Isomerization of Keggin Al₁₃ Ions Followed by Diffusion Rates

The solution chemistry of aluminum has long interested scientists due to its relevance to materials chemistry and geochemistry. The dynamic behavior of large aluminum-oxo-hydroxo clusters, specifically [Al₁₃ O₄ (OH)₂₄ (H₂ O)₁₂ ]⁷⁺ (Al₁₃ ), is the focus of this paper. ²⁷ Al NMR, ¹ H NMR, and ¹ H DOSY techniques were used to follow the isomerization of the ϵ-Al₁₃ in the presence of glycine and Ca²⁺ at 90 °C. Although the conversion of ϵ-Al₁₃ to new clusters and/or Baker-Figgis-Keggin isomers has been studied previously, new ¹ H NMR and ¹ H DOSY analyses provided information about the role of glycine, the ligated intermediates, and the mechanism of isomerization. New ¹ H NMR data suggest that glycine plays a critical role in the isomerization. Surprisingly, glycine does not bind to Al₃₀ clusters, which were previously proposed as an intermediate in the isomerization. Additionally, a highly symmetric tetrahedral signal (δ=72 ppm) appeared during the isomerization process, which evidence suggests corresponds to the long-sought α-Al₁₃ isomer in solution.