Complexation of Polyoxometalates with Cyclodextrins

Although complexation of hydrophilic guests inside the cavities of hydrophobic hosts is considered to be unlikely, we demonstrate herein the complexation between γ- and β-cyclodextrins (γ- and β-CDs) with an archetypal poly­oxo­metalate (POM)namely, the [PMo12O40]3– tri­anion​which has led to the formation of two organic–inorganic hybrid 2:1 complexes, namely [La­(H2O)9]­{[PMo12O40]⊂​​[γ-CD]2} (CD-POM-1) and [La­(H2O)9] {[PMo12O40]⊂​​[β-CD]2} (CD-POM-2), in the solid state. The extent to which these complexes assemble in solution has been investigated by (i) 1H, 13C, and 31P NMR spectroscopies and (ii) small- and wide-angle X-ray scattering, as well as (iii) mass spectrometry. Single-crystal X-ray diffraction reveals that both complexes have a sandwich-like structure, wherein one [PMo12O40]3– trianion is encapsulated by the primary faces of two CD tori through intermolecular [C–H···OMo] interactions. X-ray crystal superstructures of CD-POM-1 and CD-POM-2 show also that both of these 2:1 complexes are lined up longitudinally in a one-dimensional columnar fashion by means of [O–H···O] interactions. A beneficial nanoconfinement-induced stabilizing effect is supported by the observation of slow color changes for these supermolecules in aqueous solution phase. Electrochemical studies show that the redox properties of [PMo12O40]3– trianions encapsulated by CDs in the complexes are largely preserved in solution. The supramolecular complementarity between the CDs and the [PMo12O40]3– trianion provides yet another opportunity for the functionalization of POMs under mild conditions by using host−guest chemistry.