Fast Microwave-Assisted Synthesis of Wells-Dawson-Type 18-Tungsto-2-Phosphate [ P 2 W 18 O 62 ] 6 −

We report on a fast microwave-assisted synthetic procedure of the Wells-Dawson-type 18-tungsto-2-phosphate [P2W18O62] with a 35% yield in 24 h, rather than a week using conventional heating.

The use of household microwaves in scientific laboratories started in the 1980s, mainly for the rapid synthesis of organic compounds [11].Later, scientific microwave instruments became available and were found to be highly suitable for heating reaction mixtures fast and energy-efficiently (green chemistry); these instruments are also used in inorganic and materials chemistry.This technique has allowed enhancing reaction rates and improving yields [11][12][13][14][15].
Microwave-assisted synthesis has also been used in POM chemistry, and one of the first studies was Bonchio's synthesis of [PW 11 O 39 Ru II (DMSO)] 5− [16].More recently, Ritchie has used this heating technique for the synthesis of various polyoxomolybdates in organic solvents [17][18][19].Our group has also studied microwave-based heating in POM chemistry, and very recently we reported on the Preyssler-Pope-Jeannin polyanions [AgP 5 W 30 O 110 ] 14− and [NaP 5 W 30 O 110 ] 14− [20].
Here, we report on a fast, microwave-assisted synthetic procedure of the 18-tungsto-2-phosphate Wells-Dawson ion, P 2 W 18 .

Results
The 18-tungsto-2-phosphate [P 2 W 18 O 62 ] 6− (P 2 W 18 ) is a plenary POM comprising 18 WO 6 octahedra and two internal PO 4 tetrahedra, resulting in a structure with two caps and two belts (see Figure 1).This polyanion is the precursor for the synthesis of several interesting and useful lacunary polyanions, such as [P 2 W 17 O 61 ] ] 33− .Considering that the conventional synthesis of P 2 W 18 takes around a week [9], we decided to try and develop a significantly faster procedure by using microwave-assisted synthesis.We modified the 24 h reflux step in the conventional procedure by instead heating for 5 min in a scientific microwave oven.Then, the solution was cooled to 5 • C, and impurities were removed by filtration.KCl was added to the filtrate solid, resulting in a green-yellow precipitate, which was dissolved in hot water and then recrystallized to obtain pure P 2 W 18 with a 35% yield.The detailed synthetic procedure is shown in the Experimental Section.The yield is lower for the microwave-based procedure than for the conventional procedure, but the former is significantly faster by several days.

Results
The 18-tungsto-2-phosphate [P2W18O62] 6− (P2W18) is a plenary POM comprising 18 WO6 octahedra and two internal PO4 tetrahedra, resulting in a structure with two caps and two belts (see Figure 1).This polyanion is the precursor for the synthesis of several interesting and useful lacunary polyanions, such as [P2W17O61] 10− , [P2W15O56] 12− , [H2P2W12O48] 12− , [H6P4W24O94] 18− , and [H7P8W48O184] 33− .Considering that the conventional synthesis of P2W18 takes around a week [9], we decided to try and develop a significantly faster procedure by using microwave-assisted synthesis.We modified the 24 h reflux step in the conventional procedure by instead heating for 5 min in a scientific microwave oven.Then, the solution was cooled to 5 °C, and impurities were removed by filtration.KCl was added to the filtrate solid, resulting in a green-yellow precipitate, which was dissolved in hot water and then recrystallized to obtain pure P2W18 with a 35% yield.The detailed synthetic procedure is shown in the Experimental Section.The yield is lower for the microwave-based procedure than for the conventional procedure, but the former is significantly faster by several days.Both potassium salts of P2W18 (K-P2W18) synthesized by the microwave-based (see Exp. Section) and conventional procedures, respectively, were characterized by FT-IR, 31 P NMR, and thermogravimetric analysis (TGA).The IR spectra of both K-P2W18 products were identical (Figure 2), with the fingerprint region exhibiting signals at 1091(s), 1020(m), 960(s), 917(s), 780(s), 600(w), 568(w), 531(w), and 476(w) cm −1 (s = strong, m = medium, w = weak).Both potassium salts of P 2 W 18 (K-P 2 W 18 ) synthesized by the microwave-based (see Exp. Section) and conventional procedures, respectively, were characterized by FT-IR, 31 P NMR, and thermogravimetric analysis (TGA).The IR spectra of both K-P 2 W 18 products were identical (Figure 2), with the fingerprint region exhibiting signals at 1091(s), 1020(m), 960(s), 917(s), 780(s), 600(w), 568(w), 531(w), and 476(w) cm −1 (s = strong, m = medium, w = weak).

Results
The 18-tungsto-2-phosphate [P2W18O62] 6− (P2W18) is a plenary POM comprising 18 WO6 octahedra and two internal PO4 tetrahedra, resulting in a structure with two caps and two belts (see Figure 1).This polyanion is the precursor for the synthesis of several interesting and useful lacunary polyanions, such as [P2W17O61] 10− , [P2W15O56] 12− , [H2P2W12O48] 12− , [H6P4W24O94] 18− , and [H7P8W48O184] 33− .Considering that the conventional synthesis of P2W18 takes around a week [9], we decided to try and develop a significantly faster procedure by using microwave-assisted synthesis.We modified the 24 h reflux step in the conventional procedure by instead heating for 5 min in a scientific microwave oven.Then, the solution was cooled to 5 °C, and impurities were removed by filtration.KCl was added to the filtrate solid, resulting in a green-yellow precipitate, which was dissolved in hot water and then recrystallized to obtain pure P2W18 with a 35% yield.The detailed synthetic procedure is shown in the Experimental Section.The yield is lower for the microwave-based procedure than for the conventional procedure, but the former is significantly faster by several days.Both potassium salts of P2W18 (K-P2W18) synthesized by the microwave-based (see Exp. Section) and conventional procedures, respectively, were characterized by FT-IR, 31 P NMR, and thermogravimetric analysis (TGA).The IR spectra of both K-P2W18 products were identical (Figure 2), with the fingerprint region exhibiting signals at 1091(s), 1020(m), 960(s), 917(s), 780(s), 600(w), 568(w), 531(w), and 476(w) cm −1 (s = strong, m = medium, w = weak).The 31 P NMR spectrum of K-P 2 W 18 synthesized by the microwave-assisted procedure shows a singlet at −12.4 ppm, just like the material prepared conventionally (see Figure 3).This result is consistent with the observations of earlier reports [7][8][9][10].It should be noted that very small amounts of the beta-isomer [β-P 2 W 18 O 62 ] 6− (−10.9 and −11.6 ppm) and free phosphate (0.7 ppm) are also present.Finke's group has demonstrated how to further purify such a mixture, in order to obtain isomerically clean material [10].
The 31 P NMR spectrum of K-P2W18 synthesized by the microwave-assisted procedure shows a singlet at −12.4 ppm, just like the material prepared conventionally (see Figure 3).This result is consistent with the observations of earlier reports [7][8][9][10].It should be noted that very small amounts of the beta-isomer [β-P2W18O62] 6− (−10.9 and −11.6 ppm) and free phosphate (0.7 ppm) are also present.Finke's group has demonstrated how to further purify such a mixture, in order to obtain isomerically clean material [10].The 31 P NMR spectrum of K-P2W18 synthesized by the microwave-assisted procedure shows a singlet at −12.4 ppm, just like the material prepared conventionally (see Figure 3).This result is consistent with the observations of earlier reports [7][8][9][10].It should be noted that very small amounts of the beta-isomer [β-P2W18O62] 6− (−10.9 and −11.6 ppm) and free phosphate (0.7 ppm) are also present.Finke's group has demonstrated how to further purify such a mixture, in order to obtain isomerically clean material [10].

Figure 1 .
Figure 1.Polyhedral representation of P2W18, with the two belts of six WO6 octahedra each in red, the two caps of three WO6 octahedra each in turquoise, and the two PO4 tetrahedra in yellow.

Figure 1 .
Figure 1.Polyhedral representation of P 2 W 18 , with the two belts of six WO 6 octahedra each in red, the two caps of three WO 6 octahedra each in turquoise, and the two PO 4 tetrahedra in yellow.

Figure 1 .
Figure 1.Polyhedral representation of P2W18, with the two belts of six WO6 octahedra each in red, the two caps of three WO6 octahedra each in turquoise, and the two PO4 tetrahedra in yellow.

Figure 3 .
Figure 3. 31 P NMR spectra of solid K-P2W18 dissolved in H2O/D2O, (lower) synthesized by microwaveassisted heating and (upper) by conventional heating.See text for details.

Figure 4
Figure4shows the thermograms of K-P2W18 synthesized by microwave-assisted and conventional heating.Both samples were dried overnight in an oven at 50 °C before the TGA analysis.Between room temperature and 250 °C the microwave-synthesized K-P2W18 exhibited a weight loss of ca.4%, as compared to ca.3.5% for the conventionally synthesized K-P2W18.This results in the formulas K6[α-P2W18O62]•11H2O (microwave heating) and K6[α-P2W18O62]•9H2O (conventional heating).

Figure 3 .
Figure 3. 31 P NMR spectra of solid K-P 2 W 18 dissolved in H 2 O/D 2 O, (lower) synthesized by microwave-assisted heating and (upper) by conventional heating.See text for details.

Figure 4
Figure 4 shows the thermograms of K-P 2 W 18 synthesized by microwave-assisted and conventional heating.Both samples were dried overnight in an oven at 50 • C before the TGA analysis.Between room temperature and 250 • C the microwave-synthesized K-P 2 W 18 exhibited a weight loss of ca.4%, as compared to ca. 3.5% for the conventionally synthesized K-P 2 W 18 .This results in the formulas K 6 [α-P 2 W 18 O 62 ]•11H 2 O (microwave heating) and K 6 [α-P 2 W 18 O 62 ]•9H 2 O (conventional heating).

Figure 3 .
Figure 3. 31 P NMR spectra of solid K-P2W18 dissolved in H2O/D2O, (lower) synthesized by microwaveassisted heating and (upper) by conventional heating.See text for details.

Figure 4
Figure4shows the thermograms of K-P2W18 synthesized by microwave-assisted and conventional heating.Both samples were dried overnight in an oven at 50 °C before the TGA analysis.Between room temperature and 250 °C the microwave-synthesized K-P2W18 exhibited a weight loss of ca.4%, as compared to ca.3.5% for the conventionally synthesized K-P2W18.This results in the formulas K6[α-P2W18O62]•11H2O (microwave heating) and K6[α-P2W18O62]•9H2O (conventional heating).