Enhanced Catalytic Four-Electron Dioxygen (O2) and Two-Electron Hydrogen Peroxide (H2O2) Reduction with a Copper(II) Complex Possessing a Pendant Ligand Pivalamido Group

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Abstract
A copper complex, [(PV-tmpa)CuII](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 O2 by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CF3COOH) in acetone as compared with the corresponding copper complex without a pivalamido group, [(tmpa)CuII](ClO4)2 (2) (tmpa = tris(2-pyridylmethyl)amine). The rate constant (kobs) 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 kobs value was proportional to the concentration of catalyst 1 or 2, whereas the kobs 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 (kcat) for 1 is 4 times larger than the corresponding value determined for 2. With the pivalamido group in 1 compared to 2, the CuII/CuI 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 kcat values observed. 1 is also an excellent catalyst for the two-electron two-proton reduction of H2O2 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.

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