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. 2024 Apr 17;25(8):4417.
doi: 10.3390/ijms25084417.

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

Yoshimi Shoji  1 , Yuri Terashima  2 , Kei Ohkubo  3 , Hiromu Ito  1 , Kouichi Maruyama  2 , Shunichi Fukuzumi  4   5 , Ikuo Nakanishi  1
Affiliations

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

Yoshimi Shoji et al. Int J Mol Sci. .

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 = 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.

Keywords: Lewis acid; comproportionation; cyclic voltammetry; disproportionation; electron paramagnetic resonance; electron transfer; kinetics; radical; reaction mechanism; scandium ion.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Scheme 1
Scheme 1
Redox behavior of PTIO•.
Scheme 2
Scheme 2
Sc3+-promoted electron-transfer disproportionation of PTIO•.
Figure 1
Figure 1
Spectral change observed upon addition of Sc(OTf)3 (4.4 × 10−6 M each) to PTIO• (7.2 × 10−5 M) in deaerated MeCN. Blue and orange lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: plot of the absorbance at 361 nm vs. [Sc(OTf)3]/[PTIO•].
Figure 2
Figure 2
(a) Spectral change (interval: 70 ms) observed during the reaction of PTIO• (5.5 × 10−5 M) with Sc(OTf)3 (7.9 × 10−3 M) in deaerated MeCN at 298 K. Blue and orange lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: the second-order plot of the absorbance at 361 nm. (b) Plot of kdisp vs. [Sc(OTf)3] for the disproportionation of PTIO• (5.5 × 10−5 M) in the presence of Sc(OTf)3 in deaerated MeCN at 298 K.
Scheme 3
Scheme 3
Proposed mechanism of the Sc3+-promoted electron-transfer disproportionation of PTIO•.
Figure 3
Figure 3
Cyclic voltammograms of PTIO• (1.0 × 10−3 M) before (blue line) and after (orange line) the addition of Sc(OTf)3 (1.0 × 10−2 M) in deaerated MeCN containing 0.10 M Bu4NClO4 recorded at the scan rate of 100 mV s−1 on a glassy carbon working electrode at 298 K. Arrows denote starting points and the direction of scanning.
Scheme 4
Scheme 4
Electrochemical oxidation of [(PTIO)Sc]2+ at +1.66 V vs. SCE.
Figure 4
Figure 4
Spectral change upon the addition of H2O to a deaerated MeCN solution of PTIO• (7.3 × 10−5 M) and Sc(OTf)3 (3.7 × 10−5 M) at 298 K. Orange and blue lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes.
Figure 5
Figure 5
(a) Spectral change (interval: 0.1 s) observed after the addition of H2O (1.9 M) to a deaerated MeCN solution of PTIO• (6.9 × 10−5 M) and Sc(OTf)3 (3.5 × 10−5 M) at 298 K. Orange and blue lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: the second-order plot of the absorbance at 361 nm. (b) Plot of kcomp vs. [H2O].
Scheme 5
Scheme 5
H2O-induced regeneration of PTIO•.
Figure 6
Figure 6
EPR spectra of (a) PTIO• (7.0 × 10−5 M), (b) PTIO• (7.0 × 10−5 M) after the addition of Sc(OTf)3 (3.5 × 10−5 M), and (c) PTIO• (7.0 × 10−5 M) after the addition of Sc(OTf)3 (3.5 × 10−5 M) and H2O (5.6 M) in deaerated MeCN at room temperature.
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