How Ligand Protonation and Hydrogen Bonding Govern Oxygen Reduction Reaction Selectivity by a Dinuclear Copper...

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 [L_NHCu^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_NHCu^I---Cu^IL_NH], which binds and activates O₂ to form a peroxo-bridged intermediate [L_NHCu^II(μ-1,2-O₂^2-)Cu^IIL_NH]. A two-proton-coupled one-electron transfer step followed by a one-electron reduction yields the key intermediate [L_NHCu^I(μ-1,1-HOOH)Cu^IL_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^IIL_NH---L_NHCu^II-OH]. Notably, an extensive intramolecular hydrogen-bonding network within the [L_NHCu^I(μ-1,1-HOOH)Cu^IL_NH] 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.