Complexation of Polyoxometalates with Cyclodextrins

摘要

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-cydodextrins (γ- and β-CDs) with an archetypal polyoxometalate (POM)-namely, the [PMo_(12)O_(40)]~(3-) tri-anion-which has led to the formation of two organic-inorganic hybrid 2:1 complexes, namely [La(H_2O)_9]-{[PMo_(12)O_(40)] is contained in [γ-CD]_2} (CD-POM-1) and [La(H_2O)_9] {[PMo_(12)O_(40)] is contained in [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, ~(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 [PMo_(12)O_(40)]~(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 [PMo_(12)O_(40)]~(3-) trianions encapsulated by CDs in the complexes are largely preserved in solution. The supramolecular complementarity between the CDs and the [PMo_(12)O_(40)]~(3-) trianion provides yet another opportunity for the functionalization of POMs under mild conditions by using host-guest chemistry.