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 gamma- and beta-cyclodextrins (gamma- and beta-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]subset of[gamma-CD](2)} (CD-POM-1) and [La(H2O)(9)] {[PMo12O40]subset of[beta-CD](2)} (CD-POM-2), in the solid state. The extent to which these complexes assemble in solution has been investigated by (i) H-1, C-13, and P-31 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 center dot center dot center dot 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 center dot center dot center dot 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.