**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-Keggin polyoxotungstate.

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

Synthesis and X-Ray Crystal Structure

of -Keggin-Type Aluminum-Substituted Polyoxotungstate 607

The molecular structure of [α-PW11{Al(OH2)}O39]4- as determined by X-ray crystallography is shown in Figs. 4 and 5. The bond lengths and bond angles are summarized in appendix. The molecular structure of [α-PW11{Al(OH2)}O39]4- was identical to that of a monomeric, α-Keggin polyoxotungstate [α-PW12O40]3- (Neiwert et al., 2002; Busbongthong & Ozeki, 2009). Due to the high symmetry space group, the eleven tungsten(VI) atoms were disordered and the *mono*-aluminum-substituted site was not identified, as observed for [W9ReO32]5- (Ortéga et al., 1997), [α-PW11ReVO40]5- (Kato et al., 2010), [{SiW11O39Cu(H2O)}{Cu2(ac)- (phen)2(H2O)}]14- (phen = phenanthroline, ac = acetate) (Reinoso et al., 2006), (ANIH)5[PCu(H2O)W11O39](ANI)8H2O (ANI = aniline, ANIH+ = anilinium ion) (Fukaya et al., 2011), Cs5[PMn(H2O)W11O39]4H2O (Patel et al., 2011), and Cs5[PNi(H2O)W11O39]2H2O (T. J. R. Weakley, 1987). However, the bond lengths of [(*n*-C4H9)4N]4[-PW11{Al(OH2)}O39] were clearly influenced by the insertion of aluminum ion into the vacant site as compared with those of [CH3NH3]3[PW12O40]2H2O, [(CH3)2NH2]3[PW12O40], and [(CH3)3NH]3- [PW12O40] (Busbongthong & Ozeki, 2009) (Table 1). Thus, the lengths of the oxygen atoms belonging to the central PO4 tetrahedron (Oa) are longer than those of the three alkylammonium salts of [PW12O40]3-; whereas, the lengths of the bridging oxygen atoms between corner-sharing MO6 (M = W and Al) octahedra (Oc) and bridging oxygen atoms between edge-sharing MO6 octahedra (Oe) are shorter than those of [PW12O40]3-. For comparisons, the bond lengths of *mono*-metal-substituted α-Keggin phosphotungstates, e.g., [(CH3)2NH2]4[α-PW11ReVO40], (ANIH)5[PCu(H2O)W11O39](ANI)8H2O (ANI = aniline,

**3.2 The molecular structure of [(***n***-C4H9)4N]4[α-PW11{Al(OH2)}O39]** 

Fig. 4. The molecular structure (ORTEP drawing) of [α-PW11{Al(OH2)}O39]4-.

P(1)

O(2)

O(1) W(Al)(1)

O(3)

The sample for the elemental analysis was dried overnight at room temperature under a vacuum of 10-3 – 10-4 Torr. The elemental results for C, H, N, P, Al, and W were in good agreement with the calculated values for the formula without any absorbed or solvated molecules for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39].

Fig. 3. Profile for the potentiometric titration of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] with tetra-*n*-butylammonium hydroxide as a titrant.

The Cs analysis revealed no contamination of cesium ions from Cs7[γ-PW10O36]19H2O. The weight loss observed during the course of drying before the analysis was 2.16% for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39]; 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*-C4H9)4N]4[α-PW11{Al(OH2)}O39] had two titratable protons dissociated from the Al-OH2 group. This result was consistent with the elemental analysis result.

The sample for the elemental analysis was dried overnight at room temperature under a vacuum of 10-3 – 10-4 Torr. The elemental results for C, H, N, P, Al, and W were in good agreement with the calculated values for the formula without any absorbed or solvated

Fig. 3. Profile for the potentiometric titration of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] with

The Cs analysis revealed no contamination of cesium ions from Cs7[γ-PW10O36]19H2O. The weight loss observed during the course of drying before the analysis was 2.16% for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39]; 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

0 0.5 1.0 1.5 2.0 2.5 3.0 Equivalents of OH-

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*-C4H9)4N]4[α-PW11{Al(OH2)}O39] had two titratable protons dissociated from the Al-OH2 group. This

molecules for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39].

Potential/mV

tetra-*n*-butylammonium hydroxide as a titrant.

result was consistent with the elemental analysis result.

water molecule.

#### **3.2 The molecular structure of [(***n***-C4H9)4N]4[α-PW11{Al(OH2)}O39]**

The molecular structure of [α-PW11{Al(OH2)}O39]4- as determined by X-ray crystallography is shown in Figs. 4 and 5. The bond lengths and bond angles are summarized in appendix. The molecular structure of [α-PW11{Al(OH2)}O39]4- was identical to that of a monomeric, α-Keggin polyoxotungstate [α-PW12O40]3- (Neiwert et al., 2002; Busbongthong & Ozeki, 2009). Due to the high symmetry space group, the eleven tungsten(VI) atoms were disordered and the *mono*-aluminum-substituted site was not identified, as observed for [W9ReO32]5- (Ortéga et al., 1997), [α-PW11ReVO40]5- (Kato et al., 2010), [{SiW11O39Cu(H2O)}{Cu2(ac)- (phen)2(H2O)}]14- (phen = phenanthroline, ac = acetate) (Reinoso et al., 2006), (ANIH)5[PCu(H2O)W11O39](ANI)8H2O (ANI = aniline, ANIH+ = anilinium ion) (Fukaya et al., 2011), Cs5[PMn(H2O)W11O39]4H2O (Patel et al., 2011), and Cs5[PNi(H2O)W11O39]2H2O (T. J. R. Weakley, 1987). However, the bond lengths of [(*n*-C4H9)4N]4[-PW11{Al(OH2)}O39] were clearly influenced by the insertion of aluminum ion into the vacant site as compared with those of [CH3NH3]3[PW12O40]2H2O, [(CH3)2NH2]3[PW12O40], and [(CH3)3NH]3- [PW12O40] (Busbongthong & Ozeki, 2009) (Table 1). Thus, the lengths of the oxygen atoms belonging to the central PO4 tetrahedron (Oa) are longer than those of the three alkylammonium salts of [PW12O40]3-; whereas, the lengths of the bridging oxygen atoms between corner-sharing MO6 (M = W and Al) octahedra (Oc) and bridging oxygen atoms between edge-sharing MO6 octahedra (Oe) are shorter than those of [PW12O40]3-. For comparisons, the bond lengths of *mono*-metal-substituted α-Keggin phosphotungstates, e.g., [(CH3)2NH2]4[α-PW11ReVO40], (ANIH)5[PCu(H2O)W11O39](ANI)8H2O (ANI = aniline,

Fig. 4. The molecular structure (ORTEP drawing) of [α-PW11{Al(OH2)}O39]4-.

Synthesis and X-Ray Crystal Structure

those of [α-PW12O40]3-.

of -Keggin-Type Aluminum-Substituted Polyoxotungstate 609

oxygen atoms belonging to the central PO4 tetrahedron; Oc, bridging oxygen atoms between corner-sharing MO6 (M = Al and W) octahedra; Oe, bridging oxygen atoms between edge-

ANIH+ = anilinium ion), Cs5[PMn(H2O)W11O39]4H2O, and Cs5[PNi(H2O)W11O39]2H2O as determined by X-ray crystallography are summarized in Table 2. Although a simple comparison was difficult to draw, the following trends were observed: The W-Oa bond lengths of [PCu(H2O)W11O39]5-, [PMn(H2O)W11O39]5-, and [PNi(H2O)W11O39]5- were significantly longer than those of [α-PW12O40]3- and [α-PW11ReVO40]4-, as observed for [α-PW11{Al(OH2)}O39]4- due to the presence of a water molecule coordinated to the *mono*metal-substituted sites. The W(M)-Oc and W(M)-Oe (M = Re, Cu, Mn, and Ni) bond lengths of the four polyoxoanions mentioned in Table 2 were similar to those of [α-PW12O40]3-, whereas, the bond lengths of [-PW11{Al(OH2)}O39]4- were clearly shorter than

[(CH3)2NH2]4[-PW11ReVO40]

Cs5[PMn(H2O)W11O39]4H2O

Cs5[PNi(H2O)W11O39]2H2O

Table 2. Ranges and mean bond distances (Å) for four *mono*-metal-substituted α-Keggin phosphotungstates. The terms Oa and Ot are explained in Fig. 5. The terms Oc and Oe indicate bridging oxygen atoms between corner- and edge-sharing MO6 (M = W, Re, Cu,

To investigate the coordination sphere around the *mono*-aluminum-substituted site in [α-PW11{Al(OH2)}O39]4-, the optimized geometry was computed by means of a DFT method, as

(ANIH)5[PCu(H2O)W11O39](ANI)8H2O

sharing MO6 octahedra (M = Al and W); Ot, terminal oxygen atoms.

W(Re)-Oa 2.418 – 2.441 (2.432) W(Re)-Oc 1.896 – 1.914 (1.906) W(Re)-Oe 1.895 – 1.922 (1.907) W(Re)-Ot 1.647 – 1.694 (1.680) P-O 1.538 – 1.540 (1.539)

W(Cu)-Oa 2.4784 - 2.5044 (2.4916) W(Cu)-Oc 1.8946 - 1.9277 (1.9077) W(Cu)-Oe 1.8946 - 1.9277 (1.9077) W(Cu)-Ot 1.7163 - 1.7220 (1.7178) P-O 1.4925 - 1.5078 (1.4965)

W(Mn)-Oa 2.4220 - 2.5520 (2.4874) W(Mn)-Oc 1.9223 - 1.8698(1.9051) W(Mn)-Oe 1.8689 - 1.9620 (1.9079) W(Mn)-Ot 1.6678 - 1.752(1.6889) P-O 1.4902 - 1.602 (1.5265)

W(Ni)-Oa 2.4013 - 2.5152 (2.4792) W(Ni)-Oc 1.8628 - 1.9430 (1.8974) W(Ni)-Oe 1.8633 - 1.9421 (1.8964) W(Ni)-Ot 1.6714 - 1.7354 (1.7010) P-O 1.5150 - 1.5256 (1.5209)

Mn, Ni) octahedra. The mean values are provided in parentheses.


Table 1. Ranges and mean bond distances (Å) for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39], and the three alkylammonium salts of [PW12O40]3-. The terms Oa, Oc, Oe, and Ot are explained in Fig. 5. The mean values are provided in parentheses.

Fig. 5. The polyhedral representation of [α-PW11{Al(OH2)}O39]4-. In the polyhedral representation, the AlO6 and WO6 groups are represented by blue and white octahedra, respectively. The internal PO4 group is represented by the red tetrahedron. Further, Oa,

[CH3NH3]3[α-PW12O40]2H2O

[(CH3)2NH2]3[α-PW12O40]

[(CH3)3NH]3[α-PW12O40]

Table 1. Ranges and mean bond distances (Å) for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39], and the three alkylammonium salts of [PW12O40]3-. The terms Oa, Oc, Oe, and Ot are explained in

Fig. 5. The polyhedral representation of [α-PW11{Al(OH2)}O39]4-. In the polyhedral representation, the AlO6 and WO6 groups are represented by blue and white octahedra, respectively. The internal PO4 group is represented by the red tetrahedron. Further, Oa,

Oa

Oc

Oe Ot

W(Al)-Oa 2.466 (2.466) W(Al)-Oc 1.883 (1.883) W(Al)-Oe 1.883 (1.883) W(Al)-Ot 1.667 (1.667) P-O 1.5206 (1.5206)

W-Oa 2.4077 - 2.4606 (2.4398) W-Oc 1.8766 - 1.9407 (1.9076) W-Oe 1.8808 - 1.9448 (1.9166) W-Ot 1.6818 - 1.7068 (1.6951) P-O 1.5286 - 1.5377 (1.5324)

W-Oa 2.4273 - 2.4568 (2.4430) W-Oc 1.9044 - 1.9164 (1.9103) W-Oe 1.9029 - 1.9234 (1.9158) W-Ot 1.7000 - 1.7038 (1.7026) P-O 1.5220 - 1.5348 (1.5313)

W-Oa 2.4313 – 2.4497 (2.4313) W-Oc 1.8840 – 1.9286 (1.9127) W-Oe 1.8996 – 1.9437 (1.9186) W-Ot 1.6890 - 1.6970 (1.6933) P-O 1.5296 – 1.5355 (1.5340)

Fig. 5. The mean values are provided in parentheses.

[(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39]

corner-sharing MO6 (M = Al and W) octahedra; Oe, bridging oxygen atoms between edgesharing MO6 octahedra (M = Al and W); Ot, terminal oxygen atoms.

ANIH+ = anilinium ion), Cs5[PMn(H2O)W11O39]4H2O, and Cs5[PNi(H2O)W11O39]2H2O as determined by X-ray crystallography are summarized in Table 2. Although a simple comparison was difficult to draw, the following trends were observed: The W-Oa bond lengths of [PCu(H2O)W11O39]5-, [PMn(H2O)W11O39]5-, and [PNi(H2O)W11O39]5- were significantly longer than those of [α-PW12O40]3- and [α-PW11ReVO40]4-, as observed for [α-PW11{Al(OH2)}O39]4- due to the presence of a water molecule coordinated to the *mono*metal-substituted sites. The W(M)-Oc and W(M)-Oe (M = Re, Cu, Mn, and Ni) bond lengths of the four polyoxoanions mentioned in Table 2 were similar to those of [α-PW12O40]3-, whereas, the bond lengths of [-PW11{Al(OH2)}O39]4- were clearly shorter than those of [α-PW12O40]3-.


Table 2. Ranges and mean bond distances (Å) for four *mono*-metal-substituted α-Keggin phosphotungstates. The terms Oa and Ot are explained in Fig. 5. The terms Oc and Oe indicate bridging oxygen atoms between corner- and edge-sharing MO6 (M = W, Re, Cu, Mn, Ni) octahedra. The mean values are provided in parentheses.

To investigate the coordination sphere around the *mono*-aluminum-substituted site in [α-PW11{Al(OH2)}O39]4-, the optimized geometry was computed by means of a DFT method, as

Synthesis and X-Ray Crystal Structure

optimized [α-PW11{Al(OH2)}O39]4-.

of -Keggin-Type Aluminum-Substituted Polyoxotungstate 611

H

Al

Oa(Al)

Oe(Al)

Ot(Al)

H

Oe(Al)

Fig. 7. The coordination sphere around the *mono*-aluminum-substituted site in DFT-

Oc(Al)

W-Oa 2.4422 – 2.5140 (2.4702) 2.4568 – 2.4579 (2.4574) W-Oc 1.8311 – 1.9828 (1.9206) 1.9202 – 1.9216 (1.9209) W-Oe 1.8373 – 1.9918 (1.9267) 1.9262 – 1.9276 (1.9267) W-Ot 1.7196 – 1.7246 (1.7210) 1.7103 – 1.7106 (1.7105) P-O 1.5450 – 1.5654 (1.5517) 1.5530 – 1.5535 (1.5533)

Table 3. Ranges and mean bond distances (Å) for [α-PW11{Al(OH2)}O39]4- and [α-PW12O40]3 optimized by DFT calculations. The terms Oa, Oc, Oe, and Ot are explained in Fig. 5.

Oa (W) -0.7356 – -0.8445 (-0.7734) -0.8951 – -0.8990 (-0.8968) Oc (W) -1.226 – -1.345 (-1.317) -1.353 – -1.355 (-1.353) Oe (W) -1.030 – -1.160 (-1.074) -1.085 – -1.087 (-1.086) Ot (W) -0.6757 – -0.6991 (-0.6882) -0.6273 – -0.6277 (-0.6275)

Table 4. Mulliken charges computed for [α-PW11{Al(OH2)}O39]4- and [α-PW12O40]3-. The terms Oa(M), Oc(M), Oe(M), and Ot(M) (M = Al and W) are explained in Figs. 6 and 7.

[α-PW11{Al(OH2)}O39]4- [α-PW12O40]3-

P

Oc(Al)

[α-PW11{Al(OH2)}O39]4- [α-PW12O40]3-

P 7.255 (7.255) 9.256 (9.256) W 2.101 – 2.343 (2.257) 2.343 – 2.346 (2.345)

Oa(Al) -0.1495 (-0.1495) - Oc(Al) -0.3332, -0.5920 (-0.4626) - Oe(Al) -0.4910, -0.7848 (-0.6379) - Ot(Al) -0.5553 (-0.5553) - Al -0.5307 (-0.5307) - H 0.5754, 0.5796 (0.5775) -

Al-Oa 1.9487 (1.9487) – Al-Oc 1.8519, 1.8955 (1.8737) – Al-Oe 1.8723, 1.9215 (1.8969) – Al-OH2 2.0983 (2.0983) –

The average values are provided in parentheses.

The average values are provided in parentheses.

shown in Figs. 6 and 7. The ranges and mean bond distances, and the Millken charges for the DFT-optimized [α-PW11{Al(OH2)}O39]4- are summarized in Tables 3 and 4. It was noted that the *mono*-aluminum-substituted site was uniquely concave downward, which caused the extension of the P-O bond linkaged to the aluminum atom (1.5654 Å), whereas the Al-O bond linkaged to the internal phosphorus atom was shortened due to the insertion of the Al3+ ion that has a smaller ionic radius (0.675 Å) than that of W6+ (0.74 Å) into the *mono*vacant site (Shannon, 1976). 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 [α-PW11{Al(OH2)}O39]4- were more positive than those linkaged to tungsten atoms in [α-PW12O40]3-; whereas the charges of oxygen atoms linkaged to tungsten atoms in [α-PW11{Al(OH2)}O39]4- were similar to those in [α-PW12O40]3-. In addition, the atomic charge of the phosphorus atom in [α-PW11{Al(OH2)}O39]4- was more negative than that in [α-PW12O40]3-. In the case of *mono*-vanadium(V)-substituted Keggin silicotungstate [SiW11VO40]5-, the net charge associated with the inner tetrahedron was very similar to that supported by SiO4 in [SiW12O40]4- (Maestre et al., 2001). Thus, the difference in the charge on the internal phosphorus atom for [α-PW11{Al(OH2)}O39]4- and [α-PW12O40]3- might be due to the gravitation of aluminum atoms towards the internal PO4 group.

Fig. 6. The DFT-optimized geometry of [α-PW11{Al(OH2)}O39]4-. The phosphorus, oxygen, aluminum, tungsten, and hydrogen atoms are represented by orange, red, pink, blue, and white balls, respectively.

shown in Figs. 6 and 7. The ranges and mean bond distances, and the Millken charges for the DFT-optimized [α-PW11{Al(OH2)}O39]4- are summarized in Tables 3 and 4. It was noted that the *mono*-aluminum-substituted site was uniquely concave downward, which caused the extension of the P-O bond linkaged to the aluminum atom (1.5654 Å), whereas the Al-O bond linkaged to the internal phosphorus atom was shortened due to the insertion of the Al3+ ion that has a smaller ionic radius (0.675 Å) than that of W6+ (0.74 Å) into the *mono*vacant site (Shannon, 1976). 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

The Mulliken charges of all oxygen atoms linkaged to aluminum atoms in [α-PW11{Al(OH2)}O39]4- were more positive than those linkaged to tungsten atoms in [α-PW12O40]3-; whereas the charges of oxygen atoms linkaged to tungsten atoms in [α-PW11{Al(OH2)}O39]4- were similar to those in [α-PW12O40]3-. In addition, the atomic charge of the phosphorus atom in [α-PW11{Al(OH2)}O39]4- was more negative than that in [α-PW12O40]3-. In the case of *mono*-vanadium(V)-substituted Keggin silicotungstate [SiW11VO40]5-, the net charge associated with the inner tetrahedron was very similar to that supported by SiO4 in [SiW12O40]4- (Maestre et al., 2001). Thus, the difference in the charge on the internal phosphorus atom for [α-PW11{Al(OH2)}O39]4- and [α-PW12O40]3- might be due to

Al

W

Oc(W)

Ot(W)

Oe(W)

Oa(W)

P

Fig. 6. The DFT-optimized geometry of [α-PW11{Al(OH2)}O39]4-. The phosphorus, oxygen, aluminum, tungsten, and hydrogen atoms are represented by orange, red, pink, blue, and

the gravitation of aluminum atoms towards the internal PO4 group.

X-ray crystallography.

white balls, respectively.

Fig. 7. The coordination sphere around the *mono*-aluminum-substituted site in DFToptimized [α-PW11{Al(OH2)}O39]4-.


Table 3. Ranges and mean bond distances (Å) for [α-PW11{Al(OH2)}O39]4- and [α-PW12O40]3 optimized by DFT calculations. The terms Oa, Oc, Oe, and Ot are explained in Fig. 5. The average values are provided in parentheses.


Table 4. Mulliken charges computed for [α-PW11{Al(OH2)}O39]4- and [α-PW12O40]3-. The terms Oa(M), Oc(M), Oe(M), and Ot(M) (M = Al and W) are explained in Figs. 6 and 7. The average values are provided in parentheses.

Synthesis and X-Ray Crystal Structure

vacant site.

PW11{Al(OH2)}O39]4-.

in the solution.

of -Keggin-Type Aluminum-Substituted Polyoxotungstate 613

[(*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

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


observed by X-ray crystallography and DFT calculations, as mentioned above.

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-


ppm

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
