Isomerization of Keggin Al₁₃Ions 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 [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺(Al₁₃), is the focus of this paper.²⁷Al NMR,¹H NMR, and¹H DOSY techniques were used to follow the isomerization of the ϵ-Al₁₃in the presence of glycine and Ca²⁺at 90 °C. Although the conversion of ϵ-Al₁₃to new clusters and/or Baker–Figgis–Keggin isomers has been studied previously, new¹H NMR and¹H DOSY analyses provided information about the role of glycine, the ligated intermediates, and the mechanism of isomerization. New¹H NMR data suggest that glycine plays a critical role in the isomerization. Surprisingly, glycine does not bind to Al₃₀clusters, 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-Al₁₃isomer in solution.