Metal Substitution in Keggin-Type Tridecameric Aluminum-Oxo-Hydroxy Clusters

Area of Science:

• Inorganic Chemistry
• Materials Science
• Solution Chemistry

Background:

• The Keggin tridecamer, Al13 ([AlO4Al12(OH)24(OH2)12]7+), is a fundamental polyoxometalate species.
• Understanding metal substitution in Al13 is crucial for developing novel materials with tailored properties.
• Previous studies suggested geometric factors influence the stability of metal-substituted Al13 hybrids.

Purpose of the Study:

• To examine the formation and stability of metal-substituted Al13 hybrids.
• To determine which metal ions can be incorporated into the Al13 structure under specific preparation conditions.
• To compare the structural and crystallographic properties of the Al13 and GaAl12 hybrids.

Main Methods:

• Synthesis of metal-substituted Al13 precursors via forced hydrolysis.
• Monitoring of Al13 formation and polymerization using 27Al NMR spectroscopy.
• Characterization of precipitated sulfate salts using powder X-ray diffraction (XRD) and solid-state 27Al NMR.
• Single-crystal XRD analysis of Al13 and GaAl12 sulfate salts.

Main Results:

• Al13 units polymerize to form AlP2 upon aging at 80°C (k = 4.8 × 10^-2 h^-1).
• Gallium (Ga3+) and tin (Sn2+) ions accelerated Al13 conversion rates.
• Only the GaAl12 hybrid was successfully prepared and characterized by XRD and NMR; its crystal structure is cubic (a = 17.869 Å) and highly similar to Al13.
• The enhanced thermal stability of GaAl12 could not be explained by geometric considerations alone.

Conclusions:

• The preparation method is effective for synthesizing the GaAl12 hybrid.
• The structural similarity between Al13 and GaAl12 suggests subtle electronic or bonding differences contribute to GaAl12's stability.
• Further investigation is needed to understand the factors governing the stability of metal-substituted polyoxometalates.