Keggin-type polyaluminum cations belong to a unique class of compounds with their large positive charge, hydroxo bridges, and divergent isomerization/oligomerization. Previous reports indicated that oligomerization of this species can only occur through one isomer (δ), but herein we report the isolation of largest Keggin-type cluster that occurs through self-condensation of four ϵ-isomers ϵ-GeAl12 8+ to form [Ge4 O16 Al48 (OH)108 (H2 O)24 ]20+ cluster (Ge4 Al48 ). The cluster was crystallized and structurally characterized by single-crystal X-ray diffraction (SCXRD) and the elemental composition was confirmed by ICP-MS and SEM-EDS. Additional dynamic light scattering experiments confirms the presence of the Ge4 Al48 in thermally aged solutions. DFT calculations reveal that a single atom Ge substitution in tetrahedral site of ϵ-isomer is the key for the formation of Ge4 Al48 because it activates deprotonation at key surface sites that control the self-condensation process.
Keywords: DFT calculations; Keggin clusters; aluminum; polycations; self-condensation.
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