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

Hydrogen-Bonded Ribbons, Tapes and Sheets as Motifs for Crystal Engineering 115
Fig. 19. Sheets formed by 23 and 24
23 24
compounds [57, 58]. They have found that the solid state structure formed by
compounds of 25 and 26 is dependent upon steric interactions.Variations in the
structure of each of the monomers were shown to have a significant effect on the
architecture of the assembled system. For example, when the substituents on the
phenyl rings in the melamine subunit are small (CH 3, F, Cl, Br, I) (Fig. 20a) tapes
are formed. In the case of methyl and ethyl esters the components aggregate as
ribbons (Fig. 20b). However, when the substituents are t-butyl groups a cyclic
hexamer is formed (Fig. 20c) [59].
Lehn and coworkers designed a supramolecular cyclic aggregate based on subunits
25 and 27 [60].The butyl substituent on the triaminopyrimidine was not bulky
enough to induce cyclic aggregate formation and instead formed the tape shown in
Fig. 21 (see also the article by A. Nangia and G.R. Desiraju in this volume).
Based on the previous result, Lehn et al. developed an elegant strategy for promoting
the formation of cyclic aggregates based on hydrogen bonding function-