Influence of Hemilabile Arm and Amide Functionality in the Ligand Backbone on Chemical and Electrochemical Dioxygen Reduction Catalyzed by Mononuclear Copper(II) Complexes

Four mononuclear copper(II) complexes, [(DPA-OH)Cu(CH₃OH)(ClO₄)](ClO₄) (1), [(DPA-OMe)Cu(CH₃OH)(ClO₄)](ClO₄) (2), [(6-Amide-DPA-OMe)Cu](ClO₄)₂ (3) and [(6-Amide₂-DPA-OMe)Cu](ClO₄)₂ (4), of flexidentate ligands bearing hemilabile (hydroxy)methoxyethyl and/or amide group on DPA (di(2-picolyl)amine) backbone were isolated to explore their potency in catalyzing chemical and electrochemical oxygen reduction reaction (ORR). The role of a hemilabile arm, as well as amide functionality on the ligand backbone in affecting the rate-determining step (RDS) of the overall catalytic cycle has been explored and compared with that of the analogous [(tmpa)Cu](ClO₄)₂ (tmpa = tris(2-pyridylmethyl)amine) and [(PV-tmpa)Cu](ClO₄)₂ (PV-tmpa = bis(pyrid-2-ylmethyl){[6-(pivalamido)pyrid-2-yl]methyl}-amine) complexes. The hemilabile arm in these complexes results in an overall third-order rate, with k_cat values ranging from 10³ to 10⁴ M^–2s^–1 during dioxygen reduction catalysis. All the complexes except complex 4 selectively reduce dioxygen via the 4e^–/4H⁺ reduction pathway to water (H₂O) using decamethylferrocene (Fc*) as a sacrificial reductant in acidic acetone at 298 K. In contrast, altering the reaction conditions from a chemical to an electrochemical ambiance in phosphate buffer displays a reverse order of ORR activity of the complexes while maintaining the same product selectivity as observed in chemical ORR catalysis in acetone.