Organic Crystal Engineering with 1,4-Piperazine-2,5-diones. 8. Synthesis, Crystal Packing, and Thermochemistry of Piperazinediones Derived from 2‑Amino-4,7-dialkoxyindan-2-carboxylic Acids

1,4-Piperazine-2,5-diones possessing C2h molecular symmetry and bearing four
methoxy, ethoxy, butoxy, hexyloxy, octyloxy, nonyloxy, dodecyloxy,
or octadecyloxy groups were synthesized from 2-amino-4,7-dialkoxyindan-2-carboxylic
acids. Where possible, the piperazinediones were crystallized from
appropriate solvents and the supramolecular organizations were determined
by X-ray crystallography. In each case so studied, crystal assembly
via three mutually orthogonal interactions, namely, R22(8) hydrogen bonding, arene perpendicular edge-to-center interactions,
and alkyl chain interdigitation, was observed. The thermochemical
properties of these compounds were studied by modulated differential
scanning calorimetry. In most cases, one or more reversible thermal
events were observed between room temperature and melting to an isotropic
liquid. In the cooling cycle, freezing point temperatures were observed
to decrease with increasing hydrocarbon chain length. This may be
due to greater entropic penalties for hydrogen-bond association of
molecules with longer hydrocarbon chains.