Metformin-based copper(II) complexes: synthesis, structural characterization and mimicking activity of catechol oxidase and phenoxazinone synthase

Synthesis and characterization of the ternary copper(II) complexes containing metformin-based mixed ligand system (LL′) in which L is metformin and L′ is glycine or iminodiacetic acid (IDAH2) is the initial goal of the present study. Analytical investigations, mass spectra, electrochemical, thermal, magnetic and spectroscopic measurements assigned the suggested molecular formulae of the current copper(II) chelates. Magnetic and spectral studies suggested square-pyramidal stereochemistry for the three complexes. Furthermore, X-ray structural analysis results demonstrated the square-pyramidal geometry around copper(II) center of complexes 1, 2 and 3. Catechol oxidase and phenoxazinone synthase mimetic activity examinations displayed that the current copper(II) oxidase models efficiently catalyze the oxidative coupling of o-aminophenol (o-APH3) to the corresponding oxidation product aminophenoxazin-3-one core (APX). As well, the current metformin copper(II)-based chelates showed excellent catalytic performance toward the air conversion of the model substrate 3,5-di-tertbutylcatechol (3,5-DTBCH2) to the biologically significant 3,5-di-tertbutylquinone. The catalytic reaction rate dependence on the substrate concentration is consistent with two orders profiles: first order at low substrate concentration and saturation kinetic at the high concentration of substrate. The catalytic efficiency was described by the ratio kcat/KM which follows the order 2 > 1 > 3, and the same order holds for the geometry irregularity of the studied copper(II) complexes. Electrochemical data were employed to describe parameters that determine the rate of intramolecular reactions including the driving force of the reaction (i.e., the free energy change, − ΔGo), the reorganization energy, λ of the reacting redox centers during the aerobic catalytic oxidation of the studied substrates. Based on the kinetic measurements by using stopped-follow spectrophotometer, the probable mechanistic implications of the studied catalytic reactions are discussed.