Comparative study of the peroxidase-like activity of copper(II) complexes with N4 and N2O2 coordination environments. Application to the oxidation of phenol at moderate temperature

Abstract

The oxidation of phenol with H2O2 was investigated in the presence of mononuclear copper(II) complexes differing in the first coordination sphere, total charge, redox potential and/or geometry, under mild conditions. Among the tested complexes, [Cu(py2pn)]2+, where py2pn = 1,3-bis(pyridin-2-ylmethyleneamino)propane, proved to be a good catalyst for the para-oxidation of phenol at 25 °C and neutral pH, while [Cu(phen)2]2+ shows the highest phenol conversion at pH 9. The kinetic study of H2O2 oxidation of phenol catalyzed by [Cu(py2pn)]2+ revealed that phenol oxidation strongly depends on pH and temperature and competes with H2O2 dismutation at high catalyst concentration, while overoxidation becomes significant at high pH and temperature. Increasing the temperature to 50 °C, noticeably improves phenol conversion but decreases regioselectivity. The comparison of the performance of the copper complexes with their manganese analogues, showed the copper complexes are better catalysts at 25 °C. Encapsulation of the cationic Cu(II) complexes into mesoporous SBA-15 silica by ion exchange allows for isolation of the complex within the pores avoiding the formation of the peroxo-diCu dimer through which the competitive H2O2 dismutation occurs and boosts the phenol conversion at room temperature.