The crystal structure of the 1:1 inclusion complex of β-cyclodextrin with squaric acid

Abstract

The crystal and molecular structure of the 1:1 inclusion complex of β-cyclodextrin (cyclomaltoheptaose) with squaric acid (3,4-dihydroxycyclobutene-1,2-dione) was determined by X-ray diffraction. The complex crystallizes in the monoclinic P2 1 space group and belongs to the monomeric cage-type, characterized by a herringbone-like packing motif. Co-crystallized water molecules are present on seven sites, of which six are fully occupied. The guest molecule is placed inside the β-cyclodextrin cavity, perpendicular to the plane defined by the glycosidic O-4 n atoms, and held in place by direct and water-mediated hydrogen bonds mainly involving symmetry-related β-cyclodextrin molecules. The accommodation of the planar guest molecule into the β-cyclodextrin cavity determines a significant distortion of the latter from the sevenfold symmetry.