Crystallization of Keggin-Type Polyaluminum Species by Supramolecular Interactions with Disulfonate Anions

The hydrolysis of aluminum and formation of polynuclear species, such as the Keggin-type polycations, impacts the chemical and physical properties of the resulting aluminum oxide and hydroxide materials. Despite years of study, only a handful of Keggin-type species have been identified, hampering efforts toward a molecular-level understanding of the mechanisms of condensation. To improve the crystallization of Keggin-type polyaluminum cations, a supramolecular approach using 2,6-napthalene disulfonate (2,6-NDS) was proposed herein for the isolation of novel compounds. The present study describes the successful synthesis and structural characterization of three Keggin-type polyaluminum compounds, including (Na(Al(μ4-O4)Al12(μ-OH)24(H2O))12(2,6NDS)4(H2O)13.5 (δ-Al13), (Al2(μ4-O8)(Al28(μ2-OH)56(H2O)26)(2,6NDS)8Cl2(H2O)40 (Al30), and a new polycation, (Al2(μ4-O8)(Al24(μ2-OH)50(H2O)20)(2,6NDS)6(H2O)12.4 (Al26). Additional chemical characterization of the compounds, particularly 27Al-NMR, suggests that identifying the Al26 polycation in aqueous solutions may be difficult due to structural similarities to the δ-Al13 moiety. The structural characterization of novel Keggin-type aluminum polycations is important for a complete understanding of aluminum hydrolysis in aqueous solutions, and organosulfonates represent a viable approach for the crystallization of new polynuclear species.