Catechol oxidation promoted by bridging phenoxo moieties in a bis(μ-phenoxo)-bridged dicopper(ii) complex

Abstract

A dinuclear copper(II) complex [Cu₂(papy)₂(CH₃OH)₂] has been synthesized by reaction of one equiv. of Cu(OAc)₂·2H₂O with one equiv. of the tetradentate tripodal ligand H₂papy [N-(2-hydroxybenzyl)-N-(2-picolyl)glycine] and has been characterized by various spectroscopic techniques and its solid state structure has been confirmed by X-ray crystal structure analysis. The single-crystal structure of the complex reveals that the two copper centers are hexa-coordinated and bridged by two O-atoms of the phenoxo moieties. The variable temperature magnetic susceptibility measurement of the complex reveals weak ferromagnetic interactions among the Cu(II) ions with a J value of 1.1 cm⁻¹. The catecholase activity of the complex has been investigated spectrophotometrically using 3,5-di-tert-butyl catechol as a model substrate in methanol solvent under aerobic conditions. The Michaelis–Menten kinetic treatment has been applied using different excess substrate concentrations. The parameters obtained from the catecholase activity by the complex are K_M 2.97 × 10⁻⁴ M, V_max 2 × 10⁻⁴ M s⁻¹, and k_cat 7.2 × 10³ h⁻¹. A reaction mechanism has been proposed based on experimental findings and theoretical calculations. The catechol substrate binds to dicopper(II) centers and subsequently two electrons are transferred to the metal centers from the substrate. The bridging phenoxo moieties participate as a Brønsted base by accepting protons from catechol during the catalytic cycle and thereby facilitating the catechol oxidation process.