Kinetic studies of carbohydrate oxidation catalyzed by novel isatin-Schiff base copper(II) complexes

With the aim of elucidating the mechanism of oxidative processes initiated by copper(II)-carbohydrate interactions, new isatin-Schiff base copper(II) complexes, [Cu(isaen)H2O]ClO(4)(.)2H(2)O (1), and [Cu(isaepy)(2)](ClO4)(2)(.)2H(2)O (2), where isaen = N-[(3-indolin-2-one)]-1,3-ethylenediamine and isaepy = N-[(3-indolin-2-one)]-2-aminoethyl pyridine were isolated, and characterized by elemental analyses, UV-vis, FTIR, EPR, and molar conductivity measurements. These imine ligands are capable of modifying selectively the environment of the copper(II) ion in a pH controlled process, through keto-enolic equilibria similar to those occurring with carbohydrates. Therefore, the obtained complexes had their catalytic activity in the oxidation of common carbohydrates (glucose, fructose and galactose) by molecular oxygen, compared to that of an analogous complex [Cu(isapn)](ClO4)(2) (3), previously reported. The determined rate law, from kinetic curves of oxygen consumption, showed a pseudo-first-order dependence both on the catalyst and substrate concentrations, followed by a saturation effect, for all the compounds studied. Further, the pH profile indicated that reaction occurs significantly only in very alkaline medium (pH greater than or equal to 10), and some influence of ionic strength (controlled by carbonate buffer) was also verified. The participation of very reactive intermediates in the oxidative degradation of the substrates was monitored by EPR spin trapping, while final products were identified by capillary electrophoresis. An extensive mechanism is proposed, explaining new kinetic studies as well as earlier data. (C) 2004 Elsevier B.V. All rights reserved.