Redox-Responsive Host-Guest Association between γ-Cyclodextrin and Mixed-Metal Keggin-Type Polyoxometalates.

The complexation of Keggin-type polyoxometalates (POMs) with γ-cyclodextrin (γ-CD) leads to supramolecular inclusion assemblies in aqueous solution driven by a chaotropic effect. The strength of the interaction between γ-CD and oxidized or one-electron reduced POMs in a series of molybdenum and vanadium monosubstituted phospho- and silico-tungstates, [XWMO] Keggin-type anions where X = P or Si and M = Mo or V, has been evaluated by isothermal titration calorimetry (ITC), NMR spectroscopy, and cyclic voltammetry. Such a study reveals that the host-guest binding constant increases strongly with the decrease of the global ionic charge of the POM unit. There is an almost one magnitude order of variation in per charge unit, where falls down from about 10 M to values close to zero as ionic charge varies from 3- to 6-. Such POMs with molybdenum and vanadium addenda offer the possibility of tuning the host-guest association strength by the simple addition/removal of one electron to POMs, opening a new avenue for the design of smart materials through redox stimuli.