Formation of the [Ge4O16Al48(OH)108(H2O)24]20+ Tetramer from Condensation of ε-GeAl12 Keggin Polycations

Keggin-type polyaluminum cations belong to a unique class of polyoxometalates (POMs) with their large positive charge, hydroxo bridges, and divergent isomerization/oligomerization. Previously reported oligomerizations of the polyaluminum cations were driven solely by the δ-Keggin isomer, which created Al₂₆, Al₃₀, and Al₃₂ dimeric species. We herein report the isolation of largest ever Keggin-type structure for this system through a unique mode of self-condensation among four ε-GeAl₁₂⁸⁺ to form [NaGe₄O₁₆Al₄₈(OH)₁₀₈(H₂O)₂₄]²¹⁺(Ge₄Al₄₈). Elemental analysis confirms the Ge⁴⁺ substitution, and dynamic light scattering experiments indicated that these larger species exist in the thermally aged solutions. DFT calculations have revealed that a single atom Ge substitution in tetrahedral site of ε-Al₁₃⁷⁺ is the key for the formation this cluster because it activates the deprotonation at certain octahedral sites to assist self-condensation in a specific mode.