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.