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

The species resulting from a typical preparation for metal-substituted hybrids of the Keggin tridecamer, Al or [AlOAl(OH)(OH)], were examined by performing Al NMR on the solutions during aging and by studying the precipitated sulfate salts via solid state Al NMR and powder X-ray diffraction (XRD). Aqueous mixtures (0.25 mol L) of AlCl and another metal ion (M), in a 12:1 mole ratio (Al:M), where M = Fe, Zn, Ga, In, Sn, La, and Bi, were subjected to forced hydrolysis by addition of NaOH (1.0 mol L) until OH/(Al + M) = 2.25, and the kinetics of Al formation and disappearance with aging at 80 °C was monitored by Al NMR spectroscopy. Al units polymerize on aging with an apparent rate constant (k) of 4.8(8) × 10 h to form a species referred to as AlP. Only the solutions containing Ga and Sn exhibited faster Al conversion rates. GaAl forms quickly at 80 °C (k = 0.54 h) and is more stable than AlP. Sn apparently promotes AlP formation (k = 0.38 h). XRD and solid state NMR reveal that only the Ga hybrid can be prepared by this method. No hybrid formation was evidenced using M = Mg, Fe, Co, Ni, Cu, Zn, In, La, or Ce at 25 °C or M = Co or La under reflux conditions. Isostructural (cubic symmetry) single crystals were obtained for the sulfate salts of Al and GaAl. 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 Al and GaAl are as follows:  NaAlOAl(OH)(HO)·10HO, cubic, F4̄3m, a = 17.856(2) Å, Z = 4; NaGaOAl(OH)(HO)·10HO, cubic, F4̄3m, a = 17.869(3) Å, Z = 4. Thus, the greater thermal stability of GaAl 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.