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 large excess CF3COOH and O2 obeyed first-order kinetics. The kobs value was proportional to the catalyst concentrations, 1 or 2 whereas the kobs value remained constant irrespective of the concentrations 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 four times larger than the corresponding value determined for 2. Separate studies reveal that electron transfer from Fc* to the CuII complex is the rate-determining step in the case of PV-tmpa ligand, because the carbonyl oxygen of the pivalamido group inhibits the coordination of CF3COO− to copper, in direct contrast to the case for the tmpa ligand-complex. 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 overall four-electron O2-reduction to water, an important asset in the design of catalysts for the latter.

Mononuclear copper complexes, [(tmpa)CuII(CH3CN)](ClO4)2 (1, tmpa = tris(2-pyridylmethyl)amine) and [(BzQ)CuII(H2O)2](ClO4)2 (2, BzQ = bis(2-quinolinylmethyl)benzylamine)], act as efficient catalysts for the selective two-electron reduction of O2 by ferrocene derivatives in the presence of scandium triflate (Sc(OTf)3), in acetone, whereas 1 catalyzes the four-electron reduction of O2 by the same reductant in the presence of Brønsted acids such as triflic acid. Following formation of the peroxo-bridged dicopper(II) complex [(tmpa)CuII(O2)CuII(tmpa)]2+, the two-electron reduced product of O2 with Sc3+ is observed to be scandium peroxide ([Sc3+(O22−)]+). In the presence of three equiv of hexamethylphosphoric triamide (HMPA), [Sc3+(O22−)]+ was oxidized by [Fe(bpy)3]3+ (bpy = 2,2′-bipyridine) to the known superoxide species [(HMPA)3Sc3+(O2•−)]2+ as detected by EPR spectroscopy. A kinetic study revealed that the rate-determining step of the catalytic cycle for the two-electron reduction of O2 with 1 is electron transfer from Fc* to 1 to give a cuprous complex which is highly reactive toward O2, whereas the rate-determining step with 2 is changed to the reaction of the cuprous complex with O2 following electron transfer from ferrocene derivatives to 2. The explanation for the change in catalytic O2-reaction stoichiometry from four-electron with Brønsted acids to two-electron reduction in the presence of Sc3+ and also for the change in the rate-determining step is clarified based on a kinetics interrogation of the overall catalytic cycle as well as each step of the catalytic cycle with study of the observed effects of Sc3+ on copper-oxygen intermediates.