Redetermination of K2Mg3(OH)2(SO4)3(H2O)2 from single-crystal X-ray data revealing the correct hydrogen-atom positions

A single-crystal X-ray diffraction study of K2Mg3(OH)2(SO4)3(H2O)2 revealed the locations of all H atoms and hence a revised hydrogen-bonding scheme in comparison with a previous powder X-ray diffraction study.

In comparison with the previous structure determination of K 2 Mg 3 (OH) 2 -(SO 4 ) 3 (H 2 O) 2 , dipotassium trimagnesium dihydroxide tris(sulfate) dihydrate, from laboratory powder X-ray diffraction data [Kubel & Cabaret-Lampin (2013). Z. Anorg. Allg. Chem. 639, 1782-1786, the present redetermination against CCD single-crystal data has allowed for the modelling of all non-H atoms with anisotropic displacement parameters. As well as higher accuracy and precision in terms of bond lengths and angles, the clear localization of the Hatom positions leads also to a reasonable hydrogen-bonding scheme for this hydroxy hydrate. The structure consists of (100) sheets composed of corner-and edge-sharing [MgO 6 ] octahedra and sulfate tetrahedra. Adjacent sheets are linked by the potassium cations and a hydrogen bond of medium strength involving the water molecule. The title compound is isotypic with its Co II and Mn II analogues: the three K 2 M 3 (OH) 2 (SO 4 ) 3 (H 2 O) 2 (M = Mg, Co, Mn) structures are quantitatively compared.

Chemical context
In our recent projects focused on hydrothermal phaseformation studies in the systems M/X VI /Te IV /O/H (X = S, Se), it was tested whether tetrahedral sulfate or selenate anions can be incorporated into oxidotellurates(IV) of different divalent metals M. So far, this concept proved to be successful for M = Hg (Weil & Shirkhanlou, 2015), M = Ca, Cd, Sr (Weil & Shirkhanlou, 2017a), M = Pb (Weil & Shirkhanlou, 2017b) as well as for M = Zn, Mg (Weil & Shirkhanlou, 2017c). However, in nearly all cases multi-phase formation was observed under the given hydrothermal conditions, and the target compounds, i.e. metal oxidochalcogenates(IV,VI) with both oxidosulfate(VI) or oxidoselenate(VI) and oxidotellurate(IV) building units, appeared only as minority phases next to other different phases. The same holds for the Mg/S/Te/O/H system when working at pH $10 by using potassium hydroxide as a base. From one of the reaction batches, high-quality single crystals of the title compound, K 2 Mg 3 (OH) 2 (SO 4 ) 3 -(H 2 O) 2 , could be isolated as one of the products. A crystalstructure refinement of this phase has already been performed by Rietveld refinement against laboratory powder X-ray diffraction data (Kubel & Cabaret-Lampin, 2013). In the corresponding structure model, H-atom positions were estimated and optimized by energy minimization, but the resulting hydrogen-bonding pattern was not discussed in detail. A close check of this model revealed chemically implausible O-H bond lengths and O-HÁ Á ÁO angles (Table 1). For example, H1 is more tightly bonded to O7 than ISSN 2056-9890 to the actual hydroxide O atom (O8); the second hydroxyl group (O9) shows a too large O-H distance accompanied with large DÁ Á ÁA distances or a too small O9-H2Á Á ÁO8 angle; the water molecule (O10) shows likewise either unreasonable HÁ Á ÁA distances or D-HÁ Á ÁA angles. Hence a redetermination of the crystal structure of K 2 Mg 3 (OH) 2 (SO 4 ) 3 (H 2 O) 2 to establish a more reasonable hydrogen-bonding pattern by using single crystal X-ray diffraction CCD data seemed appropriate and is reported here.

Structural commentary
Of the 19 atoms in the asymmetric unit (1 K, 2 Mg, 2 S, 10 O, 4 H), eight (Mg1, S2, O5, O7, O8, O9, H1 and H2) are located on a crystallographic mirror plane at x = 0 (Wyckoff position 4 a); all other atoms in the asymmetric unit are on general sites (8 b). Both Mg II atoms are octahedrally coordinated by oxygen atoms. Mg1 is bonded to four O atoms belonging to sulfate groups (O5, O1 and its symmetry-related counterpart, O7) and to O atoms of two OH groups (O8, O9), whereas Mg2 is bonded to three sulfate O atoms (O4, O6, O2), two OH groups (O8, O9) and an O atom belonging to a water molecule (O10). Two [Mg2(H 2 O)(OH) 2 O 3 ] octahedra build up a {Mg2(H 2 O) 1/1 O 3/1 (OH) 2/2 } 2 dimer by edge-sharing the two OH groups. These dimers are linked to the [Mg1(OH) 2 O 4 ] octahedra by corner-sharing the two OH groups, which leads to the formation of zigzag chains running parallel to [001]. Sulfate tetrahedra join neighbouring chains into sheets extending parallel to (100). Adjacent sheets are linked into a threedimensional network by potassium cations (irregular ninecoordination), together with a hydrogen bond involving the water molecule (O10) and a sulfate O atom (O3) (Fig. 1).
The hydrogen-bonding pattern derived from the single crystal study is chemically plausible (Table 3, Fig. 1 (2) Å ]. The other hydroxy group involving O9 appears not to be involved in hydrogen bonding: the next nearest O atoms that could act as acceptor atoms are two symmetry-related O6 atoms (Àx, Ày + 1, z + 1 2 ; x, Ày + 1, z + 1 2 ), both at a distance of 3.405 (2) Å from O9. Such a long DÁ Á ÁA distance is usually not considered as relevant for hydrogen bonding but was discussed for the K 2 Co 3 (OH) 2 (SO 4 ) 3 (H 2 O) 2 structure as part of a bifurcated O-HÁ Á Á(O,O) hydrogen bond of very weak nature, here with DÁ Á ÁA = 3.370 (9) Å (Effenberger & Langhof, 1984). K 2 Mg 3 (OH) 2 (SO 4 ) 3 (H 2 O) 2 is isotypic with its Co (Effenberger & Langhof, 1984) and Mn (Yu et al., 2007) analogues. The three isotypic K 2 M 3 (OH) 2 (SO 4 ) 3 (H 2 O) 2 (M = Mg, Co, Mn) structures were quantitatively compared using the compstru program (de la Flor et al., 2016), available at the Bilbao Crystallographic Server (Aroyo et al., 2006). For this purpose, the hydrogen atoms were not taken into account. In relation to the title Mg structure, the Co and Mn structures show the following values for evaluation of the structural similarity. Co: the degree of lattice distortion is 0.0034, the maximum displacement between atomic positions of paired atoms is 0.0553 Å for the pair O9, the arithmetic mean of the distance between paired atoms is 0.0295 Å , and the measure of similarity is 0.010. Corresponding values for the Mn structure are: 0.0126, 0.1343 Å for pair O2, 0.0768 Å and 0.013, respectively. The two value sets indicate a higher similarity between the Mg and Co structures compared to the Mn structure. This is most probably related to the ionic radii (Shannon, 1976) of the six-coordinate metal cations that differ only marginally for Mg (0.72 Å ) and Co (0.745 Å , assuming a high-spin 3d 7 configuration), whereas Mn (0.83 Å for a highspin 3d 5 state) is considerately greater.

Synthesis and crystallization
A mixture of 380 mg of MgSO 4 Á7H 2 O, 100 mg of TeO 2 and 70 mg of KOH was placed in a 5 ml Teflon container that was subsequently filled with 2 ml of water and sealed with a Teflon lid. The closed container was placed in a steel autoclave and was heated at 413 K for one week at autogenous pressure and then cooled down to room temperature within 5 h. The recovered solids consisted of Mg 2 Te 3 O 8 (Lin et al., 2013) as the main product (checked by powder X-ray diffraction of the bulk), besides minor amounts of caminite, Mg 2 (SO 4 )(OH) 2 (Keefer et al., 1981), the sulfate tellurite Mg 3 (SO 4 )(TeO 3 )-(OH) 2 (H 2 O) 2 (Weil & Shirkhanlou, 2017c) and the title compound (the latter phases determined by single-crystal X-ray diffraction).

Dipotassium trimagnesium dihydroxide tris(sulfate) dihydrate
Crystal data 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 )