Influence of Ligand Denticity and Nuclearity on the Catalytic Activity and Product Selectivity in Oxygen Reduction Reaction Catalyzed by Copper(II) Complexes

Abstract Two mononuclear Cu(II) complexes, [(L1)Cu](ClO4)2 (1), containing the pentadentate BnTPEN (L1 = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane), and [(L2)Cu(ClO4)](ClO4) (2), supported by the tetradentate BnBPEN (L2 = N,N'-dibenzyl-N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine) ligands, were isolated to assess the role of the coordination environment and denticity on the oxygen reduction reaction (ORR). Complex 1 exhibits about 17-fold higher ORR activity than that of 2, highlighting the role of ligand denticity in ORR. The kinetic studies indicate that a proton-coupled electron transfer (PCET) step is involved in the rate-determining step (RDS) for both complexes. Electrochemical and spectroscopic analyses suggest that protonation of the dissociated pyridyl arm in the reduced state of complex 1 establishes an internal proton relay that facilitates ORR. Complex 1 selectively produces H2O2, whereas complex 2 promotes selective 4e–/4H+ reduction of O2 to H2O, supporting distinct protonation pathways of the Cu(II)-hydroperoxo intermediate governed by ligand denticity. Subsequently, the pentadentate ligand framework was extended to generate di- and trinuclear Cu(II) complexes to investigate the influence of multiple metal centers and steric factors on their catalytic ORR activity. The Cu(II) complexes, [(L3)Cu2](ClO4)4 (3) with a bis(pentadentate) and [(L4)Cu3](ClO4)6 (4) of tris(pentadentate) ligands, display comparable overall ORR rates, while both the complexes catalyze 2e–/2H+ reduction of dioxygen. In contrast, the trinuclear complexes, [(L5)Cu3](ClO4)6 (5) and [(L6)Cu3](ClO4)6 (6), on tris(pentadentate) BnTPEN derivatives containing three methyl or ethyl groups on the benzene platform, show substantially diminished ORR activity due to higher reorganization energies and inefficient O2 binding at the Cu(I) center. Kinetic studies reveal that, irrespective of nuclearity, all multinuclear complexes show the same RDS as the mononuclear complexes 1 and 2. Overall, this study highlights the crucial roles of ligand denticity, multimetallic centers, and steric effects in regulating ORR activity and product selectivity in Cu(II)-based catalysts.