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 = 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.