Synthesis and Characterization of 8-Yttrium ( III )-Containing 81-Tungsto-8-Arsenate ( III ) , [ Y 8 ( CH 3 COO ) ( H 2 O ) 18 ( As 2 W 19 O 68 ) 4 ( W 2 O 6 ) 2 ( WO 4 )

The 8-yttrium(III)-containing 81-tungsto-8-arsenate(III) [Y8(CH3COO)(H2O)18(As2W19O68)4(W2O6)2(WO4)]43− (1) has been synthesized in a one-pot reaction of yttrium(III) ions with [B-α-AsW9O33]9− in 1 M NaOAc/HOAc buffer at pH 4.8. Polyanion 1 is composed of four {As2W19O68} units, two {W2O10} fragments, one {WO6} group, and eight YIII ions. The hydrated cesium-sodium salt of 1 (CsNa-1) was characterized in the solid-state by single-crystal XRD, FT-IR spectroscopy, thermogravimetric and elemental analyses.


Results and Discussion
Synthesis and Structure.The polyanion [Y8(CH3COO)(H2O)18(As2W19O68)4(W2O6)2(WO4)] 43− (1) has been synthesized using a simple, one-pot procedure by reacting the trilacunary POM precursor [B-α-AsW9O33] 9− with Y III ions in 1 M NaOAc/AcOH buffer at pH 4.8, and was characterized in the solid state by IR spectroscopy, as well as thermogravimetric and elemental analysis.The title polyanion crystallizes as a hydrated mixed sodium-cesium salt (CsNa-1) in the triclinic space group P1̅ .Singlecrystal XRD on CsNa-1 revealed that polyanion 1 is composed of four {As2W19O68} units, two {W2O10} fragments, one {WO6} group, and eight yttrium(III) ions (see Figures 1 and 2).The mono-and di-tungsten fragments are most likely formed in situ by fragmentation of some of the lacunary [AsW9O33] 9− precursor during the course of the reaction, also seen previously for other large POM architectures [18,21].The {As2W19} units, also formed in situ, are composed of two {AsW9} units connected by an octahedral {WO6} group via corner-shared oxo-bridges.The structure of the {As2W19} units in 1 is different from the known [As2W19O67(H2O)] 14− POM precursor, where the linking tungsten center has terminal transoxo-aqua ligands [48].The linking tungsten(VI) in 1 bridges the two {AsW9} units in a way that the terminal oxo groups are cis to each other, non-protonated, and linking the tungsten(VI) to two Y III ions.A total of eight such oxo-bridges are present in 1, corresponding to the number of yttrium(III) ions.These yttrium(III) ions are all eight-coordinate with distorted square-antiprismatic coordination geometries and Y-O bond distances in the range of 2.3-2.5 Å.The coordination sphere of each Y III center is composed of bridging oxo and terminal aqua ligands (vide infra).Two {Y2As2W19} units are linked to each other by an edge-shared {W2O6} group to form two dimeric subunits (see Figures 1 and  2).Finally, a tungsten center (W81, corresponding to the orange octahedron in Figures 1 and 2) links the two subunits to each other via two W-O-W (namely to W79 and W80) and four W-O-Y bridges (one to Y6 and Y7, and two to Y8), leading to a 'dimer of dimers'-type assembly.The two subunits are also linked by two Y-O-W bridges, namely Y7-O-W30 and Y4-O-W25.The yttrium centers are all octacoordinated as stated above, with the coordination sphere filled by Y-O-W bridges or terminal oxo ligands.The two bridging yttriums Y7 and Y8, which are connected to W81, have two terminal aqua ligands each.The rest of the yttrium ions, apart from Y1, have three terminal aqua ligands each.For Y1, an acetate group (from the buffer medium) and two aqua ligands complete the coordination sphere.The existence of such kind of acetate groups has been previously observed, for example in [Gd6As6W65O229(OH)4(H2O)12(OAc)2] 38− and [Yb10As10W88O308(OH)8(H2O)28(OAc)4] 40− [18].Bond valence sum (BVS) calculations indicate that no bridging oxygen atoms in 1 are protonated [49].The total charge of 1 is therefore 43-, and is balanced in the solid state by 34.5 sodium and 8.5 cesium counter cations, hence resulting in the formula unit Cs8.5Na34.5[Y8(CH3COO)(H2O)18(As2W19O68)4(W2O6)2(WO4)] .230H2O.This formula is supported by elemental and thermogravimetric analyses (see Figure 3).The synthetic procedure of 1 contains several crucial parameters.An excess of yttrium ions over and above the stoichiometric ratio was needed for optimal yield.Furthermore, the type and pH of the buffer are essential for obtaining the desired product, as well as the presence of cesium ions.In fact, 1 could not be isolated in the absence of cesium ions, and a sufficient amount was needed to induce precipitation, before filtration and crystallization of the product salt.On the other hand, CsNa-1 proved to be only slightly soluble in D2O, and hence our solution studies by 183 W and 89 Y NMR spectroscopy were unsuccessful.NMR measurements on a freshly prepared reaction solution also remained inconclusive.Attempts to prepare isostructural lanthanide derivatives of 1 resulted in reported structures [20].Finally, to the best of our knowledge, 1 incorporates the largest number of yttrium ions, together with , respectively, should also be mentioned here [38].
Infrared Spectroscopy.The Fourier transform infrared (FTIR) spectrum of CsNa-1 (see Figure 4) displays a fingerprint region characteristic for the tungsten-oxo framework, indicating the presence of {AsW9} units.The bands in the range of 940-945 cm −1 are associated with the antisymmetric stretching vibrations of the terminal W═O bonds, whereas the bands in the range of 850-900 cm −1 can be mainly attributed to the antisymmetric stretching vibrations of the As-O(W).The two bands at about 780 cm −1 and 700 cm −1 arise from antisymmetric stretching of the W-O(W) bridges, whereas the bands below 650 cm −1 are due to bending vibrations of the As-O(W) and the W-O(W) bridges [50].Furthermore, the bands in the range of 1350 -1620 cm −1 can be assigned to vibrations of the bridging acetate group in the polyanion.The broad band at 3440 cm −1 and the strong one at 1620 cm −1 correspond to crystal waters.
Thermogravimetric Analysis.Thermogravimetric analysis of CsNa-1 was performed between 25 and 1000 °C under a nitrogen atmosphere to determine the number of crystal waters (see Figure 3).The weight loss of about 92% between 25 and 230 °C can be assigned to the loss of 230 crystal waters per formula unit.In addition, the second continuous weight loss step from 250 to 550 °C corresponds to the removal of 18 coordinated water molecules and decomposition of the acetate group.

Experimental Section
General Methods and Materials.All reagents were used as purchased without further purification.The trilacunary POM precursor Na9[B-α-AsW9O33]•27H2O was prepared according to the published procedure, and its purity was confirmed by infrared spectroscopy .X-ray Crystallography.A single crystal of CsNa-1 was mounted on a Hampton cryoloop in light oil for data collection at 173 K. Indexing and data collection were performed on a Bruker D8 SMART APEX II CCD diffractometer with kappa geometry and Mo-Kα radiation (graphite monochromator, λ = 0.71073 Å).Data integration was performed using SAINT [52].Routine Lorentz and polarization corrections were applied.Multiscan absorption corrections were performed using SADABS [53].Direct methods (SHELXS97) successfully located the tungsten atoms, and successive Fourier syntheses (SHELXL2014) revealed the remaining atoms [54].Refinements were full-matrix least-squares against |F 2 | using all data.In the final refinement, all non-disordered heavy atoms (As, W, Y, Cs, Na) were refined anisotropically; oxygen atoms and disordered counter cations were refined isotropically.No hydrogen atoms were included in the models.Crystallographic data are summarized in Table 1.We observed an extra W atom (W82) with an occupancy of 16.67%, but could not model its coordination environment due to serious disorder (see CIF file for details).This result implies that there is a small amount of {Y8As8W82} polyanion impurity present, which we could not avoid during synthesis or eliminate afterwards, in spite of many attempts.

Figure 1 .
Figure 1.Combined polyhedral/ball-and-stick representation of 1. Color code: WO6 octahedra pale blue/dark blue/orange, As pink balls, Y green balls, O red balls, C grey balls.

Figure 2 .
Figure 2. Combined polyhedral/ball-and-stick representation of the two half-units in 1.The turquoise balls represent bridging oxygens (Obridge).The color code is the same as in Figure 1.

Table 1 .
Crystal Data for CsNa