title: Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives.
abstract: Superoxide ion (O2˙-) forms a stable 1 : 1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)(3)(3+)], which can be detected in solution by ESR spectroscopy. Electron transfer from O2˙- -Sc(HMPA)(3)(3+) complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)(3)(3+) to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)(3)(3+) complexes. The 1 : 1 and 1 : 2 complexes between semiquinone radical anions and Sc(HMPA)(3)(3+) depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)(3)(3+)-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)(3)(3+)-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA)(3)(3+) to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)(3)(3+). The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA)(3)(3+) occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)(3)(3+) depending the number of Sc(HMPA)(3)(3+) (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, Osaka University, SORST, Japan Science and Technology Agency, Suita, Osaka 565-0871, Japan.
authors.forename: Tomonori
authors.initials: T
authors.lastname: Kawashima
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D016227:Benzoquinones / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D012538:Scandium / Q000737:chemistry*; D013481:Superoxides / Q000737:chemistry*
chemical_list: D016227:Benzoquinones; D056831:Coordination Complexes; D007477:Ions; D013481:Superoxides; C004532:quinone; D012538:Scandium