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.