Comparison of mononuclear and dinuclear copper(ii) biomimetic complexes: spectroelectrochemical mechanistic study of their catalytic pathways

Abstract

Two catecholase-like biomimetic catalysts, namely, two dinuclear copper complexes [Cu₂(L1)(OH)(H₂O)(EtOH)][ClO₄]₂ (C1) and [Cu₂Ac₂O(L1)ClO₄] (C2) with the 2,6-bis(4-methyl piperazin-1-yl-methyl)-4-formyl-phenoxy ligand (L1) together with the mononuclear complex Cu(ClO₄)₂(L2) (C3) containing ligand 1,2-(C₅H₄N-6-OCH₃-2-CHN)₂CH₂CH₂ (L2), were synthesized. Their catalytic pathways were investigated and compared. The evaluation of the catalytic activity of compound C1 (and C2, C3) using the Michaelis–Menten model was represented by values of K_M = 272.93 (223.02; 1616) μmol L⁻¹ and V_max of 0.981 (1.617; 1.689) μmol L⁻¹ s⁻¹. The role of water content in the solvent is also discussed. The dinuclear complexes C1 and C2 were found to be more efficient catalysts than mononuclear complex C3. The mode of catalytic action was characterized via cyclic voltammetry, spectrophotometry, and UV-Vis spectroelectrochemistry. The catalytic mechanism of 3,5-di-tert butyl catechol oxidation in the presence of oxygen was proposed. The reaction circle was proved by the confirmation of the chemical reversibility of complex reduction. The advantage of the in situ spectroelectrochemical measurement enabled to control the reduction of quinone formed by the chemical reaction of catechol with oxygen in solution. At this step, the simultaneous change in the absorption spectrum indicated a change in the copper redox state of the catalyst.