Complexation of polyoxometalates with cyclodextrins

Abstract

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 [PMo<inf>12</inf>O<inf>40</inf>]^3- trianion - which has led to the formation of two organic-inorganic hybrid 2:1 complexes, namely [La(H<inf>2</inf>O)<inf>9</inf>]{[PMo<inf>12</inf>O<inf>40</inf>]âŠ⊂[γ-CD]<inf>2</inf>} (CD-POM-1) and [La(H<inf>2</inf>O)<inf>9</inf>] {[PMo<inf>12</inf>O<inf>40</inf>]âŠ⊂[β-CD]<inf>2</inf>} (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 [PMo<inf>12</inf>O<inf>40</inf>]^3- trianion is encapsulated by the primary faces of two CD tori through intermolecular [C-H···Oî - 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<inf>12</inf>O<inf>40</inf>]^3- trianions encapsulated by CDs in the complexes are largely preserved in solution. The supramolecular complementarity between the CDs and the [PMo<inf>12</inf>O<inf>40</inf>]^3- trianion provides yet another opportunity for the functionalization of POMs under mild conditions by using host-guest chemistry.