Hydroperoxy Complex Research Articles

Promotive Effect of Bicarbonate Ion on Two-Electron Water Oxidation to Form H2 O2 Catalyzed by Aluminum Porphyrins.

Two-electron water oxidation initiated by one-electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four-electron pathway and could help to avoid the bottleneck subject of photon-flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two-electron water oxidation to form H2 O2 is reported. An addition of sodium carbonate (Na2 CO3 ) controlled both catalytic current and product selectivity of the two-electron water oxidation to enhance the activity of AlTMPyP at pH≈10-11. Controlled potential electrolysis experiments at different concentrations of Na2 CO3 (10-100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na2 CO3 ] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H2 O2 from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H2 O2 from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na2 CO3 on the catalytic reactivity was observed.

One Electron-Initiated Two-Electron Oxidation of Water by Aluminum Porphyrins with Earth's Most Abundant Metal.

We report herein a new molecular catalyst for efficient water splitting, aluminum porphyrins (tetra-methylpyridiniumylporphyrinatealuminum: AlTMPyP), containing earth's most abundant metal as the central ion. One-electron oxidation of the aluminum porphyrin initiates the two-electron oxidation of water to form hydrogen peroxide as the primary reaction product with the lowest known overpotential (97 mV). The aluminum-peroxo complex was detected by a cold-spray ionization mass-spectrometry in high-resolution MS (HRMS) mode and the structure of the intermediate species was further confirmed using laser Raman spectroscopy, indicating the hydroperoxy complex of AlTMPyP to be the key intermediate in the reaction. The two-electron oxidation of water to form hydrogen peroxide was essentially quantitative, with a Faradaic efficiency of 99 %. The catalytic reaction was found to be highly efficient, with a turnover frequency up to ∼2×104 s-1 . A reaction mechanism is proposed involving oxygen-oxygen bond formation by the attack of a hydroxide ion on the oxyl-radical-like axial ligand oxygen atom in the one-electron-oxidized form of AlTMPyP(O- )2 , followed by a second electron transfer to the electrode.

Oxidation of diatrizoate in aqueous phase by advanced oxidation processes based on solar radiation

Abstract The aim of this study was to investigate the use of solar radiation to treat recalcitrant pollutants, using sodium diatrizoate (DTZ), an ionic X-ray contrast media, as reference compound. The effectiveness in DTZ removal was compared among solar radiation, H2O2, solar radiation/H2O2, K2S2O8, solar radiation/K2S2O8, Fenton, solar radiation/Fenton, Fenton-Like, and solar radiation/Fenton-Like systems. Using direct DTZ photolysis, the percentage DTZ degradation is low and independent of the medium pH, but not of the irradiance level. However, in the presence of radical-promoting species, 100% DTZ degradation is achieved. Results obtained with solar radiation/Fenton and solar radiation/Fenton-like systems suggest that the formation of peroxo and iron hydroperoxy complexes susceptible of absorbing radiation in the solar radiation area represents a new source of OH and/or OH2 radicals in the medium, favoring DTZ degradation. The byproducts formed in each process and the post-treatment medium toxicity were studied, observing an increased toxicity after all oxidation treatments, indicating that the degradation products generated are more toxic than DTZ. Moreover, a direct relationship was found between percentage DTZ degradation and aqueous medium toxicity.

Theoretical investigation of the ring opening process of verdoheme to biliverdin in the presence of dioxygen

The conversion of ferrous verdoheme to ferric biliverdin in the presence of O(2) was investigated using the B3LYP method. Both 6-31G and 6-31G (d) basis sets were employed for geometry optimization calculation as well as energy stabilization estimation. Three possible pathways for the conversion of iron verdoheme to iron biliverdin were considered. In the first route oxygen and reducing electron were employed. In this path formation of ferrous verdoheme-O(2) complex was followed by the addition of one electron to the ferrous-oxycomplex to produce ferric peroxide intermediate. The ferric peroxide intermediate experienced an intramolecular nucleophilic attack to the most positive position at 5-oxo carbons on the ring to form a closed ring biliverdin. Subsequently the ring opening process took place and the iron (III) biliverdin complex was formed. Closed ring iron biliverdin intermediate and open ring iron biliverdin formed as a product of verdoheme cleavage were respectively 13.20 and 32.70 kcal mol(-1) more stable than ferric peroxide intermediate. Barrier energy for conversion of ferric peroxide to closed ring Fe (III) biliverdin and from the latter to Fe (III) biliverdin were respectively 8.67 and 3.35 kcal mol(-1). In this path spin ground states are doublet except for iron (III) biliverdin in which spin state is quartet. In the second path a ferrous-O(2) complex was formed and, without going to a one electron reduction process, nucleophilic attack of iron superoxide complex took place followed by the formation of iron (III) biliverdin. This path is thermodynamically and kinetically less favorable than the first one. In addition, iron hydro peroxy complex or direct attack of O(2) to macrocycle to form an isoporphyrin type intermediate have shown energy surfaces less favorable than aforementioned routes.

A new insight into Fenton and Fenton-like processes for water treatment

In this study, two Fenton (Fe 2+/H 2O 2) and Fenton-like (Fe 3+/H 2O 2) reactions were compared to clarify their roles in phenol degradation under varying H 2O 2 concentrations, iron dosages and pHs, as well as in the presence of radical scavenger. The results of this study showed that a Fenton-like reaction must proceed concurrently with a classic Fenton reaction, and the concurrent Fenton reaction played a major role in the degradation of pollutants. For the Fenton-like reaction, some oxidation intermediates of phenolic compounds may promote the conversion of Fe(III) to Fe(II) in addition to the uni-molecular decomposition of the Fe(III)–hydroperoxy complexes. The results also showed that varying H 2O 2 concentrations exerted identical effects on the two reactions, and that phenol degradation in both reactions could be correlated to the decomposition of H 2O 2. At low levels of iron concentration, the Fenton reaction appeared to be more efficient than the Fenton-like reaction in terms of the phenol degradation and H 2O 2 decomposition. Additionally, the Fenton reaction had an effective pH range of 2.5–6.0, while the Fenton-like reaction was limited to a narrow pH range of 2.8–3.8. Although the Fenton-like reaction was much slower than that of the Fenton reaction, the overall extent of phenol degradation and H 2O 2 decomposition at the optimal conditions was equivalent.

Catalytic oxidative degradation of s-triazine and phenoxyalkanoic acid based herbicides with metalloporphyrins and hydrogen peroxide: Identification of two distinct reaction schemes

Oxidative degradation of the herbicides atrazine ( 1), atraton ( 2), ametryn ( 3) and mecoprop ( 4), was carried out with hydrogen peroxide and metalloporphyrins as catalysts. Two different reaction conditions were studied, the first involving Mn(TDCPP)Cl in an aprotic solvent with buffer ( S-I), and the second using Fe(TPFPP)Cl in a protic solvent ( S-II). Reaction products were characterized, and based on these it is shown that there are two distinct reaction schemes. In the case of the S-I conditions, it is suggested that the s-triazines were oxidized through hydroxylation of the alkyl side chains followed by dealkylation, while S-II was ineffective for these reactions. In contrast, mecoprop, was oxidized with high efficiency by S-II, leading to decarboxylation and further oxidation, while in the presence of S-I, low substrate conversion was observed, and reaction resulted mainly from oxidation at the benzyl position. Sulfoxidation of ametryn was observed with both systems. The different reactivity shown by the two systems supports the involvement of different reactive species, which we assign to the oxo and hydroperoxy complexes. These routes show similarities with metabolic pathways, with the reactivity pattern of S-I analogous to the reported metabolism of these pollutants with cytochrome P450 enzymes, while S-II catalyses mecoprop decarboxylation via a similar pathway to that seen with peroxidase catalysed reactions.

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.

Novel Mn(III)chlorins as versatile catalysts for oxyfunctionalisation of hydrocarbons under homogeneous conditions

The novel fluorinated metallochlorin 5,10,15,20-tetrakis(pentafluorophenyl)-tetrahydro-1 H- N-methyl-pyrrolo[3,4- b]-porphyrinato manganese(III), Mn(chlor)-1, and its methylated derivative, Mn(chlor)-2, have been studied as catalysts in the oxyfunctionalisation of cyclohexane with two oxidants, namely iodosylbenzene (PhIO) and H 2O 2. For comparison reasons two metalloporphyrins have been also used: the neutral 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato manganese(III), Mn(porph)-1, and the cationic 5,10,15,20-tetrakis(2,3,5,6-tetrafluoro-4-trimethylammoniumphenyl)porphyrinato manganese(III), Mn(porph)-2. It has been possible to verify a different reactivity in the H 2O 2-oxyfunctionalisation of alkanes from studies using the radical inhibitor bromotrichloromethane and competitive cyclooctane/ cis-cyclooctene oxidations. The results suggest that the formation of alcohols and ketones from the oxidation of cyclohexane occurs mainly from a free alkyl radical mechanism, which requires a Mn V O species to abstract a hydrogen atom from the alkane to generate the alkyl radical. On the other hand, the Mn(chlor)/H 2O 2 system possibly yields, in a first step, the Mn III OOH species, but this is not reactive enough to generate Mn V O. The Mn(chlor) hydroperoxy complex shows low efficiency in the production of the alkyl radical, leading to small amounts of alcohol and ketone. However, it seems to be an efficient species for selective epoxidation, as can be observed in the competitive cyclooctane/ cis-cyclooctene oxidation reaction.

Spectrophotometric study of the formation of iron(III)-hydroperoxy complexes in homogeneous aqueous solutions

The complexation of Fe III in homogeneous aqueous solution by hydrogen peroxide (H 2O 2) was investigated spectrophotometrically. Experiments were carried out at 25°C between pH 1 and 3, with ferric perchlorate in perchloric acid/sodium perchlorate solution (ionic strength: 0.1 M). Spectrophotometric analyses showed that the addition of H 2O 2 into a solution of Fe III immediately led to the formation of species absorbing in the region 300–600 nm. Calculations made from the spectral data gave evidence of the formation of two ferric hydroperoxy complexes, formulated as [Fe III] 3+, (HO 2 −) (or [Fe III(HO 2)] 2+; I 1) and [Fe III(OH)] 2+, (HO 2 −) (or [Fe III(OH)(HO 2)] +; I 2), which are in acid–base equilibrium ( K I 1/I 2 =1.8×10 −4 mol l −1). Equilibrium constants for the formation of I 1 and I 2 and the spectrum of each ferric hydroperoxy complex at various wavelengths have also been determined in this study.

Catalytic Decomposition of Hydrogen Peroxide by Fe(III) in Homogeneous Aqueous Solution: Mechanism and Kinetic Modeling

This paper describes a kinetic model for the decomposition of hydrogen peroxide by ferric ion in homogeneous aqueous solution (pH < 3). The reaction was investigated experimentally at 25.0 °C and I = 0.1 M (HClO4/NaClO4), in a completely mixed batch reactor and under a wide range of experimental conditions (1 ≤ pH ≤ 3; 0.2 mM ≤ [H2O2]0 ≤ 1 M; 50 μM ≤ [Fe(III)]0 ≤ 1 mM; 1 ≤ [H2O2]0/[Fe(III)]0 ≤ 5000). The results of this study demonstrated that the rate of decomposition of hydrogen peroxide by Fe(III) could be predicted very accurately by a kinetic model which takes into account the rapid formation and the slower decomposition of Fe(III)−hydroperoxy complexes (FeIII(HO2)2+ and FeIII(OH)(HO2)+). The rate constant for the unimolecular decomposition of the Fe(III)−hydroperoxy complexes was determined to be 2.7 × 10-3 s-1. The use of the kinetic model allows a better understanding of the effects of operational parameters (i.e., pH and [H2O2]0/[Fe(III)]0) on the complex kinetics of decomposition of H2O2 by Fe(III).

Homogeneous oxidation of 1-octene by dioxygen catalyzed by rhodium species: comparison of the ‘rhodium-copper-lithium’ and ‘rhodium-bismuth-lithium’ systems

Complexes of the type [Rh III(H 2O) 6](A −) 3 (A − = ClO 4 −, BF 4 −,…) have been tested with other rhodium compounds as catalyst precursors in the oxidation of terminal olefins to methyl ketones by dioxygen. The effect of a cocatalyst (Cu(II) or Bi(III) salt) is reported and new systems are devised. It is possible to obtain good activities as well as good selectivities with substrate/catalyst ratios of 120 ( P(O 2) = 1 atm, 40 ⩽ T(°C) ⩽ 70). Novel homogeneous systems based on rhodium(III), copper(II) complexes and lithium chloride are more effective in the oxidation of 1-olefins dissolved in ethanol than the ‘RhCl 3·3H 2O’/Cu(ClO 4) 2·4HMPA system, (the latter gives a precipitate of cuprous chloride). Together with rhodium-copper based systems, rhodium complexes in association with bismuth chloro-complexes and/or LiCl were found to be equally efficient in the oxidation of 1-alkenes in alcoholic media, but there is less solvent cooxidation in the ‘Rh III/Cu II/Li I’ systems. The yields and the rate of reaction are not sensitive to phosphine concentration ( e.g. P(C 6F 5) 3 and P(n-Bu) 3, within the limits Rh:P =1:1 to 1:2). Cycloocta-1,5-diene is mono-oxygenated by the ‘Rh III/ Cu II/Li’ system at 60 °C, in a reaction leading to cyclooct-4-en-1-one (yield 82%). Our experimental results and those of other groups suggest that the key intermediates are a hydrido-rhodium complex: ‘[HRh IIICl xL y ]’ and a hydroperoxy complex ‘[HOORhCl xL y]’. The reaction of dioxygen with HRhCl 2[PPh 2(n-Bu)] 3 in dry ethanol-oct-1-ene mixtures affords octane-one (yield 45% based on rhodium, T = 60 °C).

The ESR evidence of VO 3+-hydroperoxy complex radical

The ESR evidence of VO 3+-hydroperoxy complex radical