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 a ging 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 subjec ted 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. A l-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 s olutions containing Ga3+ and Sn2+ exhibited faster Al-13 conversion ra tes. 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 p repared by this method. No hybrid formation was evidenced using M = Mg 2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, In3+, La3+, or Ce3+ at 25 degrees C or M = Co2+ Or La3+ under reflux conditions. Isostructural (cubic symm etry) single crystals were obtained for the sulfate salts of Al-13 and GaAl12. Single-crystal XRD analysis of these two polyoxocations provi des the first rigorous comparison between them and shows they have ver y similar structures. The main crystallographic data for Al-13 and GaA l12 are as follows: Na[AlO4Al12(OH)(24)(H2O)(12)](SO4)(4) . 10H(2)O, c ubic, <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 = 17.86 9(3) Angstrom, Z = 4. Thus, the greater thermal stability of GaAl12 ca nnot be rationalized in terms of the overall geometric considerations, as suggested by others. Solid state NMR also shows the coordination s ymmetries of the outer 12 Al nuclei in both clusters to be similar.