Endo/exo binding of alkyl and aryl diammonium ions by cyclopentanocucurbit[6]uril

The binding interactions of CyP(6)Q[6] with a series of alkyl diammonium (+H3N(CH2)(n)NH3+, n = 2, 4, 6, 8, 10, 12) and aryl diammonium (p-phenylenediammonium and p-xylenediammonium) ions both in aqueous solution and in the solid state have been studied by H-1 NMR spectroscopy, X-ray crystallography, and isothermal titration calorimetry (ITC) techniques. H-1 NMR data indicate that all guests exhibit endo binding with CyP(6)Q[6] except for the ethanediammonium ion, which bound exo to CyP(6)Q[6]. p-Xylenediammonium ions show mixed behavior (exo and endo binding with CyP(6)Q[6]). X-ray crystallography clearly displays the extended and contorted conformations of the guests when bound within CyP(6)Q[6]. The ITC study points out that the CyP(6)Q[6] complex formations with all guests are mainly driven by enthalpy, which arises from the ion-dipole interactions and the hydrophobic effects.