**2.2 Instrumentation/analytical procedures**

The elemental analysis was carried out by using Mikroanalytisches Labor Pascher (Remagen, Germany). The sample was dried overnight at room temperature under pressures of 10-3 – 10-4 Torr before analysis. Infrared spectra were recorded on a Parkin Elmer Spectrum100 FT-IR spectrometer in KBr disks at room temperature. Thermogravimetric (TG) and differential thermal analyses (DTA) data were obtained using a Rigaku Thermo Plus 2 series TG/DTA TG 8120. TG/DTA measurements were performed in air by constantly increasing the temperature from 20 to 500 °C at a rate of 4 °C per min. The 31P nuclear magnetic resonance (NMR) (242.95 MHz) spectra in acetonitrile-*d*3 solution were recorded in tubes (outer diameter: 5 mm) on a JEOL ECA-600 NMR spectrometer. The 31P NMR spectra were referenced to an external standard of 85% H3PO4 in a sealed capillary. Negative chemical shifts were reported on the δ scale for resonance upfields of H3PO4 (δ 0). The 27Al NMR (156.36 MHz) spectrum in acetonitrile-*d*3 was recorded in tubes (outer diameter: 5 mm) on a JEOL ECA-600 NMR spectrometer. The 27Al NMR spectrum was referenced to an external standard of saturated AlCl3-D2O solution (substitution method). Chemical shifts were reported as positive on the δ scale for resonance downfields of AlCl3 (δ 0). The 183W NMR (25.00 MHz) spectra were recorded in tubes (outer diameter: 10 mm) on a JEOL ECA-600 NMR spectrometer. The 183W NMR spectra measured in acetonitrile-*d*3 were referenced to an external standard of saturated Na2WO4-D2O solution (substitution method).

Fig. 2. The polyhedral representation of K6H3[ZnW11O40Al]9.5H2O (left), TBA3H[γ-SiW10- O36{Al(OH2)}2(μ-OH)2]4H2O (TBA = tetra-*n*-butylammonium) (center), and K6Na[(A-PW9- O34)2{W(OH)(OH2)}{Al(OH)(OH2)}{Al(μ-OH)(OH2)2}2]19H2O (right). The aluminum groups are represented by the blue octahedra. The WO6 groups are represented by white octahedra. The internal ZnO4, SiO4, and PO4 groups are represented by green, yellow, and red tetrahedra, respectively.

Synthesis and X-Ray Crystal Structure

*I* > 2σ(*I*)). CCDC number 851155.

**2.6 Computational details** 

**3. Results and discussion** 

Keggin polyoxotungstate.

[(CH3)2NH2]10[Hf(PW11O39)2]8H2O (Hou et al., 2007).

analysis.

**2.5 Crystal data for [(***n***-C4H9)4N]4[α-PW11{Al(OH2)}O39]** 

of -Keggin-Type Aluminum-Substituted Polyoxotungstate 605

C64H146AlN4O40PW11; M = 3692.17, *cubic*, space group *Im-3m (#229)*, *a* = 17.665(2) Å, *V* = 5512.2(8) Å3, *Z* = 2, *D*c = 2.224 g/cm3, μ(Mo-Kα) = 115.313 cm-1. *R1* = 0.0220 (*I* > 2σ(*I*)) and *wR*2 = 0.0554 (for all data). GOF = 1.093 (22662 total reflections, 652 unique reflections where

The optimal geometry of [α-PW11{Al(OH2)}O39]4- was computed by means of a DFT method. First, we optimized the crystal geometries and followed this up with single-point calculations with larger basis sets. All calculations were performed by a spin-restricted B3LYP on Gaussian09 program package (Frisch et al., 2009). The basis sets used for the geometry optimization were LANL2DZ for W atoms, 6-31+G\* for P atoms and 6-31G\* for H, O, and Al atoms. LANL2DZ and 6-31+G\* were used for W and other atoms, respectively, for the single-point calculations. The geometry optimizations were started using the X-ray structure of [-PW12O40]3- as an initial geometry, and they were performed under the gas phase condition. The optimized geometries were confirmed to be true minima by frequency analyses. All atomic charges used in this text were obtained from Mulliken population

**3.1 Synthesis and molecular formula of [(***n***-C4H9)4N]4[α-PW11{Al(OH2)}O39]** 

The tetra-*n*-butylammonium salt of [α-PW11{Al(OH2)}O39]4- was formed by the direct reaction of aluminum nitrate with [γ-PW10O36]7- (the molar ratio of Al3+:[γ-PW10O36]7- was ca. 1.0) in an aqueous solution at 40 ºC under air, followed by the addition of excess tetra*n*-butylammonium bromide. The crystallization was performed by slow-evaporation from acetonitrile at 25 ºC. During the formation of [α-PW11{Al(OH2)}O39]4-, the decomposition of a *di*-lacunary γ-Keggin polyoxotungstate, and isomerization of γ-isomer to α-isomer occurred in order to construct the *mono*-aluminum-substituted site in an α-Keggin structure. It was noted that the polyoxoanion [α-PW11{Al(OH2)}O39]4- was easily obtained by the stoichiometric reaction of aluminum nitrate with a *mono*-lacunary α-Keggin polyoxotungstate, [α-PW11O39]7-, in an aqueous solution; however, a single species of [α-PW11{Al(OH2)}O39]4- could not be obtained as a tetra-*n*-butylammonium salt by using [α-PW11O39]7- as a starting polyoxoanion.1 Thus, single crystals that were suitable for Xray crystallography could be obtained for the crystallization of the tetra-*n*butylammonium salt of [α-PW11{Al(OH2)}O39]4- synthesized by using a *di*-lacunary γ-

1 The 31P NMR spectrum in acetonitrile-*d*3 of the tetra-*n*-butylammonium salt of [α-PW11{Al(OH2)}O39]4 prepared by the stoichiometic reaction of [α-PW11O39]7- with Al(NO3)39H2O in an aqueous solution showed two signals at -12.35 ppm and -12.48 ppm. The signal at -12.48 ppm was assigned to the internal phosphorus atom in [α-PW11{Al(OH2)}O39]4-, whereas the signal at -12.35 ppm could not be identified; however, the signal was not due to the proton isomer, as reported for

Chemical shifts were reported as negative for resonance upfields of Na2WO4 (δ 0). Potentiometric titration was carried out with 0.4 mol/L tetra-*n*-butylammonium hydroxide as a titrant under argon atmosphere (Weiner et al., 1996). The compound [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] (0.018 mmol) was dissolved in acetonitrile (30 mL) at 25 °C and the solution was stirred for approximately 5 min. The titration data were obtained with a pH meter (Mettler Toledo). Data points were obtained in milivolt. A solution of tetra-*n*butylammonium hydroxide (9.0 mmol/L) was syringed into the suspension in 0.25 equivalent intervals.
