Crystal structure of strontium thiosulfate monohydrate

The title compound represents one of the hydrous alkaline earth thiosulfates with the lowest amount of crystal water known so far. The structure consists of layers, which are connected by weak hydrogen bonds.


Chemical context
Although thiosulfuric acid and its salts are common topics in textbooks of inorganic chemistry, the preparation of the pure acid was achieved just recently by a sophisticated synthesis via reaction of Na 2 S 2 O 3 and anhydrous HF (Hopfinger et al., 2018). Its salts are much better explored, as they are naturally and geologically widely spread (Caufield & Raiswell, 1999), and Na 2 S 2 O 3 , as well as (NH 4 ) 2 S 2 O 3 , is produced on a large industrial scale (Barberá et al., 2012). To date, thiosulfates of alkaline earth metals are solely known as hydrates. For example, MgS 2 O 3 Á6H 2 O has been investigated by Elerman et al. (1983) to determine its deformation electron density, and the first example of an S-H hydrogen bond that was confirmed by a single crystal-structure determination was found in BaS 2 O 3 ÁH 2 O (Manojlović-Muir, 1969).
Next to SrS 2 O 3 Á5H 2 O (Held & Bohatý, 2004), the title compound represents the second known crystal structure of a hydrate of strontium thiosulfate. As one route of preparation, the pentahydrate has been crystallized from aqueous solutions of Na 2 S 2 O 3 and Sr(NO 3 ) 2 , whereby these solutions were reported to show a tendency to decompose, inhibiting the growth of larger single crystals (Held & Bohatý, 2004). A possible step within the decomposition process, and maybe a competing product in a later stage of crystallization, might be associated with the monohydrate, the crystal structure of which is presented here.

Structural commentary
SrS 2 O 3 ÁH 2 O crystallizes in the space group P1 with one formula unit in the asymmetric unit and all atoms on general positions. Many structural features resemble the closely related pentahydrate of SrS 2 O 3 . The thiosulfate anion adopts a slightly distorted tetrahedral shape with a mean bond angle of 109.47 where the average O-S-O angles (110.32 ) are ISSN 2056-9890 slightly larger than the S-S-O angles (108.62 ). Similar to the S-S bond length found in the pentahydrate (1.995 Å ), the S-S bond length of 2.0044 (7) Å in the monohydrate is between those of the Ca (2.008 Å ) and the Ba (1.979 Å ) salts. The S-O bond lengths are between 1.466 (2) Å and 1.478 (2) Å and are in the same range as those of other alkaline earth thiosulfate hydrates (Table 1).
The Sr 2+ cation is coordinated by five O and two S atoms, belonging to six neighbouring S 2 O 3 2anions, and one additional O atom of an H 2 O molecule. One of the anions acts as a bidentate S/O ligand, while the remaining five coordinate only via one S or O atom, respectively. These six O ligands are found in narrow Sr-O distances ranging from 2.531 (2) to 2.623 (2) Å , whereas the S atoms exhibit Sr-S distances of 3.1618 (6) and 3.2379 (6) Å . A more remote O atom at a distance of 3.305 (2) Å might also be ascribed to the first coordination sphere, although exhibiting a larger distance than the neighbouring S atoms. This [8 + 1] coordination of the Sr 2+ atom ( Fig. 1) again resembles the ninefold coordination of the cation in SrS 2 O 3 Á5H 2 O, with the difference being that in the pentahydrate no S atoms are found in the first coordination sphere of Sr 2+ , but four water molecules instead. As a consequence of the presence of the larger S atoms close to Sr 2+ , in the title structure one O atom is shifted into an outer region of the coordination shell and thus is found at a considerably longer distance.
As a characteristic feature of the crystal structure, SrS 2 O 3 ÁH 2 O is made up from layers extending parallel to the crystallographic ab plane (Fig. 2). Within the layers, the condensed coordination polyhedra are packed alternately to form double sheets in such a way that the terminal S2 atoms and water molecules are directed towards the layer boundaries (Fig. 3). The layers are linked by hydrogen bonds of medium strength between the water molecules (O4Á Á ÁO4 ii ) and the water molecules and thiosulfate anions via S2 atoms (Table 2). This involves also a bifurcated hydrogen bond O4-H1Á Á Á(O4 ii /S2 iii ). The O4-H2Á Á ÁS2 i bond as well as the DÁ Á ÁA distances are in the same range as in SrS 2 O 3 Á5H 2 O (Held & Bohatý, 2004 (2004)

Figure 2
Crystal structure of SrS 2 O 3 ÁH 2 O, in a view onto (001). Displacement ellipsoids are shown as in Fig. 1.

Figure 1
Coordination polyhedron of the Sr 2+ cation in SrS 2 O 3 ÁH 2 O. Anisotropic displacement ellipsoids are drawn at the 70% probability level; H atoms are shown with arbitrary radius. [Symmetry codes: considered, which would result in shorter and more linear O4-H1Á Á ÁO4 ii and O4-H1Á Á ÁS2 iii bonds. However, a reasonable refinement of these disordered H atoms was not possible.
A striking analogy to the packing of the pentahydrate structure ( Fig. 4a) is apparent. With the presence of five water molecules instead of one, the main packing of ions in SrS 2 O 3 Á5H 2 O is only slightly changed as a result of the coordination of additional water molecules to the Sr 2+ cation and widened by two non-coordinating and hydrogen-bonded water molecules situated between the layers. The S-S bond is nearly orthogonal to the layer plane; however, the layer boundaries are also formed by S atoms and water molecules, both forming hydrogen bonds. The very close relationship between the two crystal structures suggests a topotactical degradation of the pentahydrate. Because both hydrates were crystallized at room temperature, a temperature dependence of the crystallization does not seem to be the only possible driving force. An ageing process triggered by concentration or thermodynamic stability must be taken into account as well. The degradation process, possibly running via another so far unknown trihydrate after removal of the free water molecules, was not investigated up to now, and in addition a thermal analysis of the title compound could not been carried out because of the presence of large amounts of indistinguishable crystalline by-products, viz. NaNO 3 and Sr(NO 3 ) 2 .
The orthorhombic structure of BaS 2 O 3 ÁH 2 O is likewise found to form layers, which are separated by water molecules (Fig. 4b; Manojlović-Muir, 1975). Similar to the Sr homologue, two terminal S atoms are part of the first coordination sphere of the Ba 2+ cation which has, caused by the larger ion radius, a different environment, namely by five thiosulfate anions as bidentate ligands and one additional water molecule. Interestingly, while the number of atoms forming the first coordination sphere is higher in the Ba compound, the number of directly coordinating anions is smaller.

Database survey
Besides SrS 2 O 3 , crystal structure determinations for three further alkaline-earth thiosulfates have been reported, all of them as hydrates: BaS 2 O 3 ÁH 2 O Manojlović-Muir, 1975), CaS 2 O 3 Á6H 2 O (Held & Bohatý, 2004), and MgS 2 O 3 Á6H 2 O Baggio et al., 1969;Elerman et al., 1982Elerman et al., ,1983. Together with the known Sr compounds, SrS 2 O 3 Á5H 2 O (Held & Bohatý, 2004)      divalent transition-metal thiosulfates, as there are those of Ni as the hexahydrate (Elerman et al., 1978; isostructural with the Mg salt) and of Cd as the dihydrate (Baggio et al., 1997). The only crystal structure of a hydrate-free thiosulfate of a divalent cation is reported for Pb (Christensen et al., 1991). Table 1 collates S-S and averaged S-O bond lengths in the corresponding structures of these thiosulfates.

Synthesis and crystallization
Crystals of SrS 2 O 3 ÁH 2 O were grown from an aqueous solution of Na 2 S 2 O 3 Á5H 2 O and Sr(NO 3 ) 2 . The solution was stored at room temperature and the solvent was evaporated very slowly over several months. Single crystals were isolated from highly concentrated solutions where only a little of the mother liquor remained. Besides the title compound, crystals of NaNO 3 and surplus Sr(NO 3 ) 2 were also found, and all of these compounds were identified in the X-ray powder pattern of the reaction mixture after drying at room temperature. From all these experiments, no hints of the presence of the pentahydrate were found.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were refined with a restrained O-H distance of 0.85 (5) Å and U iso (H) = 1.5U eq (O). A free refinement of H-atom positions resulted in a reliable shape for the water molecule and orientation with respect to possible hydrogen bonds, but included one short O-H distance of only 0.5 Å .  Only H-atom coordinates refined Á max , Á min (e Å À3 ) 0.97, À0.86

Strontium thiosulfate monohydrate
Crystal data Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.085 (5) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.