Isomerization of Keggin Al₁₃ Ions Followed by Diffusion Rates

Abstract

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 [Al O (OH) (H O) ] (Al ), is the focus of this paper. Al NMR, H NMR, and H DOSY techniques were used to follow the isomerization of the ϵ-Al in the presence of glycine and Ca at 90 °C. Although the conversion of ϵ-Al to new clusters and/or Baker-Figgis-Keggin isomers has been studied previously, new H NMR and H DOSY analyses provided information about the role of glycine, the ligated intermediates, and the mechanism of isomerization. New H NMR data suggest that glycine plays a critical role in the isomerization. Surprisingly, glycine does not bind to Al 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-Al isomer in solution.

27Al NMR diffusometry of Al13 Keggin nanoclusters

“…Although this is the first 27 Al PFGSTE NMR measurement of the ϵ-Al 13 Keggin, the 27 Al diffusion measurements can be compared with: (i) 1 H PFGSTE NMR measurements of the diffusion of ϵ-Al 13 Keggin clusters by tracking chelating organic molecules, [35] (ii) dynamic light scattering of ϵ-Al 13 Keggins, [39][40][41] and (iii) theoretical calculations of the diffusivity of ϵ-Al 13 Keggins using boundary element-based methods.…”

“…In the first comparison, the binding of glycinate to the AlO 6 shell of the ϵ-Al 13 Keggin in a deuterium oxide solution was described in others' work to quantify the diffusion of ϵ-Al 13 . [35] Unbound glycinate exhibits slow exchange with glycinate bound to ϵ-Al 13 Keggin clusters. Using Equation 1, the calculated hydrodynamic radius for ϵ-Al 13 Keggins functionalized with glycinate ligands is 0.9 ± 0.1 nm, [35] which is in fair agreement with the results of this study.…”

“…[28,33,34] Despite the numerous NMR studies of ϵ-Al 13 , the translational mobility of ϵ-Al 13 and other aluminum hydroxide clusters in aqueous solution has been limited to 1 H PFGSTE NMR measurements of organic tracer molecules that covalently bind to the AlO 6 shell. [35] The lack of prior study of ϵ-Al 13 with 27 Al PFGSTE NMR is likely due to the unfavorable gyromagnetic ratio of 27 Al relative to 1 H and the large size of ϵ-Al 13 relative to monomeric ions. Although the use of 27 Al PFGSTE NMR for analysis of ϵ-Al 13 is unique to this study, 27 Al PFGSTE NMR has been used in other instances to measure the diffusion of relatively small ions in either molten salts or concentrated electrolyte solutions.…”

Isomerization of Na‐Centered Alkyltin Keggin Clusters

“…For the Al 13 Keggin ion, conversion to other isomer forms was promoted by addition of glycine, and the process was monitored in solution by 27 Al NMR [23] . Chemical shifts for α, γ, δ, and ϵ isomers of Al 13 have all been observed in the same solution by 27 Al NMR [23] …”

“…W and Mo Keggins typically favor the α isomer, with the β isomer becoming more stable upon reduction and with increasing charge of the {XO 4 } n− central tetrahedron (X=central cation) [16–20] . On the other hand, Sb and Al have an inverted stability order with the ϵ isomer being most stable [21–24] . For the Al 13 Keggin ion, conversion to other isomer forms was promoted by addition of glycine, and the process was monitored in solution by 27 Al NMR [23] .…”