Tuning the catecholase activity of bis(pyrazolyl)methane-based copper(II) complexes by substitutions of the ligand core: unraveling a dual O2/H2O2 oxidation mechanism

Abstract

Five new dinuclear copper(II) complexes based on dicompartmental ligands of the general formula R1R2R1-C6(OH)[C(O)-NH-CH2-CH(pz)2]2 (pz = pyrazolyl ring, R1 = R2 = H: HHH-Cu2; R1 = H, R2 = t-Bu: HButH-Cu2; R1 = Me, R2 = H: MeHMe-Cu2; R1 = Me, R2 = Br: MeBrMe-Cu2; R1 = Me, R2 = NO2: MeNO2Me-Cu2) were prepared and characterized in both solution and solid state. The phenoxide moiety acts as a bridge between two copper(II) centers; the remaining coordination sites of the metals are filled by the deprotonated N-amide atoms, the nitrogen atoms originating from the bis(pyrazolyl)methane groups and one exogenous hydroxide bridge. The catecholase-type activity of these dicopper complexes was studied in methanol, using a combination of aerobic and anaerobic methods. These investigations revealed that the substitution pattern of the central arene ring has an impact on the catalytic activity of our complexes, with the catalytic activity increasing in the following order: HHH-Cu2 <HButH-Cu2 <MeNO2Me-Cu2 <MeBrMe-Cu2 <MeHMe-Cu2. H2O2 was identified as the byproduct of O2 reduction. Its influence on the catalytic mechanism has been examined and found to have an impact on the catecholase-type reaction using these catalysts. Based on these findings, a dual mechanism for the catechol oxidation is proposed, where both molecular oxygen and the formed hydrogen peroxide act concurrently as oxidants.