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 [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) (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.

Luminescent Carrageenan Hydrogels Containing Lanthanopolyoxometalates

“…A possible explanation for the above observations is that the K + ions will compensate the negative charge of the sulfate groups, and therefore other interactions between the biopolymer chains and the EuPOM anions can be determinant, namely hydrogen bonding between the octahedra WO 6 and hydroxyl groups of the galactopyranose units. This is in line with observations made by other authors that have pointed out the relevance of hydrophobic interactions between negatively charged POMs and uncharged molecular components, such as in lipids and cyclodextrins , . For example, the adsorption of POMs onto lipid monolayers depends on the interplay between electrostatic and hydrophobic interactions .…”

The Chaotropic Effect as an Assembly Motif in Chemistry

“…The studied chaotropic anions included not only those in Table 1 but also N 3 − , CNO − , SeCN − , IO 4 − , ReO 4 − , and SbF 6 − . Noteworthy, the thermodynamic fingerprint (enthalpically driven processes with a negative entropic contribution) is identical in all cases, regardless of which type of superchaotropic anion and which macrocycle are being used 10, 12, 22, 23, 26, 97, 98. Finally, the inclusion preference of metallo‐macrocyclic cage compounds towards chaotropic anions stands out as well but has not been mechanistically rationalized in comparable detail 142, 143, 152…”

Self-Assembly in Polyoxometalate and Metal Coordination-Based Systems: Synthetic Approaches and Developments

“…An important distinction, though, is that the interior cavity of CDs is hydrophobic, while the exterior side is hydrophilic; due to this architectural conformation, they can incorporate organic and inorganic guest molecules of appropriate size. The main driving force in this case are dipole-dipole, electrostatic forces, hydrogen bonding and van der Waals interactions [46,47]. Very recently, Cadot et al reported the isolation of the 1:1, 1:2, 1:3 adducts, which are formed due to the attractive In a similar approach, cyclodextrins (CDs) are cyclic oligosacharides comprised of 6 to 8 glucose units exhibiting torus-shaped ring structure comparable to the CB moieties discussed above.…”

“…An important distinction, though, is that the interior cavity of CDs is hydrophobic, while the exterior side is hydrophilic; due to this architectural conformation, they can incorporate organic and inorganic guest molecules of appropriate size. The main driving force in this case are dipole-dipole, electrostatic forces, hydrogen bonding and van der Waals interactions [46,47] …”