Transition-metal Complexes of Pyrrole Pigments. XXI. One-electron Oxidation of Water Mediated by a Cobalt(III)-Tetradehydrocorrin Complex

Article abstract of DOI:10.1246/bcsj.54.2302

The cobalt(III) complex of tetradehydrocorrin (TDHC), having pK_a1 6.15 and pK_a2 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 2-><*>1.0 * 10^-2 mol dm^-3 for III(TDHC)> of 7.62 * 10^-5 -1.27*10^-5 mol dm^-3, 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^III(TDHC) is responsible for one-electron oxidation of the hydroxo ligand.Reduction of Co^III(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 t-butyl alcohol.