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 <i>E</i>°’s between the first and second electron transfer (Δ<i>E</i>₁₂° = <i>E</i>₁° – <i>E</i>₂°) 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 Δ<i>E</i>₁₂° 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