ESR spectra of γ-irradiated samples of iron(iii) binuclear complexes

-Irradiation of -oxo-bridged binuclear iron complexes Fe^III ₂OL _n in a glycerol or dimethylformamide matrix at 77 K affords unstable mixed-valence Fe^IIF^III forms resulting from the transfer of a mobile electron generated by the ionizing radiation. These nonequilibrium forms retain the ligand environment of the original complexes, and their ESR spectra at 77–200 K are characterized by an asymmetric signal with an axially anisotropicg-factor, which is in agreement with the spectra of the Fe^IIFe syu forms obtained by chemical reduction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 869–871, April, 1996.     

New models of monooxygenase on the basis of Sn(II) compounds

The data on the selectivity of various C–H bonds and the stereochemical course of the reaction in the known chemical systems simulating monooxygenase, do not agree with the oxenoid mechanism postulated for these reactions, but correspond to the interaction of saturated hydrocarbons with the OH radical intermediate. On the basis of organic compounds of Sn(II) in aprotic media, new systems have been developed which selectively oxidize alkanes to alcohols in the presence of O₂ in the coupled process and display many characteristics of the oxenoid reactions.

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C–H , , , , . (II) , .

     

Structure and catalytic mechanism of methane monooxygenase and approaches to its modelling

The mechanisms of activation of O₂ and CH₄ by methane monooxygenase (MMO) are discussed. A new concept for the catalytic cycle of MMO is suggested, and approaches to its chemical modelling are considered.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1011–1020, June, 1995.The author is grateful to Academician A. E. Shilov for interest in this work and helpful discussions. The work was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08529), International Scientific Foundation (Grant REU000), and INTAS (Project No. 93-315).     

Physical properties of carbon films obtained by methane pyrolysis in an electric field

A method of synthesizing carbon films on single-crystal silicon substrates by methane pyrolysis in an electrical field is suggested. The pressure and temperature arising in a working chamber when the substrate is exposed to C^–4 ions during pyrolysis are measured. Ion bombardment generates nuclei in the form of fibers about 2 μm in diameter providing the growth of a polycrystalline film. The resulting material is examined using electron microscopy and photo- and cathodoluminescence. Synthesized films are a composite material the matrix of which contains nanoclusters of a dissimilar crystalline nature. The effect of considerable two-stage decrease in the resistivity of the film material with increasing temperature from 300 to 1750 K is discovered. This points to the semiconducting properties of thick carbon films.     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

Pt(II) complexes in activation of saturated hydrocarbons

The rate constants of deuterium exchange for cyclohexane in CH₃COOD/D₂O have been measured in the presence of 12 complexes of Pt(II) with various ligands (Cl, H₂O, NH₃, Br, NO₂, py, DMSO, PPh₃, etc.). The change in the rate constant is shown to follow the reverse series of the trans-effect of Pt(II) ligands. The rate constant of the interaction of PtCl₂S_n (S is H₂O, CH₃COOH, n=1, 2) with cyclohexane in CH₃COOD/D₂O (11), k, equals 2.5×10⁸ exp (–18200/RT) 1 mol^–1 sec^–1 (at 354–385 K). The ratio of rate constants for PtCl₂S₂, PtCl₃S^– and PtCl ₄ ^2– is 10060.5 at 100°C.

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CH₃COOD–D₂O 12- Pt(II) (Cl, H₂O, NH₃, Br, NO₂, , DMCO, PPh₃ .). , Pt(II). PtCl₂S_n (S–H₂O, CH₃COOH, n=1,2) 11 CH₃COOD–D₂O k=2,5·10⁸ exp(–18200/RT). ^{–1 –1} (354–385°). 100°C PtCl₂S₂, PtCl₃S^– PtCl ₄ ^2– 10060,5.

     

Oxo complexes of high-valence iron in oxidation catalysis

This review deals with oxo complexes of high-valence iron (oxoferryl, or simply ferryl complexes), which play an important role in the selective oxidation of hydrocarbons and other organic compounds. The efficiency and high selectivity of natural oxygenases stimulates researchers to design chemical analogues of these systems. Since the advantages of biological and biomimetic oxidation are due to the participation of ferryl intermediates, we will consider the ways of obtaining relatively stable oxo complexes of high-valence iron and their geometric and electronic structure and reactivity.     

Structure and catalytic activity of a new iron(II) complex with a tetradentate carboxamide ligand: The effect of the outer-sphere donor on the chemoselectivity of the metal complex catalyst

The interaction between Fe(ClO₄)₂ or Fe(OTf)₂(MeCN)₂ and 2 equiv of the potentially tetradentate ligand bis(2-pyridyl)methyl-2-pyridinecarboxamide (Py₂CHNHCOPy, tpcaH) yields the iron(II) complex [Fe^II(tpcaH)₂]X₂ (X = ClO₄, OTf), in which, according to X-ray crystallography data, tpcaH is facially coordinated to the iron atom as a tridentate ligand through the carbonyl group and two pyridyl donors of the bis(2-pyridyl)methylcarboxamide moiety and the third pyridyl group is uninvolved in coordination. The oxidation of saturated and unsaturated hydrocarbons with hydrogen peroxide involving this complex has been investigated. The presence of the uncoordinated nitrogen donor in the outer coordination sphere of the complex exerts a crucial effect on it catalytic properties.