Observation and Interpretation of Anomalous Inorganic Anion Binding with α- and β-Cyclodextrins in Aqueous Media

Abstract

The binding constants for hexafluorophosphate, perchlorate and triflate with α- and β-cyclodextrins were quantified using calorimetry and fluorescence spectroscopy experiments in aqueous media as well as molecular orbital calculations. The association of α- and β-cyclodextrin with three commonly used counterions of large organic cations or as supporting electrolyte systems, proved to be large enough to produce significant interferences in complexation studies. In particular, the binding constant of hexafluorophosphate and β-cyclodextrin was measured to be ten times larger than the previously reported value. The enthalpies and entropies of complexation of the two receptors with the three anions under study were directly evaluated using calorimetric measurements in aqueous media. The PM3 semiempirical molecular orbital method was employed to rationalize the enhanced binding between β-cyclodexrin and hexafluorophosphate. The computed results from several energy minimizations show that inclusion complexes of β-cyclodextrins have a complicated energy surface with many possible energy minima. The binding of octahedral hexafluorophosphate originates from a loose yet complementary host-guest geometric fit that provides for three strong hydrogen bonds with primary hydroxyls of β-cyclodextrin.