**2.3 Synthesis of [(***n***-C4H9)4N]4[α-PW11{Al(OH2)}O39]**

Cs7[γ-PW10O36]19H2O (2.00 g; 0.538 mmol) was dissolved in water (600 mL) at 40 °C, and solid Al(NO3)39H2O (0.250 g, 0.666 mmol) was added to the colorless clear solution. After stirring for 1 h at 40 ºC, a solid [(*n*-C4H9)4N]4Br (12.14 g; 37.7 mmol) was added to the solution, followed by stirring at 25 °C for 3 days. The white precipitate was collected on a glass frit (G4) and washed with water (ca. 1 L). At this stage, a crude product was obtained in a 1.662 g yield. The crude product (1.662 g) was dissolved in acetonitrile (10 mL), followed by filtering through a folded filter paper (Whatman #5). After the product was left standing for a week at 25 °C, colorless platelet crystals were formed. The obtained crystals weighted 0.752 g (the yield calculated considering that [mol of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39]/[mol of Cs7[γ-PW10O36]19H2O] 100 was 36.9%). The elemental analysis results were as follows: C, 20.73; H, 4.00; N, 1.58; P, 0.84; Al, 0.77; W, 54.6; Cs, <0.1%. The calculated values for [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] = C64H146AlN4O40PW11: C, 20.82; H, 3.99; N, 1.52; P, 0.84; Al, 0.73; W, 54.77; Cs, 0%. A weight loss of 2.16% was observed in the product during overnight drying at room temperature under 10-3–10-4 Torr before the analysis, thereby suggesting the presence of two weakly solvated or adsorbed acetonitrile molecules (2.18%). TG/DTA under atmospheric conditions showed a weight loss of 31.0% with an exothermic peak at 337 °C was observed in the temperature range from 25 to 500 °C; our calculations indicated the presence of four [(C4H9)4N]+ ions, two acetonitrile molecules, and a water molecule (calcd. 28.4%). The results were as follows: IR soectroscopy results (KBr disk): 1078s, 964s, 887s, 818s, 749m, 702w, 518w cm–1; 31P NMR (25C, acetonitrile-*d*3): δ -12.5; 27Al NMR (25 C, acetonitrile-*d*3): δ 16.1; 183W NMR (25 C, acetonitrile-*d*3): -56.2 (2W), -93.1 (2W), -108.6 (2W), -115.8 (2W), -118.5 (1W), -153.9 (2W).
