Crystal Structures of β -Cyclodextrin Complexes with Formic Acid and Acetic Acid (original) (raw)

β-Cyclodextrin (β-CD)-formic acid (1) and β-CD-acetic acid (2) inclusion complexes crystallize as β-CD·0.3HCOOH·7.7H 2 O and β-CD·0.4CH 3 COOH·7.7H 2 O in the monoclinic space group P 2 1 with comparable unit cell constants. Anisotropic refinement of atomic parameters against X-ray diffraction data with F 2 o > 2σ (F 2 o ) (986/8563 and 991/8358) converged at R-factors of 0.051 and 0.054 for 1 and 2, respectively. In both complexes, the β-CD molecular conformation, hydration pattern and crystal packing are similar, but the inclusion geometries of the guest molecules are different. The β-CD macrocycles adopt a "round" conformation stabilized by intramolecular, interglucose O3(n)· · ·O2(n + 1) hydrogen bonds and their O6-H groups are systematically hydrated by water molecules. In the asymmetric unit, each complex contains one β-CD, 0.3 formic acid (or 0.4 acetic acid), and 7.7 water molecules that are distributed over 9 positions. Water sites located in the β-CD cavity hydrogen bond to the guest molecule. In the crystal lattice, β-CD molecules are packed in a typical "herringbone" fashion. In 1, the formic acid (occupancy 0.3) is entirely included in the β-CD cavity such that its C atom is shifted from the O4-plane center to the β-CD O6-side by 2.90 Å and C=O, C-O bonds point to this side. In 2, the acetic acid (occupancy 0.4) is completely embedded in the β-CD cavity, in which the carboxylic C atom is displaced from the O4-plane center to the β-CD O6-side by 0.87 Å; the C=O bond directs to the β-CD O6-side and makes an angle of 15 • to the β-CD molecular axis. Furthermore, both dimethyl-β-CD-acetic acid and β-CD-acetic acid complexes form a cage structure, showing that the small guests enclosed entirely in the cavity either in β-CD or in dimethyl-β-CD do not affect the packing of the host macrocycles.