Organic Crystal Engineering with Piperazine-2,5-diones. 4. Crystal Packing of Piperazinediones Derived from 2-Amino-7-cyano-4-methoxyindan-2-carboxylic Acid

Piperazine-2,5-diones derived from 2-amino-7-cyano-4-methoxyindan-2-carboxylic acid were prepared
in meso, racemic, and enantiomerically pure forms, crystallized from dimethyl sulfoxide (DMSO), and the
supramolecular organization in the crystals determined by X-ray crystallography. These molecules were designed
to bear two strongly polar p-methoxybenzonitrile moieties either aligned or opposed. In contrast with nonpolar or
weakly polar piperazinediones of similar size and shape, which exclude DMSO to form ladder-like tapes by reciprocal
intermolecular amide-to-amide hydrogen bonding, these more strongly polar piperazinediones participate in hydrogen
bonding with included DMSO. In cocrystals of the enantiomerically pure compounds, the p-methoxybenzonitrile
dipoles were aligned in the three-dimensional lattice but opposed by the dipoles of included DMSO. These cocrystals
exhibited second harmonic properties comparable to urea. A second cocrystal possessing aligned p-methoxybenzonitrile
dipoles was observed when the enantiomerically pure compound was crystallized from dipropylsulfoxide. Compensating dipropylsulfoxide dipoles were absent in this cocrystal.