Stability of Superoxide Ion in Phosphonium-Based Ionic Liquids

Abstract

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 t...