Hydrogen-Bonded Tapes Based on Symmetrically Substituted Diketopiperazines:  A Robust Structural Motif for the Engineering of Molecular Solids

Abstract

A series of eight symmetrically substituted diketopiperazines (DKPs) derived from 1-amino-1-carboxycycloalkanes (n = 3−7; 3,3,5,5-tetramethylcyclohexane; 4,4-dimethylcyclohexane; 2-indan) were synthesized and their crystal structures determined. In the solid state, all eight compounds form two pairs of hydrogen bonds with two adjacent molecules to form a one-dimensional structure that we refer to as “tapes”. These molecules represent a range of volumes and shapes that contain a common molecular fragment (DKP ring). We examined this series of compounds with three objectives in mind: (i) to establish the ability of the hydrogen-bonded “tape” motif to persist through these differences in volume and shape; (ii) to provide a series of structurally related compounds to use to test computational methods of predicting crystal structure from molecular structure; (iii) to search for qualitative correlations between molecular structure and crystal packing. All compounds form tapes and with one exception, all tapes pack with their long axes parallel. When viewed down their long axis, two types of tapes emerge: planar and nonplanar. The type of tape that forms reflects the conformation adapted by the DKP ringplanar or boat. Planar tapes form when the angle (α) between the two planes defined by the cis-amides in the DKP ring is 180°; nonplanar tapes form when α < 180°. Five of the eight compounds studied form planar tapes, the remaining three compounds form nonplanar tapes. Despite the variability in volume and shape represented by this series of molecules, the persistence of the tape motif in their crystalline solids suggests that the hydrogen-bonding interactions between DKPs dominate the packing arrangement of these molecules. Void space in the crystalline solid is minimized by parallel alignment of tapes that pack in a manner that permits the interdigitation of substituents on adjacent tapes.

Influence of structure on the kinetics of assembly of cyclic dipeptides into supramolecular tapes

“…5). 38 Our goal is to use supramolecular tapes as scaffolds with which to position guest molecules within the crystalline lattice for the purpose of creating a solid with tunable properties that depend on the arrangement and structure of the guest molecules. To achieve this goal requires the development of methods for predicting which guest molecules can be incorporated into a crystalline lattice of cyclic dipeptides and the development of methods for measuring how molecular structure influences the kinetics of assembly.…”

“…38 Despite the variability in volume and shape represented by these molecules, the persistence of the tape motif in their crystalline solids suggests that the hydrogen-bonding interactions between cyclic dipeptides (i.e. amide-amide interactions) dominate the packing arrangement of these molecules.…”

“…One of eight symmetrically substituted cyclic dipeptides (spirocyclopentyl-2,5-diketopiperazine) that selfassemble into hydrogen-bonded tapes38 The cyclic dipeptide of S-aspartic acid (S,S-ASPDKP) assembles into hydrogen-bonded tapes, which function as scaffolds for positioning guest molecules such as derivatives of bis-pyridine56 …”

Use of Different Unsaturated Dicarboxylates Toward the Design of New 3D and 2D Networks of Copper(II)

“…Recent progress in this area has generated examples in which the design of network structures has produced crystals with (a) one-dimensional (1D) molecular networks composed of hydrogen bonding, [8,9] metalϪligand coordination, [10,11] or a combination of these and other interactions, referred to as hybrid interactions; [12] (b) 2D networks comprising either hydrogen bonding [13] / metalϪligand coordination [14] or hybrid interactions; and (c) 3D networks composed of hydrogen bonding [15] or metalϪligand coordination. [16] These approaches seek to establish structural relationships between the molecular and bridging modes of terephthalate ions in a single molecule is unprecedented.…”

Synthesis of Polyamide Dendrons Bearing Pyridine Groups as Multiple Hydrogen Bonding Parts on the Periphery

“…Initial supramolecular systems based on multiple hydrogen bonding were reported by Lehn, 5 Kato and Fréchet, [6][7][8] and Whitesides. [9][10][11][12] These diverse efforts focused on the selective formation of discrete supramolecules from complementary receptor-substrate hydrogen bonding pairs, such as barbituric acid and triaminopyrimidine units, 2,6-diaminopyridine and 2,6-pyridinedicarbonyl units, and carboxylic acid and pyridine units.…”