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 [Cu2L2] (L = polypyridine-polyamide) in neutral aqueous solution. The ligand of complex 1 undergoes a protonation step to form the active species [LNHCuII---CuII]+ 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 [LNHCuI---CuILNH], which binds and activates O2 to form a peroxo-bridged intermediate [LNHCuII(μ-1,2-O22-)CuIILNH]. A two-proton-coupled one-electron transfer step followed by a one-electron reduction yields the key intermediate [LNHCuI(μ-1,1-HOOH)CuILNH], 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-CuIILNH---LNHCuII-OH]. Notably, an extensive intramolecular hydrogen-bonding network within the [LNHCuI(μ-1,1-HOOH)CuILNH] intermediate kinetically suppresses the release of hydrogen peroxide, governing the high ORR selectivity of the catalyst. This study provides atomistic insights into O2 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.