Influence of Nitrogen Donor Type on Electrocatalytic Performance of Copper Complexes: A Detailed Theoretical Analysis of Experimental Results

In this work, the electroreduction of molecular oxygen was studied using Cu(II) complexes with two slightly different ligands: one pyridine-based (dpba) and the other imidazole-based (dipa). Cyclic voltammetry in MeOH/H2O at pH = 9 showed that CuIIdpba exhibits higher redox reversibility and better-defined oxidation peaks than CuIIdipa, indicating faster electron-transfer kinetics and more efficient regeneration of the reduced species. In the presence of atmospheric O2, CuIIdpba produced higher cathodic currents, indicating superior electrocatalytic activity. Scan-rate-dependent studies revealed the formation of a Cu(II)–O2 intermediate, whose accumulation depends on the ligand type and timescale. Foot-of-the-Wave Analysis (FOWA) yielded a kobs of 33.43 s−1 for CuIIdpba, compared to 0.10 s−1 for CuIIdipa, confirming the pyridine-based system’s higher kinetic efficiency. DFT calculations indicated that CuIIdpba maintains a stable geometry upon reduction, facilitating O2 interaction, while CuIIdipa undergoes distortions, forming less reactive Cu(II)–O2 adducts. Frontier orbital and electrostatic potential analyses revealed that the stronger π-acceptor character of dpba enhances Cu(I) polarization and long-range electron transfer, unlike dipa. These results demonstrate that ligand electronics critically control the electrocatalytic oxygen reduction activity, thereby explaining the superior performance of the pyridine-based Cu(II) complex.