Metal-Stabilized Thiyl Radicals as Scaffolds for Reversible Alkene Addition via C-S Bond Formation/Cleavage

The one-electron oxidation of metal thiolates results in an increased oxidation state of the metal ion or the formation of a sulfur-based, thiyl radical in limiting extremes. For complexes with highly covalent M-S bonds, the unpaired electron may be delocalized over the metal and the sulfur, yielding a metal-stabilized thiyl radical. Oxidation of the metal thiolate precursors [Ru(DPPBT)(3)](-) [Ru-1](-), and Re(DPPBT)(3), Re-1 (DPPBT = diphenylphosphinobenzenethiolate), generates metal-stabilized thiyl radicals that react with alkenes to yield dithioether-metal products. Alkene addition to [Ru-1](+) and [Re-1](+) is symmetry-allowed due to the meridional arrangement of the DPPBT chelates. Combined bulk electrolysis and cyclic voltammetry experiments reveal the addition of alkenes to [Ru-1](+) as an irreversible process with experimentally determined rate constants ranging from 4.6(5) x 107 M-1 s(-1) for electron-rich alkenes to 2.7(2) x 10(4) M-1 s(-1) for electron-poor alkenes. Rate constants for cyclic alkenes range from 4(2) x 10(7) to 2.9(3) x 10(3) M-1 s(-1). Chemical oxidation of [Ru-1](-) by ferrocenium hexafluorophosphate (FcPF(6)) in the presence of m-methylstyrene or p-methylstyrene yields the dithioether complexes [Ru-1 center dot m-methylstyrene](+) and [Ru-1 center dot p-methylstyrene](+) respectively. Each complex was crystallized and the structure determined by single-crystal X-ray diffraction. P-31 NMR of the samples reveals a major and minor product, each displaying a second-order spectrum. The oxidized intermediate [Re-1](+) binds alkenes reversibly with equilibrium binding constants that vary with the complex charge from 1.9 x 10(-11) M-1 for n = 0 to 4.0 M-1 for n = +1 to 2.5 x 10(9) M-1 for n = +2. The three binding regimes are separated by 240 mV. Crystalline samples of [Re-1 center dot C2H4](2+) are obtained upon chemical oxidation of Re-1 with silver hexafluorophosphate (AgPF6) in the presence of ethylene. Strategies for the addition of alkenes to other metal-stabilized thiyl radicals are suggested.