Host properties of alpha-cyclodextrin and a water-soluble calix[6]arene probed with dimeric bipyridinium guests

The complexation properties of two complementary hosts, a-cyclodextrin and a water-soluble sulfonated calixarene, with dimeric viologen guests, in which the viologen units are linked by an undecyl chain, have been investigated. While the complexation behavior of the cyclodextrin host is driven by hydrophobic interactions, that of the calixarene is largely electrostatic in nature. Both hosts show high affinity for the viologen guests, but for completely different reasons. H-1 NMR spectroscopic evidence demonstrates the inclusion complexation of the alkyl chain connecting the two viologen subunits by the hydrophobic cyclodextrin cavity. This interaction is strongly pH dependent, and the complexation can be switched by protonating the two unquaternized nitrogen atoms of the guest, thereby making it more hydrophilic and resulting in partial unthreading of the cyclodextrin host. Calorimetric data shows the association constant to be quite high and the complexation to be enthalpically driven, in accord with nonclassical hydrophobic interactions. Cyclic voltammetric experiments demonstrate the solubilization of the reduced viologen form by the cyclodextrin, lessening its deposition on the electrode surface. The strong electrostatically driven association between the cationic viologen guests and the hexaanionic calixarene host was also examined. The binding constants determined from NMR data were on the order of 10(3) M-l. The data suggest that these complexes are also of the inclusion type. Unlike alpha-CD, calixarene complexation makes the guests more prone to deposit on the electrode surface during voltammetric experiments.