Dalton Trans. 2026 Jul 6. doi: 10.1039/d6dt00632a. Online ahead of print.
ABSTRACT
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.
PMID:42405825 | DOI:10.1039/d6dt00632a