The Structure of the Cyclodextrin Complex. X. Crystal Structure of α-Cyclodextrin-Benzaldehyde (1 : 1) Complex Hexahyderate

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Abstract
The crystal structure of the α-cyclodextrin–benzaldehyde (1:1) complex hexahydrate, C36H60O30·C7H6O·6H2O, was determined by the X-ray method. The crystal is monoclinic, the space group being P21 with Z=2. The cell dimensions are a=7.932(1), b=13.500(1), c=24.704(2) Å, and β=90.85(1)°. The structure was solved by means of a Patterson map and a trial-and-error method combined with the rigid-body least-squares technique. Refinement was carried out by the block-diagonal least-squares method to the final R-value of 0.057 for 4565 reflections. α-Cyclodextrin molecules are stacked along the a axis in a head-to-tail fashion to form a channel-type structure. The α-cyclodextrin ring is tilted by an angle of 11.5° against the channel axis. Adjacent α-cyclodextrin molecules along the channel are linked by hydrogen bonds between the primary hydroxyl groups and the secondary hydroxyl groups, and O(2)···water···O(6) hydrogen-bond bridges. The guest benz-aldehyde molecules are aligned inside the α-cyclodextrin column. The benzene ring is inserted into the α-cyclodextrin ring from the secondary hydroxyl side, while the carbonyl group is in the van der Waals contact with the primary hydroxyl side of the next α-cyclodextrin. The guest plane is nearly parallel to the channel axis, making an angle of 71.2° against the plane through six glycosidic oxygen atoms. Spaces between α-cyclodextrin columns are filled with water molecules. Many hydrogen bonds are observed among hydroxyl groups of α-cyclodextrin and water molecules, forming a hydrogen-bond network in the crystal.

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