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-13 or [AlO4Al12(OH)(24)(OH2)(12)](7+), were examined by performing Al-27 NMR on the solutions during aging and by studying the precipitated sulfate salts via solid state Al-27 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 Al-13 formation and disappearance with aging at 80 degrees C was monitored by Al-27 NMR spectroscopy. Al-13 units polymerize on aging with an apparent rate constant (k) of 4.8(8) x 10(-2) h(-1) to form a species referred to as AlP2. Only the solutions containing Ga3+ and Sn2+ exhibited faster Al-13 conversion rates. GaAl12 forms quickly at 80 degrees 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 degrees C or M = Co2+ Or La3+ under reflux conditions. Isostructural (cubic symmetry) single crystals were obtained for the sulfate salts of Al-13 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 Al-13 and GaAl12 are as follows: Na[AlO4Al12(OH)(24)(H2O)(12)](SO4)(4) . 10H(2)O, cubic, <F(4)over bar 3m>, a = 17.856(2) Angstrom, z = 4; Na[GaO4Al12(OH)(24)(H2O)(12)](SO4)(4) . 10H(2)O, cubic, <F(4)over bar 3m>, a = 17.869(3) Angstrom, 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.