Engineering Crystalline Architecture with Diketopiperazines:  An Investigation of the Strength of Hydrogen-Bonded Tapes Based on the Cyclic Dipeptide of (S)-Aspartic Acid

A molecular building block, (S,S)-3,6-dicarboxymethyl-2,5-diketopiperazine (ASPDKP), was designed to assemble into hydrogen-bonded tapes, a one-dimensional motif in the solid state. Tapes made with ASPDKP possess pendant carboxylic acid groups that can be used to position guest molecules at well-defined intervals along the backbone of the tapes. Guest molecules can be used to control the spacing between adjacent tapes and the topology of layers composed of tapes. Several derivatives of pyridine were cocrystallized with ASPDKP, and the resulting molecular solids were characterized. Eight crystalline solids are described. These structures reveal both the strengths and limitations of hydrogen-bonded tapes to function as scaffolds with which to engineer crystalline architecture. The strengths of tapes to function as scaffolds are made evident by the six solids in which tapes are present. First, hydrogen-bonded tapes are tolerant to the presence of other functional groups capable of forming hydrogen bonds such as pyridine and carboxylic acid groups. Second, the position of guest molecules that range in size, shape, and number of hydrogen-bond acceptors can be controlled precisely at 6 Å intervals by tethering them to the edge of tapes via hydrogen bonds. Third, guest molecules of different lengths can be used to vary systematically the distance between adjacent tapes. The limitation of tapes to function as scaffolds is revealed by the two solids in which tapes are absent. Guest molecules with two hydrogen-bond acceptors must be flexible conformationally or tapes will not form. This limitation does not hold for guest molecules with only one hydrogen-bond acceptor. A mechanism by which this limitation occurs is proposed and may prove useful for predicting when tapes made from ASPDKP will assemble in the presence of other guest molecules.