**1. Introduction**

600 Advances in Crystallization Processes

[129] Gholivand, K., Dorosti, N., Ghaziany, F., Mirshahi, M. & Sarikhani, S. (2012).

Aluminum and its derivatives such as alloys, oxides, organometallics, and inorganic compounds have attracted considerable attention because of their extreme versatility and unique range of properties, including acidity, hardness, and electroconductivity (Cotton & Wilkinson, 1988). Since the properties and activities of an aluminum species are strongly dependent on the structures of the aluminum sites, the syntheses of aluminum compounds with structurally well-defined aluminum sites are considerably significant for the development of novel and efficient aluminum-based materials. However, the use of these well-defined aluminum sites is slightly limited by the conditions resulting from the hydrolysis of the aluminum species by water (Djurdjevic et al., 2000; Baes & Mesmer, 1976; Orvig, 1993; Akitt, 1989).

Polyoxometalates have been of particular interest in the fields of catalytic chemistry, surface science, and materials science because their chemical properties such as redox potentials, acidities, and solubilities in various media can be finely tuned by choosing appropriate constituent elements and countercations (Pope, 1983; Pope & Müller, 1991, 1994). In particular, the coordination of metal ions to the vacant site(s) of lacunary polyoxometalates is one of the most effective techniques used for constructing efficient and well-defined active metal centers. Among various lacunary polyoxometalates, a series of Keggin-type phosphotungstates is one of the most useful types of lacunary polyoxometalates. Fig. 1 shows some examples of lacunary Keggin-type phosphotungstates, i.e., *mono*-lacunary α-Keggin [α-PW11O39]7- (Contant, 1987), *di*-lacunary γ-Keggin [γ-PW10O36]7- (Domaille, 1990; Knoth, 1981), and *tri*-lacunary α-Keggin [A-α-PW9O34]9- (Domaille, 1990) phosphotungstates. Knoth and co-workers first synthesized the Keggin derivative (Bu4N)4(H)ClAlW11PO39 by the reaction of *mono*-lacunary α-Keggin phosphotungstate with AlCl3 in dichloroethane (Knoth et al., 1983). However, only a few aluminum-coordinated polyoxometalates (determined by X-ray crystallographic analysis) have been reported, e.g., a monomeric, *di*-aluminum-substituted γ-Keggin polyoxometalate TBA3H[γ-SiW10O36{Al(OH2)}2(μ-

Synthesis and X-Ray Crystal Structure

**2.2 Instrumentation/analytical procedures** 

of -Keggin-Type Aluminum-Substituted Polyoxotungstate 603

[(CH3)2NH2]4[α-PW11ReVO40] (Kato et al., 2010) was resolved by SHELXS-97 (direct methods) and re-refined by SHELXL-97 (Sheldrick, 2008). The crystal data are as follows: C8H32N3O4PReW11: *M* = 3063.87, *trigonal*, space group *R-3m*, *a* = 16.53(2) Å, *c* = 25.21(4) Å, *V* = 5963(12) Å3, *Z* = 6, *D*c = 5.119 g/cm3, *R1* = 0.0559 (*I* > 2(*I*)) and *wR2* = 0.1513 (for all data). The four dimethylammonium ions could not be identified due to the disorder (Nomiya et al., 2001, 2002; Weakley & Finke, 1990; Lin et al., 1993). CCDC number 851154.

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.

OH)2]4H2O (TBA = tetra-*n*-butylammonium) (Kikukawa et al., 2008), a monomeric, *mono*aluminum-substituted α-Keggin polyoxometalate K6H3[ZnW11O40Al]9.5H2O (Yang et al., 1997), and a dimeric aluminum complex having *mono*- and *di*-aluminum sites sandwiched by *tri*-lacunary α-Keggin polyoxometalate K6Na[(A-PW9O34)2{W(OH)(OH2)}{Al(OH)(OH2)} {Al(μ-OH)(OH2)2}2]19H2O (Kato et al., 2010); these structures are shown in Fig. 2.

Fig. 1. Some examples of lacunary phosphotungstates. The polyhedral representations of *mono*-lacunary α-Keggin [α-PW11O39]7- (left), *di*-lacunary γ-Keggin [γ-PW10O36]7- (center), and *tri*-lacunary α-Keggin [A-α-PW9O34]9- (right) phosphotungstates. The WO6 and internal PO4 groups are represented by the white octahedra and red tetrahedron, respectively.

In this study, we successfully obtained a monomeric, α-Keggin *mono*-aluminum-substituted polyoxotungstate in the form of crystals (suitable for X-ray structure analysis) of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] that were fully characterized by X-ray crystallography; elemental analysis; thermogravimetric/differential thermal analysis; Fourier transform infrared spectroscopy; and solution 31P, 27Al, and 183W nuclear magnetic resonance spectroscopies. Although the X-ray crystallography of [α-PW11{Al(OH2)}O39]4- showed that the *mono*-aluminum-substituted site was not identified because of the high symmetry in the compound, the bonding mode (bond lengths and bond angles) were significantly influenced by the insertion of aluminum ions into the *mono*-vacant sites. In addition, density-functionaltheory (DFT) calculations showed a unique coordination sphere around the *mono*aluminum-substituted site in [α-PW11{Al(OH2)}O39]4-; this was consistent with the X-ray crystal structure and spectroscopic results. In this paper, we report the complete details of the synthesis, molecular structure, and characterization of [(*n*-C4H9)4N]4[α-PW11 {Al(OH2)}O39].
