Screening Stability and Kinetics of Superoxide Radical Anion in Dimethyl Sulfoxide Containing Ionic Liquids

Abstract

The hunt for sustainable solvents has been an expansive pursuit ever since the genesis of green chemistry and has yielded more ecological approaches involving new organic compounds that are simultaneously used as reaction media and catalysts, such as neoteric solvents. This work primarily investigated the suitability of various ionic liquids (ILs) as a reaction medium for chemical generation of superoxide radical anions (\({\text{O}}_{2}^{{\bullet - }}\)). This was achieved by monitoring the stability of \({\text{O}}_{2}^{{\bullet - }}\) over a period of 3 h via kinetic studies in the presence of ILs comprising ammonium, pyrrolidinium, imidazolium, guanidinium, morpholinium, and sulfonium-based cations combined with different anions, such as [TFSI]⁻, [TfO]⁻, [DCA]⁻, [DMP]⁻, [TCB]⁻, [FAP]⁻, [E₃FAP]⁻, [EtSO₄]⁻, [SCN]⁻, [Cl]⁻, and [I]⁻. Potassium superoxide (KO₂) salt, generated by dissolution of KO₂ in DMSO, was used as the source of \({\text{O}}_{2}^{{\bullet - }}\). Pursuant to our recent familiarity with literature, this work represents the first use of a guanidinium-based IL in the study of \({\text{O}}_{2}^{{\bullet - }}\) stability. Following screening, it was found that consumption of generated \({\text{O}}_{2}^{{\bullet - }}\) was highest in the presence of [C₄DMIm][I], at 35.25%. This highlights its instability in that medium. In contrast, \({\text{O}}_{2}^{{\bullet - }}\) was generated as a well-stabilized species in DMSO containing [BMIm][Cl], [BMIm][TfO], [BMIm][TFSI], [PMIm][TFSI], [EMIm][TFSI], [TBAmm][TFSI], [EDMPAmm][TFSI], [N₂₁₁,mom][E₃FAP], [BMPyrr][TCB], [MOPyrr][TFSI], and [MOEMMo][TFSI]. Of the 22 ILs analyzed in this study, the lowest total consumption of \({\text{O}}_{2}^{{\bullet - }}\) was observed in morpholinium-based IL at 0.04%, positioning it as the best medium for the chemical generation of \({\text{O}}_{2}^{{\bullet - }}\).