Phase Behavior and Proton Conductivity in Crown Ether-Based Supramolecular Sodium Hydrogen Sulfate Complexes

This study deals with the preparation and solid-state characterization, as well as structural and phase transition features of supramolecular complexes composed of sodium hydrogen sulfate (NaHSO₄) and two crown ether ligands, namely, 15-crown-5 and benzo-15-crown-5. Single crystals for each compound were grown, and their structures were elucidated via single-crystal X-ray diffraction (XRD) analysis, which highlighted the following compositions: [15-crown-5·Na]-HSO₄ (1) and [benzo-15-crown-5·Na]-HSO₄ (2). Microcalorimetric analyses, hot-stage microscopy, and variable-temperature powder X-ray diffraction were employed to analyze thermal stability and phase transition behaviors. Variable-temperature ¹H T ₁ solid-state NMR measurements were also used to monitor proton dynamics and to determine activation energies associated with motion across phase transitions. Formation of supramolecular complexes is crucial for inducing solid-solid transitions, leading to superprotonic phases, namely, crystalline solids exhibiting an enhanced ability to conduct protons, as demonstrated through electrochemical impedance spectroscopy measurement.