Transition-metal complexes of pyrrole pigments. XXI. One-electron oxidation of water mediated by a cobalt(III)-tetradehydrocorrin complex.
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概要
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The cobalt(III) complex of tetradehydrocorrin (TDHC), having p<I>K</I><SUB>a1</SUB> 6.15 and p<I>K</I><SUB>a2</SUB> 11.50 for successive acid-dissociation of axial aqua ligands, undergoes auto-reduction to the cobalt(II) complex in aqueous carbonate solutions. Kinetic analysis of the reaction indicates the following mechanism: a one-electron transfer from the axial hydroxo ligand to the metal site, subsequent homolytic cleavage of the Co–OH bond, and finally scavenging of the resulting hydroxyl radical with the carbonate ion. At relatively low concentrations of the carbonate ion, the scavenging step is rate-determining; while at [CO<SUB>3</SUB><SUP>2−</SUP>]≥1.0×10<SUP>−2</SUP> mol dm<SUP>−3</SUP> for [Co<SUP>III</SUP>(TDHC)] of 7.62×10<SUP>−5</SUP>−1.27×10<SUP>−5</SUP> mol dm<SUP>−3</SUP>, the initial bond homolysis becomes rate-determining. Analysis of the kinetic data in reference to relevant electrode potentials indicates that the powerful oxidation ability of Co<SUP>III</SUP>(TDHC) is responsible for one-electron oxidation of the hydroxo ligand. Reduction of Co<SUP>III</SUP>(TDHC) also takes place in the absence of the carbonate ion, the apparent rate increasing with rise of pH in the range 11–12; deprotonation of the hydroxyl radical, which may prevent the hydroxyl radical and the cobalt(II) species from backward recombination, takes place. The one-electron oxidation of the hydroxo ligand was also significantly accelerated by addition of acetonitrile or <I>t</I>-butyl alcohol.
- 公益社団法人 日本化学会の論文
著者
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Murakami Yukito
Department Of Chemical Science And Technology Faculty Of Engineering Kyushu University
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Aoyama Yasuhiro
Department Of Chemistry Nagaoka University Of Technology
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Tada Toshito
Department of Organic Synthesis, Faculty of Engineering, Kyushu University
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