Synthesis and Structures of Novel Molecular Ionic Compounds Based on Encapsulation of Anions and Cations

The novel coordinate compounds T8[(CH2)n-EWG]8F–-18-crown-6-M+ were prepared by reaction of EWG-(CH2)n-Si­(OEt)3 (where n = 1–3 and the electron-withdrawing group, EWG, contains an Si–C­(sp3) bond) with an 18-crown-6-M+F– complex (where M+ is K+, Cs+, and Rb+) in the presence of a limited amount of water. The EWG facilitates the formation and encapsulation of the fluoride anion in a silsesquioxane cage. The reaction takes place by complexing an alkali metal in the crown ether, catalyzing silsesquioxane cage formation, and concomitant encapsulation of a fluoride ion within the final T8 cage. The reaction is simple and takes place in a single step with excellent yields of 81–95%. These novel materials have been studied by 19F and 29Si NMR spectroscopy and X-ray crystallography as well as by MALDI-TOF and ESI mass spectrometry. The T8[(CH2)n-EWG]8F–-18-crown-6-M+ compounds can be also synthesized by a facile reaction of 18-crown-6-M+F– with the corresponding T8[(CH2)n-EWG]8 cage in nearly quantitative yield. Surprisingly, the single-crystal X-ray diffraction analysis has revealed the presence of 1D and 3D polymeric complexes of T8[(CH2)n-EWG]8F–-18-crown-6-M+. To the best of our knowledge, this represents the first dual-encapsulation host–octasilsesquioxane system. The results from thermostability experiments monitored by 19F NMR and 1H NMR and DSC suggest that compound T8[CH2CH2(CF2)3CF3]8F–-18-crown-6-K+ is a potential ionic liquid (IL).