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The influence of solvent on the reaction between iron(II), (III) and hydrogen peroxide
Abstract
It has been found that in acetonitrile, in contrast to water, iron(III) is reduced by hydrogen peroxide, according to 2:1 stoichiometry. The reaction when performed by cyclic-voltammetry is an example of electrochemical catalytic processes of reductants. For the [Fe(III)]/[HOOH] ratios greater than 1, 1mol of dioxygen is produced from 1mol of hydrogen peroxide. The non-radical versus radical mechanism of the process has been discussed.
... The substantial difference between the oxidation potential of the iron(III) complex and the reduction potential of the iron(IV) oxo complex is probably caused by the presence of acetonitrile in coordination sphere of iron(III) and iron(II) complexes and its absence in coordination sphere of iron(IV) oxo complex. It has been shown [77,83] that the presence of acetonitrile in coordination sphere of an iron complex causes its redox potential to shift toward more positive values. Moreover, the generation of [(N4Py)Fe IV =O] 2+ by the electrochemical oxidation of [(N4Py)Fe II ] 2+ in acetonitrile has been reported [76]. ...
The use of dioxygen as an oxidant in fine chemicals production is an emerging problem in chemistry for environmental and economical reasons. In acetonitrile, the [(N4Py)FeII]2+ complex, [N4Py—N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine] in the presence of the substrate activates dioxygen for the oxygenation of cyclohexene and limonene. Cyclohexane is oxidized mainly to 2-cyclohexen-1-one, and 2-cyclohexen-1-ol, cyclohexene oxide is formed in much smaller amounts. Limonene gives as the main products limonene oxide, carvone, and carveol. Perillaldehyde and perillyl alcohol are also present in the products but to a lesser extent. The investigated system is twice as efficient as the [(bpy)2FeII]2+/O2/cyclohexene system and comparable to the [(bpy)2MnII]2+/O2/limonene system. Using cyclic voltammetry, it has been shown that, when the catalyst, dioxgen, and substrate are present simultaneously in the reaction mixture, the iron(IV) oxo adduct [(N4Py)FeIV=O]2+ is formed, which is the oxidative species. This observation is supported by DFT calculations.
... However, S2 and S3 are the studied magnetite specimens with the highest structural content of Fe II ( Table 1). The link between Fe II content and photo-Fenton or photo-Fenton-like reactivity is only apparently straightforward (Fe II is the most reactive Fe species toward peroxides), 32 because the studied magnetites were unreactive in the dark with 0.5 mM S 2 O 8 2− ...
We show that phenol can be effectively degraded by magnetite in the presence of persulfate (S2O8(2-)) under UVA irradiation. The process involves the radical SO4(-•), formed from S2O8(2-) in the presence of Fe(II). Although magnetite naturally contains Fe(II), the air-exposed oxide surface is fully oxidized to Fe(III) and irradiation is required to produce Fe(II). The magnetite + S2O8(2-) system was superior to the corresponding magnetite + H2O2 one in the presence of radical scavengers and in a natural water matrix, but it induced phenol mineralization in ultrapure water to a lesser extent. The leaching of Fe from the oxide surface was very limited, and much below the wastewater discharge limits. The reasonable performance of the magnetite/persulfate system in a natural water matrix and the low levels of dissolved Fe are potentially important for the removal of organic contaminants in wastewater.
... A similar behavior has been proposed previously for reactions involving different concentrations of Fe and H2O2: dioxygen evolution is observed in water when Fe:H2O2 e 1 while no O2 is observed when Fe:H2O2 > 1 (23). ...
The electrochemical behavior of 2,3-dihydroxybenzoic acid (2,3-DHBA) and the electron-transfer characteristics between Cu(II) and 2,3-DHBA were studied in aqueous solutions using cyclic voltammetry (CV). The overall electrochemical oxidation process of 2,3-DHBA by Cu(II) may be classified as a chemical reaction involving one electron oxidation of 2,3 DHBA to its semiquinone radical in solution, followed by an electron transfer reaction involving the oxidation of the semiquinone radical to a quinone at the electrode surface. In the presence of H2O2, oxidation of
2,3-DHBA by Cu(II) is enhanced due to the regeneration of Cu(II) by H2O2 oxidizing Cu(I). The redox cycling between Cu(I)/Cu(II) and H2O2 also produces hydroxyl radicals (OH•). Even though the presence of OH• may not be detected at the surface of a glassy carbon electrode, production
of electroactive dissolved oxygen (O2) suggests the presence of OH•. The production of O2 is dependent on Cu(II):H2O2 concentration ratio. At the electrode surface and when the initial Cu(II):H2O2 is less than 1, O2 is produced, suggesting that H2O2 may act as a scavenger for OH•; at initial Cu-(II):H2O2 > 1, the production of O2 is not favored, and OH• will be involved in the oxidation of Cu(I) and the organic ligand. The reaction mechanisms proposed in this study indicate that OH• production by chelator-mediated Fenton reactions is favorable under conditions found in the wood cell wall.
... A similar behavior has been proposed previously for reactions involving different concentrations of Fe and H2O2: dioxygen evolution is observed in water when Fe:H2O2 e 1 while no O2 is observed when Fe:H2O2 > 1 (23). ...
The electrochemical behavior of 2,3-dihydroxybenzoic acid (2,3-DHBA) and the electron-transfer characteristics between Cu(II) and 2,3-DHBA were studied in aqueous solutions using cyclic voltammetry (CV). The overall electrochemical oxidation process of 2,3-DHBA by Cu(II) may be classified as a chemical reaction involving one-electron oxidation of 2,3-DHBA to its semiquinone radical in solution, followed by an electron-transfer reaction involving the oxidation of the semiquinone radical to a quinone at the electrode surface. In the presence of H2O2, oxidation of 2,3-DHBA by Cu(II) is enhanced due to the regeneration of Cu(II) by H2O2 oxidizing Cu(I). The redox cycling between Cu(I)/Cu(II) and H2O2 also produces hydroxyl radicals (OH). Even though the presence of OH may not be detected at the surface of a glassy carbon electrode, production of electroactive dissolved oxygen (O2) suggests the presence of OH. The production of O2 is dependent on Cu(II):H2O2 concentration ratio. At the electrode surface and when the initial Cu(II):H2O2 is less than 1, O2 is produced, suggesting that H2O2 may act as a scavenger for OH; at initial Cu(II):H2O2 > 1, the production of O2 is not favored, and OH will be involved in the oxidation of Cu(I) and the organic ligand. The reaction mechanisms proposed in this study indicate that OH production by chelator-mediated Fenton reactions is favorable under conditions found in the wood cell wall.
The major aim of our work is to explore various ways to achieve regioselective and stereoselective oxidations of hydrocarbons under ambient conditions with high efficiency and atom economy. When researchers seek to account for differences in the chemical selectivity of metallo‐enzymes or monooxygenases, they typically invoke differences in shape selection of substrate pockets as well as differences in specific nonbonding interactions, such as hydrogen bonding, electrostatic forces, and/or van der Waals interactions, in the interactions of the hydrocarbon substrates with the active sites of the metalloenzymes/monooxygenases. To explore these effects in depth, we have redesigned the substrate binding pockets of alkane hydroxylase (AlkB) and cytochrome P450 BM3 using a heterogeneous Escherichia coli recombinant system. These two redesigned proteins are deployed to achieve efficient oxidations of n‐butane at its C‐1 and C‐2 positions, respectively. These studies have enabled us to propose mechanisms used by AlkB and cytochrome P450 BM3 to selectively oxidize inert CH bonds and oxyfunctionalize a variety of hydrocarbons. The insights gained are expected to contribute to the design of biomimetics or artificial catalysts that selectively oxidize alkanes, including nanomaterial platforms of these biomimetics.
Different oxidizing particles were shown to form in the decomposition of hydrogen peroxide in acetonitrile catalyzed by the Fe(ClO4) 3 and FeCl3 salts, namely, the HO• radical and, evidently, the ferryl ion (FeIV = O)2+, respectively. This difference was established for the examples of oxygenation of alkanes and olefins. Clear-cut differences in the kinetic behavior of the two catalytic systems and their oxidation selectivity were observed.
It has been found that tert-butyl hydroperoxide is analogous to hydrogen peroxide with respect to their reactivity towards iron(II) and iron(III). In water iron(II) is oxidized by tert-butyl hydroperoxide, whereas in acetonitrile iron(III) is reduced by tert-butyl hydroperoxide. The last reaction when performed by cyclic-voltammetry is an example of electrochemical catalytic processes of reductants.
The synthesis, photophysical and electrochemical properties as well as theoretical calculation studies of a newly designed triphenylamine derivative are described. This original compound displays one neutral form, three oxidized forms, and two protonated forms with distinct photophysical characteristics. The interplay of the emission with the protonation or the redox state (electrofluorochromism) has been explored and an on-off-on-off fluorescence switching was observed in the case of oxidation and an on-on-off fluorescence switching in the case of protonation.
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Fenton- and nodified Fenton systeme such as H2O2/HOONO/Fe2+ have been studied for oxidative degradation of 1,4-dioxane, morpholine, cyclohexanone and bentazone. Both ferrous sulfate (FeSO4 . 7 H2O) and Mohr´s salt (NH4)2Fe(SO4)2 . 6 H2O have been used as Fe2+ ion sources. With Mohr´s salt the Fenton reaction was successfully carried out under both acid (pH 3) and neutral (pH 7) conditions. The degradation ability of the new Fenton-like system H2O2/HOONO/Fe2+ has also been tested. The main results of this study are the use of Mohr´s salt under neutral conditions and practical utilization of the H2O2/HOONO/Fe2+ system in the Fenton reaction. Some mechanistic spects of the degradation initiated by H-atom abstraction with HO. radical are also discussed.
The catalytic activity of the aqua complexes [M(H2O)(n)](2+) (1) (M = Be, n = 4; M = Zn, Cd, n = 6) toward the radical decomposition of H2O2 and generation of the Ha radicals was investigated in detail by theoretical (OFT) methods. It was predicted and then confirmed by preliminary experiments that complexes 1 are able to catalyze this process despite the metals have only one stable non-zero oxidation state and, hence, cannot change it during the reaction as proposed in the classical Fenton chemistry. The mechanism of the H2O2 decomposition includes the substitution of a water ligand for H2O2, protolysis of the coordinated H2O2, second H2O-for-H2O2 substitution, elimination of one ligated water molecule (for Zn and Cd), and the homolytic HO-OH bond cleavage in complexes [M(H2O),(H2O2)(OOH)(+). The principal factors affecting the Ha formation are lability of the complexes, their acidity, and their ability to activate H2O2 toward the homolytic HO-OH bond cleavage. The participation of two H2O2 molecules is necessary for an efficient HO. generation. The water substitution steps occur via concerted I-a (Be, Cd) or dissociative D (Zn, Cd) mechanisms.
The mechanism of the rate-limiting stage of alkanes oxidation with hydrogen peroxide, that is, formation of the hydroxyl radicals, catalyzed by aqua complexes, [M(H2O)](3+) (1), of the group III metals exhibiting a unique stable non-zero oxidation state (M = Ga, In, Sc, Y, or La) was theoretically studied in detail at the DFT level. The mechanism involves the substitution of a H2O ligand for H2O2, protolysis of the coordinated H2O2, substitution of another H2O ligand for H2O2, generation of the hydroxyl radical upon the homolytic O-O bond cleavage in the key complex [M(H2O)((n-2))(H2O2)(OOH)](2+), and closure of the catalytic cycle. The substitution steps proceed via the dissociative mechanism D (M = Ga and Y) or the associative mechanisms A [M = Sc, La, or In (second substitution)] or I-a [M = In (first substitution)]. The general catalytic activity of 1 is determined by three main factors, that is, (i) lability of the complexes, (ii) acidity of the metal-bound ligands, and (iii) the H2O2 activation toward the homolytic O-O bond cleavage. The H2O2 activation, in turn, depends on the strength of the M-O2H2 bond, delocalization of the spin density within a coligand (OOH-), and ability of the coligand to be easily oxidized. The calculations predict that the catalytic activity of 1 increases along the row of the metals Al approximate to La < Y approximate to In < Sc < Ga.
Mannich aminomethylation products have been prepared from one-pot template condensation of bis(glycinato)metal(II) complexes, M(gly)2, with formaldehyde and nitromethane. The synthesized compounds were characterized by suitable spectroscopic and analytical methods such as UV–Vis, infrared spectroscopy, elemental analysis, FAB mass, ESR, cyclic voltammetry, TGA and NMR. One of the synthesized products has been used as catalyst in the hydrogen peroxide induced degradation of pyrocatechol violet (PCV) dye.
The H2O2-FeCl3-bipy system in acetonitrile efficiently oxidises alkanes predominantly to alkyl hydroperoxides. Turnover numbers attain 400 after 1 h at 60 degrees C. It has been assumed that bipy facilitates proton abstraction from a H2O2 molecule coordinated to the iron ion (these reactions are stages in the catalytic cycle generating hydroxyl radicals from the hydrogen peroxide). Hydroxyl radicals then attack alkane molecules finally yielding the alkyl hydroperoxide. (c) 2005 Elsevier Ltd. All rights reserved.
In lieu of Abstract, excerpt from Conclusion of Voltammetric Study of Interaction of Copper and Model Fungal Secreted Ligands: In summary, the cyclic voltammetry study shows evidence that the chelatormediated Fenton reaction is favored in the wood cell wall, where it may promote the degradation process. Also, reaction mechanisms between Cu(II), 2,3-DHBA and H202 have been proposed which provide at least partial explanations for the H202 cycle, and the mechanism for non-enzymatic, chelator-mediated Fenton reactions in brown rot wood decay processes. Future research in this area should explore cyclic voltarnrnetry analyses of Cu(I1) and ligands secreted by fungi, such as 2,5-DMHQ or 2,5-DMBQ in the absence or presence of H202 under a range of acidic conditions. Cyclic voltammetry experiments should also be conducted with Fe(III), oxalic acid and ligands secreted by fungi, to gain insight into the mechanism and kinetics of the relevant redox reactions in wood decay processes.
High-valent iron(IV)-oxo species was proved to be the main oxidizing species for hydroxylation of benzene to phenol by a Fenton-like reagent in triethylammonium acetate ionic liquid via UV–vis and ESI-MS characterization, while hydroxyl radical was excluded by detailed investigations. It was found that the formation of hydroxyl radical was prohibited by the reduction of redox potential of Fe(III)/Fe(II) couple in triethylammonium acetate medium, leading to a decreased over-oxidation of benzene than that in aqueous solution. The reaction mechanisms for hydroxylation of benzene, as well as for over-oxidation of phenol by iron(IV)-oxo species were proposed. The latter is partly prohibited by the hydrogen-bond interaction between as-produced phenol and acetate anion of the ionic liquid.
It has been found that hydrogen peroxide can act as an oxidant and as a reductant of transition metal complexes in acetonitrile. Therefore, the examples of catalytic currents of oxidants and reductants has been described. In the case of Co(acac)3 both the types of catalytic currents are present in a single cyclic-voltammetric scan.
Activation of hydrogen peroxide by different Cu(II)–amino acid complexes is performed and compared, with quinaldine blue as an oxidation indicator. Parameters such as pH and concentrations of Cu(II), hydrogen peroxide, and amino acids (L = glycine, alanine, and lysine) are examined to understand the activation mechanism of hydrogen peroxide. The experimental rate law determined is first order in Cu(II)–amino acid complexes and variable order in hydrogen peroxide, by Michaelis–Menten kinetics. It indicates that the formation of ligand–Cu(II)–peroxide complex may be responsible for the activation of hydrogen peroxide. The oxidation rate is also substantially enhanced in Cu(II)/amino acid/H2O2 systems with increasing pH from 6 to 9. The trend is consistent with the formation of hydroxyl radical (•OH), whose formation is favored in alkaline solutions. A mechanistic pathway that includes the formation of ligand–Cu(II)–peroxide complex and •OH is proposed. For glycine, alanine, and lysine, the maximum activation efficiencies appear at a ligand/copper molar ratio of 1.5–2.0, regardless of the change in pH values or ligand concentrations. According to the stability constants for Cu(II)–amino acid complexes, it is predicted that CuL and not CuL2 is the dominant species forming the active copper complex catalyst.
Labile iron complexes (e.g., [FeIII(bpy)2]MeCN3+, [FeII(bpy)2]MeCN2+, [FeIII(H2O)6]MeCN3+, and [FeII(H2O)6]MeCN2+) in base-free acetonitrile activate dioxygen for the direct oxygenation of N,N- and N-alkylated amines to form N-dealkylated products and corresponding aldehydes. N,N-dimethylaniline (DMA) is oxidized to N-methylaniline (MA) and formaldehyde as well as to N-methylanilide. Formaldehyde and the excess of substrate undergo condensation reaction to produce 4,4′-methylenebis(N,N-dimethylaniline) (MBDME). Small amounts of N-methylformanilide (MF) and formanilide are also formed during oxidation of N,N-dimethylaniline and N-methylaniline respectively. Iron(III) catalysts are reduced by the substrate to iron(II), which activates dioxygen. Dioxygen activation step is preceded by the equilibrium reaction between iron(II) catalyst and substrate.
In an effort to minimize the impact on the environment, removal of pollutants, such as phenolic compounds, from the industrial wastewater has great importance nowadays because of the high toxicity and low biodegradability of these compounds. This work discusses the different methods to remove these compounds from industrial wastewater, showing their advantages and disadvantages. Advanced Oxidation Process (AOPs) are presented as a promising technology for the treatment of wastewater containing phenolic compounds. Among the AOPs, photolysis, photocatalysis and the processes based on hydrogen peroxide and on ozone are discussed with emphasis on the combined processes and the oxidation mechanisms.
The tetrabutylammonium (TBA) salts of the Keggin-type polyoxotungstates [XW12O40]n−, [XW11O39](n+4)−, [XW11VO40]m− and [XW11MIII(H2O)O39]p−, X = P or Si and M = Fe or Mn, proved to be effective catalysts for the oxidation of cyclooctane with hydrogen peroxide, in acetonitrile, to the corresponding alkyl hydroperoxide, ketone, and alcohol. High turnover numbers and selectivity for cyclooctyl hydroperoxide were obtained, with values of cyclooctane conversion, after 9h, between 13 and 96%, depending on the catalyst and reaction conditions. In general tungstosilicates were less active than tungstophosphates but presented higher selectivity for cyclooctyl hydroperoxide. Results obtained with H2O2 near the stoichiometric ratio and in excess are compared. Excess of hydrogen peroxide afforded higher selectivity for cyclooctyl hydroperoxide.In the presence of (TBA)4[PW11Fe(H2O)O39], 74% of cyclooctane conversion and 80% of selectivity for cyclooctyl hydroperoxide were obtained after 2h of reaction, using an excess of H2O2. In the same conditions, 100% of cyclooctyl hydroperoxide was obtained with (TBA)4H4SiW11O39, after 9h (55% conversion). These results indicate that these are very promising systems for the oxidation of alkanes.
This paper describes an investigation of the alkane oxidation with hydrogen peroxide in acetonitrile catalyzed by iron(III) perchlorate (1), iron(III) chloride (2), iron(III) acetate (3) and a binuclear iron(III) complex with 1,4,7-triazacyclononane (4). The corresponding alkyl hydroperoxides are the main products. Nevertheless in the kinetic study of cyclohexane oxidation, the concentrations of oxygenates (cyclohexanone and cyclohexanol) were measured after reduction of the reaction solution with triphenylphosphine (which converts the cyclohexyl hydroperoxide to the cyclohexanol). Methane and ethane can be also oxidized with TONs up to 30 and 70, respectively. Chloride anions added to the oxidation solution with 1 activate the perchlorate iron derivative in acetonitrile, whereas the water as additive inactivates 2 in the H2O2 decomposition process. Pyrazine-2-carboxylic acid (PCA) added to the reaction mixture decreases the oxidation rate if 1 or 2 are used as catalysts, whereas compounds 3 and 4 are active as catalysts only in the presence of small amount of PCA. The investigation of kinetics and selectivities of the oxidations demonstrated that the mechanisms of the reactions are different. Thus, in the oxidations catalyzed by the 1, 3+PCA and 4+PCA systems the main oxidizing species is hydroxyl radical, and the oxidation in the presence of 2 as a catalyst has been assumed to proceed (partially) with the formation of ferryl ion, (FeIVO)2+. In the oxidation catalyzed by the 4+PCA system (TONs attain 240) hydroxyl radicals were generated in the rate-determining step of monomolecular decomposition of the iron diperoxo adduct containing one PCA molecule. A kinetic model of the process which satisfactorily describes the whole set of experimental data was suggested. The constants of supposed equilibriums and the rate constant for the decomposition of the iron diperoxo adduct with PCA were estimated.
Manganese(II) complexes {[MnII(bpy)2
2+]MeCN} in acetonitrile activate dioxygen for the oxidation of limonene to produce mainly carvone, carveol, limonene oxide, and
perillaldehyde. The reaction efficiencies after 24h reaction time are approximately 5-times higher than those obtained for
analogous iron(II) complexes. However, the 5h long induction period is observed for the common conditions of the reaction.
The simultaneous presence of the catalyst, dioxygen and the substrate is necessary for the active species to be formed. When
t-BuOOH is present in the reaction mixture, the induction period does not appear. In contrast, the replacement of t-BuOOH by HOOH completely inhibits the reaction. We have proposed a putative mechanism in which a manganese(IV)–hydroperoxo
adduct with incorporated substrate is formed during the induction period and it becomes an active catalyst for limonene oxidation.
Methane can directly be transformed into liquid C(1) oxygenated products with selectivities above 95% at 13% conversion by deep UV photocatalysis, in the presence of H(2)O and air. Pure silica zeolites, and more specifically, beta zeolite with a large number of internal silanol groups is active and selective, while amorphous silica with no micropores is much less efficient. Irradiation produces the homolytic cleavage of surface hydroxyl groups, leading to silyloxyl radicals that will generate methyl radicals from methane. The selectivity arises from the occurrence of the reaction in a confined space restricting the mobility of the radical intermediates that will be mostly attached to the solid surface. Energy consumption of the process is in the order of 7.2 Gcal × mol(-1) that compares very favorably with the energy required for transforming methane to synthesis gas (15.96 Gcal × mol(-1)).
A radical mechanism of hydrocarbon oxidations with the environmentally friendly and cheap homogeneous nontransition metal system [Al(H(2)O)(6)](3+)/H(2)O(2)/MeCN-H(2)O was proposed for the first time on the basis of DFT calculations. A dramatic activation of H(2)O(2) toward homolysis in the key intermediate [Al(H(2)O)(4)(OOH)(H(2)O(2))](2+) due to the presence of the easily oxidizable OOH coligand provides, without a change of metal oxidation state, the generation of HO(•) radicals, which then oxidize hydrocarbons. Nonradical mechanisms of the olefin epoxidation with the same catalytic system were also investigated.
Quantum mechanical calculations (DFT) have provided a mechanism for the oxidative C-H bond cleavage step in Fenton-like hydrocarbon hydroxylation. A transition structure for hydrocarbon oxidation by aqueous solvated cationic iron(III) hydroperoxides ((H(2)O)(n)Fe(III)OOH) is presented that involves a novel rearrangement of the hydroperoxide group (FeO-OH --> FeO...HO) in concert with hydrogen abstraction by the incipient HO* radical with activation barriers ranging from 17 to 18 kcal/mol. In every hydroperoxide examined, the activation barrier for FeO-OH isomerization, in the absence of the hydrocarbon, is significantly greater than the overall concerted activation barrier for C-H bond cleavage in support of the concept of O-O bond isomerization in concert with hydrogen abstraction. The transition structure for the oxidation step in simple anionic iron(III) hydroperoxides has been shown to bear a remarkable resemblance to model porphyrin calculations on cytochrome P450 hydroxylation.
The addition of 0.5mM catechol is shown to accelerate the degradation and mineralization of the anionic surfactant DowFax 2A1 (sodium dodecyldiphenyloxide disulfonate) under conventional Fenton reaction conditions (Fe(II) plus H(2)O(2) at pH 3). The catalytic effect causes a 3-fold increase in the initial rate (up to ca. 20 min) of conversion of the surfactant to oxidation products (apparent first-order rate constants of 0.021 and 0.061 min(-1) in the absence and presence of catechol, respectively). Although this catalytic rate increase persists for a certain amount of time after complete disappearance of catechol itself (ca. 8 min), the reaction rate begins to decline slowly after the initial 20 min towards that observed in the absence of added catechol. Total organic carbon (TOC) measurements of net mineralization and cyclic voltammetric and high performance liquid chromatographic (HPLC) measurements of the initial rate of reaction of catechol and the surfactant provide insight into the role of catechol in promoting the degradation of the surfactant and of degradation products as the eventual inhibitors of the Fenton reaction.
A study was conducted to demonstrate the relationship between the absolute proton concentration (pH value), change of pH value, and catalytic performance in important iron-catalyzed selective oxidation reactions. As per this hypothesis, the high selectivity should be independent of the pH value of the buffer and the nature of the buffer system. It clearly shows that an increased proton concentration led to higher catalyst activity and a small change of pH value during the reaction is responsible for improved selectivity. Essentially, iron oxo and radical pathways in iron-catalyzed redox reactions are switchable using the change of pH value as an on-off. The study also suggests that it is of original importance in oxidation chemistry and it should provide useful inspiration to rethink the mechanism of iron-catalyzed reactions.
For the first time, the time dependence of the amount of O2 evolved in the reaction of Fe2+ with H2O2 (the Fenton reaction) was measured by following the decrease of the total [Fe3+] formed upon quenching the reaction mixture with an excess of a strongly acidic solution of Fe2+. The disappearance of Fe2+ in the reaction was followed by quenching it with o-phenanthroline and measuring the absorbance of the Fe2+-phenanthroline complex formed at λ=510 nm. At [H2O2]<10-4 mol dm-3, and quenching the reaction with Fe2+ ions, the formation of a new intermediate was observed. It was identified as the mixed valence binuclear species {FeOFe}5+. A new mechanism for the reaction has been proposed in which FeO2+ acts as the key intermediate. Several rate constants and ratios of rate constants have been determined. The existence of free radicals in the system does not seem to be compatible with the data.
A re-examination of the data of Rush and Koppenol (J. Inorg. Biochem. 29 (1987) 199) on the competitive oxidation of C2H5OH and Fe2+ by Fenton's reagent shows that the ratio of the rate constants of the two reactions is 3.2 and not 6.3. The significance of this finding is that it is not possible to identify the active intermediate in the Fenton reaction with the .OH radical.
Endogenous proteins are highly susceptible to modification by ROS produced as byproducts of normal metabolic processes, or upon exposure to oxidative stress and atmospheric pollutants. The ROS-mediated modification of proteins may lead to loss of biological function and to conversion of the proteins to forms that are rapidly degraded by endogenous proteases, especially by the multicatalytic protease. One of many different kinds of protein modification elicited by ROS is the oxidation of some amino acid side chains to carbonyl derivatives. The carbonyl content of protein is therefore a convenient marker of ROS-mediated protein damage. By means of highly sensitive methods for the detection and quantitation of protein carbonyl groups, it has been established in several different animal models that there is an exponential increase in the level of oxidized protein during aging and that elevated levels of oxidized proteins are associated with a number of diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, rheumatoid arthritis, diabetes, muscular dystrophy, and cataractogenesis, to name a few. Although protein oxidation probably contributes to the biological dysfunction associated with these diseases, a causal relationship between protein oxidation and the etiology or progression of a disease has not been established. Nevertheless, in the case of some neurological disorders, there is a positive correlation between the loss of a particular biological function and an elevation of the level of oxidized protein in the brain (153) and in specific regions of the brain that control that function (154). Finally, because oxidized proteins are readily degraded by endogenous proteases, the steady state intracellular level of oxidized proteins is a complex function of a multiplicity of factors that govern the generation of ROS, the antioxidant systems that scavenge ROS, the susceptibility of proteins to oxidative modification, and the activities of the proteases that degrade oxidized proteins. Accordingly, the accumulation of oxidized protein that occurs during aging and in various diseases is likely attributable to the accumulated genetic damage (ROS-mediated mutations?) that affects one or more of the numerous factors that determine the balance between protein oxidation on the one hand and the degradation of oxidized proteins on the other.
AMONG THE MANY CHEMISTRY lessons that chemists hope to learn by studying enzymes, one of the more intriguing may be how to selectively oxidize hydrocarbons at ambient temperatures and pressures. It's easy to burn them up completely, releasing energy and producing carbon dioxide and water. And well-developed industrial processes can partially oxidize specific hydrocarbons, converting, for example, methane to methanol. But these processes, like hydrocarbon combustion, need high temperature, high pressure, or both. Mimicking the much gentler process that some enzymes use has turned out to be a surprisingly challenging proposition. The challenge has attracted bioinorganic chemists for more than a decade. Stephen J. Lippard, professor of chemistry at Massachusetts Institute of Technology, for example, has been probing the chemistry of an enzyme known as soluble methane monooxygenase since the late 1980s. It's produced by a number of microorganisms of a class known as methanotrophs that use methane as their only ...
Simple, but beautifully versatile. Perhaps not a description many would choose for hydrogen peroxide, but an accurate one none the less, and this unique book explains the reasons behind the description. Beginning with an historical overview, and guidelines for the safe handling of peroxygens, Applications of Hydrogen Peroxide and Derivatives goes on to cover key activation mechanisms, organic functional group oxidations and the use of hydrogen peroxide with heterogeneous catalysts. The clean-up of environmental pollutants; chemical purification; and extraction of metals from their ores are also discussed in detail, using actual examples from industry. The versatility of this reagent may well prove to be a key to integrated pollution control in the future. This book should therefore be read by academics and industrialists at all levels, to encourage wider applications of the use of hydrogen peroxide in laboratories.
Gif chemistry permits the selective functionalization of saturated hydrocarbons under very mild conditions. The formation of alkyl chlorides is shown to derive from an FeII–FeIV manifold and is distinct from the usual ketonization process (Gif chemistry) produced by an FeIII–FeV manifold. The importance of certain carboxylic acids such as picolinic acid 1 for hydrocarbon activation is highlighted. The ligand environment of the catalyst in solution is clarified using 13C NMR spectroscopy. Evidence for a µ-peroxo-dimer species as a key intermediate in solution is provided.
Various iron-catalysed procedures for the functionalisation of saturated hydrocarbons have been designated 'Gif' reactions. These were originally conceived as biomimetic models for non-haem oxidase systems, and a mechanism in which high-valent iron-ore species interact with the hydrocarbon has been adduced in support of this analogy. This review examines the evidence against an alternative free-radical mechanism. It concludes that plausible radical schemes are available for the interpretation of most, if not all, of the Gif data, but that more information is required before any unambiguous mechanistic interpretation can be justified.
In the past few years a number of iron complexes have been synthesized and characterized that model features of intermediates involved in the oxygen-activation chemistry of non-heme diiron enzymes such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases. Insights derived from these efforts are discussed in the context of a common mechanistic framework for these enzymes.
In their efforts to model high-valent intermediates in the oxygen activation cycles of nonheme diiron enzymes such as methane monooxygenase (MMOH-Q) and ribonucleotide reductase (RNR R2-X), the authors have synthesized and spectroscopically characterized a series of bis(μ-oxo)diiron(III,IV) complexes, [Feâ(μ-O)â(L)â](ClOâ)â, where L is tris(2-pyridylmethyl)amine (TPA) or its ring-alkylated derivatives. They now report the crystal structure of [Feâ(μ-O)â(5-Etâ-TPA)â](ClOâ)â (2), the first example of a structurally characterized reactive iron(IV)-oxo species, which provides accurate metrical parameters for the diamond core structure proposed for this series of complexes. Complex 2 has Fe-μ-O distances of 1.805(3) â« and 1.860(3) â«, an Fe-Fe distance of 2.683(1) â«, and an Fe-μ-O-Fe angle of 94.1(1)°. The EXAFS spectrum of 2 can be fit well with a combination of four shells: 1 O at 1.82 â«, 2--3 N at 2.03 â«, 1 Fe at 2.66 â«, and 7 C at 2.87 â«. The distances obtained are in very good agreement with the crystal structure data for 2, though the coordination numbers for the first coordination sphere are underestimated. The EXAFS spectra of MMOH-Q and RNR R2-X contain features that match well with those of 2 (except for the multi-carbon shell at 2.87 â« arising from pyridyl carbons which are absent in the enzymes), suggesting that an Feâ(μ-O)â core may be a good candidate for the core structures of the enzyme intermediates. The implications of these studies are discussed.
Solid [{l{underscore}brace}Cu(MePY2){r{underscore}brace}O]{sup 2+} is spectroscopically characterized using resonance Raman and X-ray absorption spectroscopy, for which the former technique probes the nature of the O-O bond and the latter defines the Cu-Cu interaction. In contrast to the crystal structure obtained for [{l{underscore}brace}Cu(MePY2){r{underscore}brace}O]{sup 2+}, which shows an intermediate CuO core (Cu-Cu = 3.4 and O-O = 1.6 ), resonance Raman peaks characteristic of both a side-on peroxide-bridged dicopper(II) core and bis--oxo dicopper(III) core are observed. The bis--oxo isomer is estimated to be present at approximately 5--20%. A good fit is obtained for EXAFS data for solid [{l{underscore}brace}Cu(MePY2){r{underscore}brace}O]{sup 2+} using an 80:20 ratio of Cu-Cu separations of 3.6 (characteristic of a side-on peroxide-bridged copper core) and 2.8 (associated with a bis--oxo dicopper core). Analysis of the edge region places an upper limit on the amount of bis--oxo isomer present in the solid at 40%. The factors governing the presence of bis--oxo and/or side-on peroxide cores in solution for differing ligand systems are considered, and the contribution of the bite angle of the equatorial nitrogen atom donors is explored. The reactivity of [{l{underscore}brace}Cu(MePY2){r{underscore}brace}O]{sup 2+} in solution is correlated with the presence of the bis--oxo core, using frontier molecular orbital theory.
An investigation of the ferrous ion + hydrogen peroxide system has revealed several new features. A consistent reaction mechanism involving the radicals HO and HO2 has been proposed. It appears that the reaction of HO 2 with hydrogen peroxide, hitherto generally accepted, plays no part in this system. Oxygen is evolved by reaction of HO2 with the ferric ion. Cupric ions have a pronounced effect on the system and this is interpreted in terms of the reaction scheme. A detailed analysis of kinetic and oxygen evolution data for the ferrous ion + hydrogen peroxide system under certain experimental conditions, has led to values for the ratios of the velocity constants for the reactions of ferric, ferrous and cupric ions with the HO 2 radical. The pH dependence of these reactions seems to indicate that the reactions with ferric and cupric ions proceed through the anion O 2′. In very dilute solutions minute traces of organic impurities affect the stoichiometry of the ferrous ion + hydrogen peroxide reaction and behave similarly to other organic substrates. Reaction mechanisms are discussed and shown to agree with the results obtained in the presence and absence of oxygen and with added cupric, ferric and chloride ion. A knowledge of these factors has enabled a more reliable value for the velocity constant k0 of the reaction Fe++ + H2O2 → Fe+++ + OH + OH′ to be determined. The ratio of the velocity constants for the reactions of OH radical with ferrous ion and hydrogen peroxide has been evaluated at two temperatures from gasometric and kinetic experiments at widely different concentrations. The ratio is found to be independent of pH. The numerical values obtained from the two types of experiment differ from one another by a small factor (approximately five) and a possible explanation of this in terms of the formation and reactions of a ferryl ion, Fe(OH)+++ or FeO++, is discussed.
Several dinuclear iron(III) complexes with µ-alkoxo bridges gave predominantly cyclohexyl hydroperoxide in the reaction with cyclohexane and hydrogen peroxide, and similar results were observed when linear n-alkanes, such as n-nonane and n-octane, were used instead of cyclohexane. A mechanism for selective formation of the hydroperoxide is discussed.
The complex [(terpy)(H2O)MnIII(O)2MnIV(OH2)(terpy)](NO3)3 (terpy = 2,2‘:6,2‘ ‘-terpyridine) (1) catalyzes O2 evolution from either KHSO5 (potassium oxone) or NaOCl. The reactions follow Michaelis−Menten kinetics where Vmax = 2420 ± 490 mol O2 (mol 1)-1 hr-1 and KM = 53 ± 5 mM for oxone ([1] = 7.5 μM), and Vmax = 6.5 ± 0.3 mol O2 (mol 1)-1 hr-1 and KM = 39 ± 4 mM for hypochlorite ([1] = 70 μM), with first-order kinetics observed in 1 for both oxidants. A mechanism is proposed having a preequilibrium between 1 and HSO5- or OCl-, supported by the isolation and structural characterization of [(terpy)(SO4)MnIV(O)2MnIV(O4S)(terpy)] (2). Isotope-labeling studies using H218O and KHS16O5 show that O2 evolution proceeds via an intermediate that can exchange with water, where Raman spectroscopy has been used to confirm that the active oxygen of HSO5- is nonexchanging (t1/2 1 h). The amount of label incorporated into O2 is dependent on the relative concentrations of oxone and 1. 32O2:34O2:36O2 is 91.9 ± 0.3:7.6 ± 0.3:0.51 ± 0.48, when [HSO5-] = 50 mM (0.5 mM 1), and 49 ± 21:39 ± 15:12 ± 6 when [HSO5-] = 15 mM (0.75 mM 1). The rate-limiting step of O2 evolution is proposed to be formation of a formally MnVO moiety which could then competitively react with either oxone or water/hydroxide to produce O2. These results show that 1 serves as a functional model for photosynthetic water oxidation.
We report the resonance Raman (RR) spectra of iron complexes containing the Fe2(μ-O)2 core. Frozen CH3CN solutions of the FeIIIFeIV intermediate [Fe2(μ-O)2L2](ClO4)3 (where L = TPA, 5-Me3-TPA, 5-Me2-TPA, 5-MeTPA, 5-Et3-TPA, or 3-Me3-TPA) show numerous resonance-enhanced vibrations, and among these, an oxygen-isotope-sensitive vibration around 667 cm-1 that shifts ca. 30 cm-1 when the samples are allowed to exchange with 18OH2, and whose Raman shift does not vary with methyl substitution of the TPA ligand. Spectra of iron-isotope-substituted samples of [Fe2(μ-O)2(L)2](ClO4)3 (54Fe and 57Fe for L = TPA, and 54Fe and 58Fe for L = 5-Me3-TPA) show that this vibration is also iron-isotope-sensitive. These isotopic data taken together strongly suggest that this vibration involves motion of the Fe2(μ-O)2 core that is isolated from motions of the ligand. A frozen CH3CN solution of the diiron(III) complex [Fe2(μ-O)2(6-Me3-TPA)2](ClO4)2 shows one intense resonance-enhanced vibration at 692 cm-1 that shifts −30 cm-1 with 18O labeling. Normal coordinate analysis of the Fe2(μ-O)2 core in [Fe2(μ-O)2(5-Me3-TPA)2](ClO4)3 supports the assignment of the Fermi doublet centered around 666.2 cm-1 as an A1 vibration of this core. Furthermore, we propose that this unique feature found in the region between 650 and 700 cm-1 is indicative of a diamond core structure and is the Raman signature of an iron cluster containing this core.
The reaction between dioxygen and the diiron(II) complex [FeII2(μ-OH)2(6-Me3-TPA)2]2+, where 6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine, yielding a diiron(III) peroxo complex has an associative rate-limiting step as suggested by first order in [FeII2] and [O2], small activation enthalpy (ΔH‡ = 17 kJ mol-1), significantly negative activation entropy (ΔS‡ = −175 J mol-1 K-1), and the strong sensitivity of the reaction rate to the nature of incoming ligand (kNO/kO2 = 103).
The review deals with the chemistry concerning the processes of mild and selective oxidation of alkanes and cycloalkanes by using synthetic iron complexes as catalytic systems. These cover a number of coordinated species of iron ions including the so-called Gif systems up to the polydentated Que’s models, aimed to mimic or emulate the non-haem enzymatic oxidation of alkanes. Special attention is paid to the mechanisms proposed by the various Authors and to the arguments set out for and against the free-radical interpretation to account for the activation of the CH bonds.The schemes of the free-radical chemistry of Fenton reagents in the presence of organic substrates are quoted as a reference basis for the discussion.
The hydroxylation of alkanes by hydrogen peroxide catalysed by a chiral μ-oxo diferric complex has been demonstrated to be stereospecific and partially enantioselective indicating a metal-based mechanism.
Ferric chloride in pyridine behaves as an efficient model for the catalase enzyme. It converts H2O2 nearly quantitatively into water and oxygen (2 H2O2 → 2 H2O + O2). The addition of Ph2S to the model system affords Ph2SO, the amount of which increases with the Ph2S added. The inverse relationship between oxygen and Ph2SO formation proves that there is an intermediate in the model catalase reaction. When di-n-butyl, di-t-butyl and diphenyl sulfides are reacted in pairs in competition for the intermediate a large steric effect of over 600 is found for the di-n-butyl versus di-t-butyl sulfoxide formation. In contrast the same number for per-acid oxidation is 8. It is concluded from this and other evidence that the intermediate is and FeV oxenoid, or equivalent, which reacts competitively with H2O2 to give oxygen and with sulfides to furnish sulfoxides. Comparison is made with the catalase enzyme in water and in water-acetonitrile. An unexpected by-product of this study is an efficient and economic procedure for the oxidation of sulfides to sulfoxides without further significant oxidation to sulfones.
The hydroxylation of benzene and phenol during oxygen electroreduction on mercury, lead, copper and silver electrodes in 0.1 M H2SO4 in the presence and absence of Fe2+ has been investigated. Evidence is presented that hydroxylation is possible in the absence of Fe2+. Further facts confirming that, in this case, hydroxyl radicals are the hydroxylating species have been found.
The state of the theory and practice in an important field of polarography — the catalytic currents of oxidant-substrates — is examined. It has found extensive applications in the last ten years in the development of highly sensitive methods for the determination of a large number of substrates, catalysts, and polarographically inactive ligands. Attention has been given to the problem of the electrocatalytic reduction of molecular nitrogen for its fixation under mild conditions. A systematic account is given of data for a series of inorganic and organic substrates and catalysts which transfer an electron to the substrate.
The bibliography includes 242 references.
The kinetics of the ferric ion catalyzed decomposition of hydrogen peroxide are deduced on the basis of the reaction steps which we have found necessary in the ferrous ion system. These lead to three different kinetic expressions depending on R, the ratio of the concentrations of hydrogen peroxide and ferric ion. Previous work is discussed in terms of these expressions. Measurements of rates of decomposition at high values of R and at various pH's have led to the evaluation of the rate constant for the chain initiation reaction, Fe +++ + HO2-, and its dependence on temperature. At 25°C about 103 molecules of peroxide are decomposed for each initiation. For values of R between unity and 0.002 the rate of decomposition is found to be proportional to [H2O2]3/2[Fe +++] and to an inverse power of [H+] slightly more than unity. These kinetics conform to the proposed reaction scheme as far as the dependence on the peroxide concentration is concerned but not for that of ferric and hydrogen ions. A method has been developed for measuring the concentration of ferrous ion in equilibrium with ferric ion and hydrogen peroxide. At high values of R the dependence of this ferrous ion concentration on the hydrogen peroxide and ferric ion concentrations is found to be qualitatively and quantitatively in agreement with the proposed reaction scheme. At very low values of R the dependence on the peroxide concentration is qualitatively consistent with the reaction scheme but the numerical values differ from those which would be predicted. Using the experimental results in this and previous work it is estimated that the sum of the electron affinity of the radical HO2 and the solvation energy of HO2- (E HO2 + SHO2-) is between 125 and 135 kcal. Similarly E O2 + SO2- is found to be between 77 and 107 kcal. The reactions of HO2- and O2- with ferric and cupric ions are discussed.
The incorporation of 18O from H218O into the product of stereospecific alkane hydroxylation by [FeII(bpmen)- (CH3CN)2](ClO4)2–H2O2 provides the first strong evidence for the participation of a high-valent iron–oxo species in the mechanism of a non-heme iron catalyst.
We have synthesized the first complexes with bis(μ-oxo)diiron(III) and (μ-oxo)(μ-hydroxo)diiron(III) cores (1 and 2, L = TPA (a), 5-Et3-TPA (b), 6-Me3-TPA (c), 4,6-Me6-TPA (d), BQPA (e), BPEEN (f), and BPMEN (g)) and found them to have novel structural properties. In particular, the presence of two single-atom bridges in these complexes constrains the Fe−Fe distances to 2.7−3.0 Å and the Fe−μ-O−Fe angles to 100° or smaller. The significantly acute Fe−O−Fe angles (e.g., 92.5(2)° for 1c and 100.2(2)° for 2f) enforced by the Fe2O2(H) core endow these complexes with UV−vis, Raman, and magnetic properties quite distinct from those of other (μ-oxo)diiron(III) complexes. Complex 1c exhibits visible absorption bands at 470 (ε = 560 M-1 cm-1) and 760 nm (ε = 80 M-1 cm-1), while complexes 2 show features at ca. 550 (ε ≈ 800 M-1 cm-1) and ca. 800 nm (ε ≈ 70 M-1 cm-1), all of which are red shifted compared to those of other (μ-oxo)diiron(III) complexes. These complexes also exhibit distinct νFe-O-Fe vibrations at ca. 600 and ca. 670 cm-1 assigned to the νsym and the νasym of the Fe−O−Fe units, respectively. The relative intensities of the νsym and νasym bands are affected by the symmetry of the Fe−O−Fe units; an unsymmetric core enhances the intensity of the νasym. Complexes 2 exhibit another band at ca. 500 cm-1, which is assigned to the Fe−(OH)−Fe stretching mode due to its sensitivity to both H218O and 2H2O. Magnetic susceptibility studies reveal J = 54 cm-1 for 1c and ca. 110 cm-1 for 2 (H = JS1·S2), values smaller than those for the antiferromagnetic interactions found in (μ-oxo)diiron(III) complexes. This weakening arises from the longer Fe−μ-O bonds and the smaller Fe−μ-O−Fe angles in the Fe2O2(H) diamond core structure. These spectroscopic signatures can thus serve as useful tools to ascertain the presence of such core structures in metalloenzyme active sites. These two core structures, Fe2(μ-O)2 (1) and Fe2(μ-O)(μ-OH) (2), can also be interconverted by protonation equilibria with pKa's of 16−18 in CH3CN. Furthermore, the Fe2(μ-O)2 core (1) isomerizes to the Fe3(μ2-O)3 core (7), while the Fe2(μ-O)(μ-OH) core (2) exhibits aquation equilibria to the Fe2(μ-O)(μ-H3O2) core (5), except for L = 6-Me3-TPA and 4,6-Me6-TPA. It is clear from these studies that electronic and steric properties of the ligands significantly affect the various equilibria, demonstrating a rich chemistry involving water-derived ligands alone.
The complex dichloro-meso-2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13, 15-trieneiron(III) tetrafluoroborate, abbreviated [Fe(CRH4)Cl2]BF4, is an effective catalyst for the decomposition of H2O2 to form H2O and O2 in aqueous solution. The solution behavior of the complex [Fe(CRH4)Cl2]BF4 was studied in order to facilitate the analysis of the rate data. The iron complex exists primarily as the aquo-hydroxo complex in the pH range of interest. This aquo-hydroxo complex slowly self-condenses in the presence of air and acetate buffers to give an antiferromagnetically coupled, μ-oxo dimer. The dimerization rate has been measured. The active species in the catalysis of H2O2 decomposition is the aquo-hydroxo complex Fe(CRH4)(OH)(H2O)2+. The dimer is catalytically inactive, as is the corresponding dimer of hemin. The kinetics of H2O2 decomposition in the pH range from 3.8 to 5.1 and in the temperature range from 8 to 32°C were studied thoroughly by oxygen evolution measurements using an apparatus which allows very rapid pressure changes to be monitored. The observed rate law is shown to be consistent with a mechanism formulated in terms of free-radical intermediates. Acetate inhibits the reaction, possibly by competing for hydroxyl radicals.
Solutions of FeII(MeCN)4(ClO4)2 in dry acetonitrile (MeCN) catalyze the rapid disproportionation of H2O2 to O2 and H2O, but all of the catalyst remains in the Fe(II) oxidation state. In the presence of organic substrates such as 1,4-cyclohexadiene, 1,2-diphenylhydrazine, catechols, and thiols, the Fe(II)-H2O2/MeCN system yields dehydrogenated products (PhH, PhN=NPh, quinones, and RSSR) with conversion efficiencies that range from 100% to 17%. Although the Fe(II) catalyst does not promote the disproportionation of Me3COOH or m-ClC6H4C(O)OOH, these hydroperoxides are activated for the dehydrogenation of organic substrates. With substrates such as alcohols, aldehydes, methylstyrene, thioethers, sulfoxides, and phosphines, the FeII(H2O2)2+ adduct promotes their monooxygenation to aldehydes, carboxylic acids, epoxide, sulfoxides, sulfones, and phosphine oxides, respectively: Fe(II) + H2O2 → FeII(H2O2)2+ + RH → Fe(II) + ROH + H2O. The reaction efficiencies for the group of substrates with the Fe(II) adducts that are formed by H2O2, Me3COOH, and m-ClC6H4C(O)OOH have been evaluated. Also, the reaction rates for the substrate-[FeII(H2O2)2+] dehydrogenations and monooxygenations relative to that for Ph2SO have been determined, as have the substituent effects for the monooxygenation of 4-XC6H4CH2OH and 4-XC6H4CH(O). The FeII(H2O2)2+ adduct is an efficient catalyst for the autooxygenation of PhCH(O) to PhC(O)OOH. Mechanisms are proposed for the Fe(II)-induced activation of hydroperoxides for the dehydrogenation and monooxygenation of organic substrates.
In pyridine/acetic acid solvent bis(picolinato)iron(II) [Fe(PA)2], (2,6-dicarboxylatopyridine)iron(II) [Fe(DPA)], and their μ-oxo dimers [(PA)2FeOFe(PA)2 and (DPA)FeOFe(DPA)] catalyze hydrogen peroxide for the selective ketonization of methylenic carbons (>CH2 → C=O) and the dioxygenation of acetylenes to α-diketones and arylolefins to aldehydes. Cyclohexane is transformed with 72% efficiency (c-C6H12 oxidized per two HOOH) to give 95% cyclohexanone and 5% cyclohexanol, ethyl benzene with 51% efficiency to give acetophenone as the only detectable product, and n-hexane with 52% efficiency to give 53% 3-hexanone, 46% 2-hexanone, and <2% 1-hexanol. Suspensions of KO2(s) or (Me4N)O2(s) in a pyridine/acetic acid solvent system are catalyzed by several iron complexes [(py)4FeCl2, (py)4Fe(OAc)2, FeCl3·6H2O, (MeCN)4Fe(ClO4)2, (Ph3PO)4Fe(ClO4)2, Fe(PA)2, and (PA)2FeOFe(PA)2] to give HOOH and transform methylenic carbons to ketones, and to dioxygenate acetylenes and arylolefins. Electrolytic reduction of dioxygen (O2) in the same solvent/catalyst systems results in analogous substrate transformations. The Fe(PA)2 complex is uniquely efficient and exhibits catalytic turnover for KO2(s) suspensions as well as for electro-reduced O2. All systems appear to produce a common reactive intermediate 3 [(PA)2FeOOOFe(PA)2] via in situ formation of HOOH and (PA)2FeOFe(PA)2 (1).
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The preparations, characterizations, structures, magnetic properties and catalase-like activities of four new alkoxy bridged dimeric five-coordinate iron(III) salicylaldimine complexes, [FeL1Cl]2 (1) (H2L1=N-(3-hydroxypropyl)(5-bromosalicylaldimine)), [FeL2Cl]2 (2) (H2L2=N-(3-hydroxypropyl)(3-methoxysalicylaldimine)), [FeL3Cl]2 (3) (H2L3=N-(3-hydroxypropyl)(4-diethylaminosalicylaldimine)), and [FeL4Cl]2 (4) (H2L4=N-(3-hydroxypropyl)(3,5-diiodosalicyaldimine)) are reported. The crystal structure of complexes 1–3 show an alkoxy bridged dimeric iron(III) complex, which occupies a site of inversion symmetry, and each iron atom is five-coordinate with square-pyramidal arrangements. Mössbauer spectra reveal the presence of high-spin iron(III) ions in complexes 1–4. The magnetic susceptibility (300–2 K) indicate an antiferromagnetic interaction between the two oxygen-bridged iron(III) ions with J=−17.5, −12.40, −19.0 and −21.0 cm−1 for the complexes of 1–4, respectively. The H2O2 disproportionation of complexes 1–4 in acetonitrile at 25°C show the rate law of kobs[H2O2]2[complex].
The presence or absence of certain chelating carboxylic acids such as picolinic acid permits the distinction between ketonization (Gif Chemistry) and oxygen formation (catalase reaction). In the presence of such an acid, evidence is provided for the possible involvement of a IIIFeOOFeIII species as a key intermediate in this hydrocarbon activation chemistry.
Benzylic oxidation using a Mn-salen complex 3 as catalyst and iodosylbenzene as oxidant was found to proceed with moderate enantioselectivity to give the corresponding benzylic alcohol in solvents of high viscosity such as chlorobenzene and fluorobenzene. For example, the oxidation of 1,1-dimethylindane with 3 gave 3-hydroxy-1,1-dimethylindane of 64% ee.
It is demonstrated that hydrogen peroxide can be produced with a current efficiency of 40–70% by the reduction of oxygen at a reticulated vitreous carbon cathode in a divided flow cell using catholytes consisting of aqueous chloride or sulfate media, pH≈2. The influence of ferrous salts, potential and electrolyte concentration on the current efficiency and rate of H2O2 production is reported; ferrous ions can lead to the homogeneous decomposition of H2O2 away from the cathode surface but their effectiveness as a catalyst for this decomposition depends on their speciation in solution which changes during an electrolysis. The conclusions are supported by voltammetry at both a rotating vitreous carbon disc and the reticulated vitreous carbon electrodes.