E. N. Golubeva

The role of the circumarctic forest–tundra ecotone for Arctic biodiversity

The arctic forest–tundra ecotone (FTE), which links species communities of the boreal forest with those of the arctic tundra, is expected to respond swiftly to climate change with a profound reduction of tundra as the dominating scenario. With its circumarctic expanse and up to several hundred kilometres in width, the zone occupies a large part of the vegetated surface at high latitudes. Relocation and structural changes of the ecotone vegetation will affect not only plant but also animal and other biological diversity. A large number of arctic species are dependent on the FTE in terms of food and habitat during parts of their life cycle or annual migration. In the ‘Arctic Species Trend Index’, developed to provide trends in arctic vertebrates, more than half of the species and data are from the FTE. However, in assessments of arctic biodiversity, only the northernmost tundra-dominated areas of the ecotone are included. This is unfortunate and somewhat problematic since the treed part that serves as a source of seeds for new seedlings and saplings in the tundra-dominated part is excluded. This inconsistency hampers monitoring efficiency and biodiversity conservation efforts. During the International Polar Year, a large international research project on the FTE established numerous sites around the circumpolar north where causes and consequences of vegetation change were analysed. This network of sites and data forms an excellent basis for necessary monitoring of the spatial and temporal complexity of forest encroachment into tundra and its relation to arctic biodiversity.

Formation of active catalysts in the system: chlorocuprates-CCl4-n-C10H22.

Transformations of anionic Cu(II) chlorocomplexes have been studied under conditions of catalytic exchange reactions between carbon tetrachloride and n-alkanes. It was shown that chlorocuprates are just precursors and are easily reduced to the genuine catalysts, that is, to the respective Cu(I) complexes. Both the composition and the geometric structure of the precursor (CuCl(4)(2-)) and, probably, the active site (CuCl(3)(2-)) have been investigated by several techniques (UV-vis spectroscopy, electron spin resonance (ESR), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and static magnetic measurements). The dependence of the metathesis velocity on the [Cl-]/[Cu] ratio was found to exhibit a maximum most likely corresponding to the highest content of trichlorocuprite CuCl(3)(2-).

Structure and Composition of the Anionic Chloride Complexes of Copper(II) as the Precursors of Catalysts for C–Cl Bond Metathesis

The chloride complexes of copper(II) (catalysts or catalyst precursors for various reactions of halogenated hydrocarbons) were characterized using electron, EPR, and EXAFS spectroscopy. It was found that chlorocuprates occur as mononuclear ([CuCl4]2–), binuclear ([Cu2Cl6]2–), and, probably, polynuclear species in chlorobenzene solutions. The Cu–Cl bond length in [CuCl4]2– is 2.25 ± 0.2 Å, which is close to the same values for crystalline tetrachlorocuprates. It was assumed that the chloride complexes of copper with counterions occur as globules in chlorobenzene.

Cu(II)-alkyl chlorocomplexes: stable compounds or transients? DFT prediction of their structure and EPR parameters.

DFT calculations were used for studying the structure and reactivity of organocuprates(II) usually considered as intermediates with very weak Cu-C bond. It was found that calculated principal g-tensor values of model compounds RCu(II)Cl(2(-)) are similar to the experimentally found values for organocopper product of photolysis of quaternary ammonium tetrachlorocuprates. The calculations confirm that the most of organocuprates(II) could be stable at ambient conditions, and short lifetimes of organocuprates(II) in solutions or soft matrices are caused by their high reactivity in various bimolecular processes; the rate of those may be close to the rate of diffusion controlled reactions. The charges, spin densities, and d-orbital populations of the Cu atom in them are typical for bivalent copper complexes. Natural bond orbital analysis of organochlorocuprates(II) confirms the formation of polar σ-bond between copper and carbon atoms.

Intermediate particles in the catalysis of radical reactions of chlorohydrocarbons

Intermediate products of radical reactions of chlorohydrocarbons catalyzed by copper chloride complexes were synthesized by the photolysis of solutions of [(C4H9)3BzN+]2[CuCl4]2− in chlorobenzene and chloroform at the wavelength corresponding to Cl → Cu charge transfer. It was shown by the EPR method that the products of photolysis were alkyl radicals formed by organic fragments of the quaternary ammonium cation and Cu(II) complexes (supposedly organocopper compounds). The total content of paramagnetic centers decreased. The mechanisms of photoreduction of quaternary ammonium tetrachlorocuprates and possible reasons for the decay of paramagnetic particles were considered.

UV–Vis Identification and DFT-Assisted Prediction of Structures of Cu(II)–Alkyl Chlorocomplexes

The structures of paramagnetic copper complexes, the products of photolysis of tetrachlorocuprates of quaternary ammonium in frozen solvents, earlier denoted as 1-Cu and 2-Cu, were established on the basis of comparison of experimental and theoretical UV-vis spectra. UV-vis spectra of photolysis products were registered at 77-116 K. Comparison with the EPR data in this temperature range allowed to assign photolysis products bands in the vis spectrum either to 1-Cu or to 2-Cu. Model structures for 1-Cu and 2-Cu were proposed. TD-DFT calculated spectra of model compounds along with CuCl(4)(2-) anion are in excellent agreement with the experiment. The comparison of UV-vis and EPR data and results of TD-DFT calculations evidence that 1-Cu and 2-Cu are paramagnetic organochlorocuprates(II) with general formulas Cu(II)Cl(2)R and Cu(II)Cl(3)R, respectively, where R is (-C(6)H(12))N(+)(C(6)H(13))(3).

Copper(II) organic compounds as intermediates of photochemical transformations of quaternary ammonium tetrachlorocuprates

171 Stable organometallic compounds with a Cu(II)–C σ bond have not been described [1]. Pulse radiolysis and photolysis of divalent copper salts in aqueous solutions [2, 3] and in n -donor solvents [4] gave short-lived copper complexes with organic radicals in which the copper oxidation state was presumably two. Dissociation [2] CuR + Cu + + R • and bimolecular processes [3] 2CuR + 2Cu + + R 2 , CuR + + H 3 O + Cu 2+ + RH + H 2 O, were considered as the key routes for the destruction of these complexes. Copper(II) Organic Compounds as Intermediates of Photochemical Transformations of Quaternary Ammonium Tetrachlorocuprates E. N. Golubeva a, A. V. Lobanov a , V. I. Pergushov a , N. A. Chumakova a, and A. I. Kokorin b Presented by Academician A.L. Buchachenko February 5, 2008

Copper (II) chloride–dmf catalytic system in solution and on silica

Abstract The connections between composition and formation conditions of catalysts based on immobilised complexes of Cu(II) and dmf as precursors and their catalytic activity in C–Cl bond metathesis in system CCl 4 – n -C 10 H 22 were studied. High activity and selectivity could be obtained providing that a lamina of dmf containing mononuclear copper complexes forms in pores of a support. It was shown that the composition and structure of surface complexes does not depend on concentrations of copper ions or dmf in precursor solution. The loss of catalytic activity when the support cannot fix the lamina (e.g., charcoal in contrast with silica) is observed. The key stage of catalysis could be a reduction of Cu(II) to Cu(I).

Photochemistry of Tetraalkylammonium Tetrachlorocuprates in Low-Temperature Matrices

The product composition and the principles of photochemical transformations of tetrahexylammonium tetrachlorocuprate [(RH)4N+]2[CuIICl4]2− (RH = C6H13) in 2-chlorobutane at 77 K have been found out by ESR spectroscopy. It has been shown that the photolysis of [(RH)4N+]2 [CuIICl4]2− results in the formation of alkyl radicals (R⊙), presumably, anions [CuICl3]2− and organic copper(II) compounds {CuIIR}. A reduction in the quantum yield of primary photolysis products during the reaction, nonequivalence of the quantum yield of the buildup of paramagnetic photolysis products to that of [CuIICl4]2− consumption, and a decrease in the total number of paramagnetic particles in the system during the photolysis have been revealed. A photolysis mechanism involving both photochemical and thermal processes is proposed.

Dimerization of Chlorocuprates in Weakly Polar Solvents

The solutions of CuCl2 and tributylbenzylammonium chloride in chlorobenzene, which are used as catalysts for a number of processes with the participation of halogenated hydrocarbons, were studied by electronic spectroscopy in visible and UV regions. It was found that copper(II) ions occurred as two anionic chlorocuprate species (CuCl2-4 and Cu2Cl2-6), which were in equilibrium, in solutions over wide ranges of concentrations and temperatures. The individual spectra of the two above chlorocuprate species were distinguished. The thermodynamic parameters of the equilibrium 2CuCl2-4 ⇄ Cu2Cl2-6 + 2Cl– were calculated.