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

Area of Science:

• Inorganic Chemistry
• Electrochemistry
• Catalysis

Background:

• Molecular electrocatalysts are crucial for efficient oxygen reduction reactions (ORR).
• Controlling the selectivity of ORR to either water or hydrogen peroxide is a significant challenge.
• The role of ligand design and proton transfer mechanisms in dictating ORR product selectivity requires further investigation.

Purpose of the Study:

• To synthesize and characterize two mononuclear copper(II) complexes with distinct ligand environments for ORR.
• To investigate the electrocatalytic activity and selectivity of these complexes for the oxygen reduction reaction.
• To elucidate the underlying mechanisms, particularly the role of proton-relay pathways, in controlling ORR product formation.

Main Methods:

• Synthesis of two mononuclear Cu(II) complexes, [Cu(L1)]+ (1+) and [Cu(L2)(H2O)]+ (2+), featuring N5 and N4 donor frameworks, respectively.
• Electrochemical studies, including cyclic voltammetry and rotating disk electrode voltammetry, at pH 7 to assess ORR activity and selectivity.
• Computational studies (Density Functional Theory) to analyze reaction intermediates, transition states, and proton transfer mechanisms.

Main Results:

• Complex 1+ (N5-ligated) selectively catalyzes the 4-electron reduction of O2 to H2O with pH-independent rates and a low solvent kinetic isotope effect (KIE=1.05).
• Computational analysis for complex 1+ reveals an intramolecular hydrogen bond facilitating internal proton delivery and O-O bond cleavage.
• Complex 2+ (N4-ligated) predominantly catalyzes the 2-electron reduction of O2 to H2O2, exhibiting pH-dependent behavior and a higher KIE (2.24), indicative of external protonation.

Conclusions:

• The proton-relay pathway (internal vs. external) is a critical determinant of product selectivity in copper-catalyzed ORR.
• Ligand design, specifically the coordination environment and presence of proton reservoir sites, significantly influences the catalytic mechanism.
• These findings offer valuable insights for designing sophisticated molecular electrocatalysts for selective oxygen reduction.