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, [XW₁₁MO₄₀] Keggin-type anions where X = P or Si and M = Mo^V/VI or V^IV/V, has been evaluated by isothermal titration calorimetry (ITC), NMR spectroscopy, and cyclic voltammetry. Such a study reveals that the host-guest binding constant K_1:1 increases strongly with the decrease of the global ionic charge of the POM unit. There is an almost one magnitude order of variation in K_1:1 per charge unit, where K_1:1 falls down from about 10⁵ M^-1 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.