The role of cation•••π interactions in capsule formation: co-crystals of resorcinarenes and alkyl ammonium salts

An unprecented dimeric capsular assembly of a tetramethylated C-hexyl resorcinarene and an expected one from unsubstituted C-butyl resorcinarene with tetramethylammonium cation are described. Surprisingly tetramethylated C-hexyl resorcinarene, with no apparent possibility for intra-capsular hydrogen bonds, forms a capsule which is held together solely by the cation center dot center dot center dot pi interactions and the complementary geometry of the spherical guest cation and the concave resorcinarene host. The C-butyl resorcinarene capsule, as in the case of dimeric resorcinarene capsules reported earlier, is mediated via solvent molecules and intra-capsular hydrogen bonds. We also report here two co-crystals of C-methyl resorcinarene and one of C-(3-hydroxy) propyl resorcinarene with R-2, R'(2)-type of alkyl ammonium halide salts. These structures manifest the difficulty of structure control and prediction due to competing and multiple interactions in the crystalline state. The design of the resorcinarene-based host-guest system so as to enable prediction or inducement of a specific crystal packing or structural arrangement of host and guest, is very difficult, if not impossible due to the cameleon nature of resorcinarenes in their ability to form weak interactions.