A new crystal form of β-cyclodextrin–ethanol inclusion complex: channel-type structure without long guest molecules Original Research Article

A new crystal form of β-cyclodextrin (β-CD)–ethanol–dodecahydrate inclusion complex [(C6H10O5)7·0.3C2H5OH·12H2O] belongs to monoclinic space group C2 (form II) with unit cell constants a=19.292(1), b=24.691(1), c=15.884(1) Å, β=109.35(1)°. The β-CD macrocycle is more circular than that of the complex in space group P21 [form I: J. Am. Chem. Soc. 113 (1991) 5676]. In form II, a disordered ethanol molecule (occupancy 0.3) is placed in the upper part of β-CD cavity (above the O-4 plane) and is sustained by hydrogen bonding to water site W-2. In form I, an ethanol molecule located below the O-4-plane is well ordered because it hydrogen bonds to surrounding O-3H, O-6H groups of the symmetry-related β-CD molecules. In the crystal lattice of form I, β-CD macrocycles are stacked in a typical herringbone cage structure. By contrast, the packing structure of form II is a head-to-head channel that is stabilized at both O-2/O-3 and O-6 sides of each β-CD by direct OCD⋯OCD and indirect OCD⋯OW⋯(OW)⋯OCD hydrogen bonds. The 12 water molecules are disordered in 18 positions both inside the channel-like cavity of β-CD dimer (W-1W-6) and in the interstices between the β-CD macrocycles (W-7W-18). The latter forms a cluster that is hydrogen bonded together and to the neighboring β-CD OH groups.