Abstract
Oxidation of the bis[l-hydroxy-2-(salicylideneamino)ethane]manganese(II) complex by molecular oxygen has been investigated in aqueous dimethylformamide and acetonitrile solutions. Kinetic EPR experiments indicated rapid oxidation 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(H)L2], preceding the Mn(III) complex formation. Manometric measurements of oxygen uptake also showed two kinetic paths for formation of the Mn(III) species, both of them exhibiting a first-order dependence on the [complex], with rate constants K obs = (1.65 ± 0.08)10-3s−1 and (5.57 ± 0.28)10−6s−1, respectively. Variation of the percentage of water in the reaction solution indicated that the rapid path is dependent on the oxygen concentration, while the slow one is almost [O] independent. The influence of added hydroxide ions and of the solvent were also verified, suggesting competitive substitution reactions. A mechanism involving a monomelic Mn(III) intermediate is proposed and discussed, corroborated by electrochemical and EPR measurements.
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