POLYMER-SUPPORTED CATALYSIS - OXIDATION OF CATECHOLAMINES BY FE(III) AND CU(II) COMPLEXES IMMOBILIZED TO CHITOSAN

The air oxidation of adrenaline (epinephrine) and dopa (3,4-dihydroxyphenylalanine) by Fe(III) and Cu(II) complexes immobilized to chitosan was studied at pH 8.50. While the structural features of copper catalysts were already reported, those of iron complexes, based on EPR (6 K) and Mossbauer (room temperature) spectral data, are presented here. The reaction proceeds by different pathways, depending on the oxidation power of the central metal ion, as determined by cyclic voltammetric measurements. The catecholamines are oxidized by iron(III) compounds through an intramolecular electron transfer within a binary adduct, with overall second-order rate constants of 1-3 M-1.s-1 (25-degrees-C). Oxidation of adrenaline by copper(II) compounds involves molecular oxygen within a ternary precursor complex, in which Cu(II) ion very likely acts as an electron mediator. The overall third-order rate constants of the reaction range from 1.10(3) to 22.10(3) M-2.s-1 (25-degrees-C), depending on the polymeric ligand used. Implications of the different reduction potential of the central metal ions and of the stereochemical features around the active sites on the observed phenomena are briefly discussed.