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

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 (O-2(center dot-)). This was achieved by monitoring the stability of O-2(center dot-) 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](-), [E(3)FAP](-), [EtSO4](-), [SCN](-), [Cl](-), and [I](-). Potassium superoxide (KO2) salt, generated by dissolution of KO2 in DMSO, was used as the source of O-2(center dot-). Pursuant to our recent familiarity with literature, this work represents the first use of a guanidinium-based IL in the study of O-2(center dot-) stability. Following screening, it was found that consumption of generated O-2(center dot-) was highest in the presence of [C(4)DMIm][I], at 35.25%. This highlights its instability in that medium. In contrast, O-2(center dot-) was generated as a well-stabilized species in DMSO containing [BMIm][Cl], [BMIm][TfO], [BMIm][TFSI], [PMIm][TFSI], [EMIm][TFSI], [TBAmm][TFSI], [EDMPAmm][TFSI], [N-211,mom][E(3)FAP], [BMPyrr][TCB], [MOPyrr][TFSI], and [MOEMMo][TFSI]. Of the 22 ILs analyzed in this study, the lowest total consumption of O-2(center dot-) was observed in morpholinium-based IL at 0.04%, positioning it as the best medium for the chemical generation of O-2(center dot-).