings with the Ttz ligand, indicating strong σ donation. The copper­(I) nitrite complex [PPN]⁺[(Ttz^tBu,Me)­Cu^INO₂]⁻ has been synthesized and characterized and allows for the stoichiometric reduction of NO₂^– to NO with H⁺ addition. Anionic Cu­(I) nitrite complexes are unusual and are stabilized here for the first time because Ttz is a good π acceptor