Polyoxometalate, Cationic Cluster, and γ‑Cyclodextrin: From Primary Interactions to Supramolecular Hybrid Materials

Herein, we report on a three-component
supramolecular hybrid system
built from specific recognition processes involving a Dawson-type
polyoxo­metalate (POM), [P2W18O62]6–, a cationic electron-rich cluster [Ta6Br12­(H2O)6]2+,
and γ-cyclodextrin (γ-CD). Such materials have been investigated
using a bottom-up approach by studying the specific interactions between
γ-CD and both types of inorganic units. Their ability to interact
has been investigated in the solid state by single-crystal X-ray diffraction
(XRD) and in solution using multinuclear NMR methods (including DOSY,
EXSY, and COSY), electrospray ionization mass and UV–vis spectroscopies,
electrochemistry, and isothermal titration calorimetry experiments.
Single-crystal XRD analysis reveals that POM:γ-CD constitutes
a highly versatile system which gives aggregates with 1:1, 1:2, and
1:3 stoichiometry. Surprisingly, these arrangements exhibit a common
feature wherein the γ-CD moiety interacts with the Dawson-type
POMs through its primary face. We present also the first structural
model involving an octahedral-type metallic cluster with γ-CD.
XRD study reveals that the cationic [Ta6Br12­(H2O)6]2+ ion is closely embedded
within two γ-CD units to give a supramolecular ditopic cation,
suitable to be used as a linker within extended structure. Solution
study demonstrates clearly that pre-associations exist in solution,
for which binding constants and thermodynamic parameters have been
determined, giving preliminary arguments about the chaotropic nature
of the inorganic ions. Finally, both building blocks, i.e., the ditopic
supramolecular cation {[Ta6Br12­(H2O)6]­@2CD}2+ and the Dawson-type
anion, react together to give a three-component, well-ordered hybrid
material derived either as a supramolecular hydrogel or single crystals.
The solid-state structure shows an unprecedented helicoidal tubular
chain resulting from the periodic alternation of POM and supramolecular
cation, featuring short hydrogen-bonding contacts between the electron-poor
POM and electron-rich cluster. The 1D tubular ionic polymer observed
in the single crystals should make it possible to understand the long-range
ordering observed within the hydrogel hybrid material. The supramolecular
chemical complementarities between the γ-CD-based ditopic cation
and POM open a wide scope for the design of hybrid materials that
accumulate synergistic functionalities.