| Abstrakt: |
The reactions of two CoIII-tetramine (N4) complexes, cis-a-Dichloro(triethylenetetramine)cobalt(III) chloride = [CoIIICl2(trien)]Cl (1) and aquabromo[(2R, 5R, 8R, 11R)-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane]cobalt(III) bromide = [CoIIIBr(Et4[12]-aneN4)(H2O)]Br2(2) with 3,5-di-t-butylcatechol (H2dbc) have been examined by visible, NMR and ESR spectroscopies to elucidate the mechanism of the activation and the oxygenation of catechol by the complex formation. The reactions proceeded in a mixed solvent (H2O : MeOH = 1 : 1) at 50 °C in the presence and absence of dioxygen. NMR showed that the complexes react with E2dbc and dioxygen, and yield oxidatively cleavaged products. ESR measurements detected a CoIII-semiquinone (dbsq) radical and a high-spin CoIIcomplex, regardless of the presence of dioxygen. Especially in the case of 1, the magnetic moment of the complex isolated from the anaerobic reaction solution, exhibited that there exists an antiferromagnetic spin-coupling between the high-spin (S= 3/2) CoIIion and the semiquinone radical in the complex. These results show that two species detected by ESR are in equilibrium with a high-spin CoII-dbsq binary complex formed by intramolecular electron transfer. It is conclusively clarified that the one-electron-transferred intermediate [CoII(N4)dbsq]+is the activated intermediate, which introduces dioxygen into the semiquinone moiety. |
