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)

Abstract

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.

Comprehensively understanding the distinct properties of crystal structure, phase transition and dielectrics in [M(crown)n]ReO4 (M+ = Na+, K+, NH4+; crown = 15-crown-5, 18-crown-6; n = 1 or 2)

“…The Na–O bond lengths ranging from 2.5845(5) to 2.8483(8) Å (Table S1†) are comparable to those in other reported Na + @18-crown-6 supramolecular compounds. 44 The Re–O bond lengths in the ReO 4 − anion span from 1.7170(5) to 1.7355(5) Å) (Table S1†), which are in agreement with the values in other ReO 4 − anions. 47 Two crystallographically distinct [Na(18-crown-6)] + cations are connected through the ReO 4 − anion, forming a one dimensional (1D) supramolecular chain (Fig.…”

“…Consequently, the solvent H 2 O molecules were introduced into the alternating 1D chain of {Na(18-crown-6) + ⋯H 2 O⋯BF 4 − } ∞ to mitigate the larger steric hindrance between two Na(18-crown-6) + cations. 44 This scenario contrasts with that observed in the ReO 4 − anion series.…”

“…In such instances, the BF 4 − anion only coordinated with the M + ion on one side of the crown ether ring, resulting in disorder due to weaker coulombic attraction between the supramolecular cation and the BF 4 − anion. 44 Moreover, the B–F bond lengths in the BF 4 − anion (1.340(5)–1.405(5) Å) are considerably shorter than the Re–O bond lengths (1.7170(5)–1.7355(5) Å) in the ReO 4 − anion. Consequently, the solvent H 2 O molecules were introduced into the alternating 1D chain of {Na(18-crown-6) + ⋯H 2 O⋯BF 4 − } ∞ to mitigate the larger steric hindrance between two Na(18-crown-6) + cations.…”

“…This is particularly pertinent as tetrafluoroborate anions of tetrahedral configuration exhibit lower rotational energy barriers, thereby facilitating an order–disorder structural phase transition. We successfully achieved a series of [M(crown)]BF 4 · x H 2 O (M + = Na + , K + ; crown = 15-crown-5, 18-crown-6; x = 0 or 1) 44 by introducing the BF 4 − anion to balance the charge. These supramolecular compounds exhibit crown-ether-ring size dependent crystal structures, phase transitions, and dielectric properties.…”

Enabling Superprotonic Phase Transitions in Solid Acids via Supramolecular Complex Formation: The Case of Crown Ethers and Alkali Hydrogen Sulfates

“…Crown ethers have been extensively studied in the past decades for their ability to coordinate alkali and various metal ions. − Many studies on alkali metal cations coordinated to the crown ethers show how these supramolecular complexes are prone to undergoing one or more solid–solid phase transitions that may present dynamic features. ,− It has also been pointed out that the cation that is selectively captured in the crown ethers’ cavities has an explicit effect on the dynamics of these complexes. , The anhydrous supramolecular complex made up of 18-crown-6 and potassium hydrogen sulfate, 18-crown-6·KHSO 4 ( 1 ), was reported to undergo an enantiotropic solid–solid transition, which was believed, but never proved, to be associated with the onset of a dynamical process affecting the crown ether ligand and/or the hydrogen sulfate anion …”