Stability of Superoxide Ion in Imidazolium Cation-Based Room-Temperature Ionic Liquids

The stability of superoxide ion (O2 •−) generated chemically by dissolving KO2 in dried dimethyl sulfoxide solutions containing imidazolium cation [e.g., 1-ethyl-3-methylimidazolium (EMI+) and 1-n-butyl-2,3-dimethylimidazolium (BMMI+)] based ionic liquids (ILs) was investigated with UV−visible spectroscopic, NMR, and voltammetric techniques and an ab initio molecular orbital calculation. UV−visible spectroscopic and cyclic voltammetric measurements reveal that the O2 •− species reacts with BMMI+ and EMI+ cations of ILs to form hydrogen peroxide. The pseudo first order rate constant for the reaction of BMMI+ and O2 •− species was found to be about 2.5 × 10−3 s−1. With a molecular orbital calculation, the O2 •− species is understood to attack the 2-position (C-2) of the imidazolium ring (i.e., BMMI+) to form an ion pair complex in which one oxygen atom is bounded to C-2 and the other to the hydrogen atom of -CH3 group attached to C-2. Eventually, the ion pair complex of BMMI+ cation and O2 •− species undergoes a ring opening reaction as evidenced with 1H NMR measurement.