How Ligand Protonation and Hydrogen Bonding Govern Oxygen Reduction Reaction Selectivity by a Dinuclear Copper Catalyst: Insights from DFT Calculations

Density functional theory (DFT) calculations are employed to investigate the mechanism and selectivity of the oxygen reduction reaction (ORR) catalyzed by a dinuclear copper complex 1 [Cu 2 L 2 ] (L = polypyridine–polyamide) in neutral aqueous solution. The ligand of complex 1 undergoes a protonation step to form the active species [L NH Cu II ---Cu II ] + which initiates the catalytic cycle. This species undergoes a sequence of one-electron reduction, proton transfer, and another one-electron reduction to generate the reduced species [L NH Cu I ---Cu I L NH ], which binds and activates O 2 to form a peroxo-bridged intermediate [L NH Cu II (μ-1,2–O 2 2– )Cu II L NH ]. A two-proton-coupled one-electron transfer step followed by a one-electron reduction yields the key intermediate [L NH Cu I (μ-1,1–HOOH)Cu I L NH ], featuring a coordinated hydrogen peroxide unit. This species undergoes intramolecular O–O bond homolysis with a low overall barrier of 7.1 kcal/mol, producing the bis-hydroxo intermediate [OH–Cu II L NH ---L NH Cu II –OH]. Notably, an extensive intramolecular hydrogen-bonding network within the [L NH Cu I (μ-1,1–HOOH)Cu I L NH ] intermediate kinetically suppresses the release of hydrogen peroxide, governing the high ORR selectivity of the catalyst. This study provides atomistic insights into O 2 activation, the structural evolution of intermediates, and critical electron/proton transfer steps, establishing a theoretical blueprint for designing nonprecious metal ORR catalysts through the strategic integration of the second-sphere hydrogen-bonding network.