The Structure of the Cyclodextrin Complex. VIII. Crystal Structures of α-Cyclodextrin Complexes with 2-Pyrrolidone and N,N-Dimethylformamide

Crystal structures of α-cyclodextrin complexes with N,N-dimethylformamide and 2-pyrrolidone were determined by the X-ray method. The pentahydrated crystals of both complexes are orthorhombic, and the space group is P212121 with Z=4. The cell dimensions are a=13.750(1), b=15.318(1), and c=24.544(2) Å for the N,N-dimethylformamide complex, and a=13.852(1), b=15.373(1), and c=24.353(2) Å for the 2-pyrrolidone complex. The structures were determined by assuming the same α-cyclodextrin packing in the crystal as that found in the isomorphous p-nitrophenol complex, and refined by the block-diagonal least-squares method to the final R-values of 0.054 for the N,N-dimethylformamide complex (5151 reflections) and 0.051 for the 2-pyrrolidone complex (5165 reflections). In both complexes, the guest molecules are included in the α-cyclodextrin cavity. The N,N-dimethylformamide molecule is in van der Waals contact with the host α-cyclodextrin molecule, while the 2-pyrrolidone molecule forms a weak N–H…O hydrogen bond with a glycosidic oxygen atom of α-cyclodextrin. The carbonyl oxygen atom of N,N-dimethylformamide and 2-pyrrolidone is hydrogen-bonded to water molecules of hydration located at the primary hydroxyl side of α-cyclodextrin. The tilt-angle of the glucose residue against the plane through six glycosidic oxygen atoms indicates clearly that the macro-cyclic conformation of α-cyclodextrin is affected by the packing as well as the size and shape of the guest molecule. The α-cyclodextrin molecules in the crystal are arranged parallel to the ac plane to form molecular layers. Three water molecules are located near the guest molecule, while the other two water molecules fill the intermolecular space.