Enhanced catalytic four-electron dioxygen (O₂) and two-electron hydrogen peroxide (H₂O₂) reduction with a copper(II) complex possessing a pendant ligand pivalamido group

A copper complex, [(PV-tmpa)Cu^II](ClO₄)₂ (1) [PV-tmpa = bis(pyrid-2-ylmethyl){[6-(pivalamido)pyrid-2-yl]methyl}amine], acts as a more efficient catalyst for the four-electron reduction of O ₂ by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CF₃COOH) in acetone as compared with the corresponding copper complex without a pivalamido group, [(tmpa)Cu ^II](ClO₄)₂ (2) (tmpa = tris(2-pyridylmethyl) amine). The rate constant (k_obs) of formation of decamethylferrocenium ion (Fc*⁺) in the catalytic four-electron reduction of O₂ by Fc* in the presence of a large excess CF₃COOH and O₂ obeyed first-order kinetics. The k _obs value was proportional to the concentration of catalyst 1 or 2, whereas the k_obs value remained constant irrespective of the concentration of CF₃COOH or O₂. This indicates that electron transfer from Fc* to 1 or 2 is the rate-determining step in the catalytic cycle of the four-electron reduction of O₂ by Fc* in the presence of CF₃COOH. The second-order catalytic rate constant (k_cat) for 1 is 4 times larger than the corresponding value determined for 2. With the pivalamido group in 1 compared to 2, the Cu ^II/Cu^I potentials are -0.23 and -0.05 V vs SCE, respectively. However, during catalytic turnover, the CF₃COO ^- anion present readily binds to 2 shifting the resulting complex's redox potential to -0.35 V. The pivalamido group in 1 is found to inhibit anion binding. The overall effect is to make 1 easier to reduce (relative to 2) during catalysis, accounting for the relative k_cat values observed. 1 is also an excellent catalyst for the two-electron two-proton reduction of H ₂O₂ to water and is also more efficient than is 2. For both complexes, reaction rates are greater than for the overall four-electron O₂-reduction to water, an important asset in the design of catalysts for the latter.