Crystal structure of strontium thio­sulfate monohydrate

SrS₂O₃·H₂O was obtained from an aqueous solution of Na₂S₂O₃ and Sr(NO₃)₂ and crystallizes in space group P with all atoms at general positions. The Sr²⁺ ion exhibits an [8 + 1] coordination defined by two terminal S and six O atoms of thio­sulfate ions, one of the latter at a longer distance, and by one O atom of a water mol­ecule. Two thio­sulfate anions act as bidentate, four as monodentate ligands. The structure consists of mainly ionically inter­acting layers lying parallel to the crystallographic ab plane. The layers are connected by O—H⋯S and O—H⋯O hydrogen bonds of moderate strength.

1. 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 Na₂S₂O₃ 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 Na₂S₂O₃, as well as (NH₄)₂S₂O₃, 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, MgS₂O₃·6H₂O 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 BaS₂O₃·H₂O (Manojlović-Muir, 1969).

Next to SrS₂O₃·5H₂O (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 Na₂S₂O₃ and Sr(NO₃)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.

2. Structural commentary

SrS₂O₃·H₂O crystallizes in the space group P with one formula unit in the asymmetric unit and all atoms on general positions. Many structural features resemble the closely related penta­hydrate of SrS₂O₃. 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 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).

Table 1
S—S and averaged S—O bond lengths (Å) in thio­sulfates of divalent cations

Cation/solvent mol­ecules	S—S	mean S—O
Ba^2+/1 H2O^a	1.979	1.477
Sr^2+/5 H2O^b	1.995	1.472
Sr^2+/1 H2O^c	2.004	1.474
Ca^2+/6 H2O^b	2.008	1.468
Mg^2+/6 H2O^d	2.019	1.471
Ni^2+/6 H2O^e	2.015	1.459
Cd^2+/2 H2O^f	2.056	1.454
Pb^2+/0 H2O^g	2.11	1.455
Notes: (a) Manojlović-Muir (1975); (b) Held & Bohatý (2004); (c) this work; (d) Elerman et al. (1983); (e) Elerman et al. (1978); (f) Baggio et al. (1997); (g) Christensen et al. (1991).

The Sr²⁺ cation is coordinated by five O and two S atoms, belonging to six neighbouring S₂O₃^2– anions, and one additional O atom of an H₂O 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 Sr²⁺ atom (Fig. 1) again resembles the ninefold coordination of the cation in SrS₂O₃·5H₂O, with the difference being that in the penta­hydrate no S atoms are found in the first coordination sphere of Sr²⁺, but four water mol­ecules instead. As a consequence of the presence of the larger S atoms close to Sr²⁺, 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.

Figure 1 Coordination polyhedron of the Sr^2+ cation in SrS2O3·H2O. Anisotropic displacement ellipsoids are drawn at the 70% probability level; H atoms are shown with arbitrary radius. [Symmetry codes: (i) x − 1, y + 1, z; (ii) −x + 1, −y + 1, −z; (iii) −x, −y + 1, −z; (iv) x, y + 1, z; (v) x − 1, y, z.]

As a characteristic feature of the crystal structure, SrS₂O₃·H₂O 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 (O4⋯O4ⁱⁱ) and the water mol­ecules and thio­sulfate anions via S2 atoms (Table 2). This involves also a bifurcated hydrogen bond O4—H1⋯(O4ⁱⁱ/S2ⁱⁱⁱ). The O4—H2⋯S2ⁱ bond as well as the D⋯A distances are in the same range as in SrS₂O₃·5H₂O (Held & Bohatý, 2004). For the H1 atom, a disorder model similar to that proposed for BaS₂O₃·H₂O (Manojlović-Muir, 1975) was considered, which would result in shorter and more linear O4—H1⋯O4ⁱⁱ and O4—H1⋯S2ⁱⁱⁱ bonds. However, a reasonable refinement of these disordered H atoms was not possible.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O4—H2⋯S2^i	0.76 (3)	2.62 (4)	3.344 (2)	163 (4)
O4—H1⋯O4^ii	0.78 (4)	2.42 (4)	2.962 (4)	128 (4)
O4—H1⋯S2^iii	0.78 (4)	2.84 (4)	3.458 (2)	138 (4)
Symmetry codes: (i) -x, -y+1, -z+1; (ii) -x, -y+2, -z+1; (iii) -x+1, -y+1, -z+1.

Figure 2 Crystal structure of SrS2O3·H2O, in a view onto (001). Displacement ellipsoids are shown as in Fig. 1.

Figure 3 A projection of the crystal structure of SrS2O3·H2O, approximately along [00]. Displacement ellipsoids are shown as in Fig. 1.

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 SrS₂O₃·5H₂O is only slightly changed as a result of the coordination of additional water mol­ecules to the Sr²⁺ 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. NaNO₃ and Sr(NO₃)₂.

Figure 4 Illustration of the structural relationship between the title compound, SrS2O3·H2O, and (a) SrS2O3·5H2O (view approximately along [00]) and (b) BaS2O3·H2O (view approximately along [011]).

The ortho­rhom­bic structure of BaS₂O₃·H₂O is likewise found to form layers, which are separated by water mol­ecules (Fig. 4b; Nardelli & Fava, 1962; Manojlović-Muir, 1975). Similar to the Sr homologue, two terminal S atoms are part of the first coordination sphere of the Ba²⁺ 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.

3. Database survey

Besides SrS₂O₃, crystal structure determinations for three further alkaline-earth thio­sulfates have been reported, all of them as hydrates: BaS₂O₃·H₂O (Nardelli & Fava, 1962; Manojlović-Muir, 1975), CaS₂O₃·6H₂O (Held & Bohatý, 2004), and MgS₂O₃·6H₂O (Nardelli et al., 1962; Baggio et al., 1969; Elerman et al., 1982,1983). Together with the known Sr compounds, SrS₂O₃·5H₂O (Held & Bohatý, 2004) and the new monohydrate, the trend of incorporating smaller amounts of water into stable crystal structures with increasing cation radius is obvious for alkaline-earth metal thio­sulfates. With the exception of two of the five water mol­ecules in SrS₂O₃·5H₂O, all water mol­ecules in these compounds coordinate to the divalent cations. This trend is confirmed by 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.

4. Synthesis and crystallization

Crystals of SrS₂O₃·H₂O were grown from an aqueous solution of Na₂S₂O₃·5H₂O and Sr(NO₃)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 NaNO₃ and surplus Sr(NO₃)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.

5. 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.

Table 3
Experimental details

Crystal data
Chemical formula	SrS2O3·H2O
Mr	217.76
Crystal system, space group	Triclinic, P
Temperature (K)	297
a, b, c (Å)	4.6858 (2), 5.9178 (3), 9.0167 (4)
α, β, γ (°)	84.889 (2), 87.284 (2), 80.785 (2)
V (Å^3)	245.68 (2)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	11.72
Crystal size (mm)	0.42 × 0.30 × 0.16
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Numerical (SADABS; Krause et al., 2015)
Tmin, Tmax	0.037, 0.264
No. of measured, independent and observed [I > 2σ(I)] reflections	8011, 1504, 1481
Rint	0.054
(sin θ/λ)max (Å^−1)	0.715
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.024, 0.061, 1.10
No. of reflections	1504
No. of parameters	71
No. of restraints	2
H-atom treatment	Only H-atom coordinates refined
Δρmax, Δρmin (e Å^−3)	0.97, −0.86
Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), DIAMOND (Brandenburg & Putz, 2012) and publCIF (Westrip, 2010).

Supporting information

Computing details

Data collection: APEX3 (Bruker, 2015); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg & Putz, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).

Strontium thiosulfate monohydrate

Crystal data

SrS2O3·H2O	Z = 2
Mr = 217.76	F(000) = 208
Triclinic, P1	Dx = 2.944 Mg m^−3
a = 4.6858 (2) Å	Mo Kα radiation, λ = 0.71073 Å
b = 5.9178 (3) Å	Cell parameters from 7034 reflections
c = 9.0167 (4) Å	θ = 2.3–30.6°
α = 84.889 (2)°	µ = 11.72 mm^−1
β = 87.284 (2)°	T = 297 K
γ = 80.785 (2)°	Block, colourless
V = 245.68 (2) Å^3	0.42 × 0.30 × 0.16 mm

Data collection

Bruker APEXII CCD diffractometer	1504 independent reflections
Radiation source: rotating anode FR591	1481 reflections with I > 2σ(I)
MONTEL optic monochromator	Rint = 0.054
Detector resolution: 16 pixels mm^-1	θmax = 30.6°, θmin = 2.3°
φ– and ω–rotation scans	h = −6→6
Absorption correction: numerical (SADABS; Krause et al., 2015)	k = −8→8
Tmin = 0.037, Tmax = 0.264	l = −12→12
8011 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	Only H-atom coordinates refined
wR(F^2) = 0.061	w = 1/[σ^2(Fo^2) + (0.0324P)^2 + 0.2036P] where P = (Fo^2 + 2Fc^2)/3
S = 1.10	(Δ/σ)max = 0.001
1504 reflections	Δρmax = 0.97 e Å^−3
71 parameters	Δρmin = −0.86 e Å^−3
2 restraints	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Sr	0.01493 (4)	0.73223 (3)	0.16811 (2)	0.01372 (10)
S1	0.47043 (11)	0.20793 (8)	0.16097 (5)	0.01113 (12)
S2	0.55500 (12)	0.41404 (9)	0.31260 (6)	0.01678 (13)
O1	0.2582 (3)	0.3477 (3)	0.06042 (17)	0.0163 (3)
O2	0.7385 (3)	0.1265 (3)	0.07573 (19)	0.0178 (3)
O3	0.3580 (4)	0.0090 (3)	0.23659 (19)	0.0221 (3)
O4	−0.0933 (5)	0.7998 (4)	0.4397 (2)	0.0309 (4)
H1	0.024 (9)	0.827 (8)	0.491 (5)	0.046*
H2	−0.223 (8)	0.774 (7)	0.487 (4)	0.046*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr	0.01592 (13)	0.01292 (12)	0.01266 (13)	−0.00269 (7)	−0.00130 (7)	−0.00167 (7)
S1	0.0116 (2)	0.0105 (2)	0.0117 (2)	−0.00225 (16)	−0.00057 (16)	−0.00182 (16)
S2	0.0202 (3)	0.0170 (2)	0.0145 (2)	−0.00505 (19)	−0.00235 (19)	−0.00465 (18)
O1	0.0159 (7)	0.0160 (7)	0.0168 (7)	0.0000 (6)	−0.0053 (6)	−0.0030 (6)
O2	0.0145 (7)	0.0194 (7)	0.0188 (7)	0.0006 (6)	0.0027 (6)	−0.0057 (6)
O3	0.0316 (9)	0.0187 (7)	0.0191 (8)	−0.0146 (7)	−0.0033 (7)	0.0023 (6)
O4	0.0339 (11)	0.0433 (11)	0.0187 (9)	−0.0151 (9)	0.0037 (8)	−0.0071 (8)

Geometric parameters (Å, º)

Sr—O4	2.531 (2)	S1—O2	1.4768 (16)
Sr—O2^i	2.5698 (16)	S1—O1	1.4776 (15)
Sr—O2^ii	2.5708 (17)	S1—S2	2.0044 (7)
Sr—O1^iii	2.5934 (15)	S1—Sr^vii	3.4816 (5)
Sr—O3^iv	2.6010 (17)	S2—Sr^viii	3.2379 (6)
Sr—O1	2.6226 (15)	O1—Sr^iii	2.5934 (15)
Sr—S2	3.1618 (6)	O2—Sr^vii	2.5698 (16)
Sr—S2^v	3.2379 (6)	O2—Sr^ii	2.5708 (16)
Sr—O3^i	3.305 (2)	O3—Sr^ix	2.6010 (17)
Sr—S1	3.4768 (5)	O3—Sr^vii	3.305 (2)
Sr—S1^i	3.4816 (5)	O4—H1	0.78 (4)
Sr—Sr^vi	4.1762 (4)	O4—H2	0.76 (3)
S1—O3	1.4663 (16)
O4—Sr—O2^i	93.29 (7)	O1^iii—Sr—S1^i	80.94 (3)
O4—Sr—O2^ii	145.14 (6)	O3^iv—Sr—S1^i	86.26 (4)
O2^i—Sr—O2^ii	71.34 (6)	O1—Sr—S1^i	150.22 (3)
O4—Sr—O1^iii	138.36 (6)	S2—Sr—S1^i	152.443 (14)
O2^i—Sr—O1^iii	75.45 (5)	S2^v—Sr—S1^i	89.500 (14)
O2^ii—Sr—O1^iii	69.30 (5)	O3^i—Sr—S1^i	24.78 (3)
O4—Sr—O3^iv	73.47 (6)	S1—Sr—S1^i	170.625 (18)
O2^i—Sr—O3^iv	77.99 (6)	O4—Sr—Sr^vi	122.71 (6)
O2^ii—Sr—O3^iv	72.79 (5)	O2^i—Sr—Sr^vi	35.68 (4)
O1^iii—Sr—O3^iv	138.90 (5)	O2^ii—Sr—Sr^vi	35.66 (4)
O4—Sr—O1	126.88 (7)	O1^iii—Sr—Sr^vi	68.15 (3)
O2^i—Sr—O1	139.15 (5)	O3^iv—Sr—Sr^vi	71.93 (4)
O2^ii—Sr—O1	77.29 (5)	O1—Sr—Sr^vi	109.27 (4)
O1^iii—Sr—O1	69.30 (5)	S2—Sr—Sr^vi	129.735 (12)
O3^iv—Sr—O1	116.91 (6)	S2^v—Sr—Sr^vi	134.574 (12)
O4—Sr—S2	80.16 (6)	O3^i—Sr—Sr^vi	80.32 (3)
O2^i—Sr—S2	152.24 (4)	S1—Sr—Sr^vi	124.721 (11)
O2^ii—Sr—S2	98.85 (4)	S1^i—Sr—Sr^vi	58.002 (9)
O1^iii—Sr—S2	126.47 (3)	O3—S1—O2	108.96 (11)
O3^iv—Sr—S2	74.28 (4)	O3—S1—O1	112.09 (10)
O1—Sr—S2	57.20 (3)	O2—S1—O1	109.91 (9)
O4—Sr—S2^v	69.28 (5)	O3—S1—S2	109.49 (7)
O2^i—Sr—S2^v	108.83 (4)	O2—S1—S2	109.77 (7)
O2^ii—Sr—S2^v	144.82 (4)	O1—S1—S2	106.59 (7)
O1^iii—Sr—S2^v	76.67 (4)	O3—S1—Sr	115.97 (8)
O3^iv—Sr—S2^v	142.38 (4)	O2—S1—Sr	134.05 (7)
O1—Sr—S2^v	82.74 (4)	O1—S1—Sr	43.98 (6)
S2—Sr—S2^v	94.134 (15)	S2—S1—Sr	64.00 (2)
O4—Sr—O3^i	65.98 (6)	O3—S1—Sr^vii	70.87 (8)
O2^i—Sr—O3^i	46.05 (5)	O2—S1—Sr^vii	41.53 (7)
O2^ii—Sr—O3^i	114.88 (5)	O1—S1—Sr^vii	141.36 (6)
O1^iii—Sr—O3^i	78.38 (4)	S2—S1—Sr^vii	108.24 (2)
O3^iv—Sr—O3^i	104.37 (6)	Sr—S1—Sr^vii	170.625 (18)
O1—Sr—O3^i	138.65 (4)	S1—S2—Sr	81.26 (2)
S2—Sr—O3^i	144.49 (3)	S1—S2—Sr^viii	109.00 (2)
S2^v—Sr—O3^i	64.83 (3)	Sr—S2—Sr^viii	94.134 (15)
O4—Sr—S1	106.66 (6)	S1—O1—Sr^iii	134.66 (9)
O2^i—Sr—S1	159.58 (4)	S1—O1—Sr	112.99 (8)
O2^ii—Sr—S1	89.34 (4)	Sr^iii—O1—Sr	110.70 (5)
O1^iii—Sr—S1	91.84 (3)	S1—O2—Sr^vii	116.07 (9)
O3^iv—Sr—S1	103.11 (4)	S1—O2—Sr^ii	134.93 (10)
O1—Sr—S1	23.03 (3)	Sr^vii—O2—Sr^ii	108.66 (6)
S2—Sr—S1	34.737 (12)	S1—O3—Sr^ix	135.58 (10)
S2^v—Sr—S1	82.985 (14)	S1—O3—Sr^vii	84.35 (8)
O3^i—Sr—S1	147.67 (3)	Sr^ix—O3—Sr^vii	104.37 (6)
O4—Sr—S1^i	75.63 (6)	Sr—O4—H1	122 (3)
O2^i—Sr—S1^i	22.40 (4)	Sr—O4—H2	128 (3)
O2^ii—Sr—S1^i	93.64 (4)	H1—O4—H2	109 (4)

Symmetry codes: (i) x−1, y+1, z; (ii) −x+1, −y+1, −z; (iii) −x, −y+1, −z; (iv) x, y+1, z; (v) x−1, y, z; (vi) −x, −y+2, −z; (vii) x+1, y−1, z; (viii) x+1, y, z; (ix) x, y−1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H2···S2^x	0.76 (3)	2.62 (4)	3.344 (2)	163 (4)
O4—H1···O4^xi	0.78 (4)	2.42 (4)	2.962 (4)	128 (4)
O4—H1···S2^xii	0.78 (4)	2.84 (4)	3.458 (2)	138 (4)

Symmetry codes: (x) −x, −y+1, −z+1; (xi) −x, −y+2, −z+1; (xii) −x+1, −y+1, −z+1.