Mechanism of selective and unselective enclathration by a host compound possessing open, flexible host frameworks

Molecular recognition of o-, m-, and p-xylenes (oX, mX, and pX) through enclathration of cholic acid (CA) is described. All of the xylenes give lattice inclusion crystals with CA, and crystallographic studies reveal that they are included in different open host frameworks. In particular, oX has two polymorphs, depending on,the recrystallization temperatures. Competitive recrystallization from mixtures of xylenes resulted in selective enclathrations and the formation of racemic mixed crystals. In the presence of an equimolar amount of oX, CA selectively includes mX or pX in the host frameworks, which are identical to those obtained from the pure mX or pX, respectively. The low affinity of oX is explained in terms of a lower stability of CA.oX than of the other two complexes, as judged from the low PCcavity, the volume ratio of the guest compound to the host cavity. Mean-while, mixtures of mX and pX yield inclusion crystals that accommodate both of the guests. These have the same open host framework as obtained from pure mX, and the guest components are disordered statically in the host cavity. The ratios of the xylene mixtures in the single crystals are similar to those in the original recrystallization mixtures, and also in the bulk crystals, indicating that CA forms mixed crystals of mX and pX. This non-selectivity is attributed to the similar stabilities of CA.mX and CA.pX, according to the moderate PCcavity. The inclusion behavior of CA from mixtures of xylenes is quite similar to chiral recognition by diastereomer-salt methods. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).