Kinetics and Mechanism of Oxidation of Catechol by Oxygenated [Co 2 (O 2 )(NH 3 ) 10 ] 5+ Complex

The kinetics and mechanisms of the redox reaction of Catechol and μ-superoxo-bis[pentaamminecobalt(III)] pentachloridemonohydrate, [(NH3)5Co(O2)Co(NH3)5]Cl5.H2O, hereafter represented as CoO2Co5+ was studied in aqueous perchloric acid under pseudo-first-order conditions at 27±1°C, I=0.35mol dm-3 (NaClO4) and λmax=660nm. μ-superoxo-bis[pentaamminecobalt(III)] pentachloridemonohydrate was used as the oxidant and it was synthesized and characterized according to reported procedure. This study was aimed at generating kinetic data with respect to the oxidation of cathecol by µ-superoxo-bridged binuclear cobalt(III) complex of ammine and to propose plausible mechanism that underpin this reaction of interest. Effect of changes in acid concentration, ionic strength, dielectric constant and temperature on the reaction rate was investigated. The reaction was found to be first-order in both reactants, second-order overall, acid dependent and showed a negative Bronsted-Debye salt effect. Decreasing the dielectric constant from 80.1–74.01 (CH3COCH3/H2O) increased the rate of reaction. A large negative value of ΔS≠ (−224Jmol−1K−1) was obtained during the thermodynamic studies and that showed that the activated complex formed is more ordered. This confirmed that the reaction proceeded by associative mechanism. There was evidence for the presence of free radicals during the course of the reaction, but non to validate the formation of an intermediate complex. In overall, the results suggest an outer-sphere mechanism for the reaction. A probable mechanism is proposed.