1,3,4-Oxadiazoles for Crystal Engineering. Convenient Synthesis and Self-Assembly: Nonchiral Chains versus Chiral Helices

A series of new 1,3,4-oxadiazoles
containing carboxylic and halogen groups and a double bond have been
synthesized in good yields and a multigram scale. This was achieved
at room temperature from readily available 1,2-diacylhydrazines using
a cheap condensation reagent (the solution of P2O5 in H2SO4). Single-crystal X-ray diffraction
analysis has shown that all studied 1,3,4-oxadiazole-containing acids
are self-assembled by intermolecular H-bonds into supramolecular zigzag
chains or helices, depending on the tecton molecular structure and
the type of H-bonding. Factors affecting helix formation have been
found, and a Cambridge Structural Database (CSD) survey has been performed
to support these findings. Moreover, it has been demonstrated that
the tuning of the crystal structure leading to spontaneous symmetry
breaking for supramolecular helices based on nonchiral molecules is
possible even by as little change in molecular structure as a shift
from an isopropyl substituent to a cyclopropyl. Subsequently, the
studied 1,3,4-oxadiazole-containing acids and related compounds are
found to be easily accessible building blocks for crystal engineering
of new chiral materials with tunable supramolecular arrangement.