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 t wo-dimensional layers is formulated and demonstrated by the synthesis and structure determination of a series of dicarboxylic acid urea deri vatives. The design strategy is based upon the selection of complement ary hydrogen bond functionalities and an accounting of the specific sy mmetry operators that must correspond to each intermolecular interacti on within the molecular crystal. Each of the molecules studied contain ed a disubstituted urea functionality that was expected to form a one- dimensional hydrogen-bonded network, an alpha-network, via urea hydrog en bonds. The molecules also contained terminal carboxylic acid or ami de 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, re tained the two-fold axis within the crystal and reliably formed beta-n etworks. When unsymmetrical ureas were studied, the anticipated beta-n etwork often failed to form. This was a particular problem when racema tes were used. However, when a single enantiomer of an unsymmetrical c hiral molecule was studied, the beta-network formed in accordance with the design. The importance of chirality for the development of strate gies for supramolecular synthesis is discussed.