Copper(II) complexes for electrocatalytic dioxygen reduction: controlling selectivity via a proton relay mode

Two mononuclear Cu(II) complexes, [Cu(L₁)]⁺ (1+) and [Cu(L₂)(H₂O)]⁺ (2+), were synthesized as molecular electrocatalysts for the oxygen reduction reaction (ORR). Ligand L₁^- provides an N₅-donor framework, whereas L₂^- furnishes an N₄-coordination environment; both incorporate a redox-active N-carboxyamidoquinolate unit that functions as a local proton reservoir and electron storage site during catalysis. Electrochemical studies at pH 7 reveal markedly different ORR selectivities for the two complexes. The N₅-ligated complex 1+ preferentially catalyzes the 4e^-/4H⁺ reduction of O₂ to H₂O with pH-independent rates and a negligible solvent kinetic isotope effect (KIE = 1.05). Computational studies identify a strong intramolecular hydrogen bond between the protonated amide N-H group and the distal oxygen atom of a putative Cu(I)-OOH intermediate, which promotes internal proton delivery and facilitates reductive O-O bond cleavage to produce H₂O. In contrast, the N₄-ligated complex 2+ predominantly catalyzes the 2e^-/2H⁺ reduction of O₂ to H₂O₂, displaying pronounced pH-dependent ORR behavior and a larger solvent KIE (2.24), consistent with a rate-limiting external protonation step. Computational analysis reveals an unfavorable geometry for internal proton transfer in the 2e^-/1H⁺ reduced form of the Cu(II)-OOH intermediate derived from 2+, accounting for its preference for partial oxygen reduction. These results, therefore, highlight how proton-relay pathways (internal vs. external) control product selectivity in oxygen reduction reactions.