Stability of superoxide ion in phosphonium-based ionic liquids

In this work the chemical generation of superoxide ion and determination of its stability in five phosphonium-based ionic liquids has been carried out. The stability of the generated superoxide ion depended on the anion. For the trihexyl(tetradecyl)phosphonium cation, the bis(2,4,4-trimethylpentyl)phosphinate anion (IL 104) has shown a relatively good stability with a rate constant of 3.34 × 10^-5 s^-1 for the reaction of the superoxide ion. Triisobutyl(methyl)phosphonium tosylate has also shown moderate stability (6.8 × 10^-5 s^-1). The order of stability, bis(2,4,4-trimethylpentyl)phosphinate > dicyanamide (6.97 × 10^-5 s^-1) > Br^- (7.72 × 10^-5 s^-1) > Cl^- (12.7 × 10^-5 s^-1), correlates well with the order of their respective ionic volumes. On application of the generated superoxide ion for the oxidation of two organic sulfur compounds, 15% conversion of thiophene was attained in 2 h while dibenzothiophene (DBT) was found to be unreactive to the ion in IL 104. This was attributed to higher electron density on the sulfur atom in DBT relative to thiophene and high nucleophilicity of the superoxide ion. Furthermore, the type of IL appears to slightly affect the conversion. The conversion of thiophene obtained was in the following order: IL 104 (15%) > [HMPyrr][TFSI] (8%) > [BMPyrr][TFSI] (7%) with the apparent differences in the magnitude of the alkyl chain length.