Preparation of Imidazolium Salt Type Ionic Liquids Containing Cyclic Siloxane Frameworks Takuya Kubo,1 Sayako Koge,2 Joji Ohshita,2 and Yoshiro Kaneko*1 Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065 2 Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8527
1
(E-mail:
[email protected])
Imidazolium salt type ionic liquids containing cyclic siloxane frameworks, Im-CyS-IL-NTf2 (Tg = ¹37 °C, flow temperature = ca. 0 °C, and Td5 = 415 °C) and Im-CyS-IL-OTf (Tg = 0 °C, flow temperature = ca. 20 °C, and Td5 = 391 °C) were successfully prepared by the hydrolytic condensation of DSMIC using the superacid catalysts HNTf2 and HOTf.
REPRINTED FROM
Vol.44 No.10
2015 p.1362–1364 CMLTAG October 5, 2015
The Chemical Society of Japan
CL-150598
Received: June 19, 2015 | Accepted: July 10, 2015 | Web Released: July 18, 2015
Preparation of Imidazolium Salt Type Ionic Liquids Containing Cyclic Siloxane Frameworks Takuya Kubo,1 Sayako Koge,2 Joji Ohshita,2 and Yoshiro Kaneko*1 Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065 2 Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8527
1
(E-mail:
[email protected]) Imidazolium salt type ionic liquids containing cyclic siloxane frameworks were successfully prepared by the hydrolytic condensation of 1-[3-(dimethoxymethylsilyl)propyl]-3methylimidazolium chloride, which was synthesized by the reaction of 3-chloropropyldimethoxymethylsilane and 1-methylimidazole, using the superacid catalysts bis(trifluoromethanesulfonyl)imide and trifluoromethanesulfonic acid. Ionic liquids are molten salts below 100 or 150 °C. Because ionic liquids exhibit superior properties such as negligible vapor pressure, high thermal stability, and high ionic conductivity, they have been widely studied for their remarkable potential as green solvents1 and electrolyte materials.2 General ionic liquids consist of organic cations with either organic or inorganic anions, i.e., most ionic liquids are regarded as organic compounds because of the presence of at least one organic ion. However, preparing ionic liquids containing inorganic frameworks is generally difficult. It was assumed that such ionic liquids could be applied in a wide range of materials research because of their significantly high thermostability and flame retardant properties derived from inorganic frameworks. Recently, some ionic liquids containing polyhedral oligomeric silsesquioxanes (POSSs) as the inorganic frameworks have been developed. A POSS ionic liquid was first prepared by Chujo, Tanaka, and co-worker.3 This POSS compound contained carboxylate anion side chains and imidazolium cations as counter ions, and its melting point (Tm) was 23 °C. In addition, a POSS ionic liquid, containing imidazolium cation side-chains, dodecyl sulfate anions as counter ions, and lower Tm (18 °C), was reported by Feng, Zhang, and co-workers.4 However, these POSS ionic liquids had relatively lower pyrolysis temperatures (Td’s were lower than 250 °C) because of large amounts of organic components in their side chains or counter ions. Other examples of POSS ionic liquids such as monofunctionalized POSSs have been reported,5 although their ion densities are lower than those of the aforementioned octafunctionalized POSS ionic liquids. This characteristic may be a disadvantage for the development of highly efficient electrolytes. Previously, we reported the facile preparation of regularly structured ionic poly- and oligosilsesquioxanes, i.e., ladder-like polysilsesquioxanes6 and POSSs,7 by the hydrolytic condensation of organotrialkoxysilanes containing functional side chain groups, which can be converted into ionic groups during the reactions. While performing these studies, we coincidentally found that a highly thermostable POSS ionic liquid containing imidazolium cation side-chains and bis(trifluoromethanesulfonyl)imide (NTf2) anions as counter ions (Im-Cage-SQ-IL) could be successfully prepared by the hydrolytic condensation of 1methyl-3-[3-(triethoxysilyl)propyl]imidazolium chloride (MTICl) using a water/methanol (1:19 v/v) mixed solution of HNTf2.8 1362 | Chem. Lett. 2015, 44, 1362–1364 | doi:10.1246/cl.150598
The pyrolysis temperature of 5% weight loss (Td5) was 436 °C for this ionic liquid. However, Tm and flow temperature of this compound were relatively high (105 and ca. 100 °C, respectively). Ionic liquids, which are fluids below room temperature, i.e., room-temperature ionic liquids, are particularly useful in applications for green solvents and electrolyte materials. Therefore, an imidazolium-type ionic liquid containing randomly structured oligosilsesquioxane (Im-Random-SQ-IL) was prepared by the hydrolytic condensation of MTICl in an aqueous HNTf2 solution.8 In addition, a quaternary ammonium-type ionic liquid containing randomly structured oligosilsesquioxane (Am-Random-SQ-IL) was also prepared from trimethyl[3(triethoxysilyl)propyl]ammonium chloride.9 As these compounds did not have a Tm owing to their amorphous structures, they had relatively low flow temperatures (ca. 0 °C for ImRandom-SQ-IL and ca. 35 °C for Am-Random-SQ-IL). In addition, these ionic liquids showed high thermal stabilities (Td5 = 437 °C for Im-Random-SQ-IL and 417 °C for AmRandom-SQ-IL). However, they also displayed high viscosities, probably because of the presence of silanol groups derived from their random structures and relatively higher degrees of polymerization (DP = ca. 10 and ca. 30, respectively). On the basis of these results, we assume that siloxane-based ionic liquids without silanol groups and with lower DP probably exhibit high thermal stability, low flow temperature, and low viscosity. To achieve the preparation of such siloxane-based ionic liquids, we selected cyclic siloxanes as the siloxane frameworks. Therefore, we referred to our previous study for the facile preparation of cationic cyclotetrasiloxane by the hydrolytic condensation of 3-aminopropylmethyltriethoxysilane using the superacid trifluoromethanesulfonic acid (HOTf).10 In this study, we found that imidazolium salt type ionic liquids containing cyclic siloxane frameworks (Im-CyS-IL-NTf2 and Im-CyS-ILOTf) were successfully prepared by the hydrolytic condensation of 1-[3-(dimethoxymethylsilyl)propyl]-3-methylimidazolium chloride (DSMIC) using superacid catalysts such as HNTf2 and HOTf. This is the first report of the preparation of ionic liquids containing cyclic siloxane frameworks. DSMIC was first synthesized by the reaction of 3chloropropyldimethoxymethylsilane and 1-methylimidazole. The structure of DSMIC was confirmed by the 1H NMR spectrum (Figure S1). Although a small amount of 1-methylimidazole remained in the resulting product, the product was used without further purification because of the difficulty in completely removing 1-methylimidazole. Im-CyS-IL-NTf2 was prepared by the following procedure (Scheme 1a): DSMIC was stirred in a water/methanol (1:19 v/v) mixed solution of HNTf2 at room temperature. The resulting solution was heated at ca. 50 °C in an open system until the solvent evaporated. The resulting crude product was
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CH3 CH 3O Si OCH3
Cl N
(a) in water/methanol (1:19 v/v) mixed solution of HNTf2 (b) in aqueous HOTf solution
N CH 3 DSMIC
CH 3 Si O
4~6
X N
N CH 3 Ionic liquids containing cyclic siloxane frameworks (a) Im-CyS-IL-NTf 2 (X = (CF 3SO2) 2N) (b) Im-CyS-IL-OTf (X = CF3SO3)
Scheme 1. Preparation of imidazolium salt type ionic liquids containing cyclic siloxane frameworks: (a) Im-CyS-IL-NTf2 and (b) Im-CyS-IL-OTf.
Figure 2. MALDI-TOF MS analyses of (a) Im-CyS-IL-NTf2 and (b) Im-CyS-IL-OTf. The matrix was 2,5-dihydroxybenzoic acid (DHB).
Figure 1. 1H NMR spectra of (a) Im-CyS-IL-NTf2 and (b) ImCyS-IL-OTf in DMSO-d6. Chemical shifts were referenced to DMSO (¤ 2.5). further heated at 100 °C for 2 h, washed with water, and then dried at 150 °C for ca. 5 h to obtain Im-CyS-IL-NTf2. The 1 H NMR spectrum of Im-CyS-IL-NTf2 in DMSO-d6 exhibited signals attributable to 1-methyl-3-propylimidazolium groups and methyl groups, but the signal for the methoxy group of DSMIC was not observed (Figure 1a), indicating that the DSMIC reagent was not present in the product. The energy-dispersive X-ray (EDX) pattern of Im-CyS-IL-NTf2 indicated the absence of Cl (2.6 and 2.8 keV) (Figure S2a). In addition, the Si:S elemental ratio of Im-CyS-IL-NTf2 was estimated to be 1.00:1.98, indicating that the molar ratio of imidazolium cations to NTf2 anions in Im-CyS-IL-NTf2 was ca. 1:1. In the matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis of Im-CyS-IL-NTf2, several peaks were observed, which corresponded to the mass of cyclic siloxane tetramer and pentamer (Figure 2a). Furthermore, the aforementioned 1H NMR spectrum showed multiplet signals a assigned to methyl groups at 0.23 ¹0.23 ppm (Figure 1a). In addition, the 29Si NMR spectrum of Im-CyS-IL-NTf2 in DMSOd6 at 40 °C also exhibited two multiplet signals in the D2 region (¹19.2 ¹19.6 ppm for cyclic tetrasiloxane and ¹21.4 ¹21.9 ppm for cyclic pentasiloxane) (Figure 3a). These results indicated that Im-CyS-IL-NTf2 was a mixture of cyclic tetrasiloxanes and cyclic pentasiloxanes, with some stereoisomers.
Chem. Lett. 2015, 44, 1362–1364 | doi:10.1246/cl.150598
Figure 3. 29Si NMR spectra of (a) Im-CyS-IL-NTf2 and (b) Im-CyS-IL-OTf in DMSO-d6. Chemical shifts were referenced to TMS (¤ 0.0). Im-CyS-IL-OTf was prepared using almost same procedure as that of Im-CyS-IL-NTf2 but using an aqueous HOTf solution as a catalyst (Scheme 1b). DSMIC was stirred in aqueous HOTf solution at room temperature. The resulting solution was heated at ca. 50 °C in an open system until the solvent evaporated. The resulting crude product was further heated at 100 °C for 2 h, washed with 2-propanol, and then dried at 150 °C for ca. 5 h to obtain Im-CyS-IL-OTf. The 1H NMR (Figure 1b) and EDX (Figure S2b) results of Im-CyS-IL-OTf indicated that the DSMIC reagent was not present in the product, and that the © 2015 The Chemical Society of Japan | 1363
and Im-CyS-IL-OTf (20%) at 800 °C were almost same the theoretical SiO2 yields (13% and 18%, respectively). In conclusion, we found that imidazolium salt type roomtemperature ionic liquids containing the cyclic siloxane frameworks, Im-CyS-IL-NTf2 and Im-CyS-IL-OTf, could be prepared by hydrolytic condensation of DSMIC in a water/methanol (1:19 v/v) mixed solution of HNTf2 and aqueous HOTf, respectively. Im-CyS-IL-NTf2 had a Tg of ¹37 °C, flow temperature of ca. 0 °C, and Td5 of 415 °C, while Im-CyS-IL-OTf had a Tg of 0 °C, flow temperature of ca. 20 °C, and Td5 of 391 °C. These results suggest that cyclic oligosiloxane structures are an important factor contributing to the high thermal stabilities and the behavior below room temperature of these ionic liquids.
Figure 4. DSC curves and photographs of (a) Im-CyS-ILNTf2 and (b) Im-CyS-IL-OTf. molar ratio of imidazolium cations to OTf anions in Im-CyS-ILOTf was ca. 1:1. The MALDI-TOF MS results indicated the existence of a mixture of cyclic siloxane tetramer, pentamer, and hexamer (Figure 2b). In addition, these compounds had some stereoisomers, as demonstrated by the 1H NMR spectrum with multiplet signals a due to the methyl groups at 0.16 ¹0.23 ppm (Figure 1b) and the 29Si NMR spectrum with three multiplet signals in the D2 region (¹19.1 ¹19.7 ppm for cyclic tetrasiloxane, ¹21.3 ¹21.9 ppm for cyclic pentasiloxane, and ¹22.2 ¹22.5 ppm for cyclic hexasiloxane) (Figure 3b). The differential scanning calorimetry (DSC) analyses of ImCyS-IL-NTf2 and Im-CyS-IL-OTf were performed at a heating rate of 10 °C min¹1 under a nitrogen flow (10 mL min¹1). The endothermic peaks assigned to Tg values for Im-CyS-IL-NTf2 and Im-CyS-IL-OTf were observed at ¹37 and 0 °C, respectively (Figure 4). Conversely, peaks due to the Tm were not detected. The flow temperatures of Im-CyS-IL-NTf2 and Im-CyS-ILOTf were confirmed as follows. Samples in glass vessels were maintained in a horizontal position at 100 °C for 15 min, and then the vessels were cooled to room temperature in the horizontal state. Next, the vessels were maintained in a horizontal position at various temperatures (with 5 °C intervals) for 10 min, and then held for 15 min with tilting at each temperature. Under this procedure, Im-CyS-IL-NTf2 and ImCyS-IL-OTf showed obvious fluidity at ca. 0 and ca. 20 °C, respectively (Figure 4, inset). On the basis of these results, it was concluded that Im-CyS-IL-NTf2 and Im-CyS-IL-OTf were room-temperature ionic liquids. The thermal stabilities of Im-CyS-IL-NTf2 and Im-CyS-ILOTf upon pyrolysis were investigated by thermogravimetric analyses (TGA) at a heating rate of 10 °C min¹1 up to 1000 °C and under a nitrogen flow (250 mL min¹1). The Td3, Td5, and Td10 values were 407, 415, and 427 °C for Im-CyS-IL-NTf2 and 380, 391, and 402 °C for Im-CyS-IL-OTf (Figure S3). These values were higher than those of 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C3mim][NTf2]) (366, 380, and 399 °C),8 which is an ionic liquid (Tg = 22 °C) that has the side chain structure of Im-CyS-IL-NTf2. These results indicate that the thermal stabilities of Im-CyS-IL-NTf2 and Im-CyS-IL-OTf were enhanced by the incorporation of the cyclic siloxane frameworks. The weights of residues of Im-CyS-IL-NTf2 (12%) 1364 | Chem. Lett. 2015, 44, 1362–1364 | doi:10.1246/cl.150598
This work was supported by JSPS KAKENHI (Grant-in-Aid for Challenging Exploratory Research) Number 15K13711. Supporting Information is available electronically on J-STAGE.
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