Sodium sulfite heptahydrate and its relation to sodium carbonate heptahydrate.

The crystal structure of Na2SO3(H2O)7 shows close structural similarities with Na2CO3(H2O)7, though the two heptahydrates belong to different crystal systems (monoclinic and orthorhombic, respectively) and contain anions with different shapes.


Introduction
Sodium sulfite is used extensively in industrial processes, for example, as an antioxidant and preservative in food industries (E number for food additives E221), as a corrosion inhibitor in aqueous media, as a bleaching agent, as a solubilizing agent for cellulose, straw and wood in the pulp and paper industry, or as an additive in dying processes. In the USA alone, the production of sodium sulfite reached 150 000 tons in 2002 (Weil et al., 2007). Solid sodium sulfite is stable in its anhydrous form and as the heptahydrate. Despite its use at industrial scales, structural details are known only for anhydrous Na 2 SO 3 that crystallizes with two formula units in the trigonal system in the space group P3 (Larsson & Kierkegaard, 1969). Bond lengths and near-neighbour distances of sodium sulfite in aqueous solution have been calculated by ab initio quantum mechanical charge field molecular dynamics (QMCF MD) studies and determined experimentally by largeangle X-ray scattering (LAXS) by Eklund et al. (2012). For crystalline Na 2 SO 3 (H 2 O) 7 , lattice parameters and the space group (P2 1 /n) have previously been determined from Weissenberg photographs without providing further structural details, except for a close metrical resemblance with orthorhombic Na 2 CO 3 (H 2 O) 7 (Dunsmore & Speakman, 1963). To obtain a more detailed picture of the relationship between the heptahydrates of Na 2 SO 3 and Na 2 CO 3 , we grew single crystals of Na 2 SO 3 (H 2 O) 7 and determined its crystal structure. Indeed, the two heptahydrates show not only a close metrical relationship (Table 1), but also structural similarities, though they belong to different crystal systems and contain differently shaped divalent anions, viz. trigonal-pyramidal SO 3 2À and trigonal-planar CO 3

Crystallization
Colourless prismatic crystals of Na 2 SO 3 (H 2 O) 7 were grown by recrystallization of a commercial anhydrous sample (Merck, p.A. grade) from an aqueous solution at room temperature by slow evaporation over the course of several days. In order to remove adherent mother liquor, the crystals were placed on filter paper and subsequently immersed in Paratone oil. The crystal under investigation was cleaved from a larger specimen.

Crystallography and refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal structure of Na 2 SO 3 (H 2 O) 7 was originally solved and refined in the space group P12 1 /n1 (No. 14), with lattice parameters a = 11.8576 (8), b = 7.2197 (5), c = 12.6965 (9) Å and = 106.7938 (17) at 100 K (full crystal data in the setting P12 1 /n1 are available in CIF format as supporting information). The values for the lattice parameters at 100 K are in good agreement with the previous study, with values of a = 11.922, b = 7.260, c = 12.765 Å and = 107.22 obtained at room temperature from Weissenberg film data (note that a and c are interchanged in the original description; Dunsmore & Speakman, 1963). For a better comparison with the reported crystal structure of -Na 2 CO 3 (H 2 O) 7 (Betzel et al., 1982), the nonconventional setting C112 1 /a was chosen for the final structural description of Na 2 SO 3 (H 2 O) 7 , using the matrix (101, 101, 010) for transformation of the primitive cell to the Ccentred cell with c as the unique axis; moreover, the atomic coordinates and the origin of the unit cell were chosen to ensure a similar packing of structural building blocks in the two unit cells of Na 2 SO 3 (H 2 O) 7 and -Na 2 CO 3 (H 2 O) 7 . All H atoms present in the crystal structure of Na 2 SO 3 (H 2 O) 7 were located in a difference Fourier map and were refined freely.

Crystal structure
In the crystal structure of Na 2 SO 3 (H 2 O) 7 , all atoms (2 Na, 1 S, 10 O and 14 H) are located on general sites. The two sodium cations are surrounded by six water molecules, defining a distorted octahedral coordination polyhedron in each case.
The Na-O distances range from 2.3690 (6) to 2.4952 (6) Å (Table 3), with mean values of 2.42 (4) Å for Na1 and 2.43 (6) Å for Na2, in fairly good agreement with the mean value for Na [6] -O of 2.44 (11) Å calculated for 5520 individual bonds (Gagné & Hawthorne, 2016). The bond valence sums (Brown, 2002) for the sodium cations, as calculated with parameters provided by Brese & O'Keeffe (1991) ). In both cases, the corresponding Na-O bonds to the shared O atoms at the edges are the shortest in the respective octahedron. The dimeric units connect adjacent chains by sharing the terminal water molecules (O9 and O7) on both sides of the chains (corner-sharing links). This way, the sodium-water octahedra are assembled by edge-and cornersharing into an infinite layer extending parallel to (100) (Fig. 1a).
dination capability of the sulfite S atom via its electron lonepair lobe at the apex of the SO 3 pyramid is not unexpected, but this capability seems to be weak in the context of hydrogen bonding because otherwise more examples with features comparable to the title compound would have been encountered already. As soon as covalent bonding comes into play, the coordination capability of the sulfite S atom is well documented by transition-metal complexes like K 2 [Pd-(SO 3 ) 2 ]ÁH 2 O (Messer et al., 1979) (Johansson et al., 1980) or K 5 (HSO 3 )-(S 2 O 5 ) (Magnusson et al., 1983) that contain HSO 3 À anions with hydrogen covalently bonded to sulfur.
The numerical values of the atomic distances for crystalline Na 2 SO 3 (H 2 O) 7 (Tables 3 and 4) are in good agreement with those of aqueous Na 2 SO 3 solutions determined from LAXS studies, with S-O = 1.53 Å for the sulfite group and Na-O = 2.41 Å for the sodium-water distances (Eklund et al., 2012). In the latter study, the SÁ Á ÁO water distance in solution was determined as 3.68 Å , which is considerably longer than in the solid state, giving further evidence for a weak but existing O-HÁ Á ÁS hydrogen bond in the crystalline material.

Comparison with Na 2 CO 3 (H 2 O) 7
The close structural relationship between monoclinic Na 2 SO 3 (H 2 O) 7 and orthorhombic Na 2 CO 3 (H 2 O) 7 (Table 1) becomes evident from the similar arrangement of the principal building units in the crystal structures. The same type of cationic sodium-water layers made up from edge-and cornersharing [Na(H 2 O) 6 ] octahedra [mean Na-O distance = 2.43 (4) Å and O-Na-O angles = 81-102 and 164-180 ; Fig. 1b] is present in the carbonate, likewise situated at x ' 0, 1 2 in the unit cell (Fig. 2b). The carbonate groups do not show pyramidalization (Zemann, 1981) and occupy the same space as the sulfite groups between adjacent layers close to the [Na(H 2 O) 2/2 (H 2 O) 4/1 ] 2 dimers.
The main difference between the two structures is related to the orientation of the [Na(H 2 O) 2/2 (H 2 O) 4/1 ] 2 dimers in the layers. Whereas in the sulfite structure, the dimers at y ' 0 and 1 2 in one layer and also the accompanying anions close to them have the same orientation relative to (100), in the carbonate structure, the orientation of every second dimer (at y ' 1 2 ) and the accompanying anions in a layer is reversed due to the presence of the c-glide plane (Fig. 2).
The hydrogen-bonding schemes in the two heptahydrates are similar (Fig. 3). In the carbonate structure, the anions are likewise hydrogen bonded to water molecules through medium-strong hydrogen bonds [OÁ Á ÁO = 2.690 (5)-3.060 (4) Å , with an additional weak interaction of 3.223 (5) Å ]. In analogy, two water-water O-HÁ Á ÁO interactions with donor-acceptor distances of 2.827 (5) and 2.766 (5) Å are also observed. However, in contrast to the central sulfite S atom with its free electron lone pair, the central C atom of the carbonate anion cannot act as a hydrogen-bond acceptor, and thus this interaction is missing in the carbonate structure. Comparison of the hydrogen bonding to the anion in (a) Na 2 SO 3 (H 2 O) 7 , with displacement ellipsoids drawn at the 50% probability level, and (b) Na 2 CO 3 (H 2 O) 7 , with atoms as arbitrary spheres; hydrogen bonds are shown as thin solid lines. Atoms O1 and O3 in the sulfite structure accept three hydrogen bonds each, whereas O8 and O10 in the carbonate structure accept four each. Likewise, O2 in the sulfite structure accepts four hydrogen bonds, whereas the corresponding O9 atom in the carbonate accepts three. Note that the arrangement of the hydrogen-bonded water molecules around SO 3 2À is approximately mirror-symmetric (e.g. O5 i and O6 i ), whereas it is less symmetric for the carbonate. Symmetry codes for atoms in Na 2 SO 3 (H 2 O) 7 involved in hydrogen bonding with the SO 3 2anion are: (i) x, y, z; (ii) x + 1 2 , y À 1 2 , z; (iii) Àx + 3 2 , Ày, z + 1 2 ; (iv) Àx + 3 2 , Ày, z À 1 2 ; (v) Àx + 1, Ày, Àz + 2; (vi) Àx + 1, Ày, Àz + 3.

Comparison with related compounds
Crystal structures with sulfite groups anchored exclusively by hydrogen bonds are at present restricted to the title compound Na 2 SO 3 (H 2 O) 7 , to NH 4 SO 3 (H 2 O) (Battelle & Trueblood, 1965;Durand et al., 1977) and to MgSO 3 (H 2 O) 6 (Andersen & Lindqvist, 1984;Bats et al., 1986). In MgSO 3 -(H 2 O) 6 , which is built up from [Mg(H 2 O) 6 ] octahedra and isolated SO 3 pyramids within a lattice of the space group type R3, and with Mg and S atoms both located on threefold rotation axes, there are two independent water molecules that donate, apart from one water-water hydrogen bond, three water-O sulfite hydrogen bonds to each sulfite O atom, comparable to O1 and O3 in Na 2 SO 3 (H 2 O) 7 , but with shorter OÁ Á ÁO distances [2.687 (3), 2.701 (3) and 2.726 (3) Å ] than in the latter. An electron deformation density study of MgSO 3 (H 2 O) 6 (Bats et al., 1986) proved the presence of an extended lone-pair lobe at the apex of the SO 3 pyramid, but neither MgSO 3 (H 2 O) 6 nor NH 4 SO 3 (H 2 O) contain O-HÁ Á ÁS or N-HÁ Á ÁS hydrogen bonds.
A further comparison with other hydrated sodium compounds comprised of related oxo anions shows no close structural relationship to the title heptahydrate. For example, Na 2 SO 4 (H 2 O) 7 (Oswald et al., 2008) (I4 1 /amd, Z = 4) has a completely different arrangement of the principal building units. Its crystal structure is comprised of [Na(H 2 O)] 6 octahedra concatenated by edge-and corner-sharing into a threedimensional network with isolated tetrahedral sulfate anions hydrogen bonded to the chains. Also, for sodium compounds with analogous trigonal-pyramidal oxoanions and the same charge, i.e. XO 3

2À
, with X = Se and Te, no phases related structurally or compositionally to Na 2 SO 3 (H 2 O) 7 are known. For Na 2 SeO 3 , the anhydrous form (P2 1 /c, Z = 4) is made up from [NaO 6 ] octahedra and trigonal-pyramidal SeO 3 2À anions (Wickleder, 2002), and is isotypic with Na 2 TeO 3 (Masse et al., 1980). Hydrated forms are known only for the pentahydrates Na 2 SeO 3 (H 2 O) 5 (Mereiter, 2013) and Na 2 TeO 3 (H 2 O) 5 (Philippot et al., 1979) that are, surprisingly, not isotypic (Pbcm, with Z = 8, and C2/c, with Z = 8, respectively). These structures are based on two-or three-dimensional assemblies of [NaO 5 ] polyhedra (Se) and [NaO 6 ] octahedra (Se and Te), to which SeO 3 /TeO 3 groups are bonded via two (Se) or one (Te) O atom. The [NaO 6 ] octahedra in these two salts share common faces and edges but no vertices. As pointed out by Philippot et al. (1979) for Na 2 TeO 3 (H 2 O) 5 and confirmed also for Na 2 SeO 3 (H 2 O) 5 (Mereiter, 2013), the electron lone pair of Se and Te in these structures shows no attracting interactions with neighbouring H atoms. This might be one reason why hydrates of Na 2 SeO 3 and Na 2 TeO 3 do not crystallize in the Na 2 SO 3 (H 2 O) 7 structure and vice versa.  (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: ATOMS (Dowty, 2006) and Mercury (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010). 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.