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 C-2h 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, R-2(2)(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.