A reinvestigation of the alkali metal thiosulfates has led to the new phase Cs2S2O3·H2O. At first cesium thiosulfate monohydrate was obtained as a byproduct of the synthesis of Cs4In2S5. Further investigations were carried out using the traditional synthesis reported by J. Meyer and H. Eggeling. Cs2S2O3·H2O crystallizes in transparent, colorless needles. The crystal structure of the title compound was determined by single crystal X-ray diffraction at room temperature: space group C2/m (No. 12), unit cell dimensions: a = 11.229(4), b = 5.851(2), c = 11.260(5) Å, β = 95.89(2)°, with Z = 4 and a cell volume of V = 735.9(5) Å3. The positions of all atoms including the hydrogen atoms were located in the structure refinement. Cs2S2O3·H2O is isotypic with Rb2S2O3·H2O. Isolated tetrahedra [S2O3]2− are coordinated by the alkali metal cations, and in addition they serve as acceptors for hydrogen bonding. For both Cs atoms the shortest distances are observed to oxygen atoms of the S2O32− anions whereas the terminating sulfur atom has its shortest contacts to the water hydrogen atoms. Thus, an extended hydrogen bonding network is formed. The title compound has also been characterized by IR spectroscopy. IR spectroscopy reveals the vibrational bands of the water molecules at 3385 cm−1. They show a red shift in the OH stretching and bending modes as compared to free water. This is due both to the S···H hydrogen bonding and to the coordination of H2O molecules to the cesium atoms.
Alkali and alkaline earth metal thiosulfates have been known for many decades. They crystallize in several different structure types containing varying amounts of water of crystallization. In case of the alkaline earth metal thiosulfates water of crystallization seems to be crucial for the formation of crystalline matter.
The structure determination including hydrogen positions from X-ray data often is challenging. The series of the heavier alkaline earth metal thiosulfates was completed with examples for Ca and Sr in 2004 by the reports on CaS2O3·6H2O, and SrS2O3·5H2O . The crystal structures of MgS2O3·6H2O [2–5] and BaS2O3·6H2O [6–8] were determined earlier. The O–H···O and O–H···S hydrogen bonding networks therein were determined by neutron diffraction studies [4, 5, 7, 8]. Typically, hydrogen bonding networks involving all water molecules in O–H···O and O–H···S bonds with S2O32− anions are observed .
So far several crystal structures of alkali metal thiosulfate hydrates are well characterized and the hydrogen bonding systems therein have been discussed: K2S2O3·1/3H2O [9–12], Na2S2O3·5/4H2O , and Na2S2O3·5H2O [13–16]. A detailed classification of the hydrogen bonds is available for Na2S2O3·5H2O. They are divided into four groups: two strong and two weak water–thiosulfate oxygen hydrogen bonds, two water–water, and three water–thiosulfate sulfur hydrogen bonds have been found .
Besides the water containing compounds also the anhydrous species were examined [17, 18]. In this context the investigation of the temperature dependent polymorphism of anhydrous Na2S2O3  should be mentioned. Röhr and co-workers recently reported on the first crystal structure determination of anhydrous K2S2O3 .
In contrast to the lighter alkali metal thiosulfates (sodium and potassium) only the crystal structure of the monoclinic Rb2S2O3·H2O has been established . The existence of Cs2S2O3·H2O has been known for more than 100 years . However, the crystal structure has not yet been reported in the literature. Herein, we present the crystal structure determination including hydrogen positions from single-crystal X-ray data. The existence of hydrogen bonds in Cs2S2O3·H2O is confirmed by IR spectroscopy.
2 Experimental section
Cesium thiosulfate monohydrate was first obtained as a byproduct of the synthesis of a cesium thioindate . The reaction product was stored in air, and after 24 h at a temperature of about 37°C and high atmospheric moisture, single crystals of Cs2S2O3·H2O were grown as colorless, transparent needles.
Thereafter cesium thiosulfate monohydrate was synthesized according to the well-known protocol of Meyer and Eggeling . An aqueous solution of cesium carbonate Cs2CO3 (99.9%, Sigma Aldrich and Rockwood Lithium) was mixed with an excess of barium thiosulfate BaS2O3 (98%, Alfa Aesar) in aqueous solution at 50°C. All starting materials were used as purchased. After stirring and heat treatment the reaction mixture at 90°C for one day, the white precipitate (BaCO3) was filtered off. Cesium thiosulfate monohydrate remained in solution. This solution was evaporated and subsequently dried in an evacuated desiccator over anhydrous CaCl2 (≥94%, Roth). Cs2S2O3·H2O was obtained as a white microcrystalline powder. The purity of the powder was checked by powder X-ray diffraction and vibrational spectroscopy.
2.2 Single-crystal X-ray diffraction and crystal structure determination
Single crystals of Cs2S2O3·H2O were mounted on glass fibers. The X-ray data were collected at room temperature on a STOE IPDS I diffractometer, using MoKα radiation (λ = 0.71073 Å, graphite monochromator). For data collection and data processing the Stoe X-Area software was used . The numerical absorption correction was done by X-Red  and X-Shape . The crystal structure was solved by Direct Methods using Shelxs-97 [25, 26], the structure refinement (full-matrix least-squares on Fo2) was done with Shelxl-97 [25, 26]. Table 1 summarizes the crystallographic data and selected experimental parameters. The atomic coordinates and isotropic displacement parameters are given in Table 2. Table 3 lists the anisotropic displacement parameters. Selected interatomic distances and angles are summarized in Table 4.
|Crystal color and shape||Transparent, colorless needles|
|Crystal system, space group||Monoclinic, C2/m (no. 12)|
|Unit cell dimensions (from powder diffraction data)|
|Volume V, Å3||735.9(5)|
|Calculated density ρ, g cm−3||3.574|
|Wavelength λ, Å||0.71073 (MoKα)|
|Diffractometer||STOE IPDS I|
|Measuring temperature T, K||293(2)|
|Measured θ range, °||3.64–31.69|
|Index ranges hkl||±16, ±7, ±16|
|Data averaging Rint||0.0435|
|Reflections with I > 2 σ(I)||920|
|Completeness to θ = 25°, %||96.9|
|Absorption coefficient μ, mm−1||10.412|
|Absorption correction||X-Red  and X-Shape |
|Structure solution||Direct Methods [25, 26]|
|Refinement||Full-matrix least-squares on F2 [25, 26]|
|Number of ref. parameters||50|
|R indicesa [I > 2σ(I)] R1/wR2||0.0263/0.0584|
|R indicesa [all Data] R1/wR2||0.0395/0.0608|
|Weighting Scheme Figure citation TAG missing...Ab||0.0346|
|GooFc on F2||0.956|
|Residual electron density ρ, e Å−3||–0.83 ≤ ρ ≤ 1.09|
aR1 = Σ||Fo| – |Fc||/Σ|Fo|, wR2 = [Σw(Fo2 – Fc2)2/Σw(Fo2)2]1/2; bw = [σ2(Fo2) + (AP)2]−1, where P = (Max(Fo2, 0) + 2Fc2)/3; cGoF = S = [Σw(Fo2 – Fc2)2/(nobs – nparam)]1/2.
aUeq is defined as one third of the trace of the orthogonalized Uij tensor; ball atom sites are fully occupied (sof = 1).
aCoefficients Uij of the anisotropic displacement factor tensor of the atoms are defined by exp[–2π2(h2a*2U11 +...+ 2hka*b*U12 +...)].
Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: email@example.com, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-430930.
2.3 Vibrational spectroscopy
Infrared spectroscopic investigations of microcrystalline Cs2S2O3·H2O were carried out by a Varian FT-IR 670 spectrometer coupled with a GladiATR (attenuated total reflection) unit. Intensities were finally evaluated by the software Varian Resolutions Pro .
3 Results and discussion
3.1 Crystal structure of Cs2S2O3·H2O
Cesium thiosulfate monohydrate belongs to the large group of compounds, which have been known for a long time and nonetheless basic information as e.g. their crystal structures have never been investigated in detail. Unfortunately, a recent report on the isotypic Rb2S2O3·H2O  does also not provide hydrogen positions. Therefore, we investigated the crystal structure of Cs2S2O3·H2O and could also obtain the hydrogen positions. Details of the structure determination are displayed in Table 1.
The crystal structure contains [S2O3]2− anions with almost perfect C3v symmetry. The sulfur–sulfur distance in the thiosulfate ion (d(S1–S2) = 2.028(2) Å) is identically with the corresponding bond length in Rb2S2O3·H2O (d(S1–S11) = 2.030(2) Å). The same holds for the S–O interatomic distances, which are d(S2–O3) = 1.470(4) Å and d(S2–O2) = 1.470(3) Å (2×).
These thiosulfate anions are arranged in layers parallel (001) and the S–S bonds are aligned along . The thiosulfate anions are oriented antiparallel in adjacent layer. Thus the S–S bonds of a pair of layers and the basal oxygen atoms of the tetrahedra point to each other, see Fig. 1a. The topology of the cesium atoms resembles octahedra. As shown in Fig. 1b, these octahedra are condensed in layers and the thiosulfate anions are embedded therein. Thus, the motif of the CdI2 structure type results and water molecules are located between the layers. These water molecules are embedded in the crystal structure at the same height in z as the exo-sulfur atoms S1. They form hydrogen bonds to two thiosulfate moieties via the S1 atoms along , see Fig. 2. The hydrogen bonds have a strong influence on the crystal growth, thus the needles grow parallel to the b axis.
Two different types of cesium are found in the crystal structure. Cs1 is embedded in the layers of the thiosulfate ions and coordinates to three of these units of one layer. These act as bi-dentate ligands via the exo-sulfur atom and one oxygen atom. In addition, two water molecules from this layer serve as ligands. The coordination sphere is completed by one sulfur atom and one further water molecule from an adjacent layer. The coordination number CN = 10 results, see Fig. 3. By contrast, Cs2 is located between the basal planes of the thiosulfate units. Thus, it is coordinated exclusively by oxygen atoms. Three thiosulfate ions of one layer act as bi-dentate ligands and in addition one water molecule from the same layer is coordinating. Three oxygen atoms of the adjacent thiosulfate layer complete the coordination sphere with CN = 10, see Fig. 4.
3.2 Extended hydrogen bonding system with an O–H···S donor-acceptor interaction, IR spectroscopic investigation of Cs2S2O3·H2O
Röhr already mentioned very short nonbonding distances between terminal sulfur atoms (S1) of adjacent layers in Rb2S2O3·H2O . These short distances were discussed as an indication for O–H···S hydrogen bonds. We could refine the hydrogen positions in isotypic Cs2S2O3·H2O. Thus, we can now prove the assumed expanded O–H···S hydrogen bonding system along the crystallographic b axis, see Fig. 2.
Especially IR spectroscopy is a valuable tool to further investigate the O–H···S donor-acceptor interaction. Lutz published many characteristic OH band shifts of solids with hydrogen bonds . Water molecules can act as both, good hydrogen bonding donors and hydrogen bonding acceptors [28, 29]. Therefore, hydrogen bonds are present in all known solid hydrates. In the presence of hydrogen bonds, the OH stretching modes of the water molecules shift to smaller wavenumbers. The reason is both the smaller force constant and the increased anharmonicity of these vibrations. The half widths of the bands increase and the intensities of the peaks grow. The strength of the hydrogen bonds is correlated with the OH stretching frequencies of the water molecules. The strongest hydrogen bonds, in which water molecules of solid hydrates are involved, are present in hydroxide hydrates because of the very strong proton acceptor strength of OH− ions . The bond length of such strong hydrogen bonds is in the range of 255–280 pm for the O···O distances, and the OH stretching modes are observed at about 2800 and 2150 cm−1. On the other hand, the weakest hydrogen bonds found in solid hydrates are those in NaClO4·H2O with OH modes at 3584 and 2641 cm−1 [28, 30].
The proton acceptor strengths of some anions present in solid hydrates can be arranged as follows ClO4− < NO3− < ClO3− < BrO3− < IO3− < H2O < SO42− < SeO42− < SO32− < PO43− < OH−, S2O32− < SbS43− < AsS43− < S2− < PF6− < BF4− < I− < Br− < Cl− < F− [28, 31–38]. The different strengths of the various proton acceptors are, at least partly, due to the different net charges of the acceptor atoms. For the same acceptors, the strength of the hydrogen bonds depends on the length and arrangement of the hydrogen bonding .
The shifts (relative IR shift ΔνXH in cm−1: 10–25%) [39, 40] of the OH stretching and bending modes of the water molecules in Cs2S2O3·H2O correspond to a moderate hydrogen bond system. The interatomic distance of d(S···H) = 2.58(9) Å between the donor atoms (S1) and the acceptor molecules (H2O) also suggests a moderate O–H···S hydrogen bond system in Cs2S2O3·H2O. The distance between S1 and O1 (d(S···O) = 3.396(3) Å) again indicates a hydrogen bond. The bond energy of such predominantly electrostatic interactions is in the range of 4–14 kcal mol−1 [39, 40]. Table 5 summarizes all IR stretching and bending modes of Cs2S2O3·H2O in comparison to Cs2S2O3·2H2O and anhydrous Cs2S2O3 [41–46].
|Lattice vibrations||50||Not specified||Not specified|
|ρ(SO3)||Broad band from 365 to 441||334|
|νas (SO)||1107||Broad band from 1120 to|
Cs2S2O3·H2O is shown to be isotypic with Rb2S2O3·H2O . The crystal structure resembles an anti-CdI2 type arrangement with Cs forming layers of octahedra with S2O32− located therein. Water molecules are located between the layers. The hydrogen positions were refined from single crystal X-ray diffraction data. Thus, the O–H···S hydrogen bond system proposed for Rb2S2O3·H2O by Röhr  can now be confirmed by experimental results for the Cs analouge. Additionally, the extended hydrogen bonding system was investigated by IR spectroscopy.
Dedicated to: Professor Wolfgang Jeitschko on the occasion of his 80th birthday.
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