Electron transfer catalysis in the activation of C-H bonds by ruthenium complexes

Article abstract of DOI:10.1016/S0022-328X(96)06568-0

Ruthenium(II) dimethyl complexes, [Ru(Me)₂(η⁶-C₆Me₆)(PR₃)] (R = Ph 1a, R₃ = MePh₂ 1b, R₃ = Me₂Ph 1c, R = Me 1d, R = Et 1e), react with C-H bonds of benzene or toluene under severe conditions (85-105°C depending on the phosphine ligand) to give methane and the new methyl aryl derivatives [Ru(Me)(Ar)(η⁶-C₆Me₆)(PR₃)]. The methyl tolyl complexes are formed as a 2/1 mixture of meta and para isomers. In contrast the reaction of 1a-1e with arenes, in the presence of [FeCp₂]PF₆, proceeds rapidly at room temperature: the corresponding methyl aryl derivatives [Ru(Me)(Ar)(η⁶-C₆Me₆)(PR₃)] (Ar = Ph, Tol) and/or the intramolecular reaction products [Ru(C₆H₄PR₂)(Me)(η⁶-C₆Me₆)] are formed depending on the steric hindrance of the phosphine. The fact that electrochemistry and ESR spectroscopy show that on oxidation the ruthenium(II) complexes give stable ruthenium(III) congeners suggests that the catalytic reaction triggered by ferrocenium ions proceeds through a different redox pathway.