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内容記述 |
In recent years, more remarkable attention has been paid to the necessity of strengthening energy efficiency against exhaustion of fossil fuels. Direct conversion of chemical (bonding) energy of organics into electrical energy such as fuel cells is expected as one of the key technologies to cope with the predictable energy shortage in near future. A prerequisite for power generation by the electrochemical oxidation of MeOH is that the oxidation reaction takes place at potentials more negative than the electrochemical reduction of dioxygen on counter electrodes. Metal complexes that work as catalysts in electrochemical oxidation of alcohols under very mild conditions have potential uses as new electrode materials in fuel cells. Introduction of a dioxolene ligand to a Ru<SUP>III</SUP>-aqua eomplex, [Ru<SUP>III</SUP>(OH<SUB>2</SUB>)(sq)(trpy)]<SUP>2+</SUP> (sq = 3,5-di-tert-butyl-1,2-benzosemiquinone, trpy =2,2':6',2"-terpyridine), enabled double deprotonation of the aqua ligand coupled with spontaneous one-electron transfer to the Ru<SUP>III</SUP>-dioxolene framework to generate an unusual Ru-oxyl radical complex, [Ru<SUP>II</SUP>(O<SUP>・-</SUP>)(sq)(trpy)]<SUP>0</SUP>, which has the ability to abstract hydrogen from cyclic dienes. As for an analogous ammine complex, proton dissociation of NH<SUB>3</SUB> ligated on a Ru<SUP>III</SUP>-sq framework may trigger generation of radical character on the nitrogen atom and the resultant Ru-radical complex may be more reactive toward oxidation of organic compounds <br /> Chapter 2 describes the preparation of new ruthenium-dioxolene-ammine complexes, [Ru<SUP>II</SUP>(NH<SUB>3</SUB>)(sq)(trpy)](ClO<SUB>4</SUB>) (1) and [Ru<SUP>III</SUP>(NH<SUB>3</SUB>)(sq)(trpy)(ClO<SUB>4</SUB>)<SUB>2</SUB> (2), and their catalytic activity toward the electrochemical oxidation of alcohols. The electronic absorption spectra of l and 2 in CH<SUB>2</SUB>Cl<SUB>2</SUB>/MeOH (99/1) exhibited a strong absorption band at 855 nm (ε1.3×10<SUP>4</SUP> M<SUP>-1</SUP>cm<SUP>-1</SUP>) and at 615 nm (ε1.3×10<SUP>4</SUP> M<SUP>-1</SUP>cm<SUP>-1</SUP>), respectively. An addition of an equimolar amount of a methanolic solution of f-BuOK to a CH<SUB>2</SUB>Cl<SUB>2</SUB>/MeOH(99/1) solution of 2 resulted in complete reduction of the complex to produce 1. The cyclic voltammogram (CV) of 1 showed two reversible redox couples at E<SUB>1/2</SUB>= +0.34 and -0.46 V (vs. SCE) in CH<SUB>2</SUB>Cl<SUB>2</SUB>. An addition of excessive amounts of a methanolic solution of t-BuOK to the CH<SUB>2</SUB>Cl<SUB>2</SUB> solution of 1 caused strong irreversible anodic (catatytic) currents at potentials more positive than ca. 0 V. This observation indicates that the complex 1 works as an active catalyst in the electrochemical oxidation of MeOH in CH<SUB>2</SUB>Cl<SUB>2</SUB>. In fact, the electrochemical oxidation of 1 (10 mM) at 0 V in CH<SUB>2</SUB>Cl<SUB>2</SUB> containing i-PrOH (200 mM), t-BuOK (20 mM), and (n-Bu)<SUB>4</SUB> NClO<SUB>4</SUB> as an electrolyte produced acetone as an oxidized product of i-PrOH in ca. 20% yield (based on 1) after 0.82 F/mol of electricity passed in the electrolysis. The CV of 1 in MeOH also showed two reversible [Ru<SUP>II</SUP>(NH<SUB>3</SUB>)(cat)]<SUP>0</SUP>/[Ru<SUP>II</SUP>(NH<SUB>3</SUB>)(sq)]<SUP>+</SUP> and [Ru<SUP>II</SUP>(NH<SUB>3</SUB>)(sq)]<SUP>+</SUP>/ [Ru<SUP>III</SUP>(NH<SUB>3</SUB>)(sq)]<SUP>2+</SUP> redox couples and the redox potentials are essentially consistent with those in CH<SUB>2</SUB>Cl<SUB>2</SUB>. An addition of t-BuOK (20 equiv) to the MeOH solution eaused strong catalytic currents at potentials more positive than +0.3 V This observation indicates that the complex 1 works as an active catalyst in the electrochemical oxidation of MeOH in MeOH as well as in CH<SUB>2</SUB>Cl<SUB>2</SUB>. Aminyl radical complexes, [Ru<SUP>II</SUP>(NH<SUB>2</SUB><SUP>・</SUP>)(sq)(type)]<SUP>+</SUP> and/or [Ru<SUP>III</SUP>(NH<SUB>2</SUB><SUP>・</SUP>)(sq)(typy)]<SUP>2+</SUP>, which would be generated by dissociation of amino proton coupled with electron transfer to the Ru<SUP>III</SUP>-sq and/or Ru<SUP>IV</SUP>-sq frameworks, are proposed as active catalysis in the oxidation of MeOH. The Ru-ammine complex 1 (and 2) showed higher reactivity than the analogous aqua one, [Ru<SUP>III</SUP>(OH<SUB>2</SUB>)(sq)(trpy)]<SUP>2+</SUP>. Additionally, a Ru-ammine complex having an acetylacetonato ligand instead of the dioxolene one revealed no catalytic activity toward the electrochemical oxidation of MeOH. These facts indicate that the existence of an amimne ligand as well as a dioxolene ligand is essential to this catalytic reaction. <br /> On the other hand, a Ru-dioxolene-aniline eomplex, [Ru<SUP>II</SUP>(NH<SUB>2</SUB>Ph)(sq)(trpy)] (ClO<SUB>4</SUB>)(4), was also newly synthesized in order to compare its reactivity toward the electrochemical oxidation of alcohols with that of the Ru-dioxolene-ammine complexes 1 and 2 (Chapter 3). In CH<SUB>2</SUB>Cl<SUB>2</SUB>/MeOH(99/1), the catalytic activity of the complex 4 toward the electrochemical oxidation of alcohols was much lower than that of the complex 1 and the coupling reaction of the deprotonated species of the complex 4 proceeded instead. The experiments of the chemical oxidation and the subsequent deprotonation of 4 in CH<SUB>2</SUB>Cl<SUB>2</SUB>/MeOH(99/1) by an addition of AgClO<SUB>4</SUB> and t-BuOK respectively, suggest that [Ru<SUP>III</SUP>(NPh<SUP>・-</SUP>(sq)(typy)]<SUP>+</SUP> or [Ru<SUP>III</SUP>(NHPh<SUP>・</SUP>)(sq)(trpy)]<SUP>2+</SUP> contributes to the dimerization as an active species. This dimerization reaction, therefore, strongly supports the generation of the unusual Ru-aminyl radieal complexes and their involvement in the catalytic cycles as active species. In MeOH, the complex 4 catalyzed the electrochemical oxidation of alcohols as in the case of the complex l. The difference in the reactivity of the complex 1 (or 2) and the complex 4 would be explained by the stability of the intermediates, [Ru<SUP>II</SUP>(NHR<SUP>・</SUP>)(sq)(trpy)]<SUP>+</SUP> and [Ru<SUP>III</SUP>(NHR<SUP>・</SUP>)(sq)(trpy)]<SUP>2+</SUP>(R= H or Ph). |
要旨・審査要旨 (298.4 kB)
