Erik Hennings

Crystal structures of hydrates of simple inorganic salts. I. Water‐rich magnesium halide hydrates MgCl2·8H2O, MgCl2·12H2O, MgBr2·6H2O, MgBr2·9H2O, MgI2·8H2O and MgI2·9H2O

The previously reported structures of the hydrates of simple inorganic salts that crystallize at room temperature are generally well determined. This is not true for water-rich hydrates, which crystallize at temperatures below 273 K. In this series, investigations of the crystal structures of water-rich hydrates crystallized from aqueous solutions at low temperatures are presented. Reported herein are the structures of a set of magnesium salts. Crystals of MgCl2·8H2O (magnesium dichloride octahydrate), MgCl2·12H2O (magnesium dichloride dodecahydrate), MgBr2·6H2O (magnesium dibromide hexahydrate), MgBr2·9H2O (magnesium dibromide nonahydrate), MgI2·8H2O (magnesium diiodide octahydrate) and MgI2·9H2O (magnesium diiodide nonahydrate) were grown from their aqueous solutions at temperatures below 298 K according to the solid-liquid phase diagrams. All structures are built up from Mg(H2O)6 octahedra. Dimensions and angles in the hexaaqua cation complexes are very similar and variation is not systematic. The anions are incorporated into a specific network of O-H...X hydrogen bonds.

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.

Magnesium chloride tetrahydrate, MgCl2·4H2O

The title compound, MgCl(2)·4H(2)O, was crystallized at 403 K and its structure determined at 200 K. The structure is built up from MgCl(2)(H(2)O)(4) octahedra with a trans configuration. Each complex is situated on a crystallographic twofold axis, with the rotation axis aligned along one H(2)O-Mg-OH(2) axis. They are connected by a complex network of O-H···Cl hydrogen bonds. The structure contains two-dimensional sections that are essentially identical to those in the reported tetrahydrates of CrCl(2), FeCl(2), FeBr(2) and CoBr(2), but they are stacked in a different manner in MgCl(2)·4H(2)O compared with the transition metal structures.

Crystal structures of Ca(ClO4)2·4H2O and Ca(ClO4)2·6H2O

The crystal structures of the tetra- and hexahydrate phases of Ca(ClO4)2 consist of Ca2+ ions in distorted square-antiprismatic environments and of perchlorate tetrahedra. O—H⋯O hydrogen bonds between water molecules and ClO4 units lead to the formation of a three-dimensional network in the structures.

Fe2(SO4)3·H2SO4·28H2O, a low-temperature water-rich iron(III) sulfate

The title compound, diiron(III) trisulfate-sulfuric acid-water (1/1/28), has been prepared at temperatures between 235 and 239 K from acid solutions of Fe2(SO4)3. Studies of the compound at 100 and 200 K are reported. The analysis reveals the structural features of an alum, (H5O2)Fe(SO4)2·12H2O. The Fe(H2O)6 unit is located on a centre of inversion at (½, 0, ½), while the H5O2(+) cation is located about an inversion centre at (½, ½, ½). The compound thus represents the first oxonium alum, although the unit cell is orthorhombic.

Crystal structure of tin(II) perchlorate trihydrate

The title compound was synthesized by the redox reaction of copper(II) perchlorate hexahydrate and metallic tin in perchloric acid. Both the pyramidal [Sn(H2O)3]2+ cations and tetrahedral perchlorate anions lie on crystallographic threefold axes.

Crystal structure of tin(IV) chloride octa-hydrate.

The title compound was crystallized according to the solid–liquid phase diagram at lower temperatures. It is built-up of SnCl4(H2O)2 octahedral units and lattice water molecules. An intricate three-dimensional network of O—H⋯O and O—H⋯Cl hydrogen bonds between the complex molecules and the lattice water molecules is formed in the crystal structure.

Crystal structures of ZnCl2·2.5H2O, ZnCl2·3H2O and ZnCl2·4.5H2O

The crystal structures of ZnCl2·xH2O (x = 2.5, 3 and 4.5) consist of Zn2+ ions both in an octahedral and tetrahedral environment. O—H⋯O hydrogen bonds between water molecules and tetrahedral ZnCl4 units lead to the formation of a three-dimensional network in each of the structures.

Crystal structure of iron(III) perchlorate nona­hydrate

Since the discovery of perchlorate salts on Mars and the known occurrence of ferric salts in the regolith, there is a distinct possibility that the title compound could form on the surface of Mars. [Fe(H2O)6](ClO4)3·3H2O was crystallized from aqueous solutions at low temperatures according to the solid-liquid phase diagram. It consists of Fe(H2O)6 octa-hedra (point group symmetry -3.) and perchlorate anions (point group symmetry .2) as well as non-coordinating water mol-ecules, as part of a second hydrogen-bonded coordination sphere around the cation. The perchlorate appears to be slightly disordered, with major-minor component occupancies of 0.773 (9):0.227 (9).

Crystal structures of hydrates of simple inorganic salts. III. Water‐rich aluminium halide hydrates: AlCl3·15H2O, AlBr3·15H2O, AlI3·15H2O, AlI3·17H2O and AlBr3·9H2O

Water-rich aluminium halide hydrate structures are not known in the literature. The highest known water content per Al atom is nine for the perchlorate and fluoride. The nonahydrate of aluminium bromide, stable pentadecahydrates of aluminium chloride, bromide and iodide, and a metastable heptadecahydrate of the iodide have now been crystallized from low-temperature solutions. The structures of these hydrates were determined and are discussed in terms of the development of cation hydration spheres. The pentadecahydrate of the chloride and bromide are isostructural. In AlI(3) · 15H2O, half of the Al(3+) cations are surrounded by two complete hydration spheres, with six H2O in the primary and 12 in the secondary. For the heptadecahydrate of aluminium iodide, this hydration was found for every Al(3+).