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New polyoxotantalate salt Na8[Ta6O19]·24.5H2O and its properties Article in Journal of Structural Chemistry · October 2011 DOI: 10.1134/S0022476611050295
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Journal of Structural Chemistry. Vol. 52, No. 5, pp. 1012-1017, 2011 Original Russian Text Copyright © 2011 by P. A. Abramov, A. M. Abramova, E. V. Peresypkina, A. L. Gushchin, S. A. Adonin, and M. N. Sokolov
NEW POLYOXOTANTALATE SALT Na8[Ta6O19]⋅24.5H2O AND ITS PROPERTIES P. A. Abramov,1,2 A. M. Abramova,3 E. V. Peresypkina,2 A. L. Gushchin,1 S. A. Adonin,1,2 and M. N. Sokolov1,2
UDC 546.88:548.736
The interaction of Ta2O5 with sodium hydroxide at 400°C yields Na8[Ta6O19], which can be recrystallized from an aqueous solution in the form of Na8[Ta6O19]⋅24.5H2O. The complex is characterized by IR spectroscopy, thermogravimetry, powder XRD, and single crystal XRD: hexagonal system, P63 space group, a = 12.3622(2) Å, c = 31.4305(7) Å, V = 4159.81(13) Å3, dx = 3.217 g/cm3, R = 0.0195. Keywords: tantalate, crystal structure, single crystal X-ray diffraction.
In aqueous solutions, niobium and tantalum are found in the form of stable hexanuclear [M6O19]8– oxocomplexes. However, depending on the environment conditions, primarily рН, oxide bridging ligands may undergo gradual hydrolysis to hydroxide ligands with the formation of [HM6O19]7– and [H2M6O19]6– and the maintenance of the anion structure [1]. The stability in a wide pH range and a high negative charge of hexanuclear tantalum oxocomplexes could be interesting from the standpoint of coordination chemistry when these complexes are used as ligands. Niobium-based polyoxometalates have been much better studied than the analogous tantalum compounds. Structural databases contain dozens of isopolyniobates and only few compounds whose structure contains the [Ta6O19]8– anion. The latter compounds known to date are complexes of the type AxBy[Ta6O19]⋅nH2O, where A and B = Na, K, Rb, Cs, x = 7, 8; y = 0, 1, and n = 0, 4, 14, 16 [2, 3]. Purely sodium as well as lithium salts are unknown (only mixed Na/K salt is known). In this work, we obtained a sodium salt of the [Ta6O19]8– anion and characterized it by a set of physicochemical methods. Experimental: instruments and materials. All the reagents used were commercially available (Sigma Aldrich),. analytical or higher grade and were used without preliminary purification. The IR spectra were recorded with a Specord IR 75 spectrometer. The thermogravimetric analysis (TGA) was performed on a TG209 F1 Iris®NETZSCH thermogravimetric analyzer. Synthesis of Na8[Ta6O19]⋅24,5H2O (1). 2.5 g (5.7 mmol) Ta2O5 was melted with 4.3 g (0.11 mol) NaOH in a glassy carbon crucible at 400°C for 5 h. The resulting melt was treated with 30 ml of cold water; the undissolved product was filtered off, washed with three 40 ml portions of cold water, and dried in vacuum. The resulting white powder was placed in a beaker with 80 ml of water and dissolved while stirring and heating to 80-90°C for 2 h. Almost all the precipitate dissolved. Then, the hot solution was vacuum filtered; the filtrate was placed in a refrigerator for a day, which led to the formation of hexagonal colorless crystals, whose composition and structure were determined by single crystal X-ray diffraction. The product mass was 3.0 g. The yield was 79%. IR: 3296 (vs), 1665 (s), 1441 (s), 842 (vw), 687 (vs), 532 (vs), 392 (vs), 212 (m).
1
Novosibirsk State University. 2A. V. Nikolaev Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences, Novosibirsk;
[email protected]. 3Ural Federal University, Ekaterinburg. Translated from Zhurnal Strukturnoi Khimii, Vol. 52, No. 5, pp. 1038-1042, September-October, 2011. Original article submitted October 28, 2010. 1012
0022-4766/11/5205-1012 © 2011
TABLE 1. Crystallographic Data and XRD Experiment Details for Compound 1 ISCD code Chemical formula Mol. weight Temperature, K Radiation (λ, Å) Crystal system Space group а, c, Å V, Å3 Z dx, g/cm3 μ, mm–1 F(000) Crystal size, mm θ range, deg Reflection index intervals Measured/independent reflections Reflections with I ≥ 2σ(I) Refinement method Number of refined parameters GOOF R1 (against |F| for reflections with I ≥ 2σ(I)) wR2 (against |F|2 for all the reflections) Residual electron density (min/max), e/Å3
422177 H49Na8O43.50Ta6 2015.01 100.0(2) 0.71073 Hexagonal P63 12.3622(2), 31.4305(7) 4159.81(13) 4 3.217 15.921 3692 0.134×0.112×0.110 1.90-31.83 –10 ≤ h ≤ 17, –17 ≤ k ≤ 18, –46 ≤ l ≤ 45 48666/8770 (Rint = 0.0289) 7974 Full-matrix LSM against F 2 354 1.047 0.0195 0.0432 –1.466/1.431
TGA. The temperature pattern (He atmosphere) is observed to have two endoeffects: at 70°C with a mass loss corresponding to 9 water molecules per formula unit and at 120°C, corresponding to 14.3 water molecules. Powder XRD. The X-ray pattern indicates that the sample is single-phase; however, the rather pronounced texture in the (00l) direction interferes with the intensities of the observed reflections. Pronounced texture of the sample is consistent with the layered structure of compound 1. Single crystal XRD. The structure of compound 1 was determined by single crystal XRD using the standard technique with a Bruker X8 Apex automatic four-circle diffractometer with a two-coordinate CCD detector at a temperature of 100 K using radiation from a molybdenum anode (λ = 0.71073 Å) and a graphite monochromator. The reflection intensities were measured by ϕ scanning of narrow (0.5°) frames to 2θ = 55°. The absorption was estimated empirically with the SADABS software [4]. The structure was solved by the direct method and refined by the full-matrix least squares method in an anisotropic approximation (for non-hydrogen atoms) using the SHELXTL software [5]. The hydrate water molecules and sodium cations are partly disordered. The hydrogen atoms are not localized. The crystal proved to be a racemic twin with a 0.73/0.27 component ratio. The CIF file with the full information on the studied structure has been deposited with the ICSD (Fachinformaionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, fax: (+49)7247-808-666; e-mail:
[email protected]) under no. 422177. The crystallographic data and refinement results are given in Table 1. The topology of anion sublattices was calculated and identified by the algorithm [6] using the TOPOS 4.0 Professional program suite for crystal chemical analysis [7]. Results and Discussion. In this study, we developed a technique to obtain Na8[Ta6O19]⋅24.5H2O sodium salt based on the reaction between tantalum oxide and sodium hydroxide melt in a 1:19 mol ratio; the technique is similar to that 1013
Fig. 1. [Ta6O19]8– anion (50% probability ellipsoids). Bond angles (deg): μ2-O–Ta–μ6-O 76.62(13)78.71(14), μ2-O–Ta–μ2-O 85.30(16)88.98(15) and 154.53(15)-156.16(15), μ2-O–Ta–O(H2O) 99.74(16)105.52(16), Ta–μ6-O–Ta 88.86(19)90.73(19), Ta–μ2-O–Ta 113.80(15)115.59(16). TABLE 2. Some Characteristics of the Known Structures of the Composition AxBy[Ta6O19]⋅nH2O, where x = 7, 8; y = 0, 1; n = 0-24.5 Structure Ta–μ6-O, Å Ta–μ2-O, Å dimensionality
Compound
Space group
Na8[Ta6O19]⋅24.5H2O
P63
Layered
2.348(4)2.388(4)
1.967(4)2.000(3)
1.787(4)1.816(4)
NaK7[Ta6O19]⋅14H2O
P21
Framework
2.31-2.43
1.91-2.09
1.80-1.83
K8[Ta6O19]⋅16H2O
Ia*
»
2.31-2.44
1.86-2.08
1.78-1.85
Rb8[Ta6O19]⋅4H2O Rb8[Ta6O19]⋅14H2O Cs8[Ta6O19]⋅14H2O Cs8[Ta6O19]
C2/c P21/n P21/n I4/m
» » » »
2.29-2.32 2.32-2.39 2.32-2.38 2.34-2.37
1.95-1.97 1.80-1.83 1.79-1.84 1.96-2.01
1.74-1.78 1.95-2.00 1.95-2.00 1.79-1.80
Ta–OO, Å
Isopolyanion packing pattern
Reference
Four-layer closest packing bcc
This work
Hexagonal primitive fcc » » »
[2]
[2]
[3] [3] [3] [3]
*The nonstandard Сс space group is given according to the original work. described in [8-10]. Note that other ratios between the alkali metal and tantalum lead to the formation of phases with different compositions and structures. Thus, a lack of the alkali metal [11] results in the formation of the Na5[TaO5] compound that does not contain [Ta6O19]n– cations, whereas an excess of the alkali metal yields NaTaO3 compounds with the perovskite-like structure [12-14]. Sodium salt Na8[Ta6O19]⋅24,5H2O (1) supplements a series of the so far known AxBy[Ta6O19]⋅nH2O compounds (A and B = Na, K, Rb, Cs) that contain oxo-centered isopolyanions [15, 16]. All these salts contain the [Ta6O19]8– isopolyanion (Fig. 1), whose geometrical characteristics are similar in the known structures (Table 2). Compound 1 is the only representative of layered structures in this series, which is likely to be due to its large hydrate number much higher than that in the other structures. The covalently bonded layers in 1 consist of two parallel levels of [Ta6O19]8– isopolyanions with 1014
Fig. 2. View of the layer in the crystal structure of Na8[Ta6O19]⋅24.5H2O along the b (a) and c (b) axes. The anions are presented in the form of coordination polyhedra; white and gray balls indicate sodium cations and the oxygen atoms of water molecules respectively. Hydrogen bonds are omitted for clarity. sodium cations in between the levels. The sodium cations coordinate both the oxygen atoms of the isopolyanions and the solvate water molecules both between the levels and inside them (Fig. 2a, b). All the sodium cations in the layer meet at the vertices, edges, and faces of the distorted NaO6 octahedra they form. The layers overlap with a displacement to avoid contacts between like-charged particles and are linked via a system of Ta–O(H2)…O(H2)–Na hydrogen bonds. The layers are approximately 13.3 Å thick, with the interlayer distance being markedly smaller and equal to the hydrogen bond length of 2.64-2.75 Å. However, packing of the isopolyanions is quite homogeneous (Fig. 3) because the distance between two anion levels in the layer, which corresponds to the distance between two faces of the NaO6 octahedra (∼3.3-3.4 Å), is comparable to the distance between the layers. The topology of isopolyanion packings in the AxBy[Ta6O19]⋅nH2O structures [2, 3] has a tendency to motifs typical of close packings (Table 2) regardless of the crystallization water content. The packing topology calculated by the coordination sequence [15] for all the previously studied compounds proved to be consistent with the estimates calculated manually by the authors of the original works [2, 3]. It is noteworthy that for the pairs Cs8[Ta6O19] and Cs8[Ta6O19]⋅14H2O, 1015
Fig. 3. Pattern of the four-layer closest packing for the anion sublattice of 1. The [Ta6O19]8– anions are presented in the form of coordination polyhedra of tantalum atoms.
Rb8[Ta6O19]⋅4H2O and Rb8[Ta6O19]⋅14H2O the fcc pattern of isopolyanion packing does not change with an increase in the cation size from A+ to A(H2O)+n , n = 7–10. For smaller cations A = Na, K, the tendency to the closest packing is weaker despite a rather large size of A(H2O)6+ aquacations. Nevertheless, the topology of the anion sublattice in the structure of 1 corresponds to the four-layer closest packing, according to [16] (Fig. 3). Complex 1 retains its composition for a long period of time, which enables its use as a gravimetric form in subsequent studies on the chemistry of polyoxotantalates. We failed to obtain an analogous lithium salt under these conditions (LiTaO3 only). However, from the standpoint of the coordination chemistry of these compounds, more promising is the possibility to produce their derivatives with organic ligands. This would help investigate the behavior of these complexes in nonaqueous media, which will be the purpose of our further studies. The authors thank P. E. Plyusnin for conducting the thermogravimetric experiment. The work was supported by Haldor Topsøe and State Contract No. 14.740.11.0273.
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