Sn(SO₄)₂·2H₂O from synchrotron powder data

Tin(IV) sulfate dihydrate, Sn(SO₄)₂·2H₂O, was prepared in a reflux of sulfuric acid under oxidizing conditions. Its crystal structure was determined from powder synchrotron X-ray diffraction data and is constructed of (100) layers of [SnO₄(H₂O)₂] octa­hedra (point group symmetry 1) corner-connected by sulfate tetra­hedra. Hydrogen bonds of moderate strength between the water mol­ecules and sulfate O atoms hold the layers together.

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Structure description

Tin sulfates and derivated compounds display an inter­esting structural chemistry. A number of tin sulfates, including an oxide sulfate, were reported by Ahmed et al. (1998) based on synthesis in 50–95%_wt H₂SO₄ at and above room temperature. In addition to the earlier reported and reasonably characterized compounds of Sn^IISO₄ (Rentzeperis, 1962), Sn^II₂OSO₄ (Lundren et al., 1982), Sn^II₃O(OH)₂SO₄ (Grimvall, 1975; Davies et al., 1975), Sn^II₆O₄(SO₄)(OH)₂ (Locock et al., 2006) and Sn₇(OH)₁₂(SO₄)₂ (Sn^II₆Sn^IV(OH)₁₂(SO₄)₂; Grimvall, 1982), the high-temperature reactions in concentrated sulfuric acid revealed the existence of two polymorphs of Sn(SO₄)₂·2H₂O (A and B), the tetra­hydrate Sn(SO₄)₂·4H₂O, the mixed-valent Sn^II/Sn^IV oxide sulfate Sn₆O(SO₄)₉ and two polymorphs of anhydrous Sn(SO₄)₂, of which one is obtained on heating to around 773 K (Ahmed et al., 1998). Since all the Sn^IV-containing compounds are highly hygroscopic, handling and characterization require inert conditions. Loss of crystallinity was rapidly observed for samples subjected to air at ambient conditions. More recently, Sn^IV(SO₄)₂ and mixed-valent Sn^IISn^IV(SO₄)₃ were reported (Hämmer et al., 2021). The former adopts a crystal structure with [SnO₆] octa­hedra corner-connected through sulfate tetra­hedra in all directions, while the latter adopts a layered structure.

We report here on the crystal structure of one of the above mentioned compounds, Sn(SO₄)₂·2H₂O (B), for which Ahmed et al. (1998) suggested a monoclinic structure with a = 9.705 (1) Å, b = 5.652 (1) Å, c = 7.033 (1) Å, β = 106.86 (1)^o based on powder X-ray diffraction data. On heating, Sn(SO₄)₂·2H₂O (B) transforms into anhydrous Sn(SO₄)₂ at about 623 K.

The crystal structure of Sn(SO₄)₂·2H₂O is shown in Figs. 1 and 2, and selected bond lengths and angles are given in Table 1. The structure can be described as being constructed of layers of slightly distorted [SnO₄(H₂O)₂] octa­hedra corner-connected by sulfate tetra­hedra. There is no bonding directly between the [SnO₄(H₂O)₂] units. The layers extend parallel to (100) and are stacked along [100], Fig. 1. The Sn^IV atom is situated at an inversion centre (multiplicity 2, Wyckoff letter b) and is surrounded by four sulfate groups that connect the [SnO₄(H₂O)₂] units, and by two water mol­ecules. Considering the Sn—O bond lengths, we find them to be in excellent agreement for Sn^IV with bond lengths from 1.968 (6) to 2.046 (6) Å (Table 1). In comparison, bond lengths of 2.016 (3) – 2.049 (3) Å are observed for Sn^IV(SO₄)₂ by Hämmer et al. (2021). The two water mol­ecules (O5) are directed towards the inter-layer space and exhibit the longest of the Sn—O bonds. The sulfate group shows a slight scatter in the S—O bond lengths, between 1.465 (6) and 1.526 (6) Å, around the ideal bond length of ∼1.49 Å (Louisnathan et al., 1977). Some deviations in the lengths are expected due to the different local environments of the sulfate group as two of its oxygen atoms are directed toward Sn, while the other two are directed toward hydrogen atoms.

Table 1
Selected geometric parameters (Å, °)

Sn1—O1	1.968 (6)	S1—O2	1.465 (6)
Sn1—O4^i	2.042 (5)	S1—O3	1.467 (6)
Sn1—O5	2.046 (6)	S1—O4	1.495 (5)
S1—O1	1.526 (6)
H1—O5—H2	103.2 (6)
Symmetry code: (i) .

Figure 1 Crystal structure of Sn(SO4)2·2H2O in a view along [010]. Purple coordination polyhedra represent [SnO4(H2O)2] octa­hedra, and the yellow polyhedra the sulfate tetra­hedra. Dashed lines indicate hydrogen bonds.

Figure 2 Crystal structure of Sn(SO4)2·2H2O in a view along [100]. Colour code is as in Fig. 1.

Intra-and inter­layer O—H⋯O hydrogen bonding between water mol­ecules and sulfate O atoms is observed. The intra­layer hydrogen bond is rather strong [O5⋯O2ⁱⁱ = 2.541 (8) Å], whereas the inter­layer hydrogen bond, which is responsible for the cohesion of the layers along [100], is of moderate strength [O5⋯O3ⁱⁱ = 2.735 (7) Å]. Other numerical values for these inter­actions are compiled in Table 2.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O5—H1⋯O2^ii	0.969 (6)	1.574 (5)	2.541 (8)	175.6 (4)
O5—H2⋯O3^iii	0.963 (6)	1.780 (4)	2.735 (7)	171.0 (4)
Symmetry codes: (ii) ; (iii) .

Synthesis and crystallization

The current sample of Sn(SO₄)₂·2H₂O (B) was obtained according to Ahmed et al. (1998), by reacting Sn powder (Fluka; 99.9%) in 85%_wt H₂SO₄ at 368 K in reflux while oxygen gas was passed through the reaction mixture. The obtained product was isolated after ten days by deca­nting, followed by washing and drying before storage in a vacuum desiccator.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. Synchrotron X-ray data of an Sn(SO₄)₂·2H₂O powder sample was collected in a 0.5 mm capillary at BM01B at the Swiss–Norwegian beamlines (SNBL), the European Synchrotron Radiation Facility (ESRF), Grenoble, France, with a wavelength of 1.00098 Å using a high-resolution detector. The data revealed the Sn(SO₄)₂·2H₂O sample to be of high purity, with two minor impurity reflections visible at about 1 and 2.05 Å⁻¹ (Fig. 3). The indexed cell given by Ahmed et al. (1998) was used as a starting point for the structure solution. For the final refinement, the a and c axes were inter­changed relative to the setting used by Ahmed et al. (1998). Le Bail refinements with the reported lattice parameters yielded a good fit, and space group P2₁/c as the space group. Charge flipping using SUPERFLIP (Palatinus & Chapuis, 2007) implemented in JANA2006 (Petříček et al., 2014) quickly gave a good model for the atomic positions for the heavier elements. Without the constraint for the S—O bond length, unrealistically large variations were obtained. The ideal bond length for the S—O bond in a sulfate group is ∼1.49 Å (Louisnathan et al., 1977). The crystal structure was refined with distance restraints on the S—O bond length by selecting the target distance to 1.49 Å, a value of 0.002 Å for allowed deviations, and a moderate penalty factor.

Table 3
Experimental details

Crystal data
Chemical formula	Sn(SO4)2·2H2O
Mr	346.86
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	293
a, b, c (Å)	7.03475 (5), 5.65165 (5), 9.70464 (7)
β (°)	106.8524 (4)
V (Å^3)	369.27 (1)
Z	2
Radiation type	Synchrotron, λ = 1.00098 Å
μ (mm^−1)	10.46
Specimen shape, size (mm)	Cylinder, 40 × 0.5
Data collection
Diffractometer	High-resolution sychrotron
Specimen mounting	Capillary
Data collection mode	Transmission
Scan method	Step
2θ values (°)	2θmin = 8.207 2θmax = 53, 2θstep = 0.007
Refinement
R factors and goodness of fit	Rp = 0.024, Rwp = 0.033, Rexp = 0.017, R(F) = 0.045, χ^2 = 3.792
No. of parameters	46
No. of restraints	4
H-atom treatment	H-atom parameters constrained
Computer programs: local program, JANA2006 and JANA2020 (Petříček et al., 2014), SUPERFLIP (Palatinus & Chapuis, 2007), VESTA (Momma & Izumi, 2011) and publCIF (Westrip, 2010).

Figure 3 Observed (black), calculated (red), and difference (blue) intensity profile from Rietveld refinement of synchrotron X-ray data for Sn(SO4)2·2H2O. Bragg reflections are marked with green bars.

Based on a previous report regarding composition and charge neutrality (Ahmed et al., 1998), we added hydrogen atoms to the structure. They were placed to have an O—H bond length of ∼0.95 Å and with an angle of ∼105° between the H atoms. The H atoms were further directed towards O2 and O3 as the lengths to these oxygen atoms indicate hydrogen bonding (Table 2). Displacement parameters were not refined for hydrogen atoms. Rietveld refinement of the final structural model is shown in Fig. 3. The refinement included lattice parameters, pseudo-Voigt peak shape, background, zero error, axial divergence correction, axial strain broadening tensors, atomic parameters, and thermal displacement parameters of non-H atoms.

Structural data

Computing details

Tin(IV) sulfate dihydrate

Crystal data

Sn(SO4)2·2H2O	Z = 2
Mr = 346.86	F(000) = 332
Monoclinic, P21/c	Dx = 3.120 Mg m^−3
Hall symbol: -P 2ycb	Synchrotron radiation, λ = 1.00098 Å
a = 7.03475 (5) Å	µ = 10.46 mm^−1
b = 5.65165 (5) Å	T = 293 K
c = 9.70464 (7) Å	White
β = 106.8524 (4)°	cylinder, 40 × 0.5 mm
V = 369.27 (1) Å^3	Specimen preparation: Prepared at 368 K

Data collection

High-resolution sychrotron diffractometer	Absorption correction: for a cylinder mounted on the φ axis
Specimen mounting: Capillary
Data collection mode: transmission	2θmin = 8.207°, 2θmax = 53°, 2θstep = 0.007°
Scan method: step

Refinement

Rp = 0.024	4 restraints
Rwp = 0.033	8 constraints
Rexp = 0.017	H-atom parameters constrained
R(F) = 0.045	Weighting scheme based on measured s.u.'s
6400 data points	(Δ/σ)max = 0.044
Profile function: Pseudo-Voigt	Background function: Manual background combined with 5 Legendre polynoms
46 parameters	Preferred orientation correction: none

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

	x	y	z	Uiso*/Ueq
Sn1	0.5	0	0	0.0331 (3)*
S1	0.2572 (4)	0.2445 (5)	0.2045 (3)	0.0339 (8)*
O1	0.4385 (6)	0.1591 (8)	0.1624 (5)	0.0244 (18)*
O2	0.1956 (6)	0.4801 (8)	0.1454 (4)	0.0324 (16)*
O3	0.1010 (6)	0.0638 (8)	0.1670 (5)	0.043 (2)*
O4	0.3254 (7)	0.2572 (8)	0.3653 (4)	0.0187 (17)*
O5	0.7294 (8)	0.2365 (9)	0.0353 (6)	0.0414 (19)*
H1	0.76	0.35	−0.03	0.01*
H2	0.855	0.16	0.078	0.01*

Geometric parameters (Å, º)

Sn1—O1	1.968 (6)	S1—O2	1.465 (6)
Sn1—O4^i	2.042 (5)	S1—O3	1.467 (6)
Sn1—O5	2.046 (6)	S1—O4	1.495 (5)
S1—O1	1.526 (6)
H1—O5—H2	103.2 (6)	O1—S1—O3	109.7 (3)
O1^ii—Sn1—O4^i	88.53 (18)	O1—S1—O4	104.7 (3)
O1^ii—Sn1—O4^iii	91.47 (18)	O2—S1—O3	114.9 (3)
O1^ii—Sn1—O5	95.0 (2)	O2—S1—O4	109.5 (3)
O1^ii—Sn1—O5^ii	85.0 (2)	O3—S1—O4	106.5 (3)
O1—S1—O2	110.9 (3)

Symmetry codes: (i) −x+1, y−1/2, −z+1/2; (ii) −x+1, −y, −z; (iii) x, −y+1/2, z−1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H1···O2^iv	0.969 (6)	1.574 (5)	2.541 (8)	175.6 (4)
O5—H2···O3^v	0.963 (6)	1.780 (4)	2.735 (7)	171.0 (4)

Symmetry codes: (iv) −x+1, −y+1, −z; (v) x+1, y, z.