Metal Substitution in Keggin-Type Tridecameric Aluminum−Oxo−Hydroxy Clusters

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Abstract
The species resulting from a typical preparation for metal-substituted hybrids of the Keggin tridecamer, Al13 or [AlO4Al12(OH)24(OH2)12]7+, were examined by performing 27Al NMR on the solutions during aging and by studying the precipitated sulfate salts via solid state 27Al NMR and powder X-ray diffraction (XRD). Aqueous mixtures (0.25 mol L-1) of AlCl3 and another metal ion (M), in a 12:1 mole ratio (Al:M), where M = Fe3+, Zn2+, Ga3+, In3+, Sn2+, La3+, and Bi3+, were subjected to forced hydrolysis by addition of NaOH (1.0 mol L-1) until OH/(Al + M) = 2.25, and the kinetics of Al13 formation and disappearance with aging at 80 °C was monitored by 27Al NMR spectroscopy. Al13 units polymerize on aging with an apparent rate constant (k) of 4.8(8) × 10-2 h-1 to form a species referred to as AlP2. Only the solutions containing Ga3+ and Sn2+ exhibited faster Al13 conversion rates. GaAl12 forms quickly at 80 °C (k = 0.54 h-1) and is more stable than AlP2. Sn2+ apparently promotes AlP2 formation (k = 0.38 h-1). XRD and solid state NMR reveal that only the Ga hybrid can be prepared by this method. No hybrid formation was evidenced using M = Mg2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, In3+, La3+, or Ce3+ at 25 °C or M = Co2+ or La3+ under reflux conditions. Isostructural (cubic symmetry) single crystals were obtained for the sulfate salts of Al13 and GaAl12. Single-crystal XRD analysis of these two polyoxocations provides the first rigorous comparison between them and shows they have very similar structures. The main crystallographic data for Al13 and GaAl12 are as follows: Na[AlO4Al12(OH)24(H2O)12](SO4)4·10H2O, cubic, F4̄3m, a = 17.856(2) Å, Z = 4; Na[GaO4Al12(OH)24(H2O)12](SO4)4·10H2O, cubic, F4̄3m, a = 17.869(3) Å, Z = 4. Thus, the greater thermal stability of GaAl12 cannot be rationalized in terms of the overall geometric considerations, as suggested by others. Solid state NMR also shows the coordination symmetries of the outer 12 Al nuclei in both clusters to be similar.