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) × 107 M–1 s–1 for electron-rich alkenes to 2.7(2) × 104 M–1 s–1 for electron-poor alkenes. Rate constants for cyclic alkenes range from 4(2) × 107 to 2.9(3) × 103 M–1 s–1. Chemical oxidation of [Ru-1]– by ferrocenium hexafluorophosphate (FcPF6) in the presence of m-methylstyrene or p-methylstyrene yields the dithioether complexes [Ru-1·m-methylstyrene]+ and [Ru-1·p-methylstyrene]+, respectively. Each complex was crystallized and the structure determined by single-crystal X-ray diffraction. 31P 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 × 10–11 M–1 for n = 0 to 4.0 M–1 for n = +1 to 2.5 × 109 M–1 for n = +2. The three binding regimes are separated by 240 mV. Crystalline samples of [Re-1·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.