Kinetic studies of the oxidation of bis[1-hydroxy-2-(salicylideneamino)-ethane]manganese(II) by molecular oxygen

Oxidation of the bis[1-hydroxy-2-(salicylideneamino)ethane] manganese(II) c omplex by molecular oxygen has been investigated in aqueous dimethylformami de and acetonitrile solutions. Kinetic EPR experiments indicated rapid oxid ation of the metal to Mn(III), followed by a slower step, in which a Mn(IV) complex is formed. This final oxidation product was isolated in 90% yield, and characterized by different techniques. EPR measurements suggested the existence of an unstable binuclear intermediate [L2Mn(II)...Mn(II)L-2], pre ceding the Mn(III) complex formation. Manometric measurements of oxygen upt ake also showed two kinetic paths for formation of the Mn(III) species, bot h of them exhibiting a first-order dependence on the [complex], with rate c onstants k(obs) =(1.65 +/- 0.08)10(-3) s(-1) and (5.57 +/- 0.28)10(-6) s(-1 ), respectively. Variation of the percentage of water in the reaction solut ion indicated that the rapid path is dependent on the oxygen concentration, while the slow one is almost [O-2] independent. The influence of added hyd roxide ions and of the solvent were also verified, suggesting competitive s ubstitution reactions. A mechanism involving a monomeric Mn(III) intermedia te is proposed and discussed, corroborated by electrochemical and EPR measu rements.