Crystal Structures of the Salts of Chiral Primary Amines with Achiral Carboxylic Acids: Recognition of the Commonly-Occurring Supramolecular Assemblies of Hydrogen-Bond Networks and Their Role in the Formation of Conglomerates

Abstract

X-ray crystallographic studies were carried out for seven conglomerates, seventeen racemic compounds, and four enantiomerically pure salts of chiral primary amines with achiral monocarboxylic acids. The crystal structures of the conglomerate salts revealed that these crystals can be regarded as being an assembly of a characteristic columnar hydrogen-bond network in which the ammonium cations and the carboxylate anions are aligned around a 2-fold screw axis (21-column). On the other hand, the crystal structures of the racemic-compound salts could be broadly classified into two types. One type is a crystal consisting of 21-columns; the other type is a crystal consisting of a different type of columnar hydrogen-bond network in which the ammonium cations and the carboxylate anions are related by inversion centers (i-column). Our results suggest that both the formation and the assembly of 21-columns are essential in the formation of conglomerates from these salts, and that the difference in the packing of 21-columns between conglomerate salts and racemic-compound salts is governed by van der Waals interaction between the 21-columns.