Alexander B. Sorokin

Efficient Oxidative Dechlorination and Aromatic Ring Cleavage of Chlorinated Phenols Catalyzed by Iron Sulfophthalocyanine

An efficient method has been developed for the catalytic oxidation of pollutants that are not easily degraded. The products of the hydrogen peroxide (H2O2) oxidation of 2,4,6,-trichlorophenol (TCP) catalyzed by the iron complex 2,9,16,23-tetrasulfophthalocyanine (FePcS) were observed to be chloromaleic, chlorofumaric, maleic, and fumaric acids from dechlorination and aromatic cycle cleavage, as well as additional products that resulted from oxidative coupling. Quantitative analysis of the TCP oxidation reaction revealed that up to two chloride ions were released per TCP molecule. This chemical system, consisting of an environmentally safe oxidant (H2O2) and an easily accessible catalyst (FePcS), can perform several key steps in the oxidative mineralization of TCP, a paradigm of recalcitrant pollutants.

Metallophthalocyanine functionalized silicas : catalysts for the selective oxidation of aromatic compounds

Abstract Metallophthalocyanines of iron, manganese and cobalt have been successfully anchored onto the surface of mesoporous and amorphous silicas and used as catalysts in the liquid phase oxidation of 2-methylnaphthalene and 2,3,6-trimethylphenol with hydrogen peroxide and t -butylhydroperoxide. The iron tetrasulfophthalocyanine showed an activity from far superior to that of the other metallic complexes and to the corresponding homogeneous catalysts. The activity and selectivity depend on the nature of the solvent, oxidizing agent and the catalyst itself. In particular, iron tetrasulfophthalocyanine anchored in the dimeric form yielded catalyst much more active and selective that those containing monomeric species, which was unexpected since dimeric species are usually considered as inactive in homogeneous systems. The catalytic properties of these materials were compared with those of Ti containing silica-based molecular sieves. The latter were active in the oxidation of 2,3,6-trimethylphenol but not in that of 2-methylnaphthalene. Iron (III) peroxo species, formed by reaction of the peroxide with iron phthalocyanine were proposed to undergo a homolytic or heterolytic cleavage of O–O bond in case of the monomeric and dimeric complexes, respectively, to explain the difference in their catalytic properties.

Selective oxidation of aromatic compounds with dioxygen and peroxides catalyzed by phthalocyanine supported catalysts

Abstract This article summarizes our research in catalytic oxidation on the design and study of supported metallophthalocyanine catalysts. The catalytic properties of these materials were studied in the oxidation of 2-methylnaphthalene (2MN) to 2-methyl-1,4-naphthoquinone (Vitamin K3, VK3), 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (precursor of Vitamin E) and in the epoxidation of olefins. Iron tetrasulfophthalocyanine (FePcS) covalently grafted in the dimeric form yielded catalyst more active and selective that those containing monomeric species but suffered from a lack of stability transforming into less selective monomer complexes during catalysis. The stabilization of supported dimer form by covalent link of two adjacent phthalocyanine molecule through appropriate diamine spacer provided more selective and stable catalysts. Trimethyl-1,4-benzoquinone was obtained with 87% yield at 97% conversion of TMP. More demanding oxidation of 2MN afforded 45% yield of VK3. Particular emphasis is placed on the mechanistic aspects of these oxidations using two mechanistic probes, 2-methyl-1-phenylpropan-2-yl hydroperoxide (MPPH) to distinguish between homolytic versus heterolytic cleavage of O–O bond during the formation of active species and thianthrene 5-oxide (SSO) to evaluate nucleophilic versus electrophilic character of formed active species. To illustrate a versatility of the phthalocyanine-based supported catalysts we prepared a novel phthalocyanine complex with eight triethoxysylil substituents which can be directly anchored to the silica without any modification of the silica support. This new catalyst shows good catalytic activity in epoxidation of olefins by dioxygen in the presence of isobutyraldehyde. The same catalytic system was also active in the oxidation of phenols to biphenols with 86% yields. This catalytic system is complementary to previous one that selectively oxidizes phenols to quinones. An appropriate choice of the reaction conditions allows selective oxidation either to quinones or to biaryl compounds.

Heterogeneous oxidation of aromatic compounds catalyzed by metallophthalocyanine functionalized silicas

Controlled procedures for the covalent anchoring of iron tetrasulfophthalocyanine onto amino-modified silicas have been developed to fix the complex either in a monomer or dimer form. Usually considered as a catalytically inert form, dimeric µ-oxo iron tetrasulfophthalocyanine grafted onto amino-modified silica or MCM-41 is shown to be a selective catalyst for the oxidation of 2-methylnaphthalene to 2-methylnaphthaquinone (vitamin K3) and of 2,3,6-trimethylphenol to trimethylbenzoquinone. Catalytic methods involving environmentally friendly oxidants are needed to perform selective oxidation of alkylaromatics. For example, 2-methylnaphthoquinone (Vitamin K3) is still prepared by stoichiometric oxidation of 2-methylnaphthalene (2MN) by CrO3 in H2SO4, thus leading to severe environmental problems. As an alternative, several homogeneous catalytic methods have been developed Oxidation of aromatic compounds to quinones is a multistep reaction and the yields are rarely high because of coupling and over-oxidation reactions. Vitamin K3 is obtained by 2MN oxidation with CrO3–H2SO4 in 38–60% yields. A 46% yield of vitamin K3 was reported in 2MN oxidation by the CH3ReO3–85% H2O2 system Recently, iron tetrasulfophthalocyanine (FePcS) has been shown to be an active catalyst in the oxidative degradation of chlorinated phenol and in the homogeneous oxidation of condensed aromatics The fixation of active catalysts onto appropriate supports is highly desirable to provide a high catalyst stability as well as facile recovery and recycling. Another reason to immobilize homogeneous complexes is that homogeneous solutions of complexes often contain several species, for example, monomers and dimers, while only some species, usually monomeric ones, are catalytically active. For example, among the several monomer and dimer forms of FePcS in aqueous solutions, monomeric FePcS has been proposed to be the catalytically active complex in the oxidative degradation of trichlorophenol Consequently, by using appropriate methods one can prepare a heterogeneous catalyst supporting only the catalytically active form of the complex. We report here the controlled covalent anchoring of phthalocyanine complexes onto mesoporous MCM-41 silica (Scheme 1) and the catalytic properties of these hybrid materials in the oxidation of 2-methylnaphthalene and 2,3,6-trimethylphenol. Recently discovered mesoporous silicas having large ordered hexagonal channels with diameters from 15 to 100 A(MCM-41) and high surface areas (above 700 g-1) are promising supports for different types of catalystsMCM-41 was prepared as previously describe and modified with 3-aminopropyltriethoxysilane to obtain 0.5 mmol of NH2 groups per gram of material. The modification was proven by the decrease of the silanol signals Q2 (-91 ppm) and Q3 (-100 ppm) and the appearance of a signal characteristic of (SiO)3Si–CH2 groups (-60 ppm) in the 29Si MAS NMR spectrum. 13C CP MAS NMR shows aminopropyl carbon signals at 5.5, 18.9 and 40.3 ppm. Textural characteristics of some solid materials are listed in Table 1. A decrease of specific surface area from 860 to 698 g-1 and average pore diameter from 40 to 36 Aas well as a decrease of the mesoporous volume from 1.30 to 1.04 cm3 g-1 also indicates a successful modification of MCM-41.FePcS was converted to iron tetrasulfochlorophthalocyanine, FePc(SO2Cl)4, by treatment with SOCl2 or PCl5. For covalent anchoring of FePc(SO2Cl)4 onto amino-modified MCM-41 two strategies were used in order to fix the phthalocyanine complex either as a monomer or as a dimer. We used UV-vis spectroscopy to identify these species.

An N-bridged high-valent diiron–oxo species on a porphyrin platform that can oxidize methane

High-valent oxo-metal complexes are involved in key biochemical processes of selective oxidation and removal of xenobiotics. The catalytic properties of cytochrome P-450 and soluble methane monooxygenase enzymes are associated with oxo species on mononuclear iron haem and diiron non-haem platforms, respectively. Bio-inspired chemical systems that can reproduce the fascinating ability of these enzymes to oxidize the strongest C-H bonds are the focus of intense scrutiny. In this context, the development of highly oxidizing diiron macrocyclic catalysts requires a structural determination of the elusive active species and elucidation of the reaction mechanism. Here we report the preparation of an Fe(IV)(µ-nitrido)Fe(IV)u2009=u2009O tetraphenylporphyrin cation radical species at -90xa0°C, characterized by ultraviolet-visible, electron paramagnetic resonance and Mössbauer spectroscopies and by electrospray ionization mass spectrometry. This species exhibits a very high activity for oxygen-atom transfer towards alkanes, including methane. These findings provide a foundation on which to develop efficient and clean oxidation processes, in particular transformations of the strongest C-H bonds.

Sequential addition of H2O2, pH and solvent effects as key factors in the oxidation of 2,4,6-trichlorophenol catalyzed by iron tetrasulfophthalocyanine

The efficiency of the H2O2 oxidation of 2,4,6-trichlorophenol (TCP) catalyzed by iron tetrasulfophthalocyanine (FePcS) is highly dependent on the pH value of the reaction mixture, the local hydrogen peroxide concentration and the organization of FePcS molecules in solution. Among the several forms of FePcS in aqueous solutions (dimer or monomer), monomeric FePcS is proposed to be the catalytically active complex. The key role of the organic co-solvent (acetonitrile, acetone, alcohol, . . .) is to shift the dimer/monomer equilibrium toward monomeric FePcS, the efficient catalyst precursor. A stepwise addition of hydrogen peroxide significantly improves the conversion of TCP and allows a low catalyst loading, below 1% with respect to the pollutant, to be used. Facteurs cle′s dans loxydation du 2,4,6-trichlorophe′nol catalyse′e par la te′trasulfophtalocyanine de fer: ajout se′quentiel deau oxyge′ne′e, effets du pH et du solvant.Lefficacite′ de loxydation du 2,4,6-trichlorophe′nol (TCP) catalyse′e par la te′trasulfophtalocyanine de fer (FePcS) est tres de′pendante de la valeur du pH du milieu re′actionnel, de la concentration locale en eau oxyge′ne′e et de lorganisation des mole′cules de FePcS en solution. Parmi les diffe′rentes formes possibles de FePcS en solution aqueuse (dimere ou monomere), la forme monome′rique de FePcS est probablement la forme active du catalyseur. Le role cle′ du co-solvant organique (ace′tonitrile, ace′tone, alcool, . . .) est de de′placer le′quilibre dimere/monomere vers la forme monome′rique de FePcS, le pre′curseur catalytique efficace. Une addition fractionne′e deau oxyge′ne′e ame′liore de maniere significative la conversion du TCP et permet dutiliser moins de 1% de catalyseur par rapport au polluant.

Hydroxylation of Aromatics with the Help of a Non-Haem FeOOH: A Mechanistic Study under Single-Turnover and Catalytic Conditions

Ferric-hydroperoxo complexes have been identified as intermediates in the catalytic cycle of biological oxidants, but their role as key oxidants is still a matter of debate. Among the numerous synthetic low-spin Fe(III)(OOH) complexes characterized to date, [(L(5)(2))Fe(OOH)](2+) is the only one that has been isolated in the solid state at low temperature, which has provided a unique opportunity for inspecting its oxidizing properties under single-turnover conditions. In this report we show that [(L(5)(2))Fe(OOH)](2+) decays in the presence of aromatic substrates, such as anisole and benzene in acetonitrile, with first-order kinetics. In addition, the phenol products are formed from the aromatic substrates with similar first-order rate constants. Combining the kinetic data obtained at different temperatures and under different single-turnover experimental conditions with experiments performed under catalytic conditions by using the substrate [1,3,5-D(3)]benzene, which showed normal kinetic isotope effects (KIE>1) and a notable hydride shift (NIH shift), has allowed us to clarify the role played by Fe(III)(OOH) in aromatic oxidation. Several lines of experimental evidence in support of the previously postulated mechanism for the formation of two caged Fe(IV)(O) and OH(·) species from the Fe(III)(OOH) complex have been obtained for the first time. After homolytic O-O cleavage, a caged pair of oxidants [Fe(IV)O+HO(·)] is generated that act in unison to hydroxylate the aromatic ring: HO(·) attacks the ring to give a hydroxycyclohexadienyl radical, which is further oxidized by Fe(IV)O to give a cationic intermediate that gives rise to a NIH shift upon ketonization before the final re-aromatization step. Spin-trapping experiments in the presence of 5,5-dimethyl-1-pyrroline N-oxide and GC-MS analyses of the intermediate products further support the proposed mechanism.

OXIDATION OF POLYCYCLIC AROMATIC HYDROCARBONS CATALYZED BY IRON TETRASULFOPHTHALOCYANINE FEPCS : INVERSE ISOTOPE EFFECTS AND OXYGEN LABELING STUDIES

Iron(III) tetrasulfophthalocyanine (FePcS) was shown to catalyze the oxidation of polycyclic aromatic hydrocarbons by H2O2. Benzo[a]pyrene and anthracene were converted to the corresponding quinones while biphenyl-2,2′-dicarboxylic acid was the main product of phenanthrene oxidation. The mechanism of the anthracene oxidation by H2O2 in the presence of FePcS or by KHSO5 with iron(III) meso-tetrakis(3,5-disulfonatomesityl)porphyrin (FeTMPS) (see Figure 1 for catalyst structures) has been investigated in details by using kinetic isotope effects (KIEs) and 18O labeling studies. KIEs measured on the substrate consumption in the competitive oxidation of [H10]anthracene and [D10]anthracene by FePcS/H2O2 and FeTMPS/KHSO5 were essentially the same, 0.75 ± 0.02 and 0.76 ± 0.06, respectively. These inverse KIEs on the first oxidation step can be explained by the sp2-to-sp3 hybridization change during the addition of an electrophilic oxoiron complex to the sp2 carbon center of anthracene to form a σ adduct (this inverse KIE being enhanced by stronger stacking interactions between the perdeuterated substrate with the macrocyclic catalyst). Although the first oxidation step seems to be the same, different distribution of the oxidation products of anthracene and very different 18O incorporation into anthrone and anthraquinone in catalytic oxidations performed in the presence of H218O suggested that different active species should be responsible for anthracene oxidation in both catalytic systems. All the results obtained are compatible with an involvement of TMPSFeV=O (or TMPS+FeIV=O), having two redox equivalents above the iron(III) state of the metalloporphyrin precursor, while PcSFeIV=O (one redox equivalent above FeIII state of FePcS) was proposed to be the active species in the metallophthalocyanine-based system.

Oxidation of cycloalkanes by H2O2 using a copper–hemicryptophane complex as a catalyst

Efficient alkane C-H bond oxidation was achieved using a newly designed Cu(II)-hemicryptophane complex. Protection of the copper site in the inner cavity of the host leads to enhanced yields and allows discriminating cyclohexane from cyclooctane or adamantane in competitive experiments.

Aerobic hydrocarbon oxidation catalyzed by the vanadomolybdophosphate polyoxometalate, H5PV2Mo10O40, supported on mesoporous MCM-41

The impregnation of H5PV2Mo10O40 polyoxometalate onto MCM‐41 and amino‐modified MCM‐41 materials provided mesoporous active catalysts with large surface areas for aerobic hydrocarbon oxidation using isobutyraldehyde as a reducing agent. The results of the oxidation of alkenes and alkanes gave product selectivities similar to those observed in the corresponding homogeneous reaction although catalytic activity was somewhat reduced. Under appropriate experimental conditions there was no leaching and the solid catalyst could be recovered and reused without loss in activity.