Chaotropic Effect-Driven Selective Anion Recognition: Exo-Binding of Cyclohexanocucurbit[5,6]uril with Hexafluorophosphate

Macrocycle cyclohexanocucurbit[6]­uril (Cy6Q[6]) is capable to be rapidly precipitated with hexafluorophosphate (PF6 –) anion in the presence of a mixture of inorganic anions, while its homologue cyclohexanocucurbit[5]­uril (Cy5Q[5]) can selectively bind with the PF6 – anion to form co-crystallization during the water evaporation process. The rapid and spontaneous co-precipitation between Cy6Q[6] and PF6 – could be exploited to separate PF6 – anion from aqueous solution. X-ray crystallographic analysis of the compounds Cy6Q[6]·2KPF6 and Cy5Q[5]·2KPF6 indicates that the instantaneous co-precipitation and the gradual co-crystallization between the macrocycles and the PF6 – anion are driven by the chaotropic effect. We also found that the supramolecular crystalline materials of the compounds Cy6Q[6]·2KPF6 and Cy5Q[5]·2KPF6 exhibit pale blue fluorescence under UV light.