Crystal structure of sodium thiosulfate dihydrate and comparison to the pentahydrate

Na2S2O3·2H2O has been known for more than a hundred years but no structural data were known to date. Now, crystals of this compound have been grown at the surface of an aqueous solution of Na2S2O3. The sodium cations are five- to seven-coordinate by thiosulfate anions and water molecules with the anions acting as mono- and bidentate ligands. The thiosulfate anions and water molecules are connected by O—H⋯O and O—H⋯S hydrogen bonds of medium strength to form corrugated layers, which are linked by sodium cations.

Na 2 S 2 O 3 Á2H 2 O has been mentioned in the literature for more than a hundred years and pure samples were prepared and investigated, however, no structural data except for a set of lattice parameters were known to date. Now crystals of this compound have been grown at the surface of an aqueous solution of Na 2 S 2 O 3 and the structure has been determined at 200 and 100 K. Na 2 S 2 O 3 Á2H 2 O crystallizes in the space group P2 1 /n with two formula units in the asymmetric unit and all atoms occupying general positions. The sodium cations are five-to seven-coordinate by thiosulfate anions and water molecules and the anions act as mono-and bidentate ligands. In the extended structure, the thiosulfate anions and water molecules are connected by O-HÁ Á ÁO and O-HÁ Á ÁS hydrogen bonds of medium strength to form corrugated layers, which are linked by sodium cations. For comparison, the crystal structure of Na 2 S 2 O 3 Á5H 2 O has been determined at the same conditions, i.e. for the first time below room temperature.

Chemical context
Thiosulfates containing the S 2 O 3 2anion have been studied for more than 150 years (Bunte, 1874). Nowadays, Na 2 S 2 O 3 and (NH 4 ) 2 S 2 O 3 are produced on an industrial scale (Barberá et al., 2012), and the applications of thiosulfates are growing (Kumar Paul et al., 2009). One of the most characteristic features of the thiosulfate anion is the enhanced reactivity including changes of the sulfur oxidation state, which hampered the preparation of pure compounds. For example, the synthesis of pure thiosulfuric acid succeeded just lately via the reaction of Na 2 S 2 O 3 and anhydrous HF (Hopfinger et al., 2018), and the first pure thiosulfate complexes of lanthanides were characterized very recently (Dalton et al., 2021).
The nature of the hydrates in the Na 2 S 2 O 3 system was intensively studied by Young & Burke (1906) and by Picon (1924), who identified either twelve or even fourteen different crystalline hydrates of Na 2 S 2 O 3 , respectively, among them two different dihydrates, by means of their crystalline appearance and by thiosulfate analysis. The pentahydrate is by far the most stable compound at ambient conditions, and all other hydrates were found to convert into this phase more or less rapidly. Extended studies of its full dehydration including thermal analyses, Raman spectroscopy and optical microscopy revealed the dihydrate as an intermediate phase (Nirsha et al., 1982;Edwards & Woolf, 1985;Guarini & Piccini, 1988). Finally, Edwards and Woolf (1985) synthesized dihydrate samples with an analytical water content of 1.999 eq. via shaking the pentahydrate in MeOH at room temperature and presented lattice parameters for a monoclinic cell (a = 11.431, b = 4.452, c = 20.368 Å , b = 93.79 , V = 1034.4 Å 3 ), but no further structural information was given. A different, but unindexed XRD powder pattern was reported for a sample without given composition, which was prepared through dehydration of the pentahydrate between 338 and 378 K (Nirsha et al., 1982). Besides these results, the large amount of defined hydrates of Na 2 S 2 O 3 , as implied by the early works, is supported by the structure determinations on single crystals of Na 2 S 2 O 3 Á2/3H 2 O (Hesse et al., 1993), Na 2 S 2 O 3 Á5/4H 2 O (Chan et al., 2008) and Na 2 S 2 O 3 Á5H 2 O (Taylor & Beevers, 1952;Padmanabhan et al., 1971;Uraz & Armaǧ an, 1977;Lisensky & Levy, 1978;Prasad & Rani, 2001). Nevertheless, despite the evidence for its existence, for the dihydrate no structure information is available to date.
For the present paper, the crystal structure of the dihydrate was characterized at 100 and 200 K. For comparison, the structure of the pentahydrate was determined at the same conditions, i.e., for the first time below ambient temperature.

Structural commentary
The crystal structure of the dihydrate of Na 2 S 2 O 3 has been determined for the first time. Although this phase has been mentioned in the respective literature for many decades and some sophisticated experiments to synthesize pure samples, usually via controlled dehydration of the pentahydrate, are described, no structural information besides a set of monoclinic lattice parameters is known to date. In the present case, the dihydrate was formed by crystallization at room temperature at the surface of a concentrated aqueous solution, and all dihydrate crystals that have been identified by indexing were isolated from this region. After disturbing the surface tension, most of these crystals subsided immediately to the bottom of the vessel, adding to the bulky crystalline precipitate, which has been identified from X-ray powder patterns as the pentahydrate without visible impurities. After indexing at room temperature, the crystals were cooled down and datasets were recorded at 200 K and 100 K. Besides slight thermal contraction of lattice parameters and a decrease of displacement parameters (see Fig. 1a), no structural change has been observed down to 100 K. The same is true for the crystal structure of the pentahydrate, Na 2 S 2 O 3 Á5H 2 O (Fig. 1b), which has been published formerly and is not discussed here in detail, but was used for comparison. All values mentioned in the structure description below are taken from the structure determinations at 100 K. Na 2 S 2 O 3 Á2H 2 O ( Fig. 2) crystallizes in space group P2 1 /n with two formula units in the asymmetric unit and all atoms (4 Na, 4 S, 10 O, and 8 H) lying on general positions. The two independent thiosulfate anions adopt slightly distorted tetrahedral shapes with average O-S-O angles (110.30 ) above and S-S-O angles (108.63 ) below the mean bond angle of 109.46 . The S-S bond lengths of 2.0047 (2) Å and 2.0078 (2) Å are similar to that found in the pentahydrate [2.0266 (1) Å ], and, thus, are shorter than the single bond of 2.055 Å in crystalline S 8 (Rettig & Trotter, 1987), but substantially longer than the double bond of 1.883 Å in S 2 O (Tiemann et al., 1974) or 1.889 Å in S 2 (Pyykkö & Atsumi, 2009). Also, the S-O bond lengths, which lie between 1.4722 (4) and 1.4841 (4) Å are in the same range as those of the pentahydrate [1.4665 (4)-1.4867 (4) Å ]. The bond-valence sums (Brown & Altermatt, 1985) for the central sulfur atoms, as calculated with the parameters of Brese & O'Keeffe (1991), are 5.87 and 5.88 valence units (v.u.) for S1 and S3, respectively, and are in good agreement with a formal charge of +VI as well as with the value of 5.86 v.u. obtained for the corres- The asymmetric units (a) of Na 2 S 2 O 3 Á2 H 2 O, and (b) of Na 2 S 2 O 3 Á5 H 2 O, with a comparison of relative positions and displacement ellipsoids of the non-hydrogen atoms obtained from structure determinations at 100 K (filled atoms) and at 200 K (contours of ellipsoids drawn around filled atoms). Ellipsoids are drawn at the 80% probability level, hydrogen bonds as dashed lines. ponding S atom in the pentahydrate. The anions coordinate to the Na + cations and form hydrogen bonds with the water molecules of crystallization: in detail the terminal S and O atoms are surrounded by one Na + and one H 2 O (O1, O4, O6), two Na + and one H 2 O (O2, O3), three Na + (O5), three Na + and two H 2 O (S2), or four Na + and one H 2 O (S4).
The four independent Na + cations are coordinated irregularly by the S 2 O 3 2À dianions in mono-or bidentate manner and by H 2 O, as illustrated in Fig. 3a-d. The shortest Na-O distances are in the range between 2.3169 (5) Å and 2.4884 (4) Å , with Na-S between 2.9296 (3) and 2.9695 Å . If these environments are considered exclusively, the resulting coordination polyhedra can be interpreted as an octahedron for Na3, mainly distorted due to two S 2 O 3 2ions coordinating as bidentate ligands, a trigonal prism with one missing corner for Na2 or an octahedron with one (Na1) or two (Na4) missing corners. This construction starting from six-vertex polyhedra seems to be justified due to the clearly favoured sixfold coordination for Na + in an environment of oxygen atoms (Gagné & Hawthorne, 2016). However, for the latter cases of open octahedra, S 2 O 3 2ions as additional ligands with longer bond distances of about 2.5 Å for Na-O and 3.2 Å for Na-S are found, resulting in seven-coordinate polyhedra around Na1 and Na4. For Na2, the H 2 O molecule located above the open side of the polyhedron can be excluded from the coordination sphere due to the too large Na-O distance of 3.52 Å and the orientation of the H atoms. The bond-valence sums for the Na cations are 1.08, 1.05, 1.15, and 1.06 v.u. with the highest value for the most conventionally coordinated Na3 ion while reduced values indicate weaker bonds in the coordination spheres of Na1 and Na4 or even an apparently incomplete coordination of Na2. This generally 'overbonded' situation for the Na cations as well as the trend to higher values for regular coordination polyhedra is similarly found in the pentahydrate, the respective values are 1.14 and 1.18 v.u. for the two independent cations in relatively regular octahedral coordinations, shown in Fig. 3e  Coordination polyhedra around the Na + cations in Na 2 S 2 O 3 Á2H 2 O (a-d) and in Na 2 S 2 O 3 Á5H 2 O (e-f). Anisotropic displacement ellipsoids of non-H atoms are drawn with 80% probability, weakly or non-coordinating distances above 2.55 Å for Na-O and 3.18 Å for Na-S as dashed lines.   The four independent water molecules show quite similar, roughly tetrahedral surroundings, as shown in Fig. 4. Each H 2 O molecule coordinates to two Na + ions, i.e., as a common vertex of neighbouring coordination polyhedra. All the H atoms form one hydrogen bond of moderate strength with O-HÁ Á ÁO or O-HÁ Á ÁS angles above 164 , see Table 1. This is another similarity to observations in the pentahydrate, where each H atom is part of one almost linear hydrogen bond ( Table 2). The highly irregular coordination of the Na + cations in the dihydrate is conspicuous with respect to other more conven-research communications Table 1 Hydrogen-bond geometry (Å , ) for Na 2 S 2 O 3 Á2H 2 O at 100 K. Symmetry codes: (i) x þ 1 2 ; Ày þ 1 2 ; z À 1 2 ; (ii) Àx þ 1; Ày; Àz þ 1; (iii) x þ 1; y; z.  (14) 3.3381 (4) 173.0 (13) Symmetry codes: (i) x þ 1; y; z; (ii) x; Ày þ 1 2 ; z þ 1 2 ; (iii) x þ 1; Ày þ 1 2 ; z þ 1 2 ; (iv) Àx þ 1; y þ 1 2 ; Àz þ 1 2 ; (v) x; Ày þ 1 2 ; z À 1 2 .

Figure 4
Environments of the crystal water molecules in Na 2 S 2 O 3 Á2H 2 O. Anisotropic displacement ellipsoids of non-H atoms are drawn with a probability of 80%, hydrogen bonds as dashed lines, and short contacts to coordinating Na + ions as thin lines. (a) A pair of S 2 O 3 2anions in Na 2 S 2 O 3 Á2H 2 O connected by two H 2 O molecules via hydrogen bonds. (b) Illustration of the hydrogen-bond network between thiosulfate anions, drawn as tetrahedra, and water molecules in Na 2 S 2 O 3 Á2H 2 O: two S 2 O 3 tetrahedra (e.g., the blue ones) are bonded by two H 2 O (blue) to form dimers, which are connected by two H 2 O (pink) with another dimer (green). These tetrameric units are interconnected by H 2 O with neighbouring tetramers (yellow and red tetrahedra). tional structural features, like the usual bond lengths in the anions or the near-linear hydrogen bonds. Obviously, the structure directing effect of the Na + cations is the weakest among the present building units, although more regular coordination polyhedra, particularly octahedra, would have been possible as found in the pentahydrate as well as in the related structures of Na 6 (S 2 O 3 ) 3 Á2H 2 O (Hesse et al., 1993) and Na 8 (S 2 O 3 ) 4 Á5H 2 O (Chan et al., 2008). Such open, or at least higher coordinated, polyhedra including weaker bonded ligands as observed in Na 2 S 2 O 3 Á2H 2 O should represent an easy possibility to incorporate further water molecules into the structure and, therefore, a hint for the low stability relative to higher hydrates and the retardation of this structure determination.

Supramolecular features
In Na 2 S 2 O 3 Á2 H 2 O the thiosulfate anions and water molecules are connected via hydrogen bonds of medium strength, see Table 1, with all H atoms forming one almost linear bond. Two S 2 O 3 2ions are connected by two H 2 O molecules to form the building units shown in Fig. 5a. These dimeric units (e.g. blue S 2 O 3 tetrahedra and H 2 O molecules in Fig. 5b) are connected via two further H 2 O molecules (pink in Fig. 5b) with a second dimer (green in Fig. 5b). The resulting tetramers are again interlinked with neighbouring tetramers (yellow and red tetrahedra in Fig. 5b) by water molecules, thereby forming corrugated layers lying parallel to (101), also shown in Fig. 2b. The number of H atoms nicely matches the number of corners of the S 2 O 3 2tetrahedra; however, by realizing this connection pattern, six of the eight possible corners of the tetrahedra dimers accept one hydrogen bond, but one corner (S2) accepts two while one corner (O5) is exclusively surrounded by Na + cations. The layers are not interconnected by hydrogen bonds but only by Na + cations. This is another difference to the pentahydrate where the S 2 O 3 2ions and H 2 O molecules form a three-dimensional framework including hydrogen bonds between water molecules, obviously due to the higher number of H 2 O molecules and, thus, possible hydrogen bonds.

Database survey
Na 2 S 2 O 3 and its hydrates have been structurally investigated several times within the second half of the last century. Besides the anhydrous phase (Sá ndor & Csordá s, 1961;Teng et al., 1984), including a thorough examination of its temperature dependent polymorphism (von Benda & von Benda, 1979)

Synthesis and crystallization
Colourless crystals of Na 2 S 2 O 3 Á2H 2 O were grown at ambient conditions from an aqueous solution of Na 2 S 2 O 3 . The crystals were found floating at the surface of the mother liquor, but sank down to the bottom of the crystallization vessel immediately after disturbing the surface tension. A batch of crystals was immersed into perfluoroether, and the crystals were found to be unscathed and stable at room temperature for days. In contrast, no crystals of the dihydrate could be found from the crystal bulk at the bottom of the vessel, but all crystals isolated later from there were pentahydrate crystals. In addition, an X-ray powder pattern of a sample prepared from this bulk did not contain any other reflections than those of the pentahydrate.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 3. In all presented structure refinements, all hydrogen atoms could be located from the difference-Fourier map and were refined with free atomic coordinates and isotropic displacement parameters.  , 2015); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg & Putz, 2012).

Sodium thiosulfate dihydrate (Na2S2O3H2O2_100K)
Crystal data T min = 0.919, T max = 1.000 52592 measured reflections 8735 independent reflections 7524 reflections with I > 2σ(I) 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Na1 0.72487 (4) 0.34133 (2)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.42 e Å −3 Δρ min = −0.28 e Å −3 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.