Insights into the generation of high-valent copper-oxo species in ligand-modulated catalytic system for oxidizing organic pollutants

Abstract

The development of coordination complexes to activate hydrogen peroxide (H2O2) selectively is a goal that has long been pursued. As highly reactive intermediates, the high-valent metal-oxo species play important roles in many biological and industrial catalytic oxidation processes. To improve the catalytic efficiency of metal complexes, the peroxide OO bond should be cleaved heterolytically to decrease the decomposition of H2O2 into O2 and H2O. In this work, we provide a simple method to obtain coordination complex catalysts based on iminodiacetic acid (IDA), copper(II) ions and pyridine derivatives. Catalytic activity tests showed that the introduction of 4-aminopyridine ligands resulted in an enhanced catalytic activity of the IDA/Cu(II)/4-ampy complex. A series of designed experiments proved that high-valent copper-oxo species ([OCuIII(IDA)(4-ampy)2] species) were formed in the IDA/Cu(II)/4-ampy complex system in the presence of hydrogen peroxide. The [OCuIII(IDA)(4-ampy)2] species were detected by in-situ high-definition electrospray ionization mass spectrometry, and has been considered as the possible active species during H2O2 activation catalyzed by the IDA/Cu(II)/4-ampy complex (with the molar ratio of IDA/Cu(II)/4-ampy being greater than 1:1:2). A mechanism has been proposed based on the results of electron paramagnetic resonance and density functional theory calculations.