Browsing by Author "Sakamoto, Izumi"

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    Redox Reactions between Halides and Reducing Metal Complexes in Non-aqueous Solution: Part 2.—Derivatives of Molybdenum Carbonyl
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Bamford, Clement H.; Sakamoto, Izumi
    Molybdenum carbonyl undergoes ligand exchange with vinyl monomers such as methyl metha crylate (mma) to produce complexes of the type Mo(CO)5fmma) (I) which readily enter into redox processes with organic halides to generate free radicals. The absolute rate coefficients (k3) for the redox reactions of Mo(CO)s(mma) with CCL, CCl3COOEt and CBr4 have been determined and the radical yield (J) for the first of these reactions. The rate coefficient (k)) for Ute spontaneous decay of (I) has also been measured. Activation entropies of the redox reactions are more positive than expected for a bimolecular reaction with a polar transition state in a polar medium and it is suggested that weakening of Mo—CO bonds in the transition state is responsible for this. Values of the ratios of rate coefficients are compared with those derived from polymerization kinetics. As found in Part 1 for reactions of Ni(tp)4(mma) the halide activity increases in the order CCL < CCl3COOEt < CBr4, but the range of reactivities with Mo(CO)5(mma) is much narrower. For the primary oxidation with CCl4. f->l at low [Mo(CO)6], consistent with the formation of Mo1 (although formation of Mom cannot be ex cluded). At higher [Mo(CO)fi],/assumes lower values and it is suggested that this effect is a reflection of the self-inhibition observed in these systems.
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    Redox Reactions between Halides and Reducing Metal Complexes in Non-aqueous Solution: Part 1.—Derivatives of Tetrakis(triphenyl phosphite)nickel(O)
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Bamford, Clement H.; Sakamoto, Izumi
    Tetrakisftriphcnyl phosphite)nickel(O) (Ni(tp)4) undergoes ligand exchange with electron donors such as methyl methacrylate (mma) to form complexes of the type Ni(tp)3(mma) which enter into redox reactions with organic halides to generate free radicals. Three techniques are described for determining the absolute rate coefficients of these latter processes. In two methods, concentrations of Ni(tp)3(mma) are monitored spectrophotometrically, while in the third the rate of radical generation is deduced from observations of the rate of the ensuing free-radical polymerization of (mma). Reactions of CHBr3, CBr4, CH2Br2, CCl3COOEt, CHCBCOOEt, CHClj, CH2C12 are reported in this paper. In general, bromides are much more reactive than the corresponding chlor ides, and the rate of reaction increases with the number of halogen atoms attached to a single carbon. Further, introduction of an electronegative group into the halide molecule greatly increases the rate of reaction. It is concluded that the electron-affinity of the halide and solvation of the transition com plex play important roles in determining the rate coefficients and activation parameters. The reactivities of the halides in these redox processes and in chain transfer reactions arc compared.