AN APPROACH TO THE DESIGN OF MOLECULAR-SOLIDS - STRATEGIES FOR CONTROLLING THE ASSEMBLY OF MOLECULES INTO 2-DIMENSIONAL LAYERED STRUCTURES

A design strategy for the synthesis of molecular crystals containing two-dimensional layers is formulated and demonstrated by the synthesis and structure determination of a series of dicarboxylic acid urea derivatives. The design strategy is based upon the selection of complementary hydrogen bond functionalities and an accounting of the specific symmetry operators that must correspond to each intermolecular interaction within the molecular crystal. Each of the molecules studied contained a disubstituted urea functionality that was expected to form a one-dimensional hydrogen-bonded network, an alpha-network, via urea hydrogen bonds. The molecules also contained terminal carboxylic acid or amide residues that were anticipated to unite the alpha-networks into two-dimensional beta-networks via additional hydrogen bonds. In practice, it was found that symmetrical ureas, with C2 point group symmetry, retained the two-fold axis within the crystal and reliably formed beta-networks. When unsymmetrical ureas were studied, the anticipated beta-network often failed to form. This was a particular problem when racemates were used. However, when a single enantiomer of an unsymmetrical chiral molecule was studied, the beta-network formed in accordance with the design. The importance of chirality for the development of strategies for supramolecular synthesis is discussed.