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, 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.