Syntheses and X-Ray Crystal Structures of Magnesium-Substituted Polyoxometalates

analysis, thermogravimetric/differential thermal analysis, Fourier–transform infrared spec‐ troscopy, solution nuclear magnetic resonance spectroscopies, and density-functional-theory (DFT) calculations. The X-ray crystallography results for Cs 5.25 H 1.75 [α-PW 11 MgO 40 2 O, n C 4 H 9 ) 4 N] 4.25 H 2.75 [α-PW 11 MgO 40 ]⋅H and [(CH 2 NH 7.5 H 2.5 Thermogravimetric (TG) and differential thermal analyses (DTA) data were obtained using Rigaku Thermo Plus 2 TG/DTA TG 8120 and Rigaku Thermo Plus EVO2 TG/DTA 81205Z instruments and were performed in air while constantly increasing the temperature from 20 to 500 °C at rates of 1 and 4 °C/min. 1 H (600.17 MHz) and 31 P-{ 1 H} (242.95 MHz) nuclear magnetic resonance (NMR) spectra were recorded in tubes (outer diameter: 5 mm) on a JEOL ECA-600 NMR spectrometer (Shizuoka University). 1 H NMR spectra were measured in dimethylsulfoxide- d 6 and referenced to tetramethylsilane (TMS). Chemical shifts were reported as positive for resonances downfield of TMS (δ 0). 31 P NMR spectra were referenced to an external standard of 85% H 3 PO 4 in a sealed capillary. Negative chemical shifts were reported on the δ scale for resonances upfield of H 3 PO 4 (δ 0). 183 W NMR (25.00 MHz) spectra were recorded in tubes (outer diameter: 10 mm) on a JEOL ECA-600 NMR spectrometer (Kyushu University). The 183 W NMR spectrum measured in 2.0 mM Mg(NO 3 ) 2 -D 2 O was referenced to an external standard of saturated Na 2 WO 4 -D 2 O solution (substitution method). Chemical shifts were reported as negative for resonances upfield of Na 2 WO 4 (δ 0). rate of 4 °C/min under atmospheric conditions showed a weight loss of 3.0% with an endothermic peak at 242 °C in the temperature range of 25 to 500 °C; our calculations indicated the presence of six water molecules (calcd. 3.07%). The results were as follows: IR spectroscopy (KBr disk): 1081s, 1058s, 961s, 888s, 830m, 808m, 769m, 724m cm –1 ; IR spectroscopy (in water): 1079m, 1056s, 1017w, 957s, 898m, 823m, 779w, 724w cm –1 ; 31 P NMR (27 °C, D 2 O): δ−10.81. atmospheric conditions at a rate of 4 °C/min showed a weight loss of 27.46% with an endo‐ thermic peak at 312.2 °C and an exothermic peak at 412.7 °C in the temperature range of 25 to 500 °C; our calculations indicated the presence of 4.25[(C 4 H 9 ) 4 N] + ions, a water molecule, and an acetonitrile molecule (calcd. 28.6%). The results were as follows: IR spectroscopy (KBr disk): 1081m, 1060s, 957s, 891s, 819s, 734s cm –1 ; IR spectroscopy (in acetonitrile): 1082m, 1059s, 955s, 889s, 811s, 733s cm –1 ; 31 P NMR (20.5 °C, acetonitrile- d 3 ): δ −10.26. 823, 786, and 736 cm –1 and 1087, 1065, 1018, 948, 919, 891, 805, 777, and 717 cm –1 , respectively. These bands were different from those of K 10 [α 2 -P 2 W 17 O 61 ]⋅14H 2 O (1084, 1051, 1016, 940, 918, 887, 811, 740, and 601 cm –1 ), which also supported that a magnesium atom was coordinated in the vacant site of [α 2 P 2 W 17 O 61 ] 10– . The spectral pattern of solid [(CH 3 ) 2 NH 2 ] 7.5 H 2.5 [α 2 -P 2 W 17 MgO 62 ]⋅6H 2 O was quite similar to that in water (1086, 1065, 1020, 945, 913, 808, 790, and 723 cm –1 ); this suggested that the molecular structure of [(CH 3 ) 2 NH 2 ] 7.5 H 2.5 [α 2 -P 2 W 17 MgO 62 ]⋅6H 2 O observed in a solid was maintained in an aqueous solution. In addition, the FT-IR spectrum of [(CH 3 ) 2 NH 2 ] 7.5 H 2.5 [α 2 P 2 W 17 MgO 62 ]⋅6H 2 O observed in water was the same as that of a liquid sample of K 8 H 2 [α 2 P 2 W 17 MgO 62 ]⋅15H 2 O (1086, 1064, 1015, 946, 914, 811, 788, and 724 cm –1 ). These results showed that the molecular structure of potassium salt was the same as that of the dimethylammonium salt, as suggested by the 31 P NMR spectra of the salts in D 2 O. mono-lacunary characterized X-ray crystallography, elemental thermogravimetric/differential thermal Fourier-transform infrared solution W nuclear magnetic resonance spectroscopy. The single-crystal X-ray structure analyses


Introduction
Polyoxometalates have attracted much attention in the fields of catalytic chemistry, surface science, and materials science because their acidity, redox property, and solubility in various media can be controlled at molecular levels [1 -3]. In particular, coordination of metal ions and organometallics to the vacant site(s) of lacunary polyoxometalates is one of the powerful techniques to construct effective and well-defined metal centers. Among the various metals and their derivatives that can be coordinated to the vacant site(s) of lacunary polyoxometalates, magnesium and magnesium derivatives are intriguing because of their efficient properties as catalysts, reagents for organic syntheses, pharmaceutical compounds, and so on [4,5]. However, magnesium-coordinated polyoxometalates (characterized by X-ray crystallography) are still one of the least reported compounds: Examples that have been reported include Mg 8 8 [7].
In this study, we first report the syntheses and molecular structures of cesium and tetra-nbutylammonium salts of α-Keggin-type mono-magnesium-substituted polyoxotungstate, i.e., Cs 5 17 MgO 62 ]⋅6H 2 O; these salts were characterized via X-ray crystallography, elemental analysis, thermogravimetric/differential thermal analysis, Fourier-transform infrared spectroscopy, solution nuclear magnetic resonance spectroscopies, and density-functional-theory (DFT) calculations. The X-ray crystallography results for Cs 5 2 O showed that the mono-magnesium-substituted sites in the α-Keggin and α-Dawson structures could not be identified because of the high symmetry of the compounds, as has been observed for mono-metal-substituted polyoxometalates; however, the bonding modes (i.e., bond lengths and angles) were significantly influenced by the insertion of magnesium ions into the vacant sites. The DFT calculation results also showed that coordination of a hydroxyl group and water molecule to the mono-magnesium-substituted site distorted the molecular structures.  [9] were prepared as described in the literature. The number of solvated water molecules was determined by thermogravimetric/differential thermal analyses. All the reagents and solvents were obtained and used as received from commercial sources.

Instrumentation/analytical procedures
Elemental analyses were carried out by Mikroanalytisches Labor Pascher (Remagen, Germany). Prior to analysis, the samples were dried overnight at room temperature under pressures of 10 -3 -10 -4 Torr. Infrared spectra of the solid samples were recorded on a Perkin Elmer Spectrum100 FT-IR spectrometer in KBr disks at around 25 °C in air. Infrared spectra of the liquid samples were recorded on a Perkin Elmer Frontier FT-IR spectrometer attached to a Universal ATR sampling accessory at around 25 °C in air. Thermogravimetric (TG) and differential thermal analyses (DTA) data were obtained using Rigaku Thermo Plus 2 TG/DTA TG 8120 and Rigaku Thermo Plus EVO2 TG/DTA 81205Z instruments and were performed in air while constantly increasing the temperature from 20 to 500 °C at rates of 1 and 4 °C/min. 1 H (600.17 MHz) and 31 P-{ 1 H} (242.95 MHz) nuclear magnetic resonance (NMR) spectra were recorded in tubes (outer diameter: 5 mm) on a JEOL ECA-600 NMR spectrometer (Shizuoka University). 1 H NMR spectra were measured in dimethylsulfoxide-d 6 and referenced to tetramethylsilane (TMS). Chemical shifts were reported as positive for resonances downfield of TMS (δ 0). 31 P NMR spectra were referenced to an external standard of 85% H 3 PO 4 in a sealed capillary. Negative chemical shifts were reported on the δ scale for resonances upfield of H 3 PO 4 (δ 0). 183 W NMR (25.00 MHz) spectra were recorded in tubes (outer diameter: 10 mm) on a JEOL ECA-600 NMR spectrometer (Kyushu University). The 183 W NMR spectrum measured in 2.0 mM Mg(NO 3 ) 2 -D 2 O was referenced to an external standard of saturated Na 2 WO 4 -D 2 O solution (substitution method). Chemical shifts were reported as negative for resonances upfield of Na 2 WO 4 (δ 0).

X-Ray crystallography
A colorless prism crystal of Cs 5 were mounted on a loop or MicroMount. The measurements for the cesium and tetra-nbutylammonium salts of α-Keggin mono-magnesium-substituted polyoxotungstate were obtained using a Rigaku VariMax with a Saturn diffractometer using multi-layer mirrormonochromated Mo Kα radiation (λ = 0.71075 Å) at 100±1 K. The measurement for the dimethylammonium salt of α-Dawson mono-magnesium-substituted polyoxotungstate was carried out using a Rigaku VariMax with an XtaLAB P200 diffractometer using multi-layer mirror-monochromated Mo Kα radiation (λ = 0.71075 Å) at 153±1 K. Data were collected and processed using CrystalClear, CrystalClear-SM Expert for Windows, and structural analysis was performed using CrystalStructure for Windows. The structure was solved by SHELXS-97, SHELXS-2013, and SIR-2004 (direct methods) and refined by SHELXL-97 and SHELXL2013 [10,11]. In these magnesium compounds, a magnesium atom was disordered over ten and twelve tungsten atoms in [α-PW 11 MgO 40 ] 7-, and six tungsten atoms at B-sites (cap units) in [α-PW 17 MgO 62 ] 10-. The occupancies for the magnesium and tungsten atoms were fixed at 1/10 and 9/10, 1/12 and 11/12, and 1/6 and 5/6, respectively. For Cs 5 11 MgO 40 ]⋅H 2 O⋅CH 3 CN, tetra-n-butylammonium ions, water molecules, and acetonitrile molecules could not be modeled because of disorder of the atoms. Accordingly, the residual electron density was removed using the SQUEEZE routine in PLATON [12]. Disordered counter-cations and solvated molecules are common in polyoxometalate chemistry [13 -16]

Computational details
The optimal geometries of [α-PW 11 {Mg(OH)}O 39 ] 6-and [α-PW 11 {Mg(OH 2 )}O 39 ] 5-were computed using a DFT method. First, we optimized the molecular geometries and then applied singlepoint calculations with larger basis sets. All calculations were performed using a spinrestricted B3LYP method with the Gaussian09 program package [17]. The solvent effect of acetonitrile was considered using the polarizable continuum model. The basis sets used for geometry optimization were LANL2DZ for the W atoms, 6-31+G* for the P atoms, and 6-31G* for the H, O, and Mg atoms. LANL2DZ and 6-31+G* were used for the W and other atoms, respectively, for the single-point calculations. Geometry optimization was started using the Xray structure of [α-PW 12 O 40 ] 3-as the initial geometry, and was performed in acetonitrile. 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. Zero-point energycorrected total energies were used to consider the structural stabilities of [α-PW 11 {Mg(OH)}O 39 ] 6-and [α-PW 11 {Mg(OH 2 )}O 39 ] 5-.
Samples for the elemental analyses were dried overnight at room temperature under a vacuum of 10 -3 -10 -4 Torr. The elemental analysis results for H, Cs, Mg, P, and W were in good agreement with the calculated values for the formula of Cs 5.25 H 1.75 [α-PW 11 MgO 40 ]. Br analysis revealed no contamination of bromide ions from MgBr 2 . The weight loss observed during the course of drying before the analysis was 3.03% for Cs 5.25 H 1.75 [α-PW 11 MgO 40 ]⋅6H 2 O; this corresponded to six weakly solvated or adsorbed water molecules. TG/DTA measurements also showed a weight loss of 3.1% in the temperature range of 25 to 500 °C, which corresponded to six water molecules. For the tetra-n-butylammonium salt, the elemental analysis results for C, H, N, P, Mg, and W were in good agreement with the calculated values for the formula of  Table 1. The molecular structures of these magnesium compounds were identical to that of a monomeric, α-Keggin polyoxotungstate [α-PW 12 O 40 ] 3- [18,19]. Due to the high-symmetry space groups of the structures, the ten or eleven tungsten(VI) atoms were disordered and the mono-magnesium-substituted site was not identified, as previously reported for [α-PW 11 Tables 2 and 3. As shown in Fig. 3, the ligands coordinated to the monomagnesium-substituted site caused remarkable distortion of the α-Keggin molecular structure: The Mg-P distance of [α-PW 11 {Mg(OH)}O 39 ] 6-was 3.652 Å, which was longer than that of [α-PW 11 {Mg(OH 2 )}O 39 ] 5-(3.330 Å). The charges of all atoms in [α-PW 11 {Mg(OH)}O 39 ] 6-and [α-PW 11 {Mg(OH 2 )}O 39 ] 5-were also influenced by the ligands, as shown in Table 3.  11 MgO 40 ]⋅6H 2 O. The corner-and edge-sharing oxygen atoms in the α-Keggin structure were disordered. W(2) was clearly identified; however, a magnesium atom was disordered over ten tungsten sites in [α-PW 11 MgO 40 ] 7-, and the occupancies for the magnesium and tungsten sites were fixed at 1/10 and 9/10 throughout the refinement.   11 MgO 40 ]⋅H 2 O⋅CH 3 CN, a signal was also observed at −10.3 ppm. These signals exhibited a shift from the signals of K 7 [α-PW 11 O 39 ] (δ −10.6) in D 2 O and the tetra-n-butylammonium salt of [α-PW 11 O 39 ] 7-(δ −12.0) in acetonitrile-d 3 , respectively. This showed that the magnesium ion was inserted into the vacant site.
salt of [α-PW11O39] 7-(δ −12.0) in acetonitrile-d3, respectively. This showed that the magnesium ion was inserted into the vacant site.       [21,22]. This also supported that the magnesium ion was coordinated to the vacant site in the polyoxometalate.  11 MgO 40 ]⋅H 2 O⋅CH 3 CN even though the counter-ions affected their spectra [21,22]. As shown in Fig. 5

Dawson-type mono-magnesium-substituted polyoxotungstate K8H2[α2-P2W17MgO62]·15H2O and [(CH3)2NH2]7.5H2.5[α2-P2W17MgO62]·6H2O
The potassium salt of [α2-P2W17MgO62] 10 -117.1, -140.4, -151.7, -181.0, -183.1, -218.1, -220.5, -224.0, and -242.6) observed in D 2 O [28]. These results also supported that a magnesium ion was coordinated to the vacant site of [α 2 -P 2 W 17 O 61 ] 10-, resulting in an overall C s symmetry. and -13.7 ppm); this was also confirmed by the two-line spectrum observed for a mixture of the potassium and dimethylammonium salts in D2O. These signals were different from those of K10[α2-P2W17O61]·14H2O (δ -6.8 and -13.9), suggesting that a magnesium ion was coordinated to the vacant site of [α2-P2W17O61] 10  . These results showed that the molecular structure of potassium salt was the same as that of the dimethylammonium salt, as suggested by the 31 P NMR spectra of the salts in D2O.  . These results showed that the molecular structure of potassium salt was the same as that of the dimethylammonium salt, as suggested by the 31 P NMR spectra of the salts in D 2 O.

Conclusion
In  ·6H2O by reacting magnesium ions with mono-lacunary α-Keggin and α-Dawson-type phosphotungstates. The compounds were characterized by X-ray crystallography, elemental analysis, thermogravimetric/differential thermal analysis, Fourier-transform infrared spectra, and solution 31 P and 183 W nuclear magnetic resonance spectroscopy. The single-crystal X-ray structure analyses of Cs5.