Crystal structures of Sr(ClO4)2·3H2O, Sr(ClO4)2·4H2O and Sr(ClO4)2·9H2O

The crystal structures of the tri-, tetra- and nonahydrate phases of Sr(ClO4)2 consist of Sr2+ ions coordinated by nine oxygen atoms from water molecules and perchlorate tetrahedra. O—H⋯O hydrogen bonds between water molecules and ClO4 units lead to the formation of a three-dimensional network in each of the structures.

The title compounds, strontium perchlorate trihydrate {di--aqua-aquadi-perchlorato-strontium, [Sr(ClO 4 ) 2 (H 2 O) 3 ] n }, strontium perchlorate tetrahydrate {di--aqua-bis(triaquadiperchloratostrontium), [Sr 2 (ClO 4 ) 4 (H 2 O) 8 ]} and strontium perchlorate nonahydrate {heptaaquadiperchloratostrontium dihydrate, [Sr(ClO 4 ) 2 (H 2 O) 7 ]Á2H 2 O}, were crystallized at low temperatures according to the solid-liquid phase diagram. The structures of the tri-and tetrahydrate consist of Sr 2+ cations coordinated by five water molecules and four O atoms of four perchlorate tetrahedra in a distorted tricapped trigonal-prismatic coordination mode. The asymmetric unit of the trihydrate contains two formula units. Two [SrO 9 ] polyhedra in the trihydrate are connected by sharing water molecules and thus forming chains parallel to [100]. In the tetrahydrate, dimers of two [SrO 9 ] polyhedra connected by two sharing water molecules are formed. The structure of the nonahydrate contains one Sr 2+ cation coordinated by seven water molecules and by two O atoms of two perchlorate tetrahedra (point group symmetry ..m), forming a tricapped trigonal prism (point group symmetry m2m). The structure contains additional non-coordinating water molecules, which are located on twofold rotation axes. O-HÁ Á ÁO hydrogen bonds between the water molecules as donor and ClO 4 tetrahedra and water molecules as acceptor groups lead to the formation of a three-dimensional network in each of the three structures.

Chemical context
The amount of research into perchlorates has increased considerably in the last few years, beginning with the Phoenix Mars mission (Kim et al., 2013;Kerr, 2013;Chevrier et al., 2009;Quinn et al., 2013;Davila et al., 2013;Gough et al., 2011;Navarro-Gonzá lez & McKay, 2011;Robertson & Bish, 2011;Schuttlefield et al., 2011;Navarro-Gonzá lez et al., 2010;Marion et al., 2010;Hecht et al., 2009). Important perchlorate salts in the martian regolith are Mg and Ca perchlorates. It seemed worthwhile to complete the chemical systematics in this series of alkaline-earth perchlorates. The solubility diagram of strontium perchlorate has been investigated by several authors (Pestova et al., 2005;Lilich & Djurinskii, 1956;Nicholson & Felsing, 1950;Willard & Smith, 1923) in different temperature and concentration regions. They reported the tetrahydrate and the hexahydrate to be stable phases. While re-investigating the phase diagram, we found at higher temperatures the trihydrate, the tetrahydrate at room temperature and the nonahydrate near the eutectic temperature. The existence of the hexahydrate could not be confirmed.

Structural commentary
The crystal structure of strontium perchlorate trihydrate contains two crystallographically distinct Sr 2+ cations. Both are coordinated by five water molecules and four monodentately bonding perchlorate tetrahedra (Fig. 1). Four of the five water molecules (O1, O6 and O3, O4) share edges between two Sr 2+ cations, resulting in chains with alternating Sr1 and Sr2 cations. The chains extend parallel to [100] (Fig. 2). The crystal structure of strontium perchlorate tetrahydrate is similar to the trihydrate, but different to the magnesium analogue (Robertson & Bish, 2010;Solovyov, 2012) or mercury perchlorate tetrahydrate (Johansson et al., 1966). Two symmetry-related Sr 2+ cations, both coordinated by five water molecules and four monodentate perchlorate tetrahedra, form dimers by sharing two water molecules. In strontium perchlorate nonahydrate, the Sr 2+ cation occupies a single crystallographic site with site symmetry m2m. It is coordinated by seven water molecules and two monodentate perchlorate tetrahedra (point group symmetry ..m; Fig. 3a) within a tricapped trigonal-prismatic oxygen coordination environment (Fig. 3b) Coordination around the Sr1 2+ cation in Sr(ClO 4 ) 2 Á3H 2 O. Atoms O3 and O4 as well as O6 and O1 are shared between two different Sr 2+ cations. Displacement ellipsoids are drawn at the 50% probability level.
such that each oxygen atom of the perchlorate anions represents a capping atom. The third cap is provided by a water oxygen atom.

Supramolecular features
In strontium perchlorate trihydrate, chains are formed with alternating Sr 2+ cations (Fig. 2). These zigzag chains are oriented parallel to [100] and are linked by edge-sharing with the perchlorate tetrahedra ( Fig. 4) into a layered arrangement parallel to (001), as shown in Fig. 5. Within the structure of the tetrahydrate, each perchlorate anion coordinates to the dimeric unit of two Sr 2+ cations ( Fig. 6). At the same time, it also coordinates to another dimeric unit. Thus, each dimeric unit is connected pairwise by perchlorate anions with four others. This yields in (001)  Zigzag chains parallel to [100] in the structure of Sr(ClO 4 ) 2 Á3H 2 O, linked by perchlorate tetrahedra into (100) layers, as viewed along [001].

Synthesis and crystallization
Crystals of Sr(ClO 4 ) 2 Á3H 2 O were used as purchased (ABCR, 98%). The isolated crystals were stored in a freezer separated and embedded in perfluorinated ether to avoid contact with humidity. Sr(ClO 4 ) 2 Á4H 2 O crystallized from an aqueous solution of 75.08 wt% Sr(ClO 4 ) 2 at 273 K after two days and Sr(ClO 4 ) 2 Á9H 2 O from an aqueous solution of 60.12 wt% Sr(ClO 4 ) 2 at 238 K after one week. For preparing these aqueous solutions, strontium perchlorate trihydrate was used. The Sr 2+ content was analyzed per complexometric titration with EDTA. The crystals are stable in the saturated aqueous solutions over a range of at least four weeks. The samples were stored in a freezer or a cryostat at low temperatures and were separated and embedded in perfluorinated ether for X-ray analysis.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Sr1 0.17700 (4) 0.73963 (3) (2) 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.    (7)