www.elsevier.nl/locate/poly Polyhedron 19 (2000) 1317 – 1328 Characteristic mechanisms of the homogeneous and heterogeneous oxidation of aromatic amines with transition metaloxalate complexes A.B. Zaki a , M.Y. El-Sheikh a , J. Evans b, *, S.A. El-Safty a a Department of Chemistry, Faculty of Science, Tanta Uniersity, Tanta, Egypt b Department of Chemistry, Southampton Uniersity, Southampton SO17 1BJ, UK Received 31 January 2000; accepted 11 April 2000 Abstract The oxidation – reduction reactions of aromatic amines {o -aminophenol (o -AP), o -phenylenediamine (o -PDA) and p -phenylene- diamine (p -PDA)} with Co(III), Mn(III), and Cu(II)oxalate complexes have been investigated in homogeneous and heteroge- neous 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. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aromatic amines; Amberlite anion-exchange resin; Oxalate anions; Metaloxalate complexes; Oxidation; Homogeneous and heteroge- neous 1. Introduction The oxidation of aromatic amines in human erythro- cytes is very useful for producing phenoxazine com- pounds via the intervention of human oxyhaemoglobin [1]. One of the main objectives has been the elucidation of the oxidation product of the aromatic amines, which has been shown to be a phenoxazine [2–4]. The phe- noxazine, also known as questiomycin A, is related to the naturally occurring antibiotic actinomycin D [5,6] that is used clinically for the treatment of certain types of cancer [7]. The oxidation of p -PDA in an aqueous sulphuric acid medium by Mn(III) acetate has been examined [8]. The reaction followed second-order kinetics, first order with respect to [amines] and [Mn(III)]. The reaction shows a positive salt effect. In aqueous solution, the redox reaction between p -PDA and Ag(I) ions has also been envisaged [9 – 11]. An induction period was depen- dent on pH and the concentration of p -PDA, but not on the Ag(I) ion concentration. Furthermore, the effect of pH on the equilibrium in the p -PDA – ferricyanide redox system has been discussed [12 – 14]. The equilibria were established very rapidly at all pH values. Finally, the oxidation of o -aminophenol (o -AP) by O 2 to amino 3-H -phenoxazine-3-one (APX) can be catalysed by cobalt(II) salts, cobalt(II)phthalocyanine derivatives and copper compounds [15–18]. Some tris-oxalato complexes are also known to be strong oxidising agents in aqueous solution [19]. The kinetics of reduction of tris-oxalatocobaltate(III) by amines have been investi- gated [20,21] in aqueous acidic media. The specific rate constants and the corresponding activation parameters have been evaluated at constant ionic strength. * Corresponding author. Tel.: +44-23-8059-3307; fax: +44-23- 8059-3781. E-mail address: je@soton.ac.uk (J. Evans). 0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0277-5387(00)00401-0