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 (ΔE ₁₂° = 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.