Mixed behavior of p-phenylenediaminium guest binding with the inverted cucurbit[6]uril host

The binding interaction between inverted cucurbit[6]uril (iQ[6]) and p-phenylenediaminium both in the solid state and in aqueous solution has been investigated by X-ray crystallography, (1)H NMR spectroscopy and isothermal titration calorimetry (ITC). The experimental results suggest that the binding interaction between iQ[6] and p-phenylenediaminium may generate two different types of complexes in the solid state: one is an inclusion structure and the other is a sandwich structure. In aqueous solution, the iQ[6] host has an ability to accommodate the p-phenylenediaminium guest and in-out guest exchange is fast on the NMR time scale. ITC data indicate that the complexation of iQ[6] with p-phenylenediaminium is entropy driven.