title: Enhanced catalytic four-electron dioxygen (O2) and two-electron hydrogen peroxide (H2O2) reduction with a copper(II) complex possessing a pendant ligand pivalamido group.
abstract: 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 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)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 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.
authors.affiliation: Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, ALCA (JST), Osaka University, Suita, Osaka 565-0871, Japan.
authors.forename: Saya
authors.initials: S
authors.lastname: Kakuda
authors.forename: Ryan L
authors.initials: RL
authors.lastname: Peterson
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Karlin
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D000577:Amides / Q000737:chemistry*; D002384:Catalysis; D003300:Copper / Q000737:chemistry*; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007202:Indicators and Reagents; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011725:Pyridines; D021241:Spectrometry, Mass, Electrospray Ionization; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000577:Amides; D007202:Indicators and Reagents; D008024:Ligands; D011725:Pyridines; C431843:tris(2-pyridylmethyl)amine; D003300:Copper; D006861:Hydrogen Peroxide; D010100:Oxygen