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