198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

Hydrogen-Bonded Ribbons, Tapes and Sheets as Motifs for Crystal Engineering 113
18
19
n = 1, 2, 3
Fig. 16. Formation of bidentate hydrogen bonded chains in 18 prevails in the presence of other
hydrogen bonding groups (19)
dimer and is forced to form a tape (Fig. 17). Both of these patterns have been
confirmed by vapor pressure osmometry as well as by X-ray crystallography.
Lehn et al. have used knowledge of the formation of the amide dimer to
design cyclic hexamers. This group synthesized and separated the (+) and (–)
forms of lactam 22, [55], which were anticipated to form the cyclic hexamer
depicted in Fig. 18a. However, the solid state structure of (–) 22 produced a
tetramer. The racemate in contrast is found to assemble into a ribbon of alternating
(+) and (–) subunits held together via hydrogen bonds (Fig. 18b).
The crystal structure of melamine (23) and cyanuric acid (24) shows, as
expected, the formation of a two-dimensional sheet (Fig. 19) [56]. Whitesides et
al. exploited the complementarity between substituted melamines and barbituric/cyanuric
acids, which leads to efficient hydrogen bonding between such