Pseudo-polymorphism of a camphor α-cyclodextrin complex

Crystalline products of vapor diffusion of an alcoholic solution of 1R,4R-camphor into an aqueous solution of α-cyclodextrin were prepared and subjected to X-ray structure analysis. Two different forms were obtained: a dimeric cavity formed by head-to-head association of cyclodextrin (phase A), and a complex set of monomeric cavities (phase B). Both contain camphor molecules orientationally disordered inside cavities. The structures are solvated by mixtures of water and ethanol. The structures significantly differ in chemical stabilities. Whilst phase A is relatively stable with respect to guest desorption, phase B rapidly reacts to change of its chemical environment manifested by cracking of crystals in solution. The phenomenon has been recorded and a short movie is included in the supplementary data. [Display omitted] •A novel crystalline modification of an inclusion complex containing camphor in α-cyclodextrin has been prepared and structurally characterized. The structure is highly solvated by a mixture of ethanol and water. A Large unit cell contains six molecules of the complex not related by crystal symmetry.•The structure readily exchanges solvent molecules when submitted to modified external conditions, thus behaving like an organic zeolite. In particular, when crystals in their mother solution are exposed to open air, alcohol molecules evaporate first leaving the solution enriched with water. Fast equilibrium with the solid leads to modification of the absorbed solvent (more water) which, in turn, makes structural changes (lattice contraction). This may be observed visually as cracking of the crystals, a phenomenon known in clathrate chemistry. This result is documented by microscope images and (in Supplementary Data) by short movies. The same effect takes place on adding water to the mother solution (documented by photographs and movies).•Novel structure of ‘open’ phase determined in mother solution is reported.