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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) 9PMo12O40⊂γ-CD2 (CD-POM-1) and La (H2O) 9 PMo12O40⊂β-CD2 (CD-POM-2), in the solid state. The extent to which these complexes assemble in solution has been investigated by (i) (1) H, (13) C, and (31) P 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.
Wu et al. (Tue,) studied this question.