The crystal structure of Cs₂S₂O₃·H₂O

2.1 Synthesis

Cesium thiosulfate monohydrate was first obtained as a byproduct of the synthesis of a cesium thioindate [21]. 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 [20]. 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 MoK_α radiation (λ = 0.71073 Å, graphite monochromator). For data collection and data processing the Stoe X-Area software was used [22]. The numerical absorption correction was done by X-Red [23] and X-Shape [24]. The crystal structure was solved by Direct Methods using Shelxs-97 [25, 26], the structure refinement (full-matrix least-squares on F_o²) 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.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, 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 [27].

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 [12] 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 C_3v 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 [001]. 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 [010], see Fig. 2. The hydrogen bonds have a strong influence on the crystal growth, thus the needles grow parallel to the b axis.

Fig. 1:

Section of the unit cell of Cs₂S₂O₃·H₂O.

Fig. 2:

Structure fragment of Cs₂S₂O₃·H₂O for highlighting the S···OH hydrogen bonds between S₂O₃²⁻ moieties and the hydrate water molecules.

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.

Fig. 3:

First coordination sphere of Cs1 with four sulfur atoms as ligand atoms.

Fig. 4:

Exclusively oxygen atoms act as ligands in the first coordination sphere of Cs2.

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 [12]. 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 [28]. 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 [29]. 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].

In the IR spectrum of Cs₂S₂O₃·H₂O (Fig. 5) a shift of the OH stretching mode is observed. The broad peak at 3385 cm⁻¹ (Fig. 5 and Table 5) corresponds to a moderate O–H···S hydrogen bond system.

Fig. 5:

IR spectrum of Cs₂S₂O₃·H₂O illustrating the red shift of the OH stretching modes at 3385 cm⁻¹.

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 [28].

The shifts (relative IR shift Δν_XH 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].