Hydroperoxo Species Research Articles

Structural characterization of a hydroperoxo nickel complex and its autoxidation: mechanism of interconversion between peroxo, superoxo, and hydroperoxo species.

Pincer-stabilized nickel(I) complexes readily react with molecular oxygen to form dinuclear 1,2-μ-peroxo-bridged nickel(II) complexes, which are the major components of a dynamic equilibrium with the corresponding mononuclear superoxo species. The peroxo complexes further react with hydrogen peroxide to give the corresponding nickel(II) hydroperoxides. One of these hitherto elusive species was characterized by X-ray diffraction for the first time [O-O bond length: 1.492(2) Å].

Spectroscopic characterization and reactivity studies of a mononuclear nonheme Mn(III)-hydroperoxo complex.

We report the first example of a mononuclear nonheme manganese(III)–hydroperoxo complex derived from protonation of an isolated manganese(III)–peroxo complex bearing an N-tetramethylated cyclam (TMC) ligand, [MnIII(TMC)(OOH)]2+. The MnIII-hydroperoxo intermediate is characterized with various spectroscopic methods as well as with density functional theory (DFT) calculations, showing the binding of a hydroperoxide ligand in an end-on fashion. The MnIII–hydroperoxo species is a competent oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of sulfides. The electrophilic character of the MnIII–hydroperoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles.

Mechanistic Study of the Oxidation of a Methyl Platinum(II) Complex with O2 in Water: PtIIMe-to-PtIVMe and PtIIMe-to-PtIVMe2 Reactivity

The mechanism of oxidation by O2 of (dpms)Pt(II)Me(OH2) (1) and (dpms)Pt(II)Me(OH)(-) (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4-14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a C1-symmetric monomethyl Pt(IV) complex (dpms)Pt(IV)Me(OH)2 (5) with high selectivity ≥97%; the reaction rate is first-order in [Pt(II)Me] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O2 activation at the Pt(II) center to form a Pt(IV) hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O2 than neutral complex 1 (pKa = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH(-) ions; the reaction order in [Pt(II)Me] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by Pt(IV) hydroperoxide 4. At pH ≥ 12, formation of a C1-symmetric dimethyl complex 6, (dpms)Pt(IV)Me2(OH), along with [(dpms)Pt(II)(OH)2](-) (7) becomes the dominant reaction pathway (50-70% selectivity). This change in the product distribution is explained by the formation of a Cs-symmetric intermediate (dpms)Pt(IV)Me(OH)2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; kH/kD = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A SN2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.

Reaction Mechanism of Homoprotocatechuate 2,3-Dioxygenase with 4-Nitrocatechol: Implications for the Role of Substrate

The reaction mechanism of the dioxygen activation by homoprotocatechuate 2,3-dioxygenase (HPCD) with the substrate 4-nitrocatechol was investigated by quantum mechanical/molecular mechanical calculations. Our results demonstrated that the experimentally determined side-on iron-oxygen complex in crystallo is a semiquinone substrate radical (SQ(•))-Fe(III)-hydroperoxo species, which could not act as the reactive species. In fact, the Fe(III)-superoxo species with a hydrogen bond between His200 and the proximal oxygen is the reactive oxygen species. The second-sphere His200 residue was found to play an important role in manipulating the orientation of the superoxide in the Fe-O2 adduct for the further reaction. The rate-limiting step is the attack of the superoxo group on the substrate with a barrier of 17.2 kcal/mol, in good agreement with the experimental value of 16.8 kcal/mol. The reaction mechanism was then compared with the one for HPCD with its native substrate homoprotocatechuate studied recently by the same methods, in which a hybrid SQ(•)-Fe(II)-O2(•-)/Fe(III)-O2(•-) was suggested to be the reactive species. Therefore, our studies suggested that the substrate plays important roles in the dioxygen activation by HPCD.

Non-heme iron hydroperoxo species in superoxide reductase as a catalyst for oxidation reactions.

The non-heme high-spin ferric iron hydroperoxo species formed in superoxide reductase catalyzes oxidative aldehyde deformylation through its nucleophilic character. This species also acts as an electrophile to catalyze oxygen atom transfer in sulfoxidation reactions, highlighting the oxidation potential of non-heme iron hydroperoxo species.

Peroxynitrite chemistry derived from nitric oxide reaction with a Cu(II)-OOH species and a copper mediated NO reductive coupling reaction.

New peroxynitrite-copper chemistry ensues via addition of nitric oxide (˙NO(g)) to a Cu(II)-hydroperoxo species. In characterizing the system, the ligand-Cu(i) complex was shown to effect a seldom observed ˙NO(g) reductive coupling reaction. Biological implications are discussed.

Alumina-coated Ag nanocrystal monolayers as surfaceenhanced Raman spectroscopy platforms for the direct spectroscopic detection of water splitting reaction intermediates

A novel Ag-alumina hybrid surface-enhanced Raman spectroscopy (SERS) platform has been designed for the spectroscopic detection of surface reactions in the steady state. Single crystalline and faceted silver (Ag) nanoparticles with strong light scattering were prepared in large quantity, which enables their reproducible self-assembly into large scale monolayers of Raman sensor arrays by the Langmuir-Blodgett technique. The close packed sensor film contains high density of sub-nm gaps between sharp edges of Ag nanoparticles, which created large local electromagnetic fields that serve as “hot spots” for SERS enhancement. The SERS substrate was then coated with a thin layer of alumina by atomic layer deposition to prevent charge transfer between Ag and the reaction system. The photocatalytic water splitting reaction on a monolayer of anatase TiO2 nanoplates decorated with Pt co-catalyst nanoparticles was employed as a model reaction system. Reaction intermediates of water photo-oxidation were observed at the TiO2/solution interface under UV irradiation. The surface-enhanced Raman vibrations corresponding to peroxo, hydroperoxo and hydroxo surface intermediate species were observed on the TiO2 surface, suggesting that the photo-oxidation of water on these anatase TiO2 nanosheets may be initiated by a nucleophilic attack mechanism.

Introducing Copper as Catalyst for Oxidative Alkane Dehydrogenation

The dehydrogenation of n-hexane and cycloalkanes giving n-hexene and cycloalkenes has been observed in the reaction of such hydrocarbons with hydrogen peroxide, in the presence of copper complexes bearing trispyrazolylborate ligands. This catalytic transformation provides the typical oxidation products (alcohol and ketones) with small amounts of the alkenes, a novel feature in this kind of oxidative processes. Experimental data exclude the participation of hydroxyl radicals derived from Fenton-like reaction mechanisms. DFT studies support a copper-oxo active species, which initiates the reaction by H abstraction. Spin crossover from the triplet to the singlet state, which is required to recover the catalyst, yields the major hydroxylation and minor dehydrogenation products. Further calculations suggested that the superoxo and hydroperoxo species are less reactive than the oxo. A complete mechanistic proposal in agreement with all experimental and computational data is proposed.

A novel thermally stable hydroperoxo–copper(ii) complex in a Cu(N2O2) chromophore of a potential N4O2 donor Schiff base ligand: synthesis, structure and catalytic studies

The generation and study of metal-hydroperoxo/metal-peroxo (LCu(II)-OOH or LCu(II)-OO˙) complexes is a fascinating area of research of many chemical and biochemical researchers, because of their involvement as active intermediates in many biological and industrial catalytic oxidation processes. For this purpose we have designed a bulky hexa-coordinating ligand with potential N4O2 donor atoms which could provide an opportunity to synthesize a mononuclear Cu(II) complex with an aim to utilize it in the catalytic oxidation of aromatic hydrocarbons by an environmentally benign oxidant, H2O2. The Cu(II) complex (1) was structurally characterized and found to have square-planar geometry with the two pyrazolyl groups remaining in dangling mode. A novel mononuclear complex [Et3NH][LCu(II)-OOH] (2) was found to form in the reaction between 1 and H2O2 in the presence of Et3N. The presence of this dangling groups favours the stability of hydroperoxo species, [LCu-OOH](-) (2) through H-bonding with the coordinated phenoxo oxygen atom, which was confirmed by ESI-MS(+) and MS(-) (m/z) mass analysis and DFT calculations. This complex was found to be thermally stable at room temperature [k(d) = (5.67 ± 0.03) × 10(-5) s(-1) at 25 °C] and may be due to the formation of O-O-H···O(phenoxo) H-bonding as delineated by the DFT calculations. Complex 1 was found to be an efficient catalyst for the oxidation of aromatic hydrocarbons to the corresponding aldehyde and alcohol in 2:1 mole ratio with TON ~300.

Active Site Models for the CuA Site of Peptidylglycine α-Hydroxylating Monooxygenase and Dopamine β-Monooxygenase

A mononuclear copper(II) superoxo species has been invoked as the key reactive intermediate in aliphatic substrate hydroxylation by copper monooxygenases such as peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and tyramine β-monooxygenase (TβM). We have recently developed a mononuclear copper(II) end-on superoxo complex using a N-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane tridentate ligand, the structure of which is similar to the four-coordinate distorted tetrahedral geometry of the copper-dioxygen adduct found in the oxy-form of PHM (Prigge, S. T.; Eipper, B. A.; Mains, R. E.; Amzel, L. M. Science2004, 304, 864-867). In this study, structures and physicochemical properties as well as reactivity of the copper(I) and copper(II) complexes supported by a series of tridentate ligands having the same N-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane framework have been examined in detail to shed light on the chemistry dictated in the active sites of mononuclear copper monooxygenases. The ligand exhibits unique feature to stabilize the copper(I) complexes in a T-shape geometry and the copper(II) complexes in a distorted tetrahedral geometry. Low temperature oxygenation of the copper(I) complexes generated the mononuclear copper(II) end-on superoxo complexes, the structure and spin state of which have been further characterized by density functional theory (DFT) calculations. Detailed kinetic analysis on the O(2)-adduct formation reaction gave the kinetic and thermodynamic parameters providing mechanistic insights into the association and dissociation processes of O(2) to the copper complexes. The copper(II) end-on superoxo complex thus generated gradually decomposed to induce aliphatic ligand hydroxylation. Kinetic and DFT studies on the decomposition reaction have suggested that C-H bond abstraction occurs unimolecularly from the superoxo complex with subsequent rebound of the copper hydroperoxo species to generate the oxygenated product. The present results have indicated that a superoxo species having a four-coordinate distorted tetrahedral geometry could be reactive enough to induce the direct C-H bond activation of aliphatic substrates in the enzymatic systems.

O2 Insertion into Group 9 Metal–Hydride Bonds: Evidence for Oxygen Activation through the Hydrogen-Atom-Abstraction Mechanism

A detailed density functional study was performed to examine the reaction of mixed-valence dirhodium and diiridium species [M(2)(0,II)(tfepma)(2)(CN(t)Bu)(2)(Cl)(2) (1, tfepma = MeN[P(OCH(2)CF(3))(2)](2), CN(t)Bu = tert-butyl isocyaninde)] with HCl and oxygen with an interest in examining the pathways for oxygen insertion into the intermediate metal hydride to form hydroperoxo species. The O(2) hydrogen atom abstraction mechanism for both the Rh and Ir was found to be feasible. This is the first time this mechanism has been applied to a Rh system and only the second time it has been examined for a system other than Pd. The competing trans HCl reductive elimination pathway was also examined and found to be greatly dependent on the stereochemistry of the starting hydride primarily due to the intermediate formed upon the loss of Cl(-). As a result, the reductive elimination pathway was more favorable by 11.5 kcal/mol for the experimentally observed Ir stereoisomer, while the two pathways were isoenergetic for the other stereoisomer of the Rh complex. All findings are consistent with the kinetics study previously performed.

Structural rearrangements and reaction intermediates in a di-Mn water oxidation catalyst

By using first-principles molecular dynamics simulations combined with metadynamics to simulate rare events we analyse competing reaction coordinates for a di-Mn water oxidation catalyst ([(bis(imino)pyridine)(H(2)O)Mn(IV)(μ-O)(2)Mn(V)(O)(bis(imino)pyridine)](3+)). The catalytic water oxidation cycle of the complex is examined by addressing the thermodynamic accessibility of the hydroperoxo species that is considered a critical and rate-limiting intermediate. To achieve this, hybrid quantum-mechanics/molecular-mechanics (QM/MM) and full QM simulations have been performed for an explicit treatment of the water environment that plays an active role in the reaction processes. Starting from a likely active species for the O-O bond formation, we observe that during the water approach to the oxo ligand a facile structural rearrangement of the complex takes place, leading to the opening of one μ-O bridge and the release of a water ligand, and resulting in two pentacoordinated Mn centers. This complex appears weakly active in the water oxidation process, since a concerted reaction is required to establish a Mn-OOH hydroperoxo intermediate. The slow kinetics of a concerted reaction can allow other processes, including linear degradation of the catalyst, to take precedence over catalytic water oxidation.

The reaction mechanism of hydroxyethylphosphonate dioxygenase: a QM/MM study

By employing ab initio quantum mechanical/molecular mechanical (QM/MM) and molecular dynamics (MD) simulations, we have provided further evidence against the previously proposed hydroperoxylation or hydroxylation mechanism of hydroxyethylphosphonate dioxygenase (HEPD). HEPD employs an interesting catalytic cycle based on concatenated bifurcations. The first bifurcation is based on the abstraction of hydrogen atoms from the substrate, which leads to a distal or proximal hydroperoxo species (Fe-OOH or Fe-(OH)O). The second and the third bifurcations refer to the carbon-carbon bond cleavage reaction. And this is achieved through a tridentate intermediate, or employing a proton-shuttle assisted mechanism, in which the residue Glu(176) or the Fe(IV)=O group serves as a general base. The reaction directions seem to be tunable and show significant environment dependence. This mechanism can provide a comprehensive interpretation for the seemingly contradicting experimental evidences and provide insight into the development of biochemistry and material sciences.

Structure and reactivity of a mononuclear non-haem iron(III)–peroxo complex

Oxygen-containing mononuclear iron species--iron(III)-peroxo, iron(III)-hydroperoxo and iron(IV)-oxo--are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes. It has been difficult to generate synthetic analogues of these three active iron-oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on iron(III)-peroxo complex, [Fe(III)(TMC)(OO)](+). We also report a series of chemical reactions in which this iron(III)-peroxo complex is cleanly converted to the iron(III)-hydroperoxo complex, [Fe(III)(TMC)(OOH)](2+), via a short-lived intermediate on protonation. This iron(III)-hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)](2+), via homolytic O-O bond cleavage of the iron(III)-hydroperoxo species. All three of these iron species--the three most biologically relevant iron-oxygen intermediates--have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)-hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)-oxo complex in C-H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)-hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes.

Formation and Reactivity of a Biomimetic Hydroperoxocopper(II) Cryptate

AbstractCopper(II)‐hydroperoxo species are proposed as key intermediates in the catalytic cycles of copper‐monooxygenase enzymes that perform C–H bond hydroxylation. Herein we report on the oxidation chemistry of a copper(II) complex with a coordinating cryptand based on a peralkylated tetradentate tris(2‐aminoethyl)amine (tren) moiety. X‐ray crystallography of the copper(II) acetate complex of this cryptand revealed that the copper(II) ion is in a square‐pyramidal environment, with the cryptand acting only as a tridentate ligand. This geometry is conserved in solution and likely results from restraints imposed by the semi‐rigid cryptand. Reaction of this complex with basic hydrogen peroxide in methanol led to the decomposition of the complex with an oxygen‐atom transfer to the ligand, as evidenced by mass spectrometry analysis after reaction and demetallation. Low‐temperature stopped‐flow experiments (down to –90 °C) support the formation of a copper(II)‐hydroperoxo intermediate, CuOOH, before ligand oxygenation occurs. It is proposed that this intermediate performs the oxygen‐atom transfer to a weak benzylic C–H bond of the cryptand, thereby mimicking the behavior of dopamine‐β‐hydroxylase.

Efficient Epoxidation of Electron‐Deficient Alkenes with Hydrogen Peroxide Catalyzed by [γ‐PW10O38V2(μ‐OH)2]3−

A divanadium-substituted phosphotungstate, [γ-PW(10)O(38)V(2)(μ-OH)(2)](3-) (I), showed the highest catalytic activity for the H(2)O(2)-based epoxidation of allyl acetate among vanadium and tungsten complexes with a turnover number of 210. In the presence of I, various kinds of electron-deficient alkenes with acetate, ether, carbonyl, and chloro groups at the allylic positions could chemoselectively be oxidized to the corresponding epoxides in high yields with only an equimolar amount of H(2)O(2) with respect to the substrates. Even acrylonitrile and methacrylonitrile could be epoxidized without formation of the corresponding amides. In addition, I could rapidly (≤10 min) catalyze epoxidation of various kinds of terminal, internal, and cyclic alkenes with H(2)O(2) under the stoichiometric conditions. The mechanistic, spectroscopic, and kinetic studies showed that the I-catalyzed epoxidation consists of the following three steps: 1) The reaction of I with H(2)O(2) leads to reversible formation of a hydroperoxo species [γ-PW(10)O(38)V(2)(μ-OH)(μ-OOH)](3-) (II), 2) the successive dehydration of II forms an active oxygen species with a peroxo group [γ-PW(10)O(38)V(2)(μ-η(2):η(2)-O(2))](3-) (III), and 3) III reacts with alkene to form the corresponding epoxide. The kinetic studies showed that the present epoxidation proceeds via III. Catalytic activities of divanadium-substituted polyoxotungstates for epoxidation with H(2)O(2) were dependent on the different kinds of the heteroatoms (i.e., Si or P) in the catalyst and I was more active than [γ-SiW(10)O(38)V(2)(μ-OH)(2)](4-). On the basis of the kinetic, spectroscopic, and computational results, including those of [γ-SiW(10)O(38)V(2)(μ-OH)(2)](4-), the acidity of the hydroperoxo species in II would play an important role in the dehydration reactivity (i.e., k(3)). The largest k(3) value of I leads to a significant increase in the catalytic activity of I under the more concentrated conditions.

Electronic spectra and photochemical reactivity of bismuth corrole complexes

The first examples of bismuth complexes with meso-substituted corrole ligands are presented. Complexation of 5,10,15-triphenylcorrole with bismuth(III) salts leads to the formation of a neutral low-valent species Bi-tpc, which is characterized by various spectroscopic techniques. In addition to the intense intraligand (IL) absorption bands of the corrole macrocycle, this bismuth(III) complex displays metal-centered (MC) sp-transitions and metal-to-ligand charge transfer (MLCT) transitions which show a substantial mixing with the intraligand absorption bands, thus resulting in an irregular hyper-type electronic spectrum. Excitation of Bi-tpc in the spectral region of the MC-bands under ambient conditions leads to dioxygen activation and concomitant photochemical formation of the high-valent species (O )Bi V(tpc), an unprecedented example of a bismuth(V) tetrapyrrole complex. As a potential reaction intermediate of this photoredox process, a labile hydroperoxo species (HOO)Bi V(tpc) + is also identified. First results on the spectroscopic features and the chemical reactivity of (O )Bi V(tpc) are reported. In solution this neutral bismuth(V) species is in equilibrium with the corresponding protonated form (HO)Bi V(tpc) + carrying an axial hydroxo ligand. Axial ligand exchange processes and the formation of adducts with solvent derived photoproducts are also identified in high-resolution mass spectra of irradiated samples. The molecular and electronic structures of the novel bismuth corrole systems are discussed in terms of supporting density functional theory (DFT) calculations, which suggest an out-of-plane structure of the Bi-tpc and (O )Bi V(tpc) species. Time-dependent DFT results are supporting the proposed assignment of electronic transitions observed in low- and high-valent bismuth tpc derivatives.

Nonheme ferric hydroperoxo intermediates are efficient oxidants of bromide oxidation

This work presents the first combined experimental and computational study that gives evidence of the electrophilic reactivity of a nonheme iron(III)-hydroperoxo species. We show that in contrast to their heme counterparts the nonheme iron(III)-hydroperoxo complexes are catalytically much more active and even more so than nonheme iron(IV)-oxo species.

Dioxygen Activation by a Non-Heme Iron(II) Complex: Formation of an Iron(IV)−Oxo Complex via C−H Activation by a Putative Iron(III)−Superoxo Species

Iron(III)-superoxo intermediates are believed to play key roles in oxygenation reactions by non-heme iron enzymes. We now report that a non-heme iron(II) complex activates O(2) and generates its corresponding iron(IV)-oxo complex in the presence of substrates with weak C-H bonds (e.g., olefins and alkylaromatic compounds). We propose that a putative iron(III)-superoxo intermediate initiates the O(2)-activation chemistry by abstracting a H atom from the substrate, with subsequent generation of a high-valent iron(IV)-oxo intermediate from the resulting iron(III)-hydroperoxo species.

Oxygen Activation with Transition-Metal Complexes in Aqueous Solution

Coordination to transition-metal complexes changes both the thermodynamics and kinetics of oxygen reduction. Some of the intermediates (superoxo, hydroperoxo, and oxo species) are close analogues of organic oxygen-centered radicals and peroxides (ROO(*), ROOH, and RO(*)). Metal-based intermediates are typically less reactive, but more persistent, than organic radicals, which makes the two types of intermediates similarly effective in their reactions with various substrates. The self-exchange rate constant for hydrogen-atom transfer for the couples Cr(aq)OO(2+)/Cr(aq)OOH(2+) and L(1)(H(2)O)RhOO(2+)/L(1)(H(2)O)RhOOH(2+) was estimated to be 10(1+/-1) M(-1) s(-1). The use of this value in the simplified Marcus equation for the Cr(aq)O(2+)/Cr(aq)OOH(2+) cross reaction provided an upper limit k(CrO,CrOH) <or= 10((-2+/-1)) M(-1) s(-1) for Cr(aq)O(2+)/Cr(aq)OH(2+) self-exchange. Even though superoxo complexes react very slowly in bimolecular self-reactions, extremely fast cross reactions with organic counterparts, i.e., acylperoxyl radicals, have been observed. Many of the intermediates generated by the interaction of O(2) with reduced metal complexes can also be accessed by alternative routes, both thermal and photochemical.