Characteristic mechanisms of the homogeneous and heterogeneous oxidation of aromatic amines with transition metal-oxalate complexes

The oxidation-reduction reactions of aromatic amines {o-aminophenol (o-AP), o-phenylenediamine (o-PDA) and p-phenylenediamine (p-PDA)} with Co(III)-, Mn(III)-, and Cu(II)-oxalate complexes have been investigated in homogeneous and heterogeneous systems. The kinetics of the redox reaction of both systems have been studied spectrophotometrically. The redox products were identified and characterised by diffuse reflectance spectroscopy (DRS), and mass spectrometry. The redox reactions follow first-order kinetics with respect to each of the reactants and first order in [amines] in homogeneous and heterogeneous (supported complexes on Amberlite IRA 904 anion-exchange resin) phases, respectively. However, the specific oxidation rate of the amines is found to follow the order, p-PDA > o-AP > o-PDA. The effect of the oxalate anion on the reaction rate of both systems was examined. There is a significant difference in the behaviour of the oxidation of o-AP and o-, p-PDA with all the metal complexes in both systems. Moreover, the oxidation of o-AP and o-, p-PDA appears to follow the outer- and inner-sphere mechanistic classification, respectively. (C) 2000 Elsevier Science Ltd.