Synthesis and Low Temperature Crystal Structure of Disodium Tetrapotasium Decavanadate

A decavanadate K4Na2V10O28.10H2O has been prepared and chractrisized by, IR and Raman spectroscopy. Single crystal x-ray analysis has been carried out on H20K4Na2O38V10 which crystallizes in the triclinic. System, space group P-1, with a=8.5584(5) Å, b=10.3041(6), c=10.9801(7) and Z=1. Structural units (V10O28) link by the interstitial contents ([K4Na2(H2O)20]).


Introduction
Polyoxometalates (POMs) are interesting nanosized inorganic clusters that have attracted much attention in recent years due to their functional properties and practiced applications in catalysis, medicine, surfaces sciences and pharmacology 1,2 .Among these compounds, polyoxovanadates are especially attractive since vanadium is important for biological metabolism (trace element) 3 and plays as essential role in some enzymatic activities 4 .Interest in synthetic and natural vanadium oxide compounds has recently been rekindled, in part because of their potential use as secondary-cathode materials for advanced Li batteries 5 and their key role in oxidative catalysis [6][7][8] .Vanadium in its higher oxidation states (V V , V IV ) gives a large number of isopolyvanadates that exhibit a wide range of structures, ranging from chain metananadates [VO 3 -] n , to layered oxides [V 2 O 5 ], compact polyanion [V 10 O 28 ] 6- and polyanionic hollow cage such as [V 15 O 36 ] 5-9 .Decavanadate is formally a [V 10 O 28 ] 6- polyanion, four K + and two Na + cations and aqua ligands.In the present work, the results derived from low temperature single crystal diffraction data at medium resolution and Infrared and Raman spectra are reported.

Experimental
All reagents for synthesis were purchased and used without purifications.Infrared spectrum was recorded as KBr pellet on a Shimadzu FT IR -8400 spectrometer.Raman spectrum was recorded on a Nicolet FT Raman spectrometer in a capillary tube.
V 2 O 5 2g (10 mmol) dissolved in 100 mL of 0.5 M CH 3 COOH/CH 3 COONa buffer solution (pH=4.8) with stirring.The turbid solution was heated and kept at 80 °C for 1 h.Then the solution was filtered off and Potassium chloride (7.0 g) was added to the solution and the mixture was stirred for 15 min.The orange single crystals suitable for crystallography were obtained from the solution after five days at room temperature.The crystal samples air-dried at room temperature.
Crystallographic data and structure refinement parameters are listed in Table 1.The intensities were corrected for Lorentz-polarization effects, and the SADABS program 10 was applied for absorption correction.The structure was refined with SHELXL-97 11

IR and Raman spectroscopy
The FT-IR spectrum shows characteristic vibrational features similar to other polyoxovanadates reported (Figure 1).The presence of well-differentiated V-O linkages in the crystal structures of compounds indicates the possibility of a large number of internal vibrations.The FT-IR spectra showing one strong band at ~949 cm -1 that can be assigned to the stretching of the terminal V-O bonds.The bridging antisymmetric vibrations of V-O-V possibly correspond to the bands at ~746, 804 and ~843 cm -1 , while the symmetric ones are probably at 521 and 596 cm -1 .IR and Raman spectra of K 4 Na 2 V 10 O 28 .20H 2 O (Figure 1) are not similar; therefore it confirms that this compound has a centrosymmetric structure.

Results and Discussion
The structural unit The [V 10 O 28 ] 6-decavanadate structural unit is the fundamental building block of the hummerite atomic arrangement.Figure 1 shows the structural unit and Figure 2 illustrates the linkage of adjacent structural units by the interstitial contents.The structural unit is centrosymmetric and formed of ten edge-sharing octahedra; there is a similar fundamental buildings block in pascoite, Ca 3 V 10 O 28 •(H 2 O) 17 12 .In the structural unit, each of the octahedra contains a single vanadyl bond with distance less than 1.683 Å.In addition to bonding to the vanadium atoms of the structural unit, the oxygen atoms of the structural unit bond to the potassium atom of the interstitial complex and to the hydrogen atoms of the (H 2 O) groups of the interstitial complex.Oxygen O3 of the structural unit is of particular interest in the structural unit.As depicted in Figure 1, O3 bonds to six vanadium atoms, an unusual coordination for oxygen, particularly when bonded to such a highly charged cation as pentavalent vanadium.The vanadium-oxygen bonds to O3 are the longest in each vanadium polyhedron, and thus the bond-valence is the lowest among the bonds.The bondvalence sum of 1.98 valence units, vu, for O3 demonstrates that despite the unusually large coordination number for oxygen, the atom arrangement maintains the valence-matching principle.The crystal structure of this compound was already reported by others 13 , but we carried out crystallography on this compound in low temperature (100 K) and synthesis method is different and the quality of our x-ray results is significantly better (e.g.R=0.0269).

The interstitial complex
The symmetry-equivalent structural units in hummerite are connected by the interstitial complex 14 .The interstitial complex in hummerite is [K 4 Na 2 (H 2 O) 20 ] 6+ .All oxygen atoms in the interstitial complex occur as (H 2 O) groups, in contrast to the structural unit, which is devoid of hydrogen atoms.The Na atom bonds to six (H 2 O) groups in a regular octahedral arrangement (c.f.Table 3) and does not bond directly to the structural unit.Potassium bonds to eight atoms of oxygen, five of them belong to the decavanadate structural unit and three of which are (H 2 O) groups of the interstitial complex.In addition to the bonding between five oxygen atoms of the structural unit and the interstitial potassium, there is an extensive network of hydrogen bonds linking the two structural components.Figure 3 depicts all hydrogen bonds with distance ≤ 2.013 Å.

Table 2 and
Bond lengths K 4 Na 2 V 10 O 28 .20H 2 O are given in Table3.

Table 1 .
Crystal data and structure refinement for K 4 Na 2 V 10 O 28 .20H 2 O

Table 2 .
Atomic coordinates (×10 4 ) and equivalent isotropic displacement parameters (Å 2 ×10 3 ) for K 4 Na 2 V 10 O 28 .20H 2 O U(eq) is defined as one third of the trace of the