Polymorphism in beta-cyclodextrin-benzoic acid inclusion complex: a kinetically controlled crystal growth according...

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 2beta-CD.1.5BA.0.7EtOH.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)A, 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 2beta-CD.2BA.0.7EtOH.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)cdots, three dots, centeredO-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.