Crown-Ether-Ring Size Dependent Crystal Structures, Phase Transition and Dielectric Properties of [M(crown)]BF₄⋅xH₂O (M⁺=Na⁺, K⁺; crown=15-crown-5, 18-crown-6; x=0 or 1)

Crown ethers demonstrate significant conformational flexibility and rotational symmetry, rendering them invaluable in the realms of supramolecular chemistry and crystal engineering. These unique natures facilitate the construction of supramolecular crystals of crown ethers, characterized by disorder-order phase transitions, imparts unique properties that hold promise for diverse applications across multiple fields. In this study, four supramolecular compounds, namely [Na(15-crown-5)]BF₄ (1), [Na(18-crown-6)]BF₄⋅H₂O (2), [K(15-crown-5)]BF₄ (3) and [K(18-crown-6)]BF₄⋅H₂O (4) were synthesized and characterized by microanalysis, thermogravimetric analysis, differential scanning calorimetry and powder X-ray diffraction techniques. Herein, 15-crown-5 and 18-crown-6 correspond to 1,4,7,10,13-pentaoxacyclopentadecane and 1,4,7,10,13,16-hexaoxacyclooctadecane, respectively. It was observed that the crystal structure, phase transition, and dielectric properties of these supramolecular compounds are significantly influenced by the size of the crown-ether rings. The research extensively discussed the correlation between the coordination mode of metal ions of K⁺ or Na⁺ with crown ethers, the compatibility between metal ions and crown-ether rings in terms of size, and the effects of crown-ether disorder on dielectric permittivity during phase transitions. Our discoveries hold significant implications for the design and development of crown-ether supramolecular functional materials.