Electrochemistry and spectroelectrochemistry of 1,4-dinitrobenzene in acetonitrile and room-temperature ionic liquids: ion-pairing effects in mixed solvents.

Room-temperature ionic liquids (RTILs) have been shown to have a significant effect on the redox potentials of compounds such as 1,4-dinitrobenzene (DNB), which can be reduced in two one-electron steps. The most noticeable effect is that the two one-electron waves in acetonitrile collapsed to a single two-electron wave in a RTIL such as butylmethyl imidazolium-BF4 (BMImBF4). In order to probe this effect over a wider range of mixed-molecular-solvent/RTIL solutions, the reduction process was studied using UV-vis spectroelectrochemistry. With the use of spectroelectrochemistry, it was possible to calculate readily the difference in E°'s between the first and second electron transfer (ΔE12° = E1° - E2°) even when the two one-electron waves collapsed into a single two-electron wave. The spectra of the radical anion and dianion in BMImPF6 were obtained using evolving factor analysis (EFA). Using these spectra, the concentrations of DNB, DNB(-•), and DNB(2-) were calculated, and from these concentrations, the ΔE12° values were calculated. Significant differences were observed when the bis(trifluoromethylsulfonyl)imide (NTf2) anion replaced the PF6(-) anion, leading to an irreversible reduction of DNB in BMImNTf2. The results were consistent with the protonation of DNB(2-), most likely by an ion pair between DNB(2-) and BMIm(+), which has been proposed by Minami and Fry. The differences in reactivity between the PF6(-) and NTf2(-) ionic liquids were interpreted in terms of the tight versus loose ion pairing in RTILs. The results indicated that nanostructural domains of RTILs were present in a mixed-solvent system.