Development of New Task-Specific Ionic Liquids with an Epoxy Group

Abstract

Ionic liquids (ILs) are a subset of molten salts, but they are liquid at room temperature. Such curious liquid salts have unique properties, e.g., negligible vapor pressure, inflammability, high thermal/chemical stability. These properties allow for their application in reusable solvents1 and safe electrolytes.2 The most interesting feature of ILs is that we can design their physiochemical properties by introducing various functional groups to ILs whatever we want. Due to the feature, a number of task-specific ILs, which have broad attention as new functional materials, are created so far. 3 However, as far as we know, there are a limited number of report on the modification of cation and anion that makes up the IL by chemical synthesis, e.g., nucleophilic addition reaction. It means that we can do very little to add a functionality to the ILs once they are synthesized. Since epoxides easily react with nucleophiles because of its polarity and ring strain, they are commonly used as a starting material to synthesize various types of polymers, e.g., polycarbonate, polyester, and polyether. Epoxides also react with H2O, ROH, RCOOH, NH3, SO2, CO2, and then different chemical compounds are yielded. In this study, we synthesized new TSILs with a reactive epoxy group that has great possibilities as starting IL materials to design novel functional ILs. Physicochemical properties of the obtained epoxy-containing ILs were evaluated. In addition, we examined the reactivity of the ILs with CO2as one of the model reactions. The synthesis of ILs with an epoxy group was conducted according to Scheme 1. The crude 1-allyl-1-(oxiran-2-ylmethyl)piperidin-1-ium bromide 1 was purified by washing with a super dehydrated diethylether for several times. Generally, strong acids such as HN(SO2CF3)2, HBF4, and HPF6 are used in an anion exchange reaction. However, in this study, LiN(SO2CF3)2, AgN(CN)2, NaBF4 and LiPF6 were used as anion materials to prevent a ring-opening reaction of the epoxy group on the cation. After the reaction, crude products were extracted with CH2Cl2 and purified by washing with an ultrapure water for several times in order to remove the starting materials. The obtained ILs were confirmed by 1H NMR, mass spectrometry, and elemental analysis. Physicochemical properties of the ILs were investigated by DSC, TG, and measurements of density, viscosity, and ionic conductivity. The yield of the each salt with [N(SO2CF3)2]−, [N(CN)2]−, BF4 −, and PF6 − was 90, 56, 88, and 60 %, respectively. We confirmed almost no impurity in the salts using 1H NMR, mass spectrometry, and elemental analysis. When the [N(SO2CF3)2]− and [N(CN)2]− were chosen as anions, the salts 2 and 3 became liquid at room temperature, although those with BF4 − and PF6 − anions 4 and 5 were solid. Higher melting point of the latter salts was likely caused by their high symmetric anion structures. All the salts were stable in water. ILs 2 and 3 showed hydrophobicity. We revealed by evaluating Walden plots that ILwith [N(CN)2]− 3 has the highest ionicity among the obtained salts 2 - 5. As to IL with [N(SO2CF3)2]− 2, it showed better thermal behavior and favorable viscosity and ionic conductivity. For this reason, 2 was used to examine the reactivity with CO2. Tetrabutylammonium iodide (TBAI) was employed as a catalyst for CO2 addition reaction. The reaction was conducted under a CO2 atmosphere (353 K, 1 atm). As the reaction proceeded, the viscosity of IL dramatically increased. Change in the viscosity would be caused by the increase in the steric hindrance of cations. The IL 2 was mostly converted to cyclic carbonate-containing IL 6 after 48 h (Figure 1). The reverse reaction partially proceeded by heating only. It is interesting to note that the synthesized ILs 2 and 3were stable against water and heat beyond our expectation. These ILs designed in this research will be useful starting IL materials to produce next-generation functional ILs. Acknowledgement Part of this research was supported by the Grant-in-Aid for Scientific Research, Grant Numbers 15H03591, 15K13287, and 15H2202 from the Japanese Ministry of Education, Culture, Sports, Science and Technology and by the ALCA-SPRING program, Japan Science and Technology Agency.