Properties of ILs containing siloxane frameworks.
Abstract
This chapter deals with our recent researches on the preparation and properties of thermally stable ionic liquids (ILs) containing siloxane frameworks. ILs containing randomly structured oligosilsesquioxanes with quaternary ammonium side-chain groups (Am-Random-SQ-IL) and with imidazolium side-chain groups (Im-Random-SQ-IL) were successfully prepared by the hydrolytic condensation of the corresponding trifunctional alkoxysilanes in aqueous bis(trifluoromethanesulfonyl)imide (HNTf2) solution. It is also reported that ILs containing cage-like oligosilsesquioxanes (POSSs) with imidazolium side-chain groups (Im-Cage-SQ-IL) and with random distribution of quaternary ammonium and imidazolium side-chain groups (Amim-Cage-SQ-IL)were obtained, when the similar hydrolytic condensations were performed in a water/methanol (1 : 19 v/v) mixed solution of HNTf2. In addition, we investigated the preparation of ILs containing cyclic oligosiloxanes with various imidazolium side-chain groups (MeIm-CyS-IL-NTf2, MeIm-CyS-IL-OTf, HIm-CyS-IL-NTf2, EtIm-CyS-IL-NTf2, PrIm-CyS-IL-NTf2, and BuIm-CyS-IL-NTf2) by the hydrolytic condensation of the corresponding difunctional alkoxysilanes in the solutions of superacids, such as HNTf2 and trifluoromethanesulfonic acid (HOTf).
Keywords
- alkoxysilane
- cyclic oligosiloxane
- hydrolytic condensation
- ionic liquid
- POSS
- siloxane
- silsesquioxane
- superacid
1. Introduction
Ionic liquids (ILs), molten salts below 100°C or 150°C, have attracted much attention because of their potential application to green solvents [1–4] and electrolyte materials [5–7]. These compounds indicate the negligible vapor pressure, high thermal stability, and high ionic conductivity. Most ILs are regarded as organic compounds because of the presence of large amount of organic components in ILs. On the other hand, ILs with relatively more inorganic components could be applied to a wide range of materials research due to their significantly higher thermostability derived from the inorganic components.
Based on such considerations, some ILs containing inorganic frameworks, such as cage-like oligosilsesquioxanes (polyhedral oligomeric silsesquioxanes: POSSs) have been developed so far. A POSS IL (melting point (
In this chapter, we would like to describe our recent work on the preparation of thermally stable ILs containing siloxane frameworks, such as randomly structured oligosilsesquioxanes, POSSs, and cyclic oligosiloxanes, by the hydrolytic condensation of the corresponding tri- and di-alkoxysilanes using superacid catalysts.
2. Preparation of a quaternary ammonium-type ionic liquid containing randomly structured oligosilsesquioxane
So far, we have prepared ionic siloxane compounds with regular structures, such as POSSs [10–12], ladder-like polysilsesquioxanes [13–19], and cyclic siloxanes [20], by the hydrolytic condensation of tri- and di-alkoxysilanes containing functional organic groups, which can be converted into ionic groups during the reactions. While performing these studies on the preparation of regularly structured ionic siloxane compounds, we fortuitously found a highly thermostable IL containing randomly structured oligosilsesquioxane, which has quaternary ammonium side-chain groups. We first describe the preparation and properties of this IL.
A quaternary ammonium-type IL containing randomly structured oligosilsesquioxane (
![](http://cdnintech.com/media/chapter/52725/1512345123/media/F1.png)
Scheme 1.
Preparation of (a)
The energy dispersive X-ray (EDX) pattern of
When the differential scanning calorimetry (DSC) measurement of
Run | IL | Flow temp. (°C)b | |||
---|---|---|---|---|---|
1 | 15 | NDd | ~40 | 417 | |
2 | NDd | 172 | ~155 | 420 | |
3 | −25 | NDd | ~0 | 437 | |
4 | −21 | 106 | ~100 | 436 | |
5 | Mixture of |
−7 | 164 | ~120 | 420 |
6 | −8 | NDd | ~30 | 420 | |
7 | −43 | NDd | ~0 | 415 | |
8 | −14 | NDd | ~20 | 391 | |
9 | −38 | NDd | ~0 | – | |
10 | −44 | NDd | ~0 | – | |
11 | −44 | NDd | ~0 | – | |
12 | −45 | NDd | ~0 | – |
Table 1.
aDetermined by DSC.
bDetermined by visual observation.
cDetermined by TGA.
dNot detected.
The flow temperature of
The thermal stability of
As described above,
3. Preparation of imidazolium-type ionic liquids containing random-structured and cage-like oligosilsesquioxanes
As described in the previous section,
![](http://cdnintech.com/media/chapter/52725/1512345123/media/F2.png)
Scheme 2.
Preparation of (a)
The EDX pattern of
The DSC analysis of
We assumed that such IL properties were probably attributed to the amorphous structure. Therefore, as well as the quaternary ammonium-type ILs as described in the previous section, a POSS compound with crystalline structure was prepared. A POSS compound (
The 29Si NMR spectrum of
The DSC curve for
The thermal stabilities of
4. Preparation of ionic liquids containing cage-like oligosilsesquioxane (POSS) with the random distribution of quaternary ammonium and imidazolium side-chain groups
As described in Section 3, a highly thermostable POSS IL containing imidazolium cationic side-chains and NTf2 anions as counter ions (
The development of POSS RT-ILs with high thermal stabilities is expected for both academic and application reasons because RT-ILs are particularly useful for many applications of green solvents and electrolyte materials. Therefore, to prepare such POSS ILs, we focused on our previous studies on the preparation of low-crystalline POSS [11] and amorphous POSS-linking polymer [12]. Their synthesis was achieved by hydrolytic condensation of a mixture of two types of amino-group-containing organotrialkoxysilanes. The molecular symmetry of the resulting POSS derivatives was low because of the random distribution of the two types of side-chain groups. Consequently, their crystallization was suppressed. In this section, we describe the preparation of a thermally stable POSS RT-IL (
![](http://cdnintech.com/media/chapter/52725/1512345123/media/F3.png)
Scheme 3.
Preparation of
The 1H NMR spectrum of
The 29Si NMR spectrum of
The DSC curves of
Conversely, the DSC curve of
The
5. Preparation of ionic liquids containing cyclic oligosiloxanes
In the previous sections, we described that ILs containing silsesquioxane frameworks, such as randomly structured silsesquioxanes and POSSs, were successfully prepared. In particular,
To achieve the preparation of such ILs containing cyclic oligosiloxanes, we referred to our previous study for the facile preparation of cationic cyclotetrasiloxane (this is not an IL) by the hydrolytic condensation of 3-aminopropylmethyltriethoxysilane using the superacid trifluoromethanesulfonic acid (HOTf) [20]. Therefore, when the hydrolytic condensation of 1-[3-(dimethoxymethylsilyl)propyl]-3-methylimidazolium chloride (DSMIC) was performed using superacid catalysts such as HNTf2 and HOTf, we found that imidazolium salt-type ILs containing cyclic oligosiloxane frameworks (
![](http://cdnintech.com/media/chapter/52725/1512345123/media/F4.png)
Scheme 4.
Preparation of (a)
![](http://cdnintech.com/media/chapter/52725/1512345123/media/F5.png)
Figure 1.
Photographs of (a)
In the MALDI-TOF MS analysis of
The DSC curves of the resulting products indicated the baseline shifts assigned to
The viscosity of
![](http://cdnintech.com/media/chapter/52725/1512345123/media/F6.png)
Scheme 5.
Preparation of (a)
For this chapter, we newly investigated the effects of the alkyl chain length in the imidazolium groups of ILs containing cyclic oligosiloxane frameworks. Therefore, imidazolium salt-type ILs containing cyclic oligosiloxane with various lengths of alkyl chains (R = H, CH2CH3, (CH2)2CH3, and (CH2)3CH3) were prepared by the hydrolytic condensation of the corresponding imidazolium-group-containing dimethoxysilanes using the superacid HNTf2 in a water/methanol (1:19, v/v) mixed solvent (Scheme 5). Based on the results of the 29Si NMR and MALDI-TOF MS analyses, we determined that the resulting products [
The DSC curves of the resulting ILs showed the baseline shifts assigned to
![](http://cdnintech.com/media/chapter/52725/1512345123/media/F7.png)
Figure 2.
DSC curves and photographs of (a)
6. Conclusions
In this chapter, we described the preparation and properties of thermally stable ILs containing siloxane frameworks, such as randomly structured oligosilsesquioxanes (
Acknowledgments
The authors gratefully acknowledge Prof. J. Ohshita (Hiroshima University) and Dr. T. Mizumo (Samsung R & D Institute Japan) for their enthusiastic collaborations. The authors also gratefully give thanks for financial supports from JSPS KAKENHI (Grant-in-Aid for Challenging Exploratory Research) Number 15K13711.
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