Copper(ii) complexes of a polydentate imidazole-based ligand. pH effect on magnetic coupling and catecholase activity

Abstract

A potentially dodecadentate N₈O₄-donor ligand obtained from 2,2′-biimidazole and L-valine and its tetranuclear Cu(II) complexes in different degrees of protonation were characterized by chemical and spectroscopic methods. The extensive solution studies performed reveal that the rise in pH media leads successively to the formation of imidazolato (pK_a⁽¹⁾ and pK_a⁽²⁾) and hydroxido (pK_a⁽³⁾ and pK_a⁽⁴⁾) bridges. A frozen solution EPR study shows a decrease in the signal intensity until an EPR silent spectrum is observed, upon increasing the basicity of the solution. The catalytic performance of the oxidation of 3,5-di-tert-butylcatechol to its corresponding quinone was studied using UV-Vis-NIR absorption spectroscopic methods in CH₃CN–H₂O and in CH₃OH–H₂O at pH = 7.5, 8.0 and 8.5. A marked increase in activity, consistent with the formation of the hydroxide bridged species, is observed at pH = 8.5 in both solvent mixtures, but the activity is significantly higher in CH₃OH–H₂O.