Abstract
2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO(•)), a persistent nitronyl nitroxide radical, has been used for the detection and trapping of nitric oxide, as a redox mediator for batteries, for the activity estimation of antioxidants, and so on. However, there is no report on the reactivity of PTIO(•) in the presence of redox-inactive metal ions. In this study, it is demonstrated that the addition of scandium triflate, Sc(OTf)(3) (OTf = OSO(2)CF(3)), to an acetonitrile (MeCN) solution of PTIO(•) resulted in an electron-transfer disproportionation to generate the corresponding cation (PTIO(+)) and anion (PTIO(-)), the latter of which is suggested to be stabilized by Sc(3+) to form [(PTIO)Sc](2+). The decay of the absorption band at 361 nm due to PTIO(•), monitored using a stopped-flow technique, obeyed second-order kinetics. The second-order rate constant for the disproportionation, thus determined, increased with increasing the Sc(OTf)(3) concentration to reach a constant value. A drastic change in the cyclic voltammogram recorded for PTIO(•) in deaerated MeCN containing 0.10 M Bu(4)NClO(4) was also observed upon addition of Sc(OTf)(3), suggesting that the large positive shift of the one-electron reduction potential of PTIO(•) (equivalent to the one-electron oxidation potential of PTIO(-)) in the presence of Sc(OTf)(3) may result in the disproportionation. When H(2)O was added to the PTIO(•)-Sc(OTf)(3) system in deaerated MeCN, PTIO(•) was completely regenerated. It is suggested that the complex formation of Sc(3+) with H(2)O may weaken the interaction between PTIO(-) and Sc(3+), leading to electron-transfer comproportionation to regenerate PTIO(•). The reversible disproportionation of PTIO(•) was also confirmed by electron paramagnetic resonance (EPR) spectroscopy.