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

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.