Tunable Keplerate Type-Cluster "Mo132 " Cavity with Dicarboxylate Anions

Area of Science:

• Inorganic Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Keplerate-type polyoxometalates, such as Mo132, represent unique spherical inorganic structures with internal cavities.
• Internal functionalization of these capsules offers opportunities for novel host-guest chemistry and material properties.
• Understanding ligand binding and guest encapsulation is crucial for designing functional molecular systems.

Purpose of the Study:

• To achieve internal functionalization of the Keplerate-type Mo132 capsule via ligand exchange.
• To characterize the structure and binding behavior of dicarboxylate ligands (glutarate, succinate) within the Mo132 cavity.
• To investigate the influence of encapsulated guests, specifically ammonium cations, on the Mo132 system's properties.

Main Methods:

• Ligand exchange reactions to introduce glutarate and succinate into the Mo132 capsule.
• Solution Nuclear Magnetic Resonance (NMR) spectroscopy, including 1D, 2D DOSY, and EXSY experiments.
• Single-crystal X-ray diffraction analysis.
• Thermodynamic studies to determine binding constants (KL) and associated parameters (ΔrS*, ΔrH*).

Main Results:

• Formation of glutarate- and succinate-containing Mo132 species as ammonium or dimethylammonium salts.
• NMR and X-ray diffraction confirmed asymmetric inner dicarboxylate ion disposition, with one carboxylate group attached to the shell and the other towards the cavity center.
• Binding constants (KL) for acetate, succinate, and glutarate ligands were determined, showing variation attributed to entropic gain and steric effects.
• DOSY and EXSY NMR experiments indicated firm trapping of bulky guests within the cavity.
• Variable temperature NMR revealed an abrupt line broadening below 290 K, attributed to an inner-phase transition induced by encapsulated ammonium cations.

Conclusions:

• Internal functionalization of Mo132 with dicarboxylates is feasible, leading to well-defined supramolecular structures.
• The binding affinity of ligands is influenced by a balance of entropic and steric factors.
• Encapsulated ammonium cations can induce significant changes in the Mo132 system's dynamics, leading to phase transitions.