Oxidations of organic substrates by ruthenium oxo complexes on nanocrystalline thin films of titanium dioxide

Ruthenium-oxo complexes are unusually versatile oxidants. Both their success as stoichiometric reagents and their feasibility as catalysts lie largely in the fact that they can exist in a variety of oxidation states while maintaining the same coordination environment. Although important to the success of Ru-oxo complexes as oxidants, the feasibility of multiple oxidation states, makes it difficult to study the reaction mechanisms of these compounds. For example, the comproportionation of RuIV=O2+ and RuII-OH₂2+ is both thermodynamically favorable and kinetically facile, making it difficult to distinguish between initial 1 e− and 2 e− oxidation pathways. Also, side reactions by the resulting RuIII species can further complicate data.; The purpose of this research is twofold: (1) to elucidate mechanisms of organic oxidations by ruthenium oxo complexes in the ground state, and (2) to take advantage of the light harvesting ability of polypyridyl complexes to develop a photocatalytic system for organic oxidations. The strategy employed for both purposes is the adsorption of Ru-oxo precursors onto nanocrystalline thin films of TiO2.; Ruthenium complexes with phosphonate-modified polypyridyl ligands adsorb onto nanocrystalline TiO₂ thin films. This surface adsorption limits translational mobility, preventing comproportionation of Ru-oxo species and simplifies the study of reactions carried out by Ru-oxo complexes. For many of the reactions studied, initial 2 e− oxidation pathways were confirmed, whereas in solution, initial observations were obscured by comproportionation.; Adsorbing Ru-oxo precursors onto nanocrystalline thin films of TiO ₂ is also the basis for the photoelectrochemical oxidation of organic substrates. The sensitization of wide-band gap semiconductors by visible chromophores has long been a strategy employed for the conversion of sunlight to electricity. When a chromophoric RuII-OH₂ species is adsorbed onto TiO₂, light absorption followed by photoinjection results in oxidative equivalents in the form of RuIII-OH or Ru IV=O. By modifying the basic scheme of the dye-sensitized solar cell, the photochemical oxidation of organic substrates has been realized.; Surface adsorption has proven to be a useful technique for studying mechanisms of ground state oxidations by Ru-oxo complexes and in designing photoelectrosynthetic cells based on Ru-oxo precursors.