Scaling Relation between the Reduction Potential of Copper Catalysts and the Turnover Frequency for the Oxygen and...

Changes in the electronic structure of copper complexes can have a remarkable impact on the catalytic rates, selectivity, and overpotential of electrocatalytic reactions. We have investigated the effect of the half-wave potential (E_1/2) of the Cu^II/Cu^I redox couples of four copper complexes with different pyridylalkylamine ligands. A linear relationship was found between E_1/2 of the catalysts and the logarithm of the maximum rate constant of the reduction of O₂ and H₂O₂. Computed binding constants of the binding of O₂ to Cu^I, which is the rate-determining step of the oxygen reduction reaction, also correlate with E_1/2. Higher catalytic rates were found for catalysts with more negative E_1/2 values, while catalytic reactions with lower overpotentials were found for complexes with more positive E_1/2 values. The reduction of O₂ is more strongly affected by the E_1/2 than the H₂O₂ rates, resulting in that the faster catalysts are prone to accumulate peroxide, while the catalysts operating with a low overpotential are set up to accommodate the 4-electron reduction to water. This work shows that the E_1/2 is an important descriptor in copper-mediated O₂ reduction and that producing hydrogen peroxide selectively close to its equilibrium potential at 0.68 V vs reversible hydrogen electrode (RHE) may not be easy.