Scandium Ion-Promoted Electron-Transfer Disproportionation of 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-Oxide (PTIO^•) in Acetonitrile and Its Regeneration Induced by Water

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 = OSO₂CF₃), 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 Sc³⁺ 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 Bu₄NClO₄ 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 H₂O was added to the PTIO^•–Sc(OTf)₃ system in deaerated MeCN, PTIO^• was completely regenerated. It is suggested that the complex formation of Sc³⁺ with H₂O may weaken the interaction between PTIO⁻ and Sc³⁺, leading to electron-transfer comproportionation to regenerate PTIO^•. The reversible disproportionation of PTIO^• was also confirmed by electron paramagnetic resonance (EPR) spectroscopy.