Water-Induced Regeneration of a 2,2-Diphenyl-1-picrylhydrazyl Radical after Its Scandium Ion-Promoted Electron-Transfer Disproportionation in an Aprotic Medium

A neutral, stable radical, 2,2-diphenyl-1-picrylhydrazyl radical (DPPH^•), has been frequently used to estimate the activity of antioxidants for more than 60 years. However, the number of reports about the effect of metal ions on the reactivity of DPPH^• is quite limited. We have recently reported a unique electron-transfer disproportionation of DPPH^• to produce the DPPH cations (DPPH⁺) and anions (DPPH⁻) upon the addition of scandium triflate [Sc(OTf)₃ (OTf = OSO₂CF₃)] to an acetonitrile (MeCN) solution of DPPH^•. The driving force of this reaction is suggested to be an interaction between DPPH^– and Sc³⁺. In this study, it is demonstrated that the addition of H₂O to the DPPH^•–Sc(OTf)₃ system in MeCN resulted in an increase in the absorption band at 519 nm due to DPPH^•. This indicated that an electron-transfer comproportionation occurred to regenerate DPPH^•. The regeneration of DPPH^• was also confirmed by electron paramagnetic resonance (EPR) spectroscopy. The amount of DPPH^• increased with an increasing amount of added H₂O to reach a constant value. The detailed mechanism of regeneration of DPPH^• was proposed based on the detailed spectroscopic and kinetic analyses, in which the reaction of DPPH⁺ with [(DPPH)₂Sc(H₂O)₃]⁺ generated upon the addition of H₂O to [(DPPH)₂Sc]⁺ is the rate-determining step.