Synthesis and X-Ray Crystal Structure of α-Keggin-Type Aluminum-Substituted Polyoxotungstate

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).


Introduction
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).

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 31 P nuclear magnetic resonance (NMR) (242.95MHz) spectra in acetonitrile-d 3 solution were recorded in tubes (outer diameter: 5 mm) on a JEOL ECA-600 NMR spectrometer.The 31 P NMR spectra were referenced to an external standard of 85% H 3 PO 4 in a sealed capillary.

X-Ray crystallography
A colorless platelet crystal of [(n-C 4 H 9 ) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ] (0.16 × 0.16 × 0.01 mm 3 ) was mounted on a MicroMount.All measurements were made on a Rigaku VariMax with a Saturn diffractometer using multi-layer mirror monochromated Mo K radiation ( = 0.71075 Å) at 93 K. Data were collected and processed using CrystalClear for Windows, and structural analysis was performed using the CrystalStructure for Windows.The structure was solved by SHELXS-97 (direct methods) and refined by SHELXL-97 (Sheldrick, 2008).Since one aluminum atom was disordering over twelve tungsten sites in [ -PW 11 {Al(OH 2 )}O 39 ] 4-, the occupancies for the aluminum and tungsten sites were fixed at 1/12 and 11/12 throughtout the refinement.Four tetra-n-butylammonium ions could not be modelled with disordered atoms.Accordingly, the residual electron density was removed using the SQUEEZE routine in PLATON (Spek, 2009).

Computational details
The optimal geometry of [ -PW 11 {Al(OH 2 )}O 39 ] 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 [α-PW 12 O 40 ] 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 analysis.

Conclusion
The synthesis of a monomeric, mono-aluminum-substituted α-Keggin polyoxometalate is described in this study.We successfully obtained single crystals of acetonitrile , was a monomeric, -Keggin structure, and the mono-aluminum-substituted site could not be identified due to the h i g h s y m m e t r y i n t h e p r o d u c t .I n c o n t rast, the DFT-optimized geometry of [ -PW 11 {Al(OH 2 )}O 39 ] 4-showed that the mono-aluminum-substituted site was uniquely concave downward, which caused the extension of the P-O bond linkaged to the aluminum atom, whereas the Al-O bond linkaged to the phosphorus atom was shortened.This structural difference strongly influenced the bonding mode (bond lengths and bond angles) as determined by X-ray crystallography.In addition, the Mulliken charges clearly exhibited the effect caused by the insertion of aluminum atoms into the mono-vacant sites.

1
Fig. 3. Profile for the potentiometric titration of [(n-C 4 H 9 ) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ] with tetra-n-butylammonium hydroxide as a titrant.The Cs analysis revealed no contamination of cesium ions from Cs 7 [ -PW 10 O 36 ]⋅19H 2 O.The weight loss observed during the course of drying before the analysis was 2.16% for [(n-C 4 H 9 ) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ]; this corresponded to two weakly solvated or adsorbed acetonitrile molecules.On the other hand, in the TG/DTA measurement performed under atmospheric conditions, a weight loss of 31.0%observed in the temperature range from 25 to 500 °C corresponded to four tetra-n-butylammonium ions, two acetonitrile molecules, and a water molecule.From the potentiometric titration, a break point at 2.0 equivalents of added base was observed, as shown in Fig.3.The titration profile revealed that [(n-C 4 H 9 ) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ] had two titratable protons dissociated from the Al-OH 2 group.This result was consistent with the elemental analysis result.
) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ], and the three alkylammonium salts of [PW 12 O 40 ] 3-.The terms O a , O c , O e , and O t are explained in Fig. 5.The mean values are provided in parentheses.

Fig. 5 .
Fig. 5.The polyhedral representation of [ -PW 11 {Al(OH 2 )}O 39 ] 4-.In the polyhedral representation, the AlO 6 and WO 6 groups are represented by blue and white octahedra, respectively.The internal PO 4 group is represented by the red tetrahedron.Further, O a ,

Fig. 5 .
The terms O c and O e indicate bridging oxygen atoms between corner-and edge-sharing MO 6 (M = W, Re, Cu, Mn, Ni) octahedra.The mean values are provided in parentheses.
The lengths of Al-O bonds at the corner-and edge-sharing Al-O-W bondings were shorter than those of W-O bonds at the corner-and edge-sharing W-O-W bondings, which caused shortening of the average W(Al)-O bond lengths, as observed by X-ray crystallography.The Mulliken charges of all oxygen atoms linkaged to aluminum atoms in [ -PW 11 {Al(OH 2 )}O 39 ] 4-were more positive than those linkaged to tungsten atoms in [ -PW 12 O 40 ] 3-; whereas the charges of oxygen atoms linkaged to tungsten atoms in [ -PW 11 {Al(OH 2 )}O 39 ] 4-were similar to those in [ -PW 12 O 40 ] 3-.In addition, the atomic charge of the phosphorus atom in [ -PW 11 {Al(OH 2 )}O 39 ] 4-was more negative than that in [ -PW 12 O 40 ] 3-.In the case of mono-vanadium(V)-substituted Keggin silicotungstate [SiW 11 VO 40 ] 5-, the net charge associated with the inner tetrahedron was very similar to that supported by SiO 4 in [SiW 12 O 40 ] 4-(Maestre et al., 2001).Thus, the difference in the charge on the internal phosphorus atom for [ -PW 11 {Al(OH 2 )}O 39 ] 4-and [ -PW 12 O 40 ] 3-might be due to the gravitation of aluminum atoms towards the internal PO 4 group.

Table 2 .
Table 2 were similar to those of [ -PW 12 O 40 ] 3-, whereas, the bond lengths of [α-PW 11 {Al(OH 2 )}O 39 ] 4-were clearly shorter than those of [ -PW 12 O 40 ] 3-.Ranges and mean bond distances (Å) for four mono-metal-substituted -Keggin phosphotungstates.The terms O a and O t are explained in Fourier transform infrared spectra, and solution 31 P, 27 Al, and 183 W nuclear magnetic resonance spectroscopy.The single-crystal Xray structure analysis, revealed as [(n-C 4 H 9 ) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ] -soluble tetran-butylammonium salt [(n-C 4 H 9 ) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ] by reacting aluminum nitrate with a di-lacunary -Keggin phosphotungstate.The characterization of [(n-C 4 H 9 ) 4 N] 4 [ -PW 11 {Al(OH 2 )}O 39 ] was accomplished by X-ray crystallography, elemental analysis, www.intechopen.comthermogravimetric/differential thermal analysis, This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 21200055) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.Y. Kataoka acknowledges the JSPS Research Fellowship for Young Scientist.Y. Kitagawa also has been supported by Grant-in-Aid for Scientific Research on Innovative Areas ("Coordination Programming" area 2170, No. 22108515) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).This research was partially carried out using equipment at the Center for Instrumental Analysis, Shizuoka University.