Protonation and electron transfer reactions of osmium hydride and surfactant ruthenium complexes
This dissertation presents two research projects. First, the synthesis and characterization of osmium carbonyl complexes is described. The reactivity of the carbonyl hydride complexes with protons in two redox states is also presented. The second project focuses on the effect of cationic micelles on charge separation and recombination following photoinduced electron transfer with surfactant ruthenium complexes. A brief overview of artificial photosynthetic systems is presented in Chapter 1. Approaches to minimize recombination reactions are described. This introductory chapter also presents examples of mechanistic studies of proton reduction catalysts. The preparation and characterization of Os(II) diimine complexes are described in Chapter 2. The complexes cis-[Os(NN)2(CO)X]+, where NN = 3,4,7,8-tetramethyl-1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 1,10-phenanthroline or 5-chloro-1,10-phenanthroline and X = Cl- or H-, are chromophores with emissive excited states. Their electrochemical and photochemical properties are presented. Protonation, reductive quenching, and protonation following reductive quenching of the hydride complexes are discussed in Chapter 3. Protonation by trifluoromethanesulfonic acid and p-toluenesulfonic acid in acetonitrile ultimately generates the acetonitrile complexes. Intermediate species formed following protonation are dihydrogen complexes and conjugate base-coordinated complexes. Reductive quenching by a sacrificial electron donor generates the one-electron reduced hydrides. Reaction of the one-electron reduced hydride species with p-toluenesulfonic acid is fast relative to the unreduced complexes, suggesting an increased hydride basicity upon phenanthroline-localized reduction. The effect of cationic micelle incorporation on photoinduced electron transfer, charge separation and back electron transfer between an aqueous electron donor, [Ru(NH3)6]2+, and a series of Ru(II) diimine complex chromophores is presented in Chapter 4. The chromophores have the general formula [(bpy)2Ru(NN)]2+ (NN = bipyridine; 4-R-4′-methyl-2,2′-bipyridine, R = pentyl (MC5), terdecyl (MC13), heptadecyl (MC17); 4,4′-di(heptadecyl)-2,2′-bipyridine (DC17)). Of the five chromophores, the MC13, MC17, and DC17 complexes associate with the added micelle-forming surfactant, cetyltrimethylammonium bromide (CTAB). Quenching of the luminescence of the bpy and MC5 complexes by [Ru(NH3)6]2+ is unaffected by addition of surfactant, while rate constants for quenching of the MC13 and MC17 complexes are decreased. Cage escape yields following photoinduced electron transfer to generate [(bpy)2Ru(NN)]+ and [Ru(NH3)6]3+ are approximately 0.1 for all the water-soluble chromophores (excluding DC17) in the absence of added CTAB. In the presence of surfactant, the cage escape yields dramatically increase for the MC13 (0.4) and MC17 (0.6) complexes, while remaining unchanged for [Ru(bpy)3]2+ and the MC5 complex. Rates of back electron transfer of the solvent separated ions is also strongly influenced by the presence of surfactant.