Cage-type packing structure, inclusion complexes

Hexakis(2,6-di-0-methyl)-a-CyD complexes with a small guest molecule, such as iodine and 1-propanol, crystallize with the cage-type packing structure [185]. Compared with the structure of the corresponding -CyD complex, the guest molecules in the both complexes are shifted to the secondary hydroxyl side from the center of the cavity. 3-Iodopropionic acid [186], m-nitroaniline [187], and acetonitrile [188] are also fully accommodated in the host cage . A 3-0 acetylated host, hexakis(2,6-di-0-methyl-3-0-acetyl)-a-CyD, was crystallized from butylacetate [189]. In spite of the disruption of intramolecular hydrogen bonds, the host molecule is in a round shape because of the inclusion of butylacetate. 

The packing of each molecule in the crystal is shown in Fig. 3. The 3-CyD dimers are packed in the manner of the brickwork pattern which is one of the cage type packing structures and has been frequently seen in the 3-CyD inclusion complex structure of the triclinic space group PI. 

Cyclodextrins are well known [1] to form a number of crystalline adducts or inclusion complexes with a variety of substances. In the crystalline lattice, cyclodextrins form two types of packing One is a cage type where the cavity of one cyclodextrin molecule is closed on both sides by ad jacent molecules. The other is a channel-type, in which the cyclodextrin rings are packed on top of each other to produce cylinders with infinite centralcavity. Formation of one or the other type crystal structure depends on the size and molecular character of the guest molecule. 

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