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.