Ligand-mediated modulation in copper(II) complexes for four-electron oxygen reduction in neutral medium

Area of Science:

• Coordination Chemistry
• Electrocatalysis
• Sustainable Energy Technologies

Background:

• Efficient molecular catalysts for the oxygen reduction reaction (ORR) are crucial for sustainable energy applications.
• Copper-based metalloenzymes serve as inspiration for designing novel ORR catalysts.
• Understanding ligand effects on catalyst structure and activity is key to advancing electrocatalysis.

Purpose of the Study:

• To synthesize and characterize two novel copper(II) complexes with di-tert-butyl phosphate ligands.
• To investigate the electrocatalytic activity of these complexes for the oxygen reduction reaction (ORR) in neutral aqueous media.
• To elucidate the reaction mechanism, including speciation, electron transfer, and the role of ligand design.

Main Methods:

• Synthesis of copper(II) complexes with di-tert-butyl phosphate (dtbp-H) ligands and ancillary nitrogen-donor ligands (en, tmeda).
• Structural characterization using single-crystal X-ray diffraction.
• Electrochemical evaluation including cyclic voltammetry, controlled-potential electrolysis, and rotating ring-disk electrode (RRDE) analysis.

Main Results:

• Two distinct copper(II) complexes, [Cu(en)2(enH2)][(dtbp)4]·10H2O (1) and [Cu(tmeda)(dtbp)2(H2O)] (2), were synthesized and structurally characterized.
• Complex 2 exhibited superior ORR activity (onset potential, Eo = 0.31 V vs. RHE) and selectivity for a four-electron pathway, confirmed by RRDE.
• Electrochemical studies revealed dynamic speciation and a catalytic mechanism involving both solution-phase and surface-bound species.

Conclusions:

• Ligand design plays a critical role in tuning the ORR activity and selectivity of copper-based catalysts.
• Complex 2 demonstrates significant potential as an efficient and robust ORR catalyst, showing resistance to chloride poisoning.
• The study provides insights into the interplay between molecular structure, solution behavior, and surface processes in copper-catalyzed oxygen reduction.