Kinetics of two-electron oxidations by the compound I derivative of chloroperoxidase, a model for cytochrome P450 oxidants

[structure: see text] Rate constants for two-electron oxidation reactions of Compound I from chloroperoxidase (CPO) with a variety of substrates were measured by stopped-flow kinetic techniques. The thiolate ligand of CPO Compound I activates the iron-oxo species with the result that oxidation reactions are 2 to 3 orders of magnitude faster than oxidations by model iron(IV)-oxo porphyrin radical cations containing weaker binding counterions.     

Reaction of [RuIII(edta)(H2O)]- with H2O2 in aqueous solution. Kinetic and mechanistic investigation

The reaction of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with hydrogen peroxide was studied kinetically as a function of [H(2)O(2)], temperature (5-35 degrees C) and pressure (1-1300 atm) at a fixed pH of 5.1 using stopped-flow techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOH)](2-) intermediate which subsequently undergoes parallel heterolytic and homolytic cleavage to produce [(edta)Ru(V)=O](-) (45%) and [(edta)Ru(IV)(OH)](-) (55%), respectively. The water soluble trap, 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate) (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first conditions for [ABTS] > [HOBr] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS* (+). A detailed mechanism in agreement with the rate and activation parameters is presented, and the results are discussed with reference to data reported for the corresponding [Fe(III)(edta)(H(2)O)](-)/H(2)O(2) system.     

Electrochemical mass spectrometric studies on 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine

Mechanistic studies on chemical and biological one-electron oxidations of cyclic tertiary allylamines are being pursued with the aid of an electrochemical-electrospray ionization mass spectrometric based assay. The results of previous studies on the electrochemical oxidation of 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine have documented a two-electron oxidative N-decyclopropylation pathway. The present paper describes the characterization of a second pathway involving an overall four-electron oxidation of this cyclopropylamine. The results document more completely the fate of cyclopropylaminyl radical cations that are thought to be intermediates in enzyme-catalyzed oxidations of aminyl substrates and that may lead to chemically reactive metabolites.     

Molecular and electronic structures of dinuclear iron complexes incorporating strongly electron-donating ligands: implications for the generation of the one- and two-electron oxidized forms

The ligands (L(t-Bu(2)))(2-), (L(Me(2)))(2-), and (L(Cl(2)))(2-) have been employed for the synthesis of the dinuclear Fe(III) complexes [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))], [L(Me(2))Fe(μ-O)FeL(Me(2))], and [L(Cl(2))Fe(μ-O)FeL(Cl(2))]. The strongly electron-donating groups (tert-amines and phenolates) were chosen to increase the electron density at the coordinated ferric ions and thus to facilitate the oxidation of the complexes, with the possibility of fine-tuning the electronic structures by variation of the remote substituents. Molecular structures established in the solid (by single-crystal X-ray diffraction) and in solution (by X-ray absorption spectroscopy) show that the Fe ions are five-coordinate in a square-pyramidal coordination environment with the ligand adopting a trans-conformation. Spectroscopic and magnetic characterization establishes the highly covalent nature of the Fe(III)-O(oxo) and Fe(III)-O(Ph) bonds. The variations in the donor capabilities of the phenolates (due to changes in the remote substituents) are compensated for by a flexible electron donation of the Fe(III)-O(oxo) bonding. Spectroelectrochemical characterization demonstrates that [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))] can be oxidized reversibly at +0.27 and +0.44 V versus Fc(+)/Fc, whereas [L(Me(2))Fe(μ-O)FeL(Me(2))] and [L(Cl(2))Fe(μ-O)FeL(Cl(2))] exhibit irreversible oxidations at +0.29 and +0.87 V versus Fc(+)/Fc, respectively. UV-vis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and Mo?ssbauer spectroscopy show that the successive oxidations of [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))] are ligand-centered leading to the monophenoxyl radical complex [(?)L(t-Bu(2))Fe(III)(μ-O)Fe(III)L(t-Bu(2))](+) (with the oxidation primarily localized on one-half of the molecule) and the diphenoxyl radical complex [(?)L(t-Bu(2))Fe(III)(μ-O)Fe(III?)L(t-Bu(2))](2+). Both products are unstable in solution and decay by cleavage of an Fe(III)-O(oxo) bond. The two-electron oxidized species is more stable because of two equally strong Fe(III)-O(oxo) bonds, whereas in the singly oxidized species the Fe(III)-O(oxo) bond of the non-oxidized half is weakened. The decay of the monocation results in the formation of [L(t-Bu(2))Fe(III)](+) and [L(t-Bu(2))Fe(IV)=O], while the decay of the dication yields [(?)L(t-Bu(2))Fe(III)](2+) and [L(t-Bu(2))Fe(IV)=O]. Follow-up reactions of the oxidized fragments with the counteranion of the oxidant, [SbCl(6)](-), leads to the formation of [Fe(III)Cl(4)](-).     

Kinetics and mechanism of the oxidation of water soluble porphyrin FeIIITPPS with hydrogen peroxide and the peroxomonosulfate ion

The overall six-electron oxidation of water soluble porphyrin Fe(III)TPPS by hydrogen peroxide and peroxomonosulfate ion was studied by the stopped-flow method with UV-vis detection. A three-step consecutive reaction was observed with two intermediates: Fe(III)TPPS --> Int(1)--> Int(2)--> products. The products were identified as the iron(iii) complex of the biliverdin analog formed from TPPS and 4-sulfobenzoic acid. All the rate constants with both oxidizing agents were determined. Intermediate Int(1) is proposed to be the species (TPPS (+))Fe(IV)=O. Although no unambiguous proposal for the structure of Int(2) can be made, it is most probably the product of the four-electron oxidation of the original Fe(III)TPPS, contains an iron-oxo center and has a dissociable proton with a pK of around 3.1. In spite of the protolytic equilibria occuring in the pH region 2-4, the kinetic observations do not show pH dependence.     

C. fumago chloroperoxidase is also a dehaloperoxidase: oxidative dehalogenation of halophenols

We have examined the H2O2-dependent oxidative dehalogenation of 2,4,6-trihalophenols and p-halophenols catalyzed by Caldariomyces fumago chloroperoxidase (CCPO). CCPO is significantly more robust than other peroxidases and can function under harsher reaction conditions, and so its ability to dehalogenate halophenols could lead to its use as a bioremediation catalyst for aromatic dehalogenation reactions. Optimal catalysis occurred under acidic conditions (100 mM potassium phosphate solution, pH 3.0). UV-visible absorption spectroscopy, high-performance liquid chromatography, and gas chromatography/mass spectrometry clearly identified the oxidized reaction product for CCPO-catalyzed dehalogenation of 2,4,6-trihalophenols as the corresponding 2,6-dihalo-1,4-benzoquinones. This reaction has previously been reported for two His-ligated heme-containing peroxidases (see Osborne, R. L.; Taylor, L. O.; Han, K. P.; Ely, B.; Dawson, J. H. Biochem. Biophys. Res. Commun. 2004, 324, 1194-1198), but this is the first example of a Cys-ligated heme-containing peroxidase functioning as a dehaloperoxidase. The relative catalytic efficiency (turnover number) of CCPO reported herein is comparable to that of horseradish peroxidase (Ferrari, R. P.; Laurenti, E.; Trotta, F. J. Biol. Inorg. Chem. 1965, 4, 232-237). The mechanism of dehalogenation has been probed using p-halophenols as substrates. Here the major product is a dimer with 1,4-benzoquinone as the minor product. An electron-transfer mechanism is proposed that accounts for the products formed from both the 2,4,6-trihalo- and p-halophenols. Finally, we note that this is the first case of a peroxidase known primarily for its halogenation ability being shown to also dehalogenate substrates.     

Mechanism of the six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase

Cytochrome c nitrite reductase catalyzes the six-electron reduction of nitrite to ammonia without the release of potential reaction intermediates, such as NO or hydroxylamine. On the basis of the crystallographic observation of reaction intermediates and of density functional calculations, we present a working hypothesis for the reaction mechanism of this multiheme enzyme which carries a novel lysine-coordinated heme group (Fe-Lys). It is proposed that nitrite reduction starts with a heterolytic cleavage of the N-O bond which is facilitated by a pronounced back-bonding interaction of nitrite coordinated through nitrogen to the reduced (Fe(II)) but not the oxidized (Fe(III)) active site iron. This step leads to the formation of an [FeNO](6) species and a water molecule and is further facilitated by a hydrogen bonding network that induces an electronic asymmetry in the nitrite molecule that weakens one N-O bond and strengthens the other. Subsequently, two rapid one-electron reductions lead to an [FeNO](8) form and, by protonation, to an Fe(II)-HNO adduct. Hereafter, hydroxylamine will be formed by a consecutive two-electron two-proton step which is dehydrated in the final two-electron reduction step to give ammonia and an additional water molecule. A single electron reduction of the active site closes the catalytic cycle.     

Role of Fe(IV)-oxo intermediates in stoichiometric and catalytic oxidations mediated by iron pyridine-azamacrocycles

An iron(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand L1 (L1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene), 1, was prepared and characterized. Complex 1 contains low-spin iron(II) in a pseudo-octahedral geometry as determined by X-ray crystallography. Magnetic susceptibility measurements (298 K, Evans method) and M?ssbauer spectroscopy (90 K, δ = 0.50(2) mm/s, ΔE(Q) = 0.78(2) mm/s) confirmed that the low-spin configuration of Fe(II) is retained in liquid and frozen acetonitrile solutions. Cyclic voltammetry revealed a reversible one-electron oxidation/reduction of the iron center in 1, with E(1/2)(Fe(III)/Fe(II)) = 0.49 V vs Fc(+)/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex 1 catalyzed the epoxidation of cyclooctene and other olefins with H(2)O(2). Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron(IV)-oxo intermediate in the reaction of 1 with H(2)O(2) and concomitant partial ligand oxidation. A soluble iodine(V) oxidant, isopropyl 2-iodoxybenzoate, was found to be an excellent oxygen atom donor for generating Fe(IV)-oxo intermediates for additional spectroscopic (UV-vis in CH(3)CN: λ(max) = 705 nm, ε ≈ 240 M(-1) cm(-1); M?ssbauer: δ = 0.03(2) mm/s, ΔE(Q) = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (L1)Fe(IV)═O intermediate was established in rapid (k(2) = 26.5 M(-1) s(-1) for oxidation of PPh(3) at 0 °C), associative (ΔH(?) = 53 kJ/mol, ΔS(?) = -25 J/K mol) oxidation of substituted triarylphosphines (electron-donating substituents increased the reaction rate, with a negative value of Hammet's parameter ρ = -1.05). Similar double-mixing kinetic experiments demonstrated somewhat slower (k(2) = 0.17 M(-1) s(-1) at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (L1)Fe(IV)═O that could be generated from (L1)Fe(II) and H(2)O(2) or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe(IV)-oxo species, (L1)Fe(IV)═O, is a kinetically competent intermediate for cyclooctene epoxidation with H(2)O(2) at room temperature. Partial ligand oxidation of (L1)Fe(IV)═O occurs over time in oxidative media, reducing the oxidizing ability of the ferryl species; the macrocyclic nature of the ligand is retained, resulting in ferryl(IV) complexes with Schiff base PyMACs. NH-groups of the PyMAC ligand assist the oxygen atom transfer from ferryl(IV) intermediates to olefin substrates.     

Mechanistic investigation of cyclohexane oxidation by a non-heme iron complex: evidence of product inhibition by UV/vis stopped-flow studies

We report herein studies examining a binuclear non-heme iron model complex that is capable of catalytically oxidizing cyclohexane to cyclohexanol in excess of 200 turnovers, relative to the iron complex, and cyclohexanone (5 turnovers) via heterolytic cleavage of the mechanistic probe peroxide MPPH. Low-temperature stopped-flow electronic spectroscopy was utilized to investigate the mechanism of the reaction of this diiron(II) compound, Fe(2)(H(2)Hbamb)(2)(N-MeIm)(2), (H(2)Hbamb = 2,3-bis(2-hydroxybenzamido)dimethylbutane) (1) with MPPH. In the absence of substrates, the reaction proceeds in three consecutive steps starting with oxygen atom transfer to the diferrous complex to generate a putative [Fe(IV)=O species], thought to be the oxidant in the catalytic cycle. Over time, the rate of catalysis is observed to decrease without consumption of all available peroxide. By utilizing low-temperature stopped-flow UV/vis kinetic studies, the diferrous complex, 1, is shown to undergo product inhibition arising from the interaction of either cyclohexanol or MPP-OL product species to the diiron center, therefore precluding further reaction with MPPH.     

Surface control of oxidation by an adsorbed Ru(IV)-oxo complex.

When adsorbed to optically transparent, thin films of TiO(2) nanoparticles on glass, the aqua complex [Ru(II)(tpy)(bpy(PO(3)H(2))(2))(OH(2))](2+) (bpy(PO(3)H(2))(2) is 2,2'-bipyridyl-4,4'-diphosphonic acid; tpy is 2,2':6',2' '-terpyridine) is oxidized by Ce(IV)(NH(4))(2)(NO(3))(6) in 0.1 M HClO(4) to its Ru(IV)=O(2+) form as shown by UV-visible measurements and analysis of oxidative equivalents by oxidation of hydroquinone to quinone. Kinetic studies on the oxidations of cyclohexene, benzyl alcohol, phenol, and trans-stilbene by surface-bound Ru(IV)=O(2+) by UV-visible monitoring reveal direct evidence for initial 2-electron steps to give Ru(II) intermediates in all four cases. These steps are masked in solution where Ru(IV) --> Ru(II) reduction is followed by rapid reactions between Ru(II) intermediates and Ru(IV)=O(2+) to give Ru(III). Reactions between Ru(II) and Ru(IV)=O(2+) on the surface are inhibited by binding to the surface, which restricts translational mobility. Rate constants on the surface and in solution are comparable, pointing to comparable reactivities. The surface experiments give unprecedented insight into oxidation mechanism with important implications for achieving product selectivity in synthesis by limiting oxidation to two electrons.     

The reactions of AgY salts with [Fe2(η-C5H5)2(CO)4-n(CNMe)n] complexes (Y−  NO3−, BF4− or PF6−; n  0, 1 or 2). The crystal structure of [Fe(η-C5H5)2(CNMe)]BF4

The oxidative cleavage of [Fe₂(η-C₅H₅)₂(CO)_4-n(CNMe)_n] (n=0−2) by 2AgX gives mononuclear products. It is shown to be a two-electron process in most solvents but a one-electron process in acetonitrile. The two-electron oxidations proceed by way of adducts such as [Fe₂(η-C₅H₅)₂(CO)(CNMe)(μ-CO){;μ-CN(Me)AgPPh₃};]BF₄ which are isolable when n = 2, detectable when n = 1 and postulatetd when n = 0. The one-electron process gives no adducts, and 1AgX cleaves all of the substrate to [Fe(η-C₅H₅)(CO)(L)(NCMe)]⁺ and [Fe(η-C₅H₅)(CO)(L)]^. (L  CO or CNME). The latter may combine or react with added CHBr₃ to give [Fe(η-C₅H₅)(CO)(L)Br]. The structure of [Fe(η-C₅H₅)(CO)₂-(CNMe)]BF₄ has been determined by X-ray diffraction.     

Photochemical organic oxidations and dechlorinations with a mu-oxo bridged heme/non-heme diiron complex

Steady state and laser flash photolysis studies of the heme/non-heme mu-oxo diiron complex [((6)L)Fe(III)-O-Fe(III)-Cl](+) (1) have been undertaken. The anaerobic photolysis of benzene solutions of 1 did not result in the buildup of any photoproduct. However, the addition of excess triphenylphosphine resulted in the quantitative photoreduction of 1 to [((6)L)Fe(II)...Fe(II)-Cl](+) (2), with concomitant production by oxo-transfer of 1 equiv of triphenylphosphine oxide. Under aerobic conditions and excess triphenylphosphine, the reaction produces multiple turnovers (approximately 28) before the diiron complex is degraded. The anaerobic photolysis of tetrahydrofuran (THF) or toluene solutions of 1 likewise results in the buildup of 2. The oxidation products from these reactions included gamma-butyrolactone (approximately 15%) for the reaction in THF and benzaldehyde (approximately 23%) from the reaction in toluene. In either case, the O-atom which is incorporated into the carbonyl product is derived from dioxygen present under workup or under aerobic photolysis conditions. Transient absorption measurements of low-temperature THF solutions of 1 revealed the presence of an (P)Fe(II)-like [P = tetraaryl porphyrinate dianion] species suggesting that the reactive species is a formal (heme)Fe(II)/Fe(IV)=O(non-heme) pair. The non-heme Fe(IV)=O is thus most likely responsible for C-H bond cleavage and subsequent radical chemistry. The photolysis of 1 in chlorobenzene or 1,2-dichlorobenzene resulted in C-Cl cleavage reactions and the formation of [[((6)L)Fe(III)-Cl...Fe(III)-Cl](2)O](2+) (3), with chloride ligands that are derived from solvent dehalogenation chemistry. The resulting organic products are biphenyl trichlorides or biphenyl monochlorides, derived from dichlorobenzene and chlorobenzene, respectively. Similarly, product 3 is obtained by the photolysis of benzene-benzyl chloride solutions of 1; the organic product is benzaldehyde (approximately 70%). A brief discussion of the dehalogenation chemistry, along with relevant environmental perspectives, is included.     

Reactivity of electrochemically generated rhenium (II) Tricarbonyl alpha-diimine complexes: a reinvestigation of the oxidation of luminescent Re(CO)3(alpha-Diimine)Cl and related compounds

The oxidative electrochemistry of luminescent rhenium (I) complexes of the type Re(CO) 3(LL)Cl, 1, and Re(CO) 3(LL)Br, 2, where LL is an alpha-diimine, was re-examined in acetonitrile. These compounds undergo metal-based one-electron oxidations, the products of which undergo rapid chemical reaction. Cyclic voltammetry results imply that the electrogenerated rhenium (II) species 1 ( + ) and 2 ( + ) disproportionate, yielding [Re(CO) 3(LL)(CH 3CN)] (+), 7, and additional products. Double potential step chronocoulometry experiments confirm that 1 ( + ) and 2 ( + ) react via second-order processes and, furthermore, indicate that the rate of disproportionation is influenced by the basicity and steric requirements of the alpha-diimine ligands. The simultaneous generation of rhenium (I) and (III) carbonyl products was detected upon the bulk oxidation of 1 using infrared spectroelectrochemistry. The rhenium (III) products are assigned as [Re(CO) 3(LL)Cl 2] (+), 5; an inner-sphere electron-transfer mechanism of the disproportionation is proposed on the basis of the apparent chloride transfer. Chemically irreversible two-electron reduction of 5 yields 1 and Cl (-). No direct spectroscopic evidence was obtained for the generation of rhenium (III) tricarbonyl bromide disproportionation products, [Re(CO) 3(LL)Br 2] (+), 6; this is attributed to their relatively rapid decomposition to 7 and dibromine. In addition, the 17-electron radical cations, 7 ( + ), were successfully characterized using infrared spectroelectrochemistry.     

An unusual dinuclear ruthenium(III) complex with a conjugated bridging ligand derived from cleavage of a 1,4-dihydro-1,2,4,5-tetrazine ring. Synthesis,structure, and UV-vis-NIR spectroelectrochemical characterization of a five-membered redox chain incorporating two mixed-valence states

Reaction of [Ru(acac)(2)(CH(3)CN)(2)] with 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine (H(2)L) results in formation of an unexpected dinuclear complex [(acac)(2)Ru(III)(L(1))Ru(III)(acac)(2)] (1) in which the bridging ligand [L(1)](2)(-) contains an (-)HN[bond]C[double bond]N[bond]N[double bond]C[bond]NH(-) unit arising from two-electron reduction of the 1,4-dihydro-1,2,4,5-tetrazine component of H(2)L. The crystal structure of complex 1 confirms the oxidation assignment of the metal ions as Ru(III) and clearly shows the consequent arrangement of double and single bonds in the bridging ligand, which acts as a bis-bidentate chelate having two pyrazolyl/amido chelating sites. Cyclic voltammetry of the complex shows the presence of four reversible one-electron redox couples, assigned as two Ru(III)/Ru(IV) couples (oxidations with respect to the starting material) and two Ru(II)/Ru(III) couples (reductions with respect to the starting material). The separation between the two Ru(III)/Ru(IV) couples (Delta E(1/2) = 700 mV) is much larger than that between the two Ru(II)/Ru(III) couples (Delta E(1/2) = 350 mV) across the same bridging pathway, because of the better ability of the dianionic bridging ligand to delocalize an added hole (in the oxidized mixed-valence state) than an added electron (in the reduced mixed-valence state), implying some ligand-centered character for the oxidations. UV-vis-NIR spectroelectrochemical measurements were performed in all five oxidation states; the Ru(II)-Ru(III) mixed-valence state of [1](-) has a strong IVCT transition at 2360 nm whose parameters give an electronic coupling constant of V(ab) approximately 1100 cm(-1), characteristic of a strongly interacting but localized (class II) mixed-valence state. In the Ru(III)-Ru(IV) mixed-valence state [1](+), no low-energy IVCT could be detected despite the strong electronic interaction, possibly because it is in the visible region and obscured by LMCT bands.     

Ru-based oxidation catalysis

Ranging from the oxidative conversion of water to O(2) to the elegant hydroxylation of olefins and to oxidative dehydrogenation of alcohols Ru-mediated oxidations are finding increasing application due to the unique properties of this extremely versatile transition metal, whose oxidation state can vary from -II to +VIII. Covering recent developments in both homogeneously and heterogeneously catalysed oxidations (in liquid-phase as well as in novel reaction media), this tutorial review aims to provide investigators with a general picture of the chemical and structural origins of the excellent performance of many ruthenium catalysts and to promote further advancement that, it is envisaged, will soon benefit society at large.     

Theoretical investigation on the electronic structure of pentacyano(L)ferrate(II) complexes with NO(+), NO, and NO(-) ligands. Redox interconversion, protonation, and cyanide-releasing reactions

Reaction pathways for the one- and two-electron reductions of [Fe(CN)(5)NO](2)(-) have been investigated by means of a density functional theory (DFT) approach combined with the polarized continuum model (PCM) of solvation. In addition, UV-vis spectroscopic data were obtained using ZINDO/S calculations including a point-charge model simulation of solvent effects. DFT methodologies have been used to assess the thermodynamical feasibility of protonation and cyanide-release processes for the reduced species. We conclude that [Fe(CN)(5)NO](3)(-) is a stable species in aqueous solution but may release cyanide yielding [Fe(CN)(4)NO](2)(-), consistent with experimental results. On the other hand, the [Fe(CN)(5)NO](4)(-) complex turns out to be unstable in solution, yielding the product of cyanide release, [Fe(CN)(4)NO](3)(-), and/or the protonated HNO complex. All the structural and spectroscopic (IR, UV-vis) predictions for the [Fe(CN)(5)HNO](3)(-) ion are consistent with the scarce but significant experimental evidence of its presence as an intermediate in nitrogen redox interconversion chemistry. Our computed data support an Fe(II)(LS) + NO(+) assignment for [Fe(CN)(5)NO](2)(-), an Fe(II)(LS) + NO assignment for the one-electron reduction product, but an Fe(I)(LS) + NO(+) for the one-electron product after dissociation of an axial cianide, and an Fe(II) + singlet NO(-) for the two-electron reduction species.     

Mechanism of aromatic hydroxylation by an activated FeIV=O core in tetrahydrobiopterin-dependent hydroxylases

The chemical pathways leading to the hydroxylated aromatic amino acids in phenylalanine and tryptophan hydroxylases have been investigated by means of hybrid density functional theory. In the catalytic core of these non-heme iron enzymes, dioxygen reacts with the pterin cofactor and is likely to be activated by forming an iron(IV)=O complex. The capability of this species to act as a hydroxylating intermediate has been explored. Depending on the protonation state of the ligands of the metal, two different mechanisms are found to be energetically possible for the hydroxylation of phenylalanine and tryptophan by the high-valent iron-oxo species. With a hydroxo ligand the two-electron oxidation of the aromatic ring passes through a radical, while an arenium cation is involved when a water replaces the hydroxide. After the attack of the activated oxygen on the substrate, it is also found that a 1,2-hydride shift (known as an NIH shift) generates a keto intermediate, which can decay to the true product through an intermolecular keto-enol tautomerization. The benzylic hydroxylation of 4-methylphenylalanine by the Fe(IV)=O species has also been investigated according to the rebound mechanism. The computed energetics lead to the conclusion that Fe(IV)=O is capable not only of aromatic hydroxylation, but also of benzylic hydroxylation.     

Electrochemical and Spectral Characterization of Iron Corroles in High and Low Oxidation States: First Structural Characterization of an Iron(IV) Tetrapyrrole pi Cation Radical

The electrochemistry and spectroscopic properties of three iron corroles were examined in benzonitrile, dichloromethane, and pyridine containing 0.1 M tetra-n-butylammonium perchlorate or tetra-n-ethylammonium hexafluorophosphate as supporting electrolyte. The investigated compounds are represented as (OEC)Fe(IV)(C(6)H(5)), (OEC)Fe(IV)Cl, and (OEC)Fe(III)(py), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. Each iron(IV) corrole undergoes two one-electron reductions and two or three one-electron oxidations depending upon the solvent. Under the same solution conditions, the iron(III) corrole undergoes a single one-electron reduction and one or two one-electron oxidations. Each singly oxidized and singly reduced product was characterized by UV-vis and/or EPR spectroscopy. The data indicate a conversion of (OEC)Fe(IV)(C(6)H(5)) and (OEC)Fe(IV)Cl to their iron(III) forms upon a one-electron reduction and to iron(IV) corrole pi cation radicals upon a one-electron oxidation. The metal center in [(OEC)Fe(III)(C(6)H(5))](-) is low spin (S = (1)/(2)) as compared to electrogenerated [(OEC)Fe(III)Cl](-), which contains an intermediate-spin (S = (3)/(2)) iron(III). (OEC)Fe(III)(py) also contains an intermediate-spin-state iron(III) and, unlike previously characterized (OEC)Fe(III)(NO), is converted to an iron(IV) corrole upon oxidation rather than to an iron(III) pi cation radical. Singly oxidized [(OEC)Fe(IV)(C(6)H(5))](*)(+) is the first iron(IV) tetrapyrrole pi cation radical to be isolated and was structurally characterized as a perchlorate salt. It crystallizes in the triclinic space group P&onemacr; with a = 10.783(3) ?, b = 13.826(3) ?, c = 14.151(3) ?, alpha = 78.95(2) degrees, beta = 89.59(2) degrees, and gamma = 72.98(2) degrees at 293 K with Z = 2. Refinement of 8400 reflections and 670 parameters against F(o)(2) yields R1 = 0.0864 and wR2 = 0.2293. The complex contains a five-coordinated iron with average Fe-N bond lengths of 1.871(3) ?. The formulation of the electron distribution in this compound was confirmed by M?ssbauer, X-ray crystallographic, and magnetic susceptibility data as well as by EPR spectroscopy, which gives evidence for strong antiferromagnetic coupling between the iron(IV) center and the singly oxidized corrole macrocycle.     

Nickel(III) oxidation of its glycylglycylhistamine complex

The doubly-deprotonated Ni(III) complex of Gly(2)Ha (where Ha is histamine) undergoes base-assisted oxidative self-decomposition of the peptide. At 相似文献   

Oxidation state changes and electron flow in enzymatic catalysis and electrocatalysis through Wannier-function analysis

In catalysis by metalloenzymes and in electrocatalysis by clusters related in structure and composition to the active components of such enzymes transition-metal atoms can play a central role in the catalyzed redox reactions. Changes to their oxidation states (OSs) are critical for understanding the reactions. The OS is a local property and we introduce a new, generally useful local method for determining OSs, their changes, and the associated bonding changes and electron flow. The method is based on computing optimally localized orbitals (OLOs). With this method, we analyze two cases, superoxide reductase (SOR) and a proposed hydrogen-producing model electrocatalyst [FeS(2)]/[FeFe](P), a modification of the active site of the diiron hydrogenase enzymes. Both utilize an under-coordinated Fe site where a one-electron reduction (for SOR) or a two-electron reduction (for [FeFe](P)) of the substrate occurs. We obtain the oxidation states of the Fe atoms and of their critical ligands, the changes of the bonds to those ligands, and the electron flow during the catalytic cycle, thereby demonstrating that OLOs constitute a powerful interpretive tool for unraveling reaction mechanisms by first-principles computations.