Single-Crystal-to-Single-Crystal Cluster Transformation in a Microporous Molybdoarsenate(V)-Metalorganic Framework

Area of Science:

• Inorganic Chemistry
• Materials Science
• Crystallography

Background:

• Hybrid organic-inorganic compounds offer tunable properties for advanced applications.
• Polyoxometalates and metal-organic frameworks are key building blocks in materials design.
• Understanding structural transformations in crystalline materials is crucial for developing functional materials.

Purpose of the Study:

• To synthesize and characterize a novel hybrid compound incorporating copper-cyclam and molybdoarsenate units.
• To investigate the structural transformations upon dehydration and their impact on material properties.
• To explore the potential applications of the resulting material, particularly in gas adsorption.

Main Methods:

• Synthesis of the hybrid compound [Cu(cyclam)(H2O)2]0.5[{Cu(cyclam)}1.5{B-H2As2Mo6O26(H2O)}]·9H2O (1) in aqueous solution.
• Single-crystal X-ray diffraction to determine the structures of the initial and dehydrated phases.
• Thermal gravimetric analysis to study solvent evacuation and phase transitions.
• Electron paramagnetic resonance (EPR) spectroscopy to probe structural and electronic changes.
• Gas adsorption experiments (CO2/N2) to evaluate selective adsorption properties.

Main Results:

• The hybrid compound 1 exhibits a supramolecular open-framework structure with squarelike channels hosting water molecules.
• Dehydration under vacuum leads to a single-crystal-to-single-crystal transformation to anhydrous phase 2, [{Cu(cyclam)}2(A-H2As2Mo6O26)].
• This transformation involves an unprecedented solid-state B to A isomerization of the polyoxoanion and changes in Cu(II) bonding.
• The resulting microporous material 2 demonstrates selective adsorption of CO2 over N2.

Conclusions:

• The synthesized hybrid compound undergoes a unique single-crystal-to-single-crystal transformation upon dehydration, revealing a novel isomerization pathway.
• The permanent microporosity of the dehydrated phase makes it a promising candidate for selective gas separation applications.
• This study highlights the potential of designing functional hybrid materials through controlled structural modifications.