Polymorphism in β-cyclodextrin-benzoic acid inclusion complex:: a kinetically controlled crystal growth according to the Ostwald's rule

Crystal form II of the beta-cyclodextrin-benzoic acid (beta-CD-BA) inclusion complex was obtained from the 1.5-year stored aqueous EtOH solution of beta-CD and BA as 2 beta-CD center dot 1.5BA center dot 0.7EtOH center dot 21H(2)O in the monoclinic space group C2 with unit cell constants: a = 19.413(3), b = 24.306(4), c = 32.975(1) angstrom, beta = 104.41(1)degrees. By contrast, the desired crystal form I in the triclinic space group P1 that ever grew up from the fresh solution as 2 beta-CD center dot 2BA center dot 0.7EtOH center dot 20.65H(2)O was not reproducible any more [Aree, T.; Chaichit, N. Carbohydr. Res. 2003, 338, 439-446]. In the two crystal forms, beta-CDs are isostructural with a 'round' conformation stabilized by intramolecular O-3(n)center dot center dot center dot O-2(n + 1) hydrogen bonds. The BA inclusion geometries are similar with a preferred orientation, that is, BAs are situated above the O-4 plane, point their COOH groups to the beta-CD O-6 side, incline 52 degrees with respect to the O-4 plane and are mainly maintained in positions by hydrogen bonding with the surrounding water molecules. beta-CDs form dimers as structural motif of different packing modes: the screw-channel type in form II and the average of intermediate and tetrad types in form I. Polymorphism in the beta-CD-BA inclusion complex is a kinetically controlled crystal growth following the Ostwald's rule: the less stable crystal form I grew up first within one week from the fresh solution, whereas the more stable crystal form II appeared after 1.5-year storage. (C) 2008 Elsevier Ltd. All rights reserved.