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.


Chemical context
Since the discovery of perchlorate salts on the surface of Mars during the Phoenix expedition (Hecht et al., 2009;Davila et al., 2013;Kerr, 2013;Marion et al., 2010;Navarro-Gonzá lez et al., 2010), interest in the solubility and crystal structures of the perchlorate hydrate phases became more important (Chevrier, Hanley & Altheide, 2009;. Based on the red color of the planet, one can expect different iron phases, such as perchlorate and sulfate, to be important constituents of the regolith (Chevrier, Ulrich & Altheide, 2009;Chevrier & Altheide, 2008;Hennings et al., 2013). While investigating the solubility of ferric perchlorate, we obtained the nonahydrate as a stable phase in the binary salt-water system.

Structural commentary
The central Fe atom is situated on a threefold inversion axis and is octahedrally coordinated by six water molecules in the first, and by six water molecules as well as six perchlorate tetrahedra in the second coordination spheres (Fig. 1). The water molecules of the second coordination sphere (O4 and symmetry equivalents) are connected to perchlorate tetrahedra ( Fig. 2a) via hydrogen bonds (Table 1). Six O4-water molecules form a second, larger octahedron outside the octahedron of the first coordination shell (Fig. 2b). The perchlorate anion, situated on a twofold rotation axis, appears to be slightly disordered, with major:minor component occupancies of 0.773 (9):0.227 (9).

Supramolecular features
From the unit cell of ferric perchlorate nonahydrate (Fig. 3a), it is obvious that the O4 atoms form a secondary hydration shell around the Fe(H 2 O) 6 units. This becomes clearer when drawing the second octahedra as water coordination poly-

Database survey
For crystal structure determination of other perchlorate nonahydrates, see: Davidian et al. (2012) for the Al, Ga and Sc salts and Hennings et al. (2014) for the strontium salt. For crystal structure determinations of other Fe III salts with a high water content, see: Schmidt et al. (2013); Lindstrand (1936).  Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular units (a) and second coordination sphere (b) of ferric perchlorate nonahydrate. Dashed lines indicate hydrogen bonds. Displacement ellipsoids are drawn at the 50% probability limit. The minor disorder component of the ClO 4 tetrahedron has been omitted.

Figure 2
The connection scheme of water molecules of the second coordination sphere by hydrogen bonds (a) and the formation of a secondary hydration shell (yellow) around the cations (b). The minor disorder component of the ClO 4 tetrahedron has been omitted for clarity. Dashed lines indicate hydrogen bonds. [Symmetry code: (i) 2 3 À x, 1 3 À x + y, 5 6 À z.] 263 K after 2 d. To prepare this solution, ferric perchlorate nonahydrate (Fluka, pure) was used. The content of Fe III ions was analysed using gravimetric analysis by precipitation with ammonia. All crystals are stable in their saturated solution over a period of at least four weeks. The samples were stored in a freezer or a cryostat at low temperatures. The crystals were separated and embedded in perfluorinated ether for X-ray diffraction analysis

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms were placed in the positions indicated by difference Fourier maps. No further constraints were applied. Computer programs: X-AREA and X-RED (Stoe & Cie, 2009), SHELXS97 and SHELXL2012 (Sheldrick, 2008), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010).

Figure 3
The unit cell of iron(III) perchlorate nonahydrate with coordination polyhedra of the first (a) and second (b) (Sheldrick, 2008); program(s) used to refine structure: SHELXL2012 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). diffractometer Radiation source: fine-focus sealed tube Detector resolution: 6.67 pixels mm -1 rotation method scans Absorption correction: integration (Coppens, 1970)  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.