KINETIC AND MECHANISTIC ASPECTS OF SULFUR ABSTRACTION FROM PD(2)X(2)(MU-S)(MU-DPM)(2) USING DPM OR DPMME [X=HALIDE, DPM=BIS(DIPHENYLPHOSPHINO)METHANE, DPMME=1,1'-BIS(DIPHENYLPHOSPHINO)ETHANE] AND CATALYTIC CONVERSION OF H2S TO H-2

Solution kinetic and mechanistic studies were performed on the abstraction of sulfur from Pd(2)X(2)(mu-S)(mu-dpm)(2) [X = Cl (2a), Br (2b), I(2c)] using bis(diphenylphosphino)methane (dpm) to give, respectively, Pd(2)X(2)(dpm)(2) [X = Cl (1a), Br (1b), I (1c)] and dpm(S). The reaction is first-order in both 2 and dpm with the reactivity trend in CHCl3 being X = Cl > Br > I. The activation parameters for the chloride system are Delta H-double dagger = 41 +/- 3 kJ mol(-1) and Delta S-double dagger = -127 +/- 10 J K-1 mol(-1), and for the bromide, Delta H-double dagger = 38 +/- 1 kJ mol(-1) and Delta S-double dagger = -144 +/- 4 J K-1 mol(-1), showing the entropy term is dominant in governing reactivity. Low-temperature NMR studies did not reveal the presence of observable intermediates. 2b and 1b undergo rapid diphosphine ligand exchange with dpm-d(2); with 2b, prior to any S-abstraction, Pd2Br2(mu-S)(dpm)(dpm-d(2)) (7b) and Pd2Br2(mu-S)(dpm-d(2))(2) (8b) are present, while Pd2Br2(dpm)(dpm-d(2)) (3b) and Pd2Br2(dpm-d(2))(2) (4b) are formed, as well as 1b and dpm(S)-d(2), in the abstraction reaction of 2b with dpm-d(2). The distribution of products is close to statistical on the basis of the stoichiometries of the reactants. Reaction of 2b with 1,1'-bis(diphenylphosphino)ethane (dpmMe) proceeds in an analogous way generating 1b, Pd2Br2(dpm)(dpmMe) (5b), Pd2Br2(dpmMe)(2) (6b), dpm(S), and dpmMe(S). In contrast, 1b undergoes slow diphosphine ligand exchange with dpmMe to form 5b and 6b; 2b undergoes no ligand exchange with dpmMe prior to the S-abstraction reaction. The findings are rationalized in terms of postulated intermediates and transition states that include formulations with three equivalent diphosphines. No sulfur abstraction occurs on treating 2b with PPh(3), PPh(2)Me, or Ph(2)P(CH2)(3)PPh(2).