Host-Guest Complexation Between Cyclodextrins and Hybrid Hexavanadates: What are the Driving Forces?

Area of Science:

• Supramolecular Chemistry
• Coordination Chemistry
• Materials Science

Background:

• Cyclodextrins (CDs) are macrocyclic hosts known for forming inclusion complexes.
• Hybrid polyoxovanadates (POVs) are novel inorganic-organic hybrid materials with tunable properties.
• Understanding host-guest interactions is crucial for designing functional supramolecular systems.

Purpose of the Study:

• To investigate host-guest complex formation between native cyclodextrins (α-, β-, γ-CD) and functionalized Lindqvist-type polyoxovanadates (POVs).
• To elucidate the influence of the POV's functional group (R) on complex conformation, crystal packing, and binding thermodynamics in aqueous solution.
• To determine the driving forces governing the association between CDs and POVs.

Main Methods:

• Single-crystal X-ray diffraction for solid-state structural analysis.
• Isothermal Titration Calorimetry (ITC) for thermodynamic binding studies.
• Cyclic Voltammetry and Nuclear Magnetic Resonance (NMR) spectroscopy (1D 1H, 2D DOSY, ROESY) for solution-state interaction analysis.

Main Results:

• Six crystal structures revealed functional group (R) embedding within CD cavities with weak contacts.
• Solution studies demonstrated a dramatic influence of the R group on POV supramolecular properties, with association constants (K1:1) ranging from 0 to 2000 M⁻¹.
• Binding affinity of isolated functional groups to CDs was very weak.

Conclusions:

• The nature of the functional group on POVs dictates host-guest complex conformation and binding strength.
• Driving forces vary with cyclodextrin size: chaotropic effects dominate for β- and γ-CD, while hydrophobic effects are key for α-CD.
• The ability of the macrocyclic host to desolvate exposed POV moieties is the fundamental origin of the observed host-guest association.