Hydroperoxo Complex Research Articles

Hydrogen-Atom Transfer from Transition Metal Hydroperoxides, Hydrogen Peroxide, and Alkyl Hydroperoxides to Superoxo and Oxo Metal Complexes

Superoxochromium(III) complexes L(H2O)CrOO2+ (L = (H2O)4 and 1,4,8,11-tetraazacyclotetradecane) oxidize hydroperoxo complexes of rhodium and cobalt in an apparent hydrogen-atom transfer process, i.e., L(H2O)CrOO2+ + L(H2O)RhOOH2+ --> L(H2O)CrOOH2+ + L(H2O)RhOO2+. All of the measured rate constants fall in a narrow range, 17-135 M-1 s-1. These values are about 2.5-3.0 times smaller in D2O, where the hydroperoxo hydrogen is replaced by deuterium, and coordinated molecules of water by D2O. The failure of the back reaction to take place in the available concentration range places the O-H bond dissociation energy in RhOO-H2+ at <or=320 kJ/mol. The rates of oxidation of L(H2O)RhOOH2+ by CraqOO2+ are comparable to those for the oxidation of the corresponding hydrides despite the great difference (>or=80 kJ/mol) in the driving force for the two types of reactions. A chromyl ion, CrIVaqO2+, oxidizes L(H2O)RhOOH2+ and the cobalt analogs to the corresponding superoxo complexes. The rate constants are approximately 102-fold larger than those for the oxidation by CraqOO2+. The oxidation of tert-BuOOH by CrIVaqO2+ has k = 160 M-1 s-1 and exhibits an isotope effect kBuOOH/kBuOOD = 12. Hydrogen atom transfer from H2O2 to CraqOO2+ is slow, k approximately 10-3 M-1 s-1.

Fe(II) Mononuclear Complexes with a New Aminopyridyl Ligand Bearing a Pivaloylamido Arm. Preparation and Spectroscopic Characterizations of a FeIII-Hydroperoxo Complex with Oxygen and Nitrogen Donors

Two new mononuclear FeII complexes, [(L52aH)FeII](PF6)2 (1-(PF6)2) and [(L52a)FeII]BPh4 (2-(BPh4)) have been synthesized with the new aminopyridyl ligand bearing a pivaloylamido arm L52aH (2,2-dimethyl-N-[6-({[2-(methyl-pyridin-2-ylmethyl-amino)-ethyl]-pyridin-2-ylmethyl-amino}-methyl)-pyridin-2-yl]-propionamide), or its deprotonated form L52a-. The structures of the ferrous complexes have been determined by X-ray analysis. The mononuclear FeII is in a pseudo-octahedral environment in both complexes, the six positions around the metal center being occupied by five nitrogen atoms and one oxygen atom from the ligand. Whatever the protonation state of the amide function, the structures are very similar, the FeII being 6-fold coordinated by the two amines, three pyridines, and the oxygen atom from the ligand. These two complexes exhibit an acid/base equilibrium in solution that has been studied by UV-vis spectroscopy and cyclic voltammetry in acetonitrile. The reactivity of 1-(PF6)2 with H2O2 in methanol affords the formation of a new low-spin FeIII(OOH) intermediate in which the oxygen atom is retained in the coordination sphere of the metal.

Proton-Shuffle Mechanism of O−O Activation for Formation of a High-Valent Oxo−Iron Species of Bleomycin

Bleomycins (BLMs) can utilize H2O2 to cleave DNA in the presence of ferric ions. DFT calculations were used to study the mechanism of O-O bond cleavage in the low-spin FeIII-hydroperoxo complex of BLM. The following alternative hypotheses were investigated using realistic structural models: (a) heterolytic cleavage of the O-O bond, generating a Compound I (Cpd I) like intermediate, formally BLM-FeV=O; (b) homolytic O-O cleavage, leading to a BLM-FeIV=O species and an OH* radical; and (c) a direct O-O cleavage/H-abstraction mechanism by ABLM. The calculations showed that (a) is a facile and viable mechanism; it involves acid-base proton reshuffle mediated by the side-chain linkers of BLM, causing thereby heterolytic cleavage of the O-O bond and generation of Cpd I. Formation of Cpd I is found to involve a barrier of 13.3 kcal/mol, which is lower than the barriers in the alternative mechanisms (b and c) that possess respective barriers of 31 and 17 kcal/mol. The so-formed Cpd I species with a radical on the side-chain linker, methylvalerate (V), adjacent to the BLM-FeIV=O complex, resembles the formation of the active species of cytochrome c peroxidase in the Poulos-Kraut proton-shuffle mechanism in heme peroxidases (Poulos, T. L.; Kraut, J. J. Biol. Chem. 1980, 255, 8199-8205). Experimental data are discussed and shown to be in accord with this proposal. It suggests that the high-valence Cpd I species of BLM participates in the DNA cleavage. This is an alternative mechanistic hypothesis to the exclusive reactivity scenario based on ABLM (FeIII-OOH).

Iron-assisted, base-catalyzed biomimetic activation of dioxygen by dioximatoiron(II) complexes: Kinetics and mechanism of model catecholase activity

The dioximatoiron complexes [Fe(Hdmed)]+, [Fe(Hdmpd)]+ and [Fe(H2dmdt)]2+ catalyze the oxidation of 3,5-di-tert-butylcatechol (H2dtbc) by O2 to the corresponding o-benzoquinone (dtbq) at room temperature in MeOH solution. The reaction was followed by measuring the rate of dioxygen absorption as a function of catalyst, substrate and dioxygen concentration. Kinetic measurements reveal first-order dependence on the catalyst and O2 concentration and saturation type behavior with respect to the substrate. The proposed reaction mechanism involves prior binding of the substrate H2dtbc and O2 to the iron complex, forming a ternary active intermediate, decomposing in the rate-limiting step to a semiquinonato anion radical (dbsq−), detected by ESR spectroscopy. It is then rapidly oxidized to the dtbq product. Added triethylamine accelerates the reaction to an extent much greater than that expected from the parallel base-catalyzed oxidation route. The kinetic behavior is similar to the TEA-free systems except for a saturation type dependence on TEA. This feature is due to a novel iron-enhanced oxidation path in which Hdtbc− binds O2 to form the hydroperoxide HdtbcO2−, coordinating to the iron(II) complexes as a hydroperoxo ligand. Subsequently, the hydroperoxo complex eliminates dbsq−, which is directly oxidized by O2 to dtbq. According to Mössbauer spectroscopy, the catalyst species are predominantly low-spin iron(II) complexes.

QM/MM Study of Mechanisms for Compound I Formation in the Catalytic Cycle of Cytochrome P450cam

In the catalytic cycle of cytochrome P450cam, after molecular oxygen binds as a ligand to the heme iron atom to yield a ferrous dioxygen complex, there are fast proton transfers that lead to the formation of the active species, Compound I (Cpd I), which are not well understood because they occur so rapidly. In the present work, the conversion of the ferric hydroperoxo complex (Cpd 0) to Cpd I has been investigated by combined quantum-mechanical/molecular-mechanical (QM/MM) calculations. The residues Asp(251) and Glu(366) are considered as proton sources. In mechanism I, a proton is transported to the distal oxygen atom of the hydroperoxo group via a hydrogen bonding network to form protonated Cpd 0 (prot-Cpd0: FeOOH(2)), followed by heterolytic O-O bond cleavage that generates Cpd I and water. Although a local minimum is found for prot-Cpd0 in the Glu(366) channel, it is very high in energy (more than 20 kcal/mol above Cpd 0) and the barriers for its decay are only 3-4 kcal/mol (both toward Cpd 0 and Cpd I). In mechanism II, an initial O-O bond cleavage followed by a concomitant proton and electron transfer yields Cpd I and water. The rate-limiting step in mechanism II is O-O cleavage with a barrier of about 13-14 kcal/mol. According to the QM/MM calculations, the favored low-energy pathway to Cpd I is provided by mechanism II in the Asp(251) channel. Cpd 0 and Cpd I are of similar energies, with a slight preference for Cpd I.

Superoxo, Peroxo, and Hydroperoxo Complexes Formed from Reactions of Rhodium Porphyrins with Dioxygen: Thermodynamics and Kinetics

Rhodium(II) porphyrin complexes react with dioxygen to form terminal superoxo and bridged mu-peroxo complexes. Equilibrium constants for dioxygen complex formation with rhodium(II) tetramesitylporphyrin ((TMP)Rh*) and a m-xylyl-tethered dirhodium(II) diporphyrin complex (*Rh(m-xylyl)Rh*) are reported. (TMP)Rh-H reacts with oxygen to form a transient hydroperoxy complex ((TMP)Rh-OOH), which reacts on to form the rhodium(II) complex ((TMP)Rh*) and water. Kinetic studies for reactions of (TMP)Rh-H with O2 suggest a near concerted addition of dioxygen to the (TMP)Rh-H unit. Reactivity studies for mixtures of H2/O2 and CH4/O2 with the dirhodium(II) complex (*Rh(m-xylyl)Rh*) are reported.

Reactivities of Mononuclear Non-Heme Iron Intermediates Including Evidence that Iron(III)−Hydroperoxo Species Is a Sluggish Oxidant

There is an intriguing, current controversy on the involvement of iron(III)-hydroperoxo species as a "second electrophilic oxidant" in oxygenation reactions by heme and non-heme iron enzymes and their model compounds. In the present work, we have performed reactivity studies of the iron-hydroperoxo species in nucleophilic and electrophilic reactions, with in situ-generated mononuclear non-heme iron(III)-hydroperoxo complexes that have been well characterized with various spectroscopic techniques. The intermediates did not show any reactivities in the nucleophilic (e.g., aldehyde deformylation) and electrophilic (e.g., oxidation of sulfide and olefin) reactions. These results demonstrate that non-heme iron(III)-hydroperoxo species are sluggish oxidants and that the oxidizing power of the intermediates cannot compete with that of high-valent iron(IV)-oxo complexes. We have also reported reactivities of mononuclear non-heme iron(III)-peroxo and iron(IV)-oxo complexes in the aldehyde deformylation and the oxidation of sulfides, respectively.

Synthesis of Mononuclear Non-Heme Iron(III)-Hydroperoxo Complex as an Oxidative Catalyst

Mononuclear FeII complex [FeII(ppba)]2+ (ppba = (6-pivalamido-2-pyridylmethyl) (2-pyridylmethyl)tert-butylamine) (1) has been synthesized and structurally characterized. The addition of H2O2 (10eq) to an acetone solution of 1 at -78°C generated FeIIIPPBA-OOH species, accompanied by a color change from pale yellow to reddish purple, which showed an intense absorption band corresponding to LMCT band (HOO- to Fe) at 528 nm (ε = 1120 M-1cm-1). The ESR spectrum for FeIIIPPBA-OOH species (g// = 4.22, g⊥ = 7.70 in acetone at 77 K) was typical of a high-spin FeIII species, suggesting that the FeIIIPPBA-OOH species is maintained as a mononuclear complex in the solution state. The resonance Raman spectra, when H2 16O2 and H2 18O2 were used, exhibited intense bands characteristic to the O-O stretching vibrations at 880 cm-1 ν(16O-16O) and 824 cm-1 ν(18O-18O), respectively. These findings suggest that FeIIIPPBA-OOH species is [FeIII(ppba)(OOH)]2+ (2). In order to understand the catalytic reactivity of complex 2, oxidative reactions for some organic substrates by complex 2 were carried out in the presence of H2O2; thioanisole was oxidized to the corresponding sulfoxide and sulfone, respectively, and furthermore, cyclohexene was converted to cyclohexene oxide, cyclohexenol and cyclohexenone.

DFT Analysis of Interligand Vibrations in a Hydroperoxo Complex of Cobalt Bleomycin

Density functional theory (DFT) has been applied to the analysis of interligand vibrations in two chiral isomers of hydroperoxo complex of cobalt bleomycin (BLM-Co(III)-OOH, BLM = bleomycin). The DFT-based normal coordinate analysis reveals that 16O/18O isotope-sensitive modes associated with the Co-OOH moiety uniquely reflect the chiral organization of ligands around the cobalt atom. This study provides an independent probe of cobalt chirality coordinated to BLM and shows that interligand modes associated with the Co-OOH moiety could be used as a structural marker of the chiral isomers.

Aqueous rhodium(iii) hydrides and mononuclear rhodium(ii) complexes

In aqueous solutions, as in organic solvents, rhodium hydrides display the chemistry of one of the three limiting forms, i.e. {Rh(I)+ H+}, {Rh(II)+ H.}, and {Rh(III)+ H-}. A number of intermediates and oxidation states have been generated and explored in kinetic and mechanistic studies. Monomeric macrocyclic rhodium(II) complexes, such as L(H2O)Rh2+ (L = L1 = [14]aneN4, or L2 = meso-Me6[14]aneN4) can be generated from the hydride precursors by photochemical means or in reactions with hydrogen atom abstracting agents. These rhodium(II) complexes are oxidized rapidly with alkyl hydroperoxides to give alkylrhodium(III) complexes. Reactions of Rh(II) with organic and inorganic radicals and with molecular oxygen are fast and produce long-lived intermediates, such as alkyl, superoxo and hydroperoxo complexes, all of which display rich and complex chemistry of their own. In alkaline solutions of rhodium hydrides, the existence of Rh(I) complexes is implied by rapid hydrogen exchange between the hydride and solvent water. The acidity of the hydrides is too low, however, to allow the build-up of observable quantities of Rh(I). Deuterium kinetic isotope effects for hydride transfer to a macrocyclic Cr(v) complex are comparable to those for hydrogen atom transfer to various substrates.

Mononuclear copper(ii)–hydroperoxo complex derived from reaction of copper(i) complex with dioxygen as a model of DβM and PHM

A mononuclear copper(II)-hydroperoxo species has been generated by the reaction of Cu(I)-H2BPPA complex with dioxygen, which illustrates the enzymatic reaction process of the CuB site in the DbetaM and PHM.

Ligand effect on the kinetics of hydroperoxochromium(iii)–oxochromium(v) transformation and the lifetime of chromium(v)

A macrocyclic superoxochromium complex L(2)(H(2)O)CrOO(2+)(L(2)=meso-Me(6)-[14]aneN(4)) is generated from L(2)Cr(H(2)O)(2)(2+) and O(2) with k(on)=(2.80 +/- 0.07)x 10(7) M(-1) s(-1). One-electron reduction of L(2)(H(2)O)CrOO(2+) produces a transient hydroperoxo complex that readily undergoes intramolecular conversion to L(2)Cr(v), k(1)= 1.00 +/- 0.01 s(-1) in acidic aqueous solutions, and 0.273 +/- 0.010 s(-1) at pH >7, with an apparent pK(a) of 5.9. The decay of L(2)Cr(v) in the pH range 1.3-6.2 obeys the rate law, -d[L(2)Cr(v)]/dt= (0.0080 (+/- 0.0049)+ 8.19 (+/- 0.13)[H(+)])[L(2)Cr(v)]. Both the kinetics of formation and lifetime of L(2)Cr(v) are significantly different from those for the closely related [14]aneN(4) complex. The X-ray structure of the parent Cr(iii) complex, [L(2)Cr(H(2)O)(2)](ClO(4))(3).4H(2)O, shows that the macrocyclic ligand adopts the most stable, "two up-two down" configuration around the nitrogens.

Pulp bleaching by hydrogen peroxide activated with copper 2,2′-dipyridylamine and 4-aminopyridine complexes

Abstract Oxygen-prebleached kraft pulp (OKP) was bleached with H2O2 activated with a copper complex coordinated with 2,2′-dipyridylamine (dpa) or 4-aminopyridine (4-ap) under alkaline conditions. Bleaching of OKP by H2O2 activated with the Cu(II)–dpa complex decreased the kappa number (k) and viscosity ( v ) of the pulp by 26 and 0.7%, respectively. In contrast pulp bleaching without the coordinator resulted in a decrease in k and v values of 25 and 7.8%, respectively. Thus, selectivity for delignification, expressed by k / v , was increased 12-fold by the coordination with dpa. ESR demonstrated that the coordination with dpa suppressed the production of OH by 57%. These results support the involvement of a hydroperoxo complex of Cu(II) formed by the reaction of −OOH with [Cu(II)(dpa)(H2O)3]2+. When the OKP was bleached with H2O2 activated with a Cu(II)–4-ap complex, selectivity ( k / v ) in pulp bleaching increased by 2.6-fold and production of OH decreased by 47%. We conclude that Cu(II)–dpa is a catalyst potentially applicable to totally chlorine free (TCF) bleaching sequences due to its high selectivity for delignification.

Kinetics and DFT studies on the reaction of copper(II) complexes and H2O2

Copper(II) complexes supported by bulky tridentate ligands L1(H) (N,N-bis(2-quinolylmethyl)-2-phenylethylamine) and L1(Ph) (N,N-bis(2-quinolylmethyl)-2,2-diphenylethylamine) have been prepared and their crystal structures as well as some physicochemical properties have been explored. Each complex exhibits a square pyramidal structure containing a coordinated solvent molecule at an equatorial position and a weakly coordinated counter anion (or water) at an axial position. The copper(II) complexes reacted readily with H(2)O(2) at a low temperature to give mononuclear hydroperoxo copper(II) complexes. Kinetics and DFT studies have suggested that, in the initial stage of the reaction, deprotonated hydrogen peroxide attacks the cupric ion, presumably at the axial position, to give a hydroperoxo copper(II) complex retaining the coordinated solvent molecule (H(R).S). H(R).S then loses the solvent to give a tetragonal copper(II)-hydroperoxo complex (H(R)), in which the -OOH group may occupy an equatorial position. The copper(II)-hydroperoxo complex H(R) exhibits a relatively high O-O bond stretching vibration at 900 cm(-1) compared to other previously reported examples.

Formation of 8-Oxo-7,8-dihydro-2‘-deoxyguanosine (8-Oxo-dGuo) by PAH o-Quinones: Involvement of Reactive Oxygen Species and Copper(II)/Copper(I) Redox Cycling

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants and procarcinogens that require activation by host metabolism. Metabolic activation of PAHs by aldo-keto reductases (AKRs) leads to formation of reactive and redox active o-quinones, which may cause oxidatively generated DNA damage. Spectrophotometric assays showed that NADPH caused PAH o-quinones to enter futile redox cycles, which result in the depletion of excess cofactor. Copper(II) amplified NADPH-dependent redox cycling of the o-quinones. Concurrent with NADPH oxidation, molecular oxygen was consumed, indicating the production of ROS. To determine whether PAH o-quinones can cause 8-oxo-dGuo formation in salmon testis DNA, three prerequisite experimental conditions were satisfied. Quantitative complete enzymatic hydrolysis of DNA was achieved, adventitious oxidation of dGuo was eliminated by the use of chelex and desferal, and basal levels of less than 2.0 8-oxo-dGuo/10(5) dGuo were obtained. The HPLC-ECD analytical method was validated by spiking the DNA with standard 8-oxo-dGuo and demonstrating quantitative recovery. HPLC-ECD analysis revealed that in the presence of NADPH and Cu(II), submicromolar concentrations of PAH o-quinones generated >60.0 8-oxo-dGuo adducts/10(5) dGuo. The rank order of 8-oxo-dGuo generated in isolated DNA was NP-1,2-dione > BA-3,4-dione > 7,12-DMBA-3,4-dione > BP-7,8-dione. The formation of 8-oxo-dGuo by PAH o-quinones was concentration-dependent. It was completely or partially inhibited when catalase, tiron, or a Cu(I) specific chelator, bathocuproine, was added, indicating the requirement for H(2)O(2), O(2)(-), and Cu(I), respectively. Methional, which is a copper-hydroperoxo complex [Cu(I)OOH] scavenger, also suppressed 8-oxo-dGuo formation. By contrast, mannitol, sodium benzoate, and sodium formate, which act as hydroxyl radical scavengers, did not block its formation. Sodium azide, which can act as both a hydroxyl radical and a (1)O(2) scavenger, abolished the formation of 8-oxo-dGuo. These data showed that the production of 8-oxo-dGuo was dependent on Cu(II)/Cu(I) catalyzed redox cycling of PAH o-quinones to produce ROS and that the immediate oxidant was not hydroxyl radical or Cu(I)OOH and that it is more likely (1)O(2), which can produce a 4,8-endoperoxide-dGuo intermediate.

Influence of the composition of cobalt ammonia complex solutions on formation of sulphide coatings

The Co(II)–ammonia complex solution, which is used for deposition of Co sulphide coatings, has been investigated by the spectrophotometric method. The data from spectrophotometric analysis of a freshly prepared solution indicate that both the increase in intensity in the near UV region and the absorption peak at λ=360 nm are determined by intermediate Co ammonia binuclear peroxo and hydro-peroxo complexes. When these complexes dominate in solution high quality Co sulphide coatings are deposited. With solution ageing a new absorption peak emerged at λ=515 nm as a result of the formation of a final Co(III)–ammonia complex without bridge links and subsequently no Co sulphide coating formed.

ピリジルアルキルアミン系配位子を用いた銅錯体の構造と反応性の精密制御―酵素活性中心モデルを中心に―

Ligand effects on the structure and reactivity of copper complexes have been studied systematically by using several types of pyridylalkylamine ligands. Special attention has been focused on the dioxygen activation chemistry by copper (I) complexes aiming at understanding the catalytic mechanism of copper oxidases and copper monooxygenases. Different types of copper-dioxygen complexes such as a (μ-η2 : η2-peroxo) dicopper (II) complex, a (μ-η1 : η2-peroxo) dicopper (II) complex, a bis (μ-oxo) dicopper (III) complex, and a mixed-valent bis (μ3-oxo) tricopper (II, II, III) complex have been assessed by the reaction of O2 and copper (I) complexes supported by a series of pyridylalkylamine ligands. Reactivity of the generated active oxygen complexes has been also investigated in detail to get insights into the catalytic mechanism of copper enzymes such as tyrosinase and particulate methane monooxygenase. Mononuclear copper (II) -hydroperoxo complexes as well as bis (μ-oxo) dinickel (III) complexes have been also obtained using the same series of pyridylalkylamine ligands. Furthermore, redox interconversion between disulfide (RS-SR) and thiolate (2RS-) has been finely controlled by the copper complexes of pyridylalkylamine derivatives. Ligand effects on the respective reactions have been discussed based on the structures and physicochemical properties of the complexes.

Characterization of low-spin ferric hydroperoxo complexes with N,N’-dimethyl- N,N’-bis(2-pyridylmethyl)-1,2-diaminoethane

Hydroperoxo intermediates formed by the interaction of [FeII(BPMEN)(MeCN)2](ClO4)2 [BPMEN = N,N’-dimethyl-N,N’-bis-(2-pyridylmethyl)-1,2-diaminoethane] with H2O2 in a CH2Cl2–MeCN mixture were characterised using EPR spectroscopy.

Oxygen activation by a macrocyclic chromium complex. Mechanism of hydroperoxo-chromium(iii) to oxo-chromium(v) transformation

The transformation of the hydroperoxo complex L1(H2O)CrOOH2+ (L1 = 1, 4 8, 11-tetraazacyclotetradecane) to an oxo-chromium(v) species is a first-order process throughout the pH range examined, 1.7 < pH < 9.2. The pH dependence of the rate constant (k1) yielded an apparent pKa of 5.6 for L1(H2O)CrOOH2+. In the acidic range, (pH <4), the value of k1 is 0.191 s(-1). At the other extreme, pH >7.5, k(1)= 0.025 s(-1). No [H+]-dependence is observed within the two limiting regimes, clearly ruling out a simple attack by H+ at the hydroperoxo group. The temperature dependence of k1 in 0.020 M HClO4 yielded the activation parameters DeltaH++ = 53.7 kJ mol(-1) and DeltaS++ = -80.5 J mol(-1) K(-1).

Competing Coordination and Oxygen Atom Transfer in a Reaction Between Triarylphosphines and a Chromium Hydroperoxo Complex

Competing Coordination and Oxygen Atom Transfer in a Reaction Between Triarylphosphines and a Chromium Hydroperoxo Complex