[α-GeW11O39] (M = Nd, Sm, Y, Y

ABSTRACT: Five novel polyoxometalate compounds built on the lacunary Keggin-type polyanion [GeW11O39]8- and trivalent rare earth ions (M ) Nd, Sm, Y, ...
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Novel Rare Earth Germanotungstates and Organic Hybrid Derivatives: Synthesis and Structures of M/[r-GeW11O39] (M ) Nd, Sm, Y, Yb) and Sm/[r-GeW11O39](DMSO) Jing-Ping Wang,† Xian-Ying Duan,‡ Xiao-Di Du,† and Jing-Yang Niu*,†

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 10 2266-2270

Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan UniVersity, Kaifeng 475001, P. R. China, and State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed July 5, 2005; ReVised Manuscript ReceiVed July 6, 2006

ABSTRACT: Five novel polyoxometalate compounds built on the lacunary Keggin-type polyanion [GeW11O39]8- and trivalent rare earth ions (M ) Nd, Sm, Y, Yb), [(CH3)4N]2H1.50[Nd1.50(GeW11O39)(H2O)6]‚3H2O (1), [(CH3)4N]2H2.25[Sm1.25(GeW11O39) (H2O)4]‚ 3.75H2O (2), [(CH3)4N]2.50H2.50[Y(GeW11O39)(H2O)2]‚4H2O (3), [(CH3)4N]2.50H2.50Yb(GeW11O39)(H2O)2]‚3.75H2O (4), and [Sm(H2O)7]0.5H0.5[Sm2(GeW11O39)(DMSO)3(H2O)6]‚3.5H2O (5) (DMSO ) dimethyl sulfoxide) have been synthesized and characterized by elemental analysis, IR and UV spectroscopy, and single-crystal X-ray diffraction. The solid-state structures of compounds 1 and 2 consist of one-dimensional zigzag chains built of [R-GeW11O39]8- anions connected by Nd3+/Sm3+ cations, while the linkage of the building blocks by Y3+/Yb3+ centers in 3 and 4 leads to the formation of linear wires. Compound 5 displays a double-parallel chainlike structure constructed by two linear wires linked by samarium coordination cations. Introduction Polyoxometalates (POMs) are metal-oxygen clusters with a tremendous structural variety and interesting properties in different fields including catalysis, medicine, and material science.1-5 Because of the remarkable features of metal oxide surfaces and their diversity in geometric topology, there recently has been increasing interest in the assembly of polyoxometalate clusters into extended inorganic or hybrid inorganic-organic solids.6 By means of their multiple coordination requirement and oxophilicity, trivalent rare earth ions are suitable to link polyoxometalate building blocks to form new classes of materials with potentially useful magnetic and luminescent properties.7-9 In 1971, Peacock and Weakley first put forward that the monovacant Keggin anions [XW11O39]n- (X ) SiIV, PV) form both 1:1 and 1:2 compounds with rare earth ions in solution.10 For a period of time, only 1:2 complexes were isolated and characterized. Finally, in 2000, Pope et al. reported the structural characterization of one-dimensional (1D) 1:1 [Ln(R-SiW11O39)(H2O)3]5- (Ln ) LaIII, CeIII) compounds, showing that these anions are polymeric in the solid state.11 Then, a fascinating interest in rare earth/monovacant POMs system was provoked, and some studies related to the non-1:2 type have been performed.9b,11-13 In 1977, Herve´ and Te´ze´ reported on different isomers of the first monolacunary germanotungstate, [GeW11O39]8-.14 In 1980, Tourne´ et al. reported on the first example of the rare earth derivative of the monolacunary germanotungstate, Cs12[U(GeW11O39)2]‚13-14H2O.15 In 1987, Liu et al reported on a series of compounds of bis(undecatungstogermanate) lanthanides,16 in which all the anions bear a structure similar to that of Cs12[U(GeW11O39)2]‚13-14H2O; no other crystal structures of rare earth cations with the [R-GeW11O39]8- polyanion have been reported. As for the [R-GeW11O39]8- polyanion, which has larger bite angle in the vacant site than that of [R-SiW11O39]8-, when it binds rare earth ions, novel structures * To whom correspondence should be addressed. E-mail: jyniu@ henu.edu.cn. † Henan University. ‡ Nanjing University.

may appear. In the search for novel polyoxometalates that may have potential applications in catalysis and material science, five novel compounds [(CH3)4N]2H1.50[Nd1.50(GeW11O39)(H2O)6]‚3H2O (1), [(CH3)4N]2H2.25[Sm1.25 (GeW11O39)(H2O)4]‚ 3.75H2O (2), [(CH3)4N]2.50H2.50[Y(GeW11O39)(H2O)2]‚4H2O (3), [(CH3)4N]2.50H2.50[Yb(GeW11O39)(H2O)2]‚3.75H2O (4), and [Sm(H2O)7]0.5H0.5[Sm2(GeW11O39)(DMSO)3(H2O)6]‚3.5H2O (5) (DMSO ) dimethyl sulfoxide) were synthesized. The structures of these compounds exhibit three kinds of distinct polymeric chains. The solid-state structures of compounds 1 and 2 consist of 1D zigzag chains built of [R-GeW11O39]8- anions connected by Nd3+/Sm3+ cations, while the linkage of the building blocks by Y3+/Yb3+ centers in 3 and 4 leads to the formation of linear wires. In 5, the introduction of the organic molecules DMSO leads to a double-parallel chainlike structure constructed of two linear chains {[Sm(1)(GeW11O39)(DMSO)(H2O)2]5-}n linked by samarium coordination cations. Experimental Section General Methods and Materials. The precursor R-K8GeW11O39‚ nH2O was prepared according to the literature17 and confirmed by IR spectroscopy. Other reagents were purchased commercially and used without further purification. Elemental analyses (C, H, N, and S) were conducted on a Perkin-Elmer 240C analyzer. Inductively coupled plasma (ICP) analyses were carried out on a Jarrel-Ash J-A1100 spectrometer. Infrared spectra were obtained from a sample powder palletized with KBr on Nicolet AVATAR 360 FTIR spectrophotometer over the range 4000-400 cm-1. UV spectra were obtained on a Unican UV-500 spectrometer (distilled water as solvent) in the range of 400190 nm. Synthesis of [(CH3)4N]2H1.50[Nd1.50(GeW11O39)(H2O)6]‚3H2O (1). A 2.32 g (0.71 mmol) sample of R-K8[GeW11O39]‚nH2O was dissolved in 30 mL of water at 80 °C, followed by a dropwise addition of 0.35 g (1.42 mmol) of NdCl3 in 15 mL of water. After 1 h, the solution was cooled to room temperature, and the precipitate was removed by filtration. Then 0.4 g of tetramethylammonium bromide was added. The resulting clear solution was filtered and left to evaporate at room temperature. The next day, mauve pillar-like crystals of 1, suitable for X-ray diffraction, were collected. Yield: 1.96 g (85%). IR (KBr pellets, ν/cm-1): 949 (s), 873 (s), 809 (s), 705 (s), 668 (sh), 523 (m). Anal. Calcd for C8H43.50O48N2GeNd1.50W11: C, 2.96; H, 1.35; N, 0.86; Ge, 2.24; Nd, 6.66; W, 62.28. Found: C, 3.00; H, 1.30; N, 0.91; Ge, 2.12; Nd, 6.60; W, 61.20.

10.1021/cg050321s CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006

Novel Rare Earth Germanotungstates

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Table 1. Crystal Data and Structure Refinement for Compounds 1-5

formula fw crystal system space group a/Å b/Å c/Å β/deg Dc/g cm-3 Z F(000) V/Å3 µ/mm-1 independent reflns reflns collected final R indices [I > 2σ(I)] R indices (all data)

1

2

3

4

5

C8H43.50O48N2 GeNd1.50W11 3247.25 monoclinic P21/n 18.846(4) 13.443(3) 27.561(6) 105.75(3) 3.210 4 5702 6720(2) 20.401 11053 19527 R1 ) 0.0546 wR2 ) 0.1368 R1 ) 0.0715 wR2 ) 0.1445

C8H41.75O46.75N2 GeSm1.25W11 3197.06 monoclinic P21/n 18.781(4) 13.388(3) 27.534(6) 105.73(3) 3.185 4 5605 6667(2) 20.497 10788 18349 R1 ) 0.0605 wR2 ) 0.1462 R1 ) 0.0875 wR2 ) 0.1589

C10H44.50O45N2.50 GeYW11 3103.83 monoclinic C2/c 27.895(6) 22.795(5) 23.877(5) 123.36(3) 3.251 8 10936 12681(4) 21.319 10900 19595 R1 ) 0.0674 wR2 ) 0.1609 R1 ) 0.1059 wR2 ) 0.1747

C10H44O44.75N2.50 GeYbW11 3183.50 monoclinic C2/c 27.834(6) 22.706(5) 23.818(5) 123.12(3) 3.360 8 11204 12606(4) 22.003 10572 18989 R1 ) 0.0585 wR2 ) 0.1408 R1 ) 0.0711 wR2 ) 0.1476

C6H44.50O55S3 GeSm2.50W11 3563.83 monoclinic P21/c 12.8034(8) 44.520(3) 11.4867(7) 107.52(10) 3.791 4 6276 6243.8(7) 23.156 10895 30993 R1 ) 0.0416 wR2 ) 0.0975 R1 ) 0.0472 wR2 ) 0.1001

Synthesis of [(CH3)4N]2H2.25[Sm1.25(GeW11O39)(H2O)4]‚3.75H2O (2). 2 was prepared according to the procedure described for 1 but using SmCl3 (0.36 g, 1.42 mmol) as the rare earth reagent. Yield: 2.02 g (89%). IR (KBr pellets, ν/cm-1): 949 (s), 873 (s), 806 (s), 703 (s), 527 (m). Anal. Calcd for C8H41.75O46.75N2GeSm1.25W11: C, 3.00; H, 1.32; N, 0.88; Ge, 2.27; Sm, 5.88; W, 63.26. Found: C, 3.10; H, 1.22; N, 0.98; Ge, 2.48; Sm, 5.7; W, 63.80. Synthesis of [(CH3)4N]2.50H2.50[Y(GeW11O39)(H2O)2]‚4H2O (3). The syntheses procedures were similar to those of 1 but using YCl3 (0.28 g, 1.42 mmol) as the rare earth agent. Additionally, when 0.4 g of tetramethylammonium bromide was added, precipitation appeared immediately. The resulting precipitate was filtered, washed with EtOH, and dried with Et2O. The resulting white powder was dissolved in 25 mL of hot water and allowed to crystallize at room temperature. A week later, rhombus-like crystals suitable for analysis were collected. Yield: 1.7 g (78%). IR (KBr pellets, ν/cm-1): 949 (s), 877 (s), 831 (sh), 811 (s), 732 (w), 703 (m), 528 (m). Anal. Calcd for C10H44.50O45N2.50GeYW11: C, 3.87; H, 1.44; N, 1.13; Ge, 2.34; Y, 2.86; W, 65.16. Found: C, 3.98; H, 1.33; N, 1.19; Ge, 2.40; Y, 2.82; W, 65.32. Synthesis of [(CH3)4N]2.50H2.50[Yb(GeW11O39)(H2O)2]‚3.75H2O (4). 4 was prepared following the procedure described for 3 but using YbCl3 (0.4 g, 1.42 mmol) as the rare earth reagent. Yield: 1.6 g (74%). IR (KBr pellets, ν/cm-1): 949 (s), 876 (s), 829 (sh), 810 (s), 703 (s), 528 (m). Anal. Calcd for C10H44O44.75N2.50GeYbW11: C, 3.77; H, 1.39; N, 1.10; Ge, 2.28; Yb, 5.44; W, 63.52. Found: C, 3.86; H, 1.35; N, 1.18; Ge, 2.35; Yb, 5.37; W, 63.68. Synthesis of [Sm(H2O)7]0.5H0.5[Sm2(GeW11O39)(DMSO)3(H2O)6]‚ 3.5H2O (5). A 2 mL (21 mmol) sample of HClO4 (10.37 M) was added dropwise into the powder of 2 g (5.74 mmol) of Sm2O3 under heating, until the Sm2O3 was dissolved entirely. The resulting solution was added to the 30 mL water solution containing 6 g (2 mmol) of R-K8[GeW11O39]‚nH2O at 80 °C. After 1 h, the solution was cooled to room temperature, and a precipitate containing potassium perchlorate as the main product was removed by filtration. Then, 1 mL of DMSO was added, and after 0.5 h, the clear solution achieved was left to evaporate at room temperature; several days later, yellow crystals were collected. Yield: 3.9 g (55%). IR (KBr pellets, ν/cm-1): 994 (w), 946 (s), 881 (m), 812 (vs), 752 (m), 702 (m), 522 (m). Anal. Calcd for C6H44.50O55S3GeSm2.50W11: C, 2.02; H, 1.26; S, 2.70; Ge, 2.04; Sm, 10.55; W, 56.74. Found: C, 2.08; H, 1.17; S, 2.85; Ge, 2.21; Sm, 10.50; W, 57.68. Crystallographic Analysis. Intensity data were collected on a Rigaku RAXIS-IV image plate area detector using graphite monochromatized Mo KR radiation (λ ) 0.71073 Å) at 293(2) K. The structures were solved by direct methods and refined by the full-matrix leastsquares method on F2 using the SHELXTL-97 package.18 Intensity data were corrected for Lorentz and polarization effects as well as for empirical absorption. All of the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms of organic molecules were geometrically fixed. Crystal data and structural refinement parameters for 1-5 are listed in Table 1. CCDC reference numbers are 269618269622.

Results and Discussion Synthesis. The effects of pH, counterions, and stoichiometry on the formation of compounds 1-5 was investigated. In all cases, the pH was controlled and maintained between 4 and 6. During the preparation of 1, we tested adopting K+ instead of [(CH3)4N]+ as the counterion; however, good quality crystals could not be obtained. The reason may be that the larger organic counterion [(CH3)4N]+ does not bind to the surface of POM to form ion pairs, which is preferable for the formation of the chainlike structure.19 As for the stoichiometry, when the Nd3+/ [R-GeW11O39]8- ratio decreased from 2:1 to 1:1, the IR spectrum of the resulting crystals showed characteristics of the ML2 dimeric type; while it increased to 3:1, crystals having the same structure as 1 were obtained, showing that when the Nd3+/[R-GeW11O39]8- ratio is less than 2:1, it may affect the structure of the product, whereas it shows no effect as long as it is between 2:1 and 3:1. The introduction of a rare earth cation in the perchlorate form may play an important role in the formation of 5. During our experiments, when using SmCl3 instead of Sm2O3 and HClO4 as the rare earth reagent, compound 5 could not be obtained. Crystal Structures. On the basis of X-ray diffraction analysis, the solid-state structures of compounds 1-5 consist of an infinite 1D arrangement built of [R-GeW11O39]8- anions connected by rare earth cations M3+ (M ) Nd, Sm, Y, Yb). In 1 and 2, the polyanions show a 1D zigzag chainlike structure, in 3 and 4, they show linear structures, and in 5, a doubleparallel chain is formed. The [R-GeW11O39]8- polyanion is obtained by the removal of a WdOd group from the [R-GeW12O40]4- anion formed of 12 {WO6} octahedra forming four {W3O13} fragments. Compared to the isostructural monovacant species [R-SiW11O39]8-, the [R-GeW11O39]8- anion contains the larger “bite angle”, which in the lacunary site is influenced by the effective radius of the central element (Ge4+, 53 pm and Si4+, 40 pm). The larger bite angle for [R-GeW11O39]8- may allow the rare earth cation to bind deeper into the defect site.20 As shown in Figure 1, in the isostructural anion [Sm(GeW11O39)(H2O)4]5- and [Sm(SiW11O39)(H2O)4]5-,21 the Ge‚‚‚Sm distance (4.240 Å) is 0.2 Å shorter than the Si‚‚‚Sm distance (4.537 Å). In addition, the aperture of the vacant site in [R-GeW11O39]8- (dO35‚‚‚O32 ) 4.281 Å) is larger than that of the [R-SiW11O39]8- (dO14‚‚‚O34 ) 4.068 Å). The asymmetric structural unit of 1 consists of 2 [(CH3)4N]+ cations, 1.5 H+, 1 [Nd1.5(GeW11O39)(H2O)6]3.5- polyanion, and

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Wang et al.

Figure 1. Polyhedral representations of [Sm(GeW11O39)(H2O)4]5- (left) and [Sm(SiW11O39)(H2O)4]5- (right).

Figure 4. Polyhedral representation of 3 showing the one-dimensional linear arrangement of polyanions [R-GeW11O39]8-. Key: Y, black; {WO6}, cyan octahedra. Figure 2. Polyhedral representation of 1 showing the one-dimensional zigzag arrangement of the polyanions [R-GeW11O39]8-. Key: Nd(1), black; Nd(2), purple; {WO6}, cyan octahedra.

Figure 3. Representation of the molecular structure unit of 1. The cations are omitted for clarity.

3 water of crystallization. The structure of 1 is represented in Figure 2. It is interesting that there are two kinds of coordination neodymium cations in the building block [Nd1.5(GeW11O39)(H2O)6]3.5- (Figure 3), which is composed of one [Nd(2)(H2O)6]3+, one [Nd(1)(H2O)3]3+, and one [R-GeW11O39]8- subunit. The Nd3+(1) cation occupies the vacant site of an [R-GeW11O39]8- subunit and is coordinated to the four available oxygen atoms of the lacunary site (dNd-Ob,c ) 2.41Å), and connection of the neighboring [R-GeW11O39]8subunits occurs via Nd(1)-Od (O4#1) bonds (dNd-Od ) 2.50(10) Å) forming the 1D infinite zigzag chainlike structure along the a-axis. The coordination sphere of the Nd3+(1) center is completed by three water molecules (dNd(1)-O(H2O) ) 2.541 Å).

Thus, the Nd(1) center is eight-coordinated, adopting a distorted square antiprism geometry. In the coordination polyhedron around the Nd3+(1) center, the O32, O33, O34, O35 group and the O4, O1w, O2w, O3w group constitute two bottom planes of the square antiprism, and their average deviations from their ideal planes are 0.0278 and 0.1172 Å, respectively. The distances between the Nd3+(1) cation and the two bottom planes are 1.1129, 1.4314 Å, respectively. The Nd3+(2) coordination cation links to the [R-GeW11O39]8- subunit via the terminal oxygen atom (O2) (dNd(2)-O2 ) 2.419(12) Å), and its occupancy is 0.5 due to the crystallographic disorder. The Nd3+(2) center is heptacoordinated, and its coordination sphere is completed by six water molecules (each occupancy is 0.5) (dNd(2)-O(H2O) ) 2.54 Å). Along the ac plane, the Nd3+(2) ions are located on opposite sites of the chains owing to the steric hindrance (S1). The compound 2 (S2) possesses the same motif as 1 to show the 1D zigzag chainlike structure (dSm(1)-Ob,c ) 2.380 Å, dSm(1)-Od ) 2.480 (13) Å, dSm(1)-OH2O ) 2.50 Å) except for two points. First, the occupancy of the Sm3+(2) ion is 0.25 in compound 2; second, the Sm3+(2) ion is pentacoordinated. Besides the terminal oxygen atom (O4) of the polyanion, it is additionally coordinated by four water molecules (each occupancy is 0.25) (dSm(2) -O(H2O) ) 2.61 Å). As shown in Figure 4, the compounds 3 and 4 display the 1D linear structure which is closely related to that previously reported by Daniel, Francis et al. for KCs[Yb(R-SiW11O39)(H2O)2]‚24H2O.12 The rare earth cations M(1)3+ (M ) Y, Yb) reside in the vacancy of the polyanion [R-GeW11O39]8- and bond to the adjacent polyanion via the terminal oxygen atoms; additionally, two water molecules are coordinated to each rare earth cation to complete the coordination sphere. The dY-Ob,c, dYb-Ob,c are 2.281, 2.253 Å, dY-Oa, dYb-Oa are 2.716(14), 2.724(9) Å, dY-Od, dYb-Od are 2.439(14), 2.434(9) Å and dY-OH2O, dYb-OH2O, are 2.36, 2.29 Å, respectively. In compound 5, the introduction of the organic molecules DMSO results in an interesting 1D double-parallel chainlike structure (Figure 5), in which the two parallel chains {[Sm(1)(GeW11O39)(DMSO)(H2O)2]5-}n are joined together through [Sm(2)(DMSO)2(H2O)4]3+ moieties. As shown in Figure 6, in the [Sm(1)(GeW11O39)(DMSO)(H2O)2]5- subunit, the

Novel Rare Earth Germanotungstates

Figure 5. Polyhedral representation of 5. Key: Sm, pink; {WO6}, cyan octahedra; S, yellow; O, blue.

Figure 6. (a) Representation of the molecular structure unit of 5. The hydrogen, discrete Sm3+ coordination ions and crystal water molecules are omitted for clarity. (b) The connection mode between two parallel chains.

Sm3+(1) cation, which is incorporated into the vacant site of the polyanion [R-GeW11O39]8-, is eight-coordinated, showing the antiprismatic coordination geometry in which four oxygen atoms come from the vacant site (dSm(1)-Ob,c ) 2.37 Å), two from water molecules (dSm(1)-OH2O ) 2.549 Å), one from the DMSO (dSm(1)-O(DMSO) ) 2.393(13) Å), and another from the terminal oxygen atom (O4#1) of the adjacent [R-GeW11O39]8subunit (dSm(1)-Od ) 2.417 (9) Å). In such an arrangement, one linear chain {[Sm(1)(GeW11O39)(DMSO)(H2O)2]5-}n is formed. The Sm3+(2) coordination cation is also eight-coordinated with a distorted bicapped trigonal prism, defined by eight coordination oxygen atoms, four from water molecules (dSm(2)-OH2O ) 2.425 Å), two from DMSO molecules (dSm(2)-O(DMSO) ) 2.368 Å), and two from terminal oxygen atoms (O10#1,O11) of the neighboring [R-GeW11O39]8- anions coming from the separate chain (dSm(2)-O10#1 ) 2.524(9) Å, dSm(2)-O11 ) 2.492(9) Å). Thus, the two parallel chains were joined together by two

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bridging moieties [Sm(2)(DMSO)2(H2O)4]3+. In the coordination polyhedron around Sm3+(2) cation, the two terminal oxygen atoms (O10#1,O11) occupy the cap positions. Because of the effect of the steric hindrance, the bond angle of O11-Sm2O10#1 is 129.3(3)° to show “V” shape, and the coordinated DMSO molecules are stretched outward. The separation distance between the two [Sm(2)(DMSO)2(H2O)4]3+ moieties (dSm2-Sm2#2) supported by the same two heteropolyanions [R-GeW11O39]8is 6.918 (4) Å. Another important feature, compared to compounds 1-4, is that, instead of [(CH3)4N]+ counterions, disorded lanthanide cations [Sm(3)(H2O)4]3+ and [Sm(4)(H2O)3]3+ together with half H+ have been found to balance the charge. Such disorder in the countercations also is common in the hetereopolyoxometalate crystal structures.22 In addition, in 5, all the sulfur atoms S1, S2, S3 are crystallgraphically disordered. That the average bond length of Sm-ODMSO (2.376 Å) is shorter than that of Sm-Ow (2.466 Å) indicates the more stable feature of the Sm-ODMSO bonds because the electrondonating effect of DMSO is stronger than that of the water molecules. Compared to that in the free DMSO molecules (1.513 Å),23 the longer SdO bond distances are found in compound 5. The reason that these distances do not differ in a statistical sense may be that pointed out by the literature:23,24 partially caused by the disordered S atoms and the large uncertainties. The corresponding O-M-O bond angles in 1 and 2 differ within narrow ranges and confirm the analogous crystal structures (S5). The same phenomenon could also be observed in 3 and 4. The average M-O bond lengths in 1-4 decrease along with the shortening of the ionic radii (Nd > Sm > Y > Yb). And the average bond lengths of the M-Od for the oxygen atoms connecting with the neighboring polyanion [R-GeW11O39]8are longer than that for the oxygen atoms in the vacant site (dM-Ob,c). In addition, the average M-O bond lengths are comparable to the sum of M-O ionic radii for 8-coordinated M3+ and 2-coordinated O2- ions in 1 and 2 and 5 and 7-coordinate M3+ and 2-coordinate O2- ions in 3 and 4.25 As compounds 1-5 have been synthesized in similar conditions, it follows that the solid-state structures adopted by compounds 1-5 must be imposed by the nature of the rare earth ions used, which represents a scarce example in which the nature of the rare earth imposes the arrangement of the building units. It should be noted that in one [M(GeW11O39)(H2O)n]5- building block of compound 1 or 2, the WO6 octahedron which connects the neighboring [M(GeW11O39)(H2O)n]5- unit via M-Od (the terminal oxygen atom) and the “vacant” WO6 octahedron substituted by lanthanide coordination cations are located at one W3O13 triplet, whereas in 3 and 4, they belong to different W3O13 triplets. It can be interpreted as ref 12 reported that as the size of M3+ decreases, the coordination number of M(1)3+ ions decreases from eight to seven and the repulsion between the vacant polyanions [R-GeW11O39]8- increases, followed by the change of the connecting points, which further provokes the transition from a zigzag to a linear organization of the [R-GeW11O39]8- subunits. By comparing the structures of compounds 2 and 5, the difference is likely related to the formation mechanism resulting from the different forms of the introducing rare earth cations. Additionally, a different arrangement of the two isostructural polyanions [R-GeW11O39]8- and [R-SiW11O39]8- in the neodymium12 and samarium21 compounds was observed, partly due to the larger bite angle in the former polyanion. Spectroscopic Characterization. The IR spectra of compounds 1-5 show the characteristic vibration patterns of the Keggin-type structure. In comparison with the IR spectrum of

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precursor R-K8GeW11O39‚13H2O,20 the νas(W-Od) vibration frequencies (946-949 cm-1) have red shifts of 9-12 cm-1; the possible major reasons for which may be that the rare earth cations have stronger interactions to the terminal oxygen atoms of the polyanions, impairing the W-Od bonds, reducing the W-Od bond force constant, and leading to a decrease in the W-Od vibration frequency. The νas(W-Ob-W) frequencies for 1-5 show two peaks at 877 ( 4 and 809 ( 3 cm-1; as for νas(W-Oc-W), only one strong absorption peak at 703 ( 2 cm-1 could be clearly observed, and the other split peaks at about 780 cm-1 are very weak and partly overlapped by the strong peak of νas(W-Ob-W) at 809 ( 3 cm-1; the possible reason for this is that the symmetry of the polyanion [R-GeW11O39]8- in 1-5 increases as compared to that of R-K8GeW11O39. The νas(Ge-O) vibrational frequencies for 1-5 at 526 ( 4 cm-1 are almost equal to that of the precursor. In addition, a weakening stretching band at 994 cm-1 is observed in the IR spectrum of compound 5, which is assigned to ν(SdO) asymmetric stretching vibration of DMSO molecules. Compared to that of free DMSO,24 the ν(SdO) shifts from 1057 to 994 cm-1. The result suggests that the DMSO ligands are coordinated to the rare earth ions by means of the oxygen atoms,19 and it is consistent with the X-ray analysis. The UV spectra in aqueous solution for compounds 1-5 in the range of 400-190 nm reveal an absorption band at ca. 276 nm, which is assigned to the pπ dπ charge-transfer transitions of Ob(c) f W band, which is the characteristic absorption of the polyanion of [R-GeW11O39]8in 1-5, indicating the similar electronic structures. Conclusion In this paper, we report five novel polyoxometalate compounds constructed of lacunary Keggin-type polyanions [R-GeW11O39]8- and trivalent rare earth ions. Compounds 1-5 display three kinds of distinct polymeric chains, which show that the arrangement of the [R-GeW11O39]8- building units can be changed by choosing the appropriate rare earth ions or introducing organic molecules. Acknowledgment. This work was supported by Nature Science Foundation of China, Specialized Research Fund for the Doctoral Program of Higher Education, Henan Innovation Project for University Prominent Research Talents, the Foundation of Educational Department of Henan Province and Natural Science Foundation of Henan Province. Supporting Information Available: X-ray crystallographic information files (CIF) for all compounds. The packing arrangement of compound 1 viewed along the ac plane; polyhedral representation of 2 and 4; representation of the molecular structure unit of 3; tables of selected bond lengths and bond angles for 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

Wang et al.

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