Mechanistic Investigations into the Selective Reduction of Oxygen by a MCO T3 site-inspired Copper Complex

Abstract

Understanding how multicopper oxidases (MCOs) efficiently and selectively reduce oxygen in the trinuclear copper cluster (TNC) is of great importance. Previously it was reported that when the T2-site is removed from the TNC, all O2 binding activity at the dinuclear T3-site is lost. Computational studies attribute this loss of activity to the flexibility of the protein active site, where the T3-copper centers move apart to minimize electrostatic repulsions. To address the question if and how a more constrained T3-site will catalyze the reduction of oxygen, we herein report a mechanistic investigation into the oxygen reduction reaction (ORR) activity of the dinuclear copper complex [Cu2L(μ-OH)]3+ (L=2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine). This T3-inspired complex confines the Cu centers in a rigid scaffold in close proximity instead of the flexible scaffold found in the protein active site and we demonstrate that under these constraints the dinuclear copper site displays ORR activity. Compared to the ORR mechanism of MCOs, we show that electrochemical reduction of [Cu2L(μ-OH)]3+ follows a similar pathway as the reduction of the resting enzyme due to the presence of the Cu-OH-Cu motif. By identification of key intermediates along the catalytic cycle of [Cu2L(μ-OH)]3+ we provide for the first time evidence that metal-metal cooperativity takes place during electrocatalysis of the ORR by a copper-based catalyst, which is achieved by the ability of the rigid ligand framework to bind two copper atoms in close proximity. Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction (HPRR) found for [Cu2L(μ-OH)]3+ are different from the ones found for analogous mononuclear copper catalysts. In addition, the metal-metal cooperativity results in an improved selectivity for the four-electron ORR of more than 70%. This selectivity is achieved by better stabilization of reaction intermediates between both copper centers, which is also essential for the ORR mechanism observed in MCOs. Overall, the mechanism of the [Cu2L(μ-OH)]3+-catalyzed ORR in this work gives insight into the ORR activity of a T3-site and contributes to understanding of how the ORR activity and selectivity are established in MCOs.