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

The FTIR spectra measured as a KBr disk of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39], K7[α-PW11O39]11H2O, Cs7[γ-PW10O36]19H2O, and Na3[α-PW12O40]16H2O are shown in Fig. 8. For

Fig. 8. FTIR spectra (as KBr disks) in the range of 1800 – 400 cm-1 for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] (top), K7[α-PW11O39]11H2O (the second top), Cs7[γ-PW10O36]19H2O (the third top), and Na3[α-PW12O40]16H2O (bottom)

The FTIR spectra measured as a KBr disk of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39], K7[α-PW11O39]11H2O, Cs7[γ-PW10O36]19H2O, and Na3[α-PW12O40]16H2O are shown in Fig. 8. For

Fig. 8. FTIR spectra (as KBr disks) in the range of 1800 – 400 cm-1 for [(*n*-C4H9)4N]4[α-

third top), and Na3[α-PW12O40]16H2O (bottom)

PW11{Al(OH2)}O39] (top), K7[α-PW11O39]11H2O (the second top), Cs7[γ-PW10O36]19H2O (the

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

[(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39], the P-O band was observed at 1078 cm-1, and the W-O bands were observed at 964, 887, 818, 749, and 702 cm-1, these were different from those of K7[α-PW11O39]11H2O (1086, 1043, 953, 903, 862, 810, and 734 cm-1) and Cs7[γ-PW10O36]19H2O (1085, 1053, 1025, 952, 938, 893, 827, and 751 cm-1) (Rocchiccioli-Deltcheff et al., 1983; Thouvenot et al., 1984). This result suggested that the aluminum atom was coordinated into the vacant site in the polyoxometalate. It should be noted that the bands observed for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] were significantly different from those of Na3[α-PW12O40]16H2O (1080, 984, 893, and 808 cm-1). This was consistent with the results observed by X-ray crystallography and DFT calculations, as mentioned above.

The 31P NMR spectrum of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] in acetonitrile-*d*3 at ~25 °C was a clear single line spectrum at -12.5 ppm due to the internal phosphorus atom, thereby confirming the compound's purity and homogeneity, as shown in Fig. 9. The signal exhibited a shift from the signals of tetra-*n*-butylammonium salts of [α-PW12O40]3- (δ -14.6) and [α-PW11O39]7- (δ -12.0), suggesting the insertion of aluminum ion into the vacant site.

Fig. 9. 31P NMR spectrum in acetonitrile-*d*3 of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39].

The 27Al NMR spectrum (Fig. 10) of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] in acetonitrile-*d*3 at ~25 ºC showed a broad signal at 16.1 ppm due to the *mono*-aluminum-substituted site in [α-PW11{Al(OH2)}O39]4-.

The 183W NMR spectrum (Fig. 11) of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] in acetonitrile-*d*<sup>3</sup> at ~25 ºC was a six-line spectrum of (δ -56.2, -93.1, -108.6, -115.8, -118.5, -153.9) with 2:2:2:2:1:2 intensities, which were in accordance with the presence of eleven tungsten atoms with *Cs* symmetry. These spectral data were completely consistent with the X-ray structure and the optimized structure, suggesting that the solid structure was maintained in the solution.

Synthesis and X-Ray Crystal Structure

**5. Acknowledgment** 

**6. Appendix** 

of -Keggin-Type Aluminum-Substituted Polyoxotungstate 615

thermogravimetric/differential thermal analysis, Fourier transform infrared spectra, and solution 31P, 27Al, and 183W nuclear magnetic resonance spectroscopy. The single-crystal Xray structure analysis, revealed as [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39], was a monomeric, α-Keggin structure, and the *mono*-aluminum-substituted site could not be identified due to the high symmetry in the product. In contrast, the DFT-optimized geometry of [α-PW11{Al(OH2)}O39]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

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

Bond lengths (Å) of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39]: W(1)-O(1) 1.883(4); W(1)-O(1)1 1.883(4); W(1)-O(1)2 1.883(4); W(1)-O(1)3 1.883(4); W(1)-O(2) 2.465(5); W(1)-O(2)4 2.465(5); W(1)-O(3) 1.667(4); P(1)-O(2) 1.522(5); P(1)-O(2)5 1.522(5); P(1)-O(2)6 1.522(5); P(1)-O(2)7 1.522(5); P(1)-O(2)4 1.522(5); P(1)-O(2)8 1.522(5); P(1)-O(2)9 1.522(5); P(1)-O(2)10 1.522(5); Al(1)-O(1) 1.883(4); Al(1)-O(1)1 1.883(4); Al(1)-O(1)2 1.883(4); Al(1)-O(1)3 1.883(4); Al(1)-O(3) 1.667(4). Symmetry operators: (1) X,Z,Y (2) Z,Y,-X+1 (3) Z,-X+1,Y (4) Y,Z,-X+1 (5) Y,Z,X (6)

Bond angles (º) of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39]: O(1)-W(1)-O(1)1 87.5(2); O(1)-W(1)- O(1)2 87.08(18); O(1)-W(1)-O(1)3 154.8(2); O(1)-W(1)-O(2) 63.32(19); O(1)-W(1)-O(2)4 92.40(18); O(1)-W(1)-O(3) 102.58(17); O(1)1-W(1)-O(1)2 154.8(2) O(1)1-W(1)-O(1)3 87.08(18); O(1)1-W(1)-O(2) 63.32(19); O(1)1-W(1)-O(2)4 92.40(18); O(1)1-W(1)-O(3) 102.58(17); O(1)2- W(1)-O(1)3 87.5(2); O(1)2-W(1)-O(2) 92.40(18); O(1)2-W(1)-O(2)4 63.32(19); O(1)2-W(1)-O(3) 102.58(17); O(1)3-W(1)-O(2) 92.40(18); O(1)3-W(1)-O(2)4 63.32(19); O(1)3-W(1)-O(3) 102.58(17); O(2)-W(1)-O(2)4 41.76(15); O(2)-W(1)-O(3) 159.12(11); O(2)4-W(1)-O(3) 159.12(11); O(2)-P(1)- O(2)5 109.5(3); O(2)-P(1)-O(2)6 109.5(3); O(2)-P(1)-O(2)7 70.5(3); O(2)-P(1)-O(2)4 70.5(3); O(2)- P(1)-O(2)8 180.0(4); O(2)-P(1)-O(2)9 109.5(3); O(2)-P(1)-O(2)10 70.5(3); O(2)5-P(1)-O(2)6 109.5(3); O(2)5-P(1)-O(2)7 70.5(3); O(2)5-P(1)- O(2)4 70.5(3); O(2)5-P(1)-O(2)8 70.5(3); O(2)5-P(1)- O(2)9 109.5(3); O(2)5-P(1)-O(2)10 180.0(4); O(2)6-P(1)-O(2)7 180.0(4); O(2)6-P(1)-O(2)4 70.5(3); O(2)6-P(1)-O(2)8 70.5(3); O(2)6-P(1)-O(2)9 109.5(3); O(2)6-P(1)-O(2)10 70.5(3); O(2)7-P(1)-O(2)4 109.5(3); O(2)7-P(1)-O(2)8 109.5(3); O(2)7-P(1)-O(2)9 70.5(3); O(2)7-P(1)-O(2)10 109.5(3); O(2)4- P(1)-O(2)8 109.5(3); O(2)4-P(1)-O(2)9 180.0(4); O(2)4-P(1)-O(2)10 109.5(3); O(2)8-P(1)-O(2)9 70.5(3); O(2)8-P(1)-O(2)10 109.5(3); O(2)9-P(1)-O(2)10 70.5(3); O(1)-Al(1)-O(1)1 87.5(2); O(1)-

effect caused by the insertion of aluminum atoms into the *mono*-vacant sites.

using equipment at the Center for Instrumental Analysis, Shizuoka University.

Z,X,Y (7) X,Y,-Z+1 (8) Z,X,-Y+1 (9) -Z+1,X,-Y+1 (10) -Y+1,-Z+1,-X+1.

Fig. 10. 27Al NMR spectrum in acetonitrile-*d*3 of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39].

Fig. 11. 183W NMR spectrum in acetonitrile-*d*3 of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39].

## **4. Conclusion**

The synthesis of a monomeric, *mono*-aluminum-substituted -Keggin polyoxometalate is described in this study. We successfully obtained single crystals of acetonitrile-soluble tetra*n*-butylammonium salt [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] by reacting aluminum nitrate with a *di*-lacunary γ-Keggin phosphotungstate. The characterization of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] was accomplished by X-ray crystallography, elemental analysis, thermogravimetric/differential thermal analysis, Fourier transform infrared spectra, and solution 31P, 27Al, and 183W nuclear magnetic resonance spectroscopy. The single-crystal Xray structure analysis, revealed as [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39], was a monomeric, α-Keggin structure, and the *mono*-aluminum-substituted site could not be identified due to the high symmetry in the product. In contrast, the DFT-optimized geometry of [α-PW11{Al(OH2)}O39]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.
