Functional biomimetics for copper oxidases: interesting catalytic promiscuity of novel monocopper(ii) complexes†

Abstract

Employing H(L¹) [2-((pyridine-2-ylmethyl)imino)methylphenol] or H(L²) [2-((pyridine-2-ylethyl)imino)methylphenol] and phen (1,10-phenanthroline), two novel monocopper(II) complexes, [Cu(L¹/L²)(phen)](ClO₄) (1 or 2), have been produced and studied. The single-crystal structure of the complex ion in 2, as determined by X-ray structure analysis, shows a trigonal bipyramidal geometry with distortion (τ, 0.65). DFT calculations were used to investigate the molecular geometry of copper(II) complexes in solution as well as their electronic characteristics. The electronic and EPR spectra in the solid-state of 1 and 2 reveal a trigonal bipyramidal geometry, whereas the geometry in solution is square pyramidal. The positive and reversible nature of the redox pair (Cu^II/Cu^I) makes redox states easily interconvertible. The catalysts in methanol and/or the buffer induced three separate chemical changes: (i) ascorbic acid → dehydroascorbic acid, (ii) benzylamine → benzaldehyde, and (iii) 3,5-di-tert-butylcatechol → 3,5-di-tert-butylquinone. Their k_cat results show higher activities of amine oxidase (10⁵ h⁻¹). Ascorbate oxidase (10⁷ h⁻¹) and catechol oxidase (10⁶ h⁻¹) activity in the buffer yields k_cat values that are closer to those of the natural enzyme. This is due to the presence of ligand flexibility, structural distortion, an appropriate chelate ring size, a labile donor, a positive redox potential, and a persistent catalyst–substrate interaction. Therefore, the two monocopper(II) complexes serve as the most efficient promiscuous catalysts, acting as complementary agents to the activity of copper oxidase enzymes and superior models for oxidation processes.