Surface control of oxidation by an adsorbed RuIV-oxo complex

When adsorbed to optically transparent,,thin films of TiO2 nanoparticles on glass, the aqua complex [Ru-II (typ)(bpy(PO3H2)(2))(OH2)](2+) (bpy(PO3H2)(2) is 2,2'-bipyridyl-4,4'-diphosphonic acid; tpy is 2,2':6',2"-terpyridine) is oxidized by Ce-IV(NH4)(2)(NO3)(6) in 0.1 M HClO4 to its Ru-IV=O2+ form as shown by UV-visible measurements and analysis of oxidative equivalents by oxidation of hydroquinone to quinone. Kinetic studies on the oxidations of cyclohexene, benzyl alcohol, phenol, and tr-ans-stilbene by surface-bound Ru-IV=O2+ by UV-visible monitoring reveal direct evidence for initial 2-electron steps to give Ru-II intermediates in all four cases. These steps are masked in solution where Ru-IV --> Ru-II reduction is followed by rapid reactions between Ru-II intermediates and Ru-IV=O2+ to give Ru-III. Reactions between Ru-II and Ru-IV = O2(+) on the surface are inhibited by binding to the surface, which restricts translational mobility. Rate constants on the surface and in solution are comparable, pointing to comparable reactivities. The surface experiments give unprecedented insight into oxidation mechanism with important implications for achieving product selectivity in synthesis by limiting oxidation to two electrons.