Synthesis , Structure , and Characterization of In 10-Containing Open-Wells – Dawson Polyoxometalate

We have successfully synthesized K17{[{KIn2(μ-OH)2}(α,α-Si2W18O66)]2[In6(μ-OH)13 (H2O)8]} ̈ 35H2O (potassium salt of In10-open), an open-Wells–Dawson polyoxometalate (POM) containing ten indium metal atoms. This novel compound was characterized by X-ray crystallography, 29Si NMR, FTIR, complete elemental analysis, and TG/DTA. X-ray crystallography results for {[{KIn2(μ-OH)2}(α,α-Si2W18O66)]2[In6(μ-OH)13(H2O)8]}17 ́ (In10-open) revealed two open-Wells–Dawson units containing two In3+ ions and a K+ ion, [{KIn2(μ-OH)2}(α,α-Si2W18O66)]11 ́, connected by an In6-hydroxide cluster moiety, [In6(μ-OH)13(H2O)8]. In10-open is the first example of an open-Wells–Dawson POM containing a fifth-period element. Moreover, to the best of our knowledge, it exhibits the highest nuclearity among the indium-containing POMs reported to date.

In this study, we successfully synthesized an open-Wells-Dawson POM containing ten indium metal ions, In 10 -open), and characterized it by X-ray crystallography, 29 Si NMR, FTIR, complete elemental analysis, and thermogravimetric/differential thermal analyses (TG/DTA).In contrast to Al 4 -and Ga 4 -open, In 10 -open showed a dimer structure bridged by a deca-indium-hydroxide cluster.

Synthesis
The crystalline sample of potassium salt of In 10 -open, was afforded in 17.9% yield.This complex was prepared from a 1:5 molar ratio reaction of with InCl 3 .The sample was characterized using complete elemental analysis (H, In, K, O, Si, and W analyses), FTIR, TG/DTA, 29 Si NMR in D 2 O, and X-ray crystallography.
The FTIR spectrum of potassium salt of Before elemental analysis, the sample of In 10 -open was dried overnight at room temperature under vacuum (10 ´3-10 ´4 Torr).All elements (H, In, K, O, Si, and W) were observed for a total analysis of 100.37%.The recorded data were in good accordance with the calculated values for the formula without water of crystallization, K 17 [{KIn 2 (µ-OH) 2 }(α,α-Si 2 W 18 O 66 )] 2 [In 6 (µ-OH) 13 (H 2 O) 8 ] (see Experimental section).The weight loss observed during drying, before analysis, was 5.28% corresponding to ca. 35 crystalline water molecules.On the other hand, during the TG/DTA measurements carried out under atmospheric conditions, a weight loss of 6.40%, observed at temperatures below 500 ˝C, corresponding to a total of ca.42 water molecules, i.e., 8 coordinated water molecules and 34 molecules of water of crystallization (Figure S2).Thus, the elemental analysis and TG/DTA displayed a presence of a total of 34-35 water molecules for the sample under atmospheric conditions.The formula for potassium salt of In 10 -open presented herein was determined as K 17 [{KIn 2 (µ-OH) 2 }(α,α-Si 2 W 18 O 66 )] 2 [In 6 (µ-OH) 13 (H 2 O) 8 ]¨35H 2 O based on the results of the complete elemental analysis.

Solution 29 Si NMR
The solution 29

Instrumentation and Analytical Procedures
A complete elemental analysis was carried out by Mikroanalytisches Labor Pascher (Remagen, Germany).The sample was dried overnight at room temperature under a pressure of 10 ´3-10 ´4 Torr before analysis.The 29 Si NMR (119.24MHz) spectra in D 2 O solution were recorded in 5-mm outer diameter tubes, on a JEOL JNM ECP 500 FTNMR spectrometer with a JEOL ECP-500 NMR data-processing system (JEOL, Akishima, Japan).The 29 Si NMR spectrum was referenced to an internal standard of DSS.Infrared spectra were recorded on a Jasco 4100 FTIR spectrometer (Jasco, Hachioji, Japan) by using KBr disks at room temperature.TG/DTA measurements were performed using a Rigaku Thermo Plus 2 series TG8120 instrument (Rigaku, Akishima, Japan), under air flow with a temperature ramp of 4.0 ˝C per min at a temperature ranging between 26 and 500 ˝C.The crystalline sample was soluble in water, but insoluble in organic solvents such as methanol, ethanol, and diethyl ether.Elemental analysis (%) calcd.for H 33

X-Ray Crystallography
For In 10 -open, a single crystal with dimensions of 0.21 ˆ0.06 ˆ0.05 mm 3 was surrounded by liquid paraffin (Paratone-N) and analyzed at 150(2) K.All measurements were performed on a Rigaku MicroMax-007HF with a Saturn CCD diffractometer (Rigaku).The structure was solved by direct methods (SHELXS-97), followed by difference Fourier calculations and refinement by a full-matrix least-squares procedure on F 2 (program SHELXL-97) [47].
In 10 -open (FigureS1) displays peaks at 1000 and 945 cm ´1 that correspond to ν as (Si-O) and ν as (W-O t ), respectively.The characteristic bands at 900-600 cm ´1 are associated with ν(W-O c ), ν(W-O b ), and ν(W-O-W).The IR spectrum is very similar to those of the common open-Wells-Dawson POMs.

Figure 2 .
Figure 2. The metal arrangement of (a) In10-open and (b) V2-open in the open pocket.
-open has a dimeric structure composed of two indium-containing open-Wells-Dawson POM moieties bridged by In6 hydroxide clusters.Dimeric open-Wells-Dawson POMs, similar to In10-open, have also been reported for {[Zn6(μ-OH)7(H2O)(α,α-Si2W18O66)]2} 22− (Zn12-open) by Hill et al. [23].In this complex, the six zinc atoms are included in the open pocket of the open-Wells-Dawson unit, and the two Zn6-containing open-Wells-Dawson units are connected through the two edge-sharing oxygen atoms.The arrangement and the number of metal ions in the open-pocket of the In10-open are different from those of the Zn12-open reported previously.In open-Wells-Dawson POMs, the bite angle can be defined as the dihedral angle between the planes that pass through the six oxygen atoms of the lacunary site of each trilacunary Keggin unit.The bite angle varies, depending on the metal cluster included in the open pocket of the open-Wells-Dawson unit.The bite angles of In10-open are 64.363° and 65.139° (Figure 3).These values are wider than those of other open-Wells-Dawson POMs, including other group 13 ions, such as Al4-(54.274°)andGa4-open (56.110°)[28].The difference between the bite angles of Al4-, Ga4-, and In10-open is caused by the difference in the ionic radii of the Al (0.53 Å), Ga (0.76 Å), and In (0.94 Å) ions[45,46].In10-open displays the widest bite angles when compared to previously reported open-Wells-Dawson POMs, including the Co6 (60.045°)[27],Zn6 dimer (60.308°)[23],Ni5 (58.925°)[24], and Cu5 (61.663°)[15,17] clusters.The open-Wells-Dawson POM containing a Cu5 cluster (Cu5-open) exhibits a large bite angle (61.663°) due to the long bond lengths between the copper and the edge-sharing oxygen atom, caused by Jahn-Teller distortion [15,17].The bite angles of In10-open are ca.3° wider than that of Cu5-open.This increase appears to be caused by the large ionic radius of the indium ions incorporated in the open pocket.

Figure 2 .
Figure 2. The metal arrangement of (a) In 10 -open and (b) V 2 -open in the open pocket.
10 -open has a dimeric structure composed of two indium-containing open-Wells-Dawson POM moieties bridged by In 6 hydroxide clusters.Dimeric open-Wells-Dawson POMs, similar to In 10 -open, have also been reported for {[Zn 6 (µ-OH) 7 (H 2 O)(α,α-Si 2 W 18 O 66 )] 2 } 22´( Zn 12 -open) by Hill et al. [23].In this complex, the six zinc atoms are included in the open pocket of the open-Wells-Dawson unit, and the two Zn 6 -containing open-Wells-Dawson units are connected through the two edge-sharing oxygen atoms.The arrangement and the number of metal ions in the open-pocket of the In 10 -open are different from those of the Zn 12 -open reported previously.In open-Wells-Dawson POMs, the bite angle can be defined as the dihedral angle between the planes that pass through the six oxygen atoms of the lacunary site of each trilacunary Keggin unit.The bite angle varies, depending on the metal cluster included in the open pocket of the open-Wells-Dawson unit.The bite angles of In 10 -open are 64.363˝and 65.139 ˝(Figure 3).These values are wider than those of other open-Wells-Dawson POMs, including other group 13 ions, such as Al 4 -(54.274˝) and Ga 4 -open (56.110 ˝) [28].The difference between the bite angles of Al 4 -, Ga 4 -, and In 10 -open is caused by the difference in the ionic radii of the Al (0.53 Å), Ga (0.76 Å), and In (0.94 Å) ions [45,46].In 10 -open displays the widest bite angles when compared to previously reported open-Wells-Dawson POMs, including the Co 6 (60.045 ˝) [27], Zn 6 dimer (60.308 ˝) [23], Ni 5 (58.925 ˝) [24], and Cu 5 (61.663 ˝) [15,17] clusters.The open-Wells-Dawson POM containing a Cu 5 cluster (Cu 5 -open) exhibits a large bite angle (61.663 ˝) due to the long bond lengths between the copper and the edge-sharing oxygen atom, caused by Jahn-Teller distortion [15,17].The bite angles of In 10 -open are ca.3 ˝wider than that of Cu 5 -open.This increase appears to be caused by the large ionic radius of the indium ions incorporated in the open pocket.

Figure 3 .
Figure 3. Bite angles of In10-open.The previously reported open-Wells-Dawson POMs mainly accommodated the fourth-period elements.Except for the lanthanoid-containing open-Wells-Dawson POMs, whose open-pockets weakly coordinate to the lanthanoid ions, open-Wells-Dawson POMs that accommodate the larger fifth-and sixth-period elements have not been reported to date.Thus, In10-open indicates that elements (such as indium) having large ionic radii (0.94 Å) can be incorporated in the open-pocket of an open-Wells-Dawson unit.
Si NMR spectrum of In10-open in D2O displays a two-line spectrum at −82.415 and −83.159 ppm in a 1:1 ratio (Figure 4).The two Si atoms in one open-Wells-Dawson unit are nonequivalent due to the configuration of the other open-Wells-Dawson unit, even though the two units are equivalent.The adjacent two 29 Si NMR peaks are consistent with the structure of In10-open observed by X-ray crystallography.This suggests that In10-open exists as a single species and maintains its structure in solution.

Figure 4 .
Figure 4. Solution 29 Si NMR spectrum of potassium salt of In10-open dissolved in D2O.

2. 3 .of 9 Figure 3 .
Figure 3. Bite angles of In10-open.The previously reported open-Wells-Dawson POMs mainly accommodated the fourth-period elements.Except for the lanthanoid-containing open-Wells-Dawson POMs, whose open-pockets weakly coordinate to the lanthanoid ions, open-Wells-Dawson POMs that accommodate the larger fifth-and sixth-period elements have not been reported to date.Thus, In10-open indicates that elements (such as indium) having large ionic radii (0.94 Å) can be incorporated in the open-pocket of an open-Wells-Dawson unit.

2. 3 .
Solution29 Si NMR The solution29 Si NMR spectrum of In10-open in D2O displays a two-line spectrum at −82.415 and −83.159 ppm in a 1:1 ratio (Figure 4).The two Si atoms in one open-Wells-Dawson unit are nonequivalent due to the configuration of the other open-Wells-Dawson unit, even though the two units are equivalent.The adjacent two 29 Si NMR peaks are consistent with the structure of In10-open observed by X-ray crystallography.This suggests that In10-open exists as a single species and maintains its structure in solution.

Figure 4 .
Figure 4. Solution 29 Si NMR spectrum of potassium salt of In10-open dissolved in D2O.

Figure 4 .
Figure 4. Solution 29 Si NMR spectrum of potassium salt of In 10 -open dissolved in D 2 O.
summary, we prepared and characterized an open-Wells-Dawson structural POM, potassium salt of In 10 -open, containing ten indium ions, i.e., K 17 {[{KIn 2 (µ-OH) 2 }(α,α-Si 2 W 18 O 66 )] 2 [In 6 (µ-OH) 13 (H 2 O) 8 ]}¨35H 2 O (potassium salt of In 10 -open).Single-crystal X-ray analyses revealed that two open-Wells-Dawson units that include two In 3+ ions and a K + ion are connected by an In 6 -hydroxide cluster moiety to form a dimeric open-Wells-Dawson polyanion.In 10 -open displayed the widest bite angles among the previously reported open-Wells-Dawson POMs.This is mainly due to the large ionic radius of the indium ion.The solution 29 Si spectrum in D 2 O indicated that In 10 -open was obtained as a single species and that its structure was maintained in solution.In 10 -open is the first example of an open-Wells-Dawson POM containing a fifth-period element, and it exhibits the highest nuclearity of any indium-containing POM reported to date.This work can be extended to the future molecular design of novel open-Wells-Dawson POMs containing large fifth-and sixth-period elements, such as Ru, Rh, Pd, Pt.Studies of open-Wells-Dawson structural POMs containing larger metal atoms are in progress.Supplementary Materials: The following are available online at www.mdpi.com/2304-6740/4/2/16/s1, Figure S1: FT-IR spectrum of potassium salt of In 10 -open (KBr disk), Figure S2: TG/DTA data of potassium salt of In 10 -open (from 22 to 500 ˝C), Table S1: Bond valence sum (BVS) calculations of In and O atoms of the Indium-cluster moieties of In 10 -open: checkCIF/PLATON report.