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

## TL;DR

This paper investigates how crown ether-based complexes with sodium hydrogen sulfate behave in different phases and conduct protons.

## Contribution

The study introduces new supramolecular complexes that exhibit superprotonic behavior due to solid–solid phase transitions.

## Key findings

- The complexes [15-crown-5·Na]HSO4 and [benzo-15-crown-5·Na]HSO4 were structurally characterized using X-ray diffraction.
- Phase transitions and proton dynamics were analyzed using calorimetry, microscopy, and NMR.
- The complexes show enhanced proton conductivity in superprotonic phases confirmed by electrochemical measurements.

## Abstract

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.