Helical ammonium halide framework constituting polar conglomerate crystals of 2-ethylanilinium chloride†

Abstract

Crystal engineering, which periodically aligns supramolecular synthons through intermolecular interactions, is a key technology for creating a variety of functional crystalline materials such as porous, polar, chiral, and elastic/plastic crystals. Organic ammonium halides, composed of readily available amines and hydrogen halides, are typical crystalline framework motifs called long-range synthon aufbau modules. In contrast to ammonium halide frameworks such as ladder and sheet structures, chiral helical frameworks have received limited attention, despite their significant potential to exhibit useful functions owing to their unique structures. Herein, the facile synthesis of chiral and polar fibrous crystals with space group P3₂ by neutralizing 2-ethylaniline with hydrogen chloride is reported. Single-crystal X-ray diffraction reveals that the resultant 2-ethylanilinium chloride formed a helical framework through consecutive hydrogen bonding between NH₃⁺ and Cl⁻ moieties along the fiber axis, which presents helical arrays of 2-ethylanilinium moieties with the same handedness. The gas adsorption isotherms suggest that the crystals are essentially nonporous, whereas molecular hydrogen is slightly adsorbed. The fibrous crystals are elastic to some extent, which is consistent with the noncovalent packing of helices perpendicular to the fiber axis. Furthermore, the preparation of mixed crystals of 2-ethylaniline and other 2-substituted anilines is demonstrated. The study promotes the rational design of tunable crystalline materials based on helical frameworks of polar conglomerate fibrous crystals.