Inclusion compounds of plant growth regulators in cyclodextrins. V. 4-Chlorophenoxyacetic acid encapsulated in beta-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin.

The crystal structures of 4-chlorophenoxyacetic acid (4CPA) included in beta-cyclodextrin (beta-CD) and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMbetaCD) have been studied by X-ray diffraction. The 4CPA/beta-CD complex crystallizes as a head-to-head dimer in the space group C2 in the Tetrad packing mode. The packing modes of some beta-CD dimeric complexes, having unique stackings, are also discussed. The 4CPA/TMbetaCD inclusion complex crystallizes in the space group P2(1) and its asymmetric unit contains two crystallographically independent complexes, complex A and complex B, exhibiting different conformations. The host molecule of complex A is significantly distorted, as a glucosidic residue rotated about the O4'-C1 and C4-O4 bonds forms an aperture where the guest molecule is accommodated. The phenyl moiety of the guest molecule of complex B is nearly perpendicular to the mean plane of the O4n atoms. The conformations of the guest molecules of the two complexes are similar. The crystal packing consists of antiparallel columns as in the majority of the TMbetaCD complexes published so far.