Preparation of protic ionic liquids containing cyclic oligosiloxane frameworks

Abstract

Protic ionic liquids (PILs) containing cyclic oligosiloxanes with ammonium side-chain groups (Am-CyS-IL and 2Am-CyS-IL) were successfully prepared by the hydrolytic condensation of 3-aminopropyldimethoxymethylsilane and 3-(2-aminoethylamino)propyldimethoxymethylsilane, respectively, in a water/methanol (1 : 19 v/v) mixed solution of bis(trifluoromethanesulfonyl)imide (HNTf₂). The differential scanning calorimetry (DSC) curves of Am-CyS-IL and 2Am-CyS-IL exhibited baseline shifts attributable to glass transition temperatures (T_g) of −2 °C and 9 °C, respectively, and endothermic peaks at their melting temperatures (T_m) were not detected for both compounds, indicating their amorphous structures. In addition, fluidity was visually observed for each compound at ∼35 °C and ∼45 °C, respectively. On the basis of these results, we concluded that Am-CyS-IL and 2Am-CyS-IL were PILs. To investigate the influence of the kind of siloxane framework on the nature of the ionic liquid, we prepared polyhedral oligomeric silsesquioxanes (POSSs) containing the same side-chain groups and counteranions (Am-POSS and 2Am-POSS) from 3-aminopropyltrimethoxysilane and 3-(2-aminoethylamino)propyltrimethoxysilane, respectively, using an HNTf₂ catalyst. Am-POSS had a T_g of 15 °C and a T_m of 193 °C, and 2Am-POSS had a T_g of 32 °C and a T_m of 208 °C, which were estimated by DSC analyses. Furthermore, fluidity was not observed below 150 °C for both compounds, indicating that these POSS compounds were not PILs. These results suggest that the cyclic oligosiloxane frameworks are an important factor for the preparation of siloxane-based PILs exhibiting fluidity at low temperature. In addition, the present PILs containing cyclic oligosiloxane frameworks exhibited relatively high decomposition temperatures (T_d; Am-CyS-IL: T_d5 = 351 °C and T_d10 = 362 °C, 2Am-CyS-IL: T_d5 = 294 °C and T_d10 = 309 °C).