Surface Control of Oxidation by an Adsorbed Ru^IV−Oxo Complex

Abstract

When adsorbed to optically transparent, thin films of TiO(2) nanoparticles on glass, the aqua complex [Ru(II)(tpy)(bpy(PO(3)H(2))(2))(OH(2))](2+) (bpy(PO(3)H(2))(2) is 2,2'-bipyridyl-4,4'-diphosphonic acid; tpy is 2,2':6',2' '-terpyridine) is oxidized by Ce(IV)(NH(4))(2)(NO(3))(6) in 0.1 M HClO(4) to its Ru(IV)=O(2+) 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 trans-stilbene by surface-bound Ru(IV)=O(2+) 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)=O(2+) to give Ru(III). Reactions between Ru(II) and Ru(IV)=O(2+) 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.

Heterogeneous Water Oxidation Catalysts for Molecular Anodes and Photoanodes

“…Nonetheless, using these same surfaces and a chemical oxidant to prepare the Ru IV O complex—avoiding slow ET with the electrode—revealed similar rates of chemical reactivity between the surface‐bound and solution dissolved catalyst with organic substrates. [ 11 ] This similarity in reactivity, as well as redox and pK a behavior, between oxide surface‐bound and solvated analogues has been observed with other charged polypyridyl ruthenium complexes as well. [ 12 ] These studies suggest Ru–aquo complexes may behave similar at an electrode surface as under homogeneous conditions, however, the larger KIEs observed suggested a greater role for PCET and a large dependence on the surface loading concentration for dictating the chemical activity of the complex at the surface.…”

Ruthenium Catalyst for Epoxidation Reaction

“…The latter represent the main olefin epoxidation intermediate and are highly useful catalysts because they act as oxygen donor to alkenes. A variety of rutheniumcatalyzed epoxidation catalysts are supplied by homogeneous ruthenium complexes with ligands like porphyrin [18], polypyridyl [19], Schiff base [20], oxazoline [21], and pyrazolyl [22].…”

Transition Metal Complex Catalyzed Photo- and Electrochemical (De)hydrogenations Involving C=O and C=N Bonds

“…18b In later work, such an insertion of the Ru IV =O into the C-H bond was proposed as initial step in the electrochemical oxidation of benzyl alcohol mediated by [Ru II (tpy)(bpy (PO3H2)2 )(OH 2 )] 2+ on nano-ITO (Scheme 5). 21 For the oxidation of aromatic and non-aromatic alcohols with 2, kinetic measurements also suggested a 2e -/H + transfer mechanism from the -C-H bond, as benzyl alcohol oxidation exhibits a large k CH /k CD isotope effect but no isotope effect was observed arising from the solvent. 22,23 Notably, such a hydride transfer mechanism from a C-H bond to the Ru IV =O species was revised later by the Mayer group in the case of cumene.…”