Investigation of inclusion complexation of imidazolium and pyrrolidinium chlorides with water-soluble p-sulfonatocalix[6]arene: characteristic effects of external pressure, temperature, and substituents

Using the inclusion complexation of methylene blue as a chemical indicator, the inclusion constants of imidazolium and pyrrolidinium chlorides (ionic liquids (IL)) with p-sulfonatocalix[6]arene (Calix-S6) were determined in a water–methanol mixture. In the inclusion of imidazolium chlorides, we found the characteristic inclusion constants, dependent on the length of the alkyl side chain. From the temperature effect on the inclusion, the differences in the inclusion interactions of imidazolium and pyrrolidinium ring moiety with the sulfonate groups of the Calix-S6’s portal were observed. Further, from high-pressure studies on the inclusion complexation, the volume changes caused by the conformational changes of Calix-S6 and desolvation around the guest and host molecules upon inclusion were evaluated. Based on the results, we demonstrated that the volume change related to desolvation upon inclusion plays an important role in the reaction volume for the inclusion equilibrium of IL with Calix-S6. The structures of the inclusion complexes were established from 2D ROESY NMR measurements and a volumetric study of the inclusion complexation with p-sulfonated Calix-S6 having a flexible cavity was performed.