Magnesium chloride tetra­hydrate, MgCl₂·4H₂O

The title compound, MgCl₂·4H₂O, was crystallized at 403 K and its structure determined at 200 K. The structure is built up from MgCl₂(H₂O)₄ octa­hedra with a trans configuration. Each complex is situated on a crystallographic twofold axis, with the rotation axis aligned along one H₂O—Mg—OH₂ 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 CrCl₂, FeCl₂, FeBr₂ and CoBr₂, but they are stacked in a different manner in MgCl₂·4H₂O compared with the transition metal structures.

S1. Comment

Recently, Sugimoto et al. (2007) reported a crystal structure of MgCl₂.4H₂O from in situ synchrotron powder diffraction experiments during dehydration of MgCl₂.6H₂O. Under these conditions the tetrahydrate represents an intermediate phase between MgCl₂.6H₂O and lower hydrates in the dehydration process, and the degree of equilibration is dependent on the heating rate and the pressure of the water vapour. A highly disordered structure was found, where only every second MgCl₂(H₂O)₄ octahedron is occupied. We report here an ordered crystal structure for MgCl₂.4H₂O, obtained from single-crystal data under equilibrium conditions.

MgCl₂.4H₂O crystallizes as a stable phase between 390 and 454 K, in accordance with the phase diagram of the MgCl₂–H₂O system (Fanghänel et al., 1987). The structure is built up from octahedral MgCl₂(H₂O)₄ units with Cl atoms in trans positions, as shown in Fig. 1. The complexes lie on crystallographic two-fold axes, aligned along the O1—Mg1—O3 axis. Although there is only one unique crystallographic position for the Mg and Cl atoms, two different orientations of the MgCl₂(H₂O)₄ octahedra exist. Viewing the structure parallel to the b axis (Fig. 2), adjacent rows of octahedra possess alternating orientations of their Cl—Mg—Cl axes, giving rise to a zigzag pattern along the a axis. Consequently, the Mg(H₂O)₄ planes also have alternating orientations with an angle of nearly 90° between them. However, there are no hydrogen bonds between the water molecules of nearest neighbour planes. Instead, all the octahedra are interconnected by O—H···Cl hydrogen bonds (Table 1 and Fig. 3). The Mg(H₂O)₄ coordination planes contain two types of water molecules: one type (O1 and O3) in a `dipole orientation' and the other (O2) tilted from the Mg(H<sub>2</sub>O)<sub>4</sub> plane by ca 30° (Fig. 1). `Dipole orientation' means that the water molecules and Mg atoms are located in one plane, and the Mg—O axis bisects the H—O—H angle. The tilted water molecules form shorter hydrogen bonds towards adjacent Cl atoms (Table 1 and Fig. 3). Every H atom of an Mg(H₂O)₄ plane forms one hydrogen bond to an adjacent Cl atom of a different MgCl₂(H₂O)₄ octahedron, resulting in connections to eight octahedra (Fig. 3). Each Cl atom acts as an acceptor for four hydrogen bonds, as illustrated in Fig. 4. Three of these are formed with water molecules from octahedra already connected (as shown in Fig. 3), while the fourth hydrogen bond extends to an additional MgCl₂(H₂O)₄ octahedron. Thus, each octahedron is connected to ten others, yielding a complicated hydrogen-bond network.

The trans configuration observed in MgCl₂.4H₂O has also been reported for FeCl₂.4H₂O (Meunier-Piret & Van Meerssche, 1971), CrCl.4H₂O (Von Schnering & Brand, 1973), CoBr₂.4H₂O and FeBr₂.4H₂O (Waizumi et al., 1992), whereas the tetrahydrates of MnCl₂ (Hwang & Ha, 2009), MnBr₂ (Sudarsanan, 1975), CoCl₂ (Waizumi et al., 1990) and NiCl₂ (Ptasiewicz-Bak et al., 1999; Waizumi et al., 1992) crystallize with the Cl atoms in a cis configuration. For the structures with a trans configuration, MgCl₂.4H₂O provides a structure reference for a cation without any d-electron effects. FeCl₂.4H₂O (space group P2₁/c), CrCl₂.4H₂O (P2₁/c), CoBr₂.4H₂O (P2₁/a) and FeBr₂.4H₂O (P2₁/a) are isomorphous to each other, although reported with two different cell settings. By contrast, MgCl₂.4H₂O crystallizes in the orthorhombic space group Pbcn. Comparing, for example, the crystal structures of MgCl₂.4H₂O and FeBr₂.4H₂O, as shown in Fig. 5, identical layers of MX₂(H₂O)₄ octahedra can be recognized. However, there is a distinction between the stacking arrangements of the layers. In FeBr₂.4H₂O, they are stacked by translation along the lattice vector c. This type of stacking is referred to as AAA. In MgCl₂.4H₂O, the layer stacking is of the type ABAB, with each layer offset laterally by 1/2b compared with the Fe compound. This can also be viewed as a reflection of adjacent layers, with the mirror plane perpendicular to the a axis, at x = 0.25 and 0.75. This difference in stacking is expressed by the different space groups. Possibly, these different stackings are caused by small differences in the orientation of the water molecules in the MX₂(H₂O)₄ octahedra, and a more symmetric orientation is evident in the case of MgCl₂.4H₂O. The interplay between the energetic states of the d electrons in the ligand fields and the requirements for hydrogen bonding reduces the symmetry in the case of the transition-metal hydrates. A similar situation was reported recently by Schau-Magnussen et al. (2011) for hexaaquamagnesium (space group Pbca) and hexaaquanickel(II) furan-2,5-dicarboxylates (space group P2₁/c).

Related literature

For related literature, see: Fanghänel et al. (1987); Hwang & Ha (2009); Meunier-Piret & Van Meerssche (1971); Ptasiewicz-Bak, Olovsson & McIntyre (1999); Schau-Magnussen, Gorbanev, Kegnαeas & Riisager (2011); Sudarsanan (1975); Sugimoto et al. (2007); Von Schnering & Brand (1973); Waizumi et al. (1990, 1992).

S2. Experimental

Magnesium chloride hexahydrate (MgCl₂.6H₂O, Fluka, >98%) was heated in a flask at 403 K. The flask was closed with a cork stopper, which was pierced with a glass capillary for water discharge. The sample was held for 48 h at that temperature, and single crystals were formed on the surface of the dehydrated melt. The crystals were separated from a solidified hydrate melt. To prevent contact of the crystals with humidity in the air, they were covered by n-paraffin oil. The crystal selected for data collection was embedded in silicone oil before being mounted on the diffractometer.

S3. Refinement

All H atoms were located in Fourier maps and refined freely with isotropic displacement parameters.

Computing details

Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA (Stoe & Cie, 2001); data reduction: X-RED (Stoe & Cie, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. The MgCl2(H2O)4 octahedron, showing displacement ellipsoids at the 50% probability level. The labels T and D refer to tilted and dipole orientations of the water molecules, respectively, as discussed in the Comment. [Symmetry code: (i) -x + 1, y, -z + 1/2.]
	Fig. 2. A packing diagram, viewed along the b axis, showing the zigzag arrangement of Cl—Mg—Cl axes along the a axis.
	Fig. 3. The hydrogen-bonding pattern (dashed lines) for the tilted water molecules (top) and the dipole orientation (bottom). Symmetry codes are as in Table 1; additionally: (i) -x + 1, y, -z + 1/2; (vi) -x + 1/2, -y + 3/2, z + 1/2; (vii) x, -y + 2, z + 1/2; (viii) -x + 1/2, y - 1/2, z; (ix) -x + 1/2, y + 1/2, z.]
	Fig. 4. The hydrogen bonds (dashed lines) formed to the Cl atoms at one MgCl2(H2O)4 octahedron. For clarity, two octahedra corresponding to the H atoms indicated by open circles are not drawn. The central octahedron is connected in total to ten neighbouring octahedra.
	Fig. 5. A polyhedral representation of MgCl2.4H2O (top) and FeBr2.4H2O (bottom; data from Waizumi et al., 1992). The horizontal layers of octahedra are essentially identical in the two structures. Stacking of the layers in MgCl2.4H2O can be described as an ABAB sequence, compared with AAA in FeBr2.4H2O.

Magnesium chloride tetrahydrate

Crystal data

MgCl2·4H2O	F(000) = 344
Mr = 167.27	Dx = 1.645 Mg m^−3
Orthorhombic, Pbcn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2n 2ab	Cell parameters from 2665 reflections
a = 7.2557 (3) Å	θ = 3.7–35.2°
b = 8.4285 (4) Å	µ = 0.98 mm^−1
c = 11.0412 (6) Å	T = 200 K
V = 675.22 (6) Å^3	Block, colourless
Z = 4	0.5 × 0.4 × 0.3 mm

Data collection

Stoe IPDS 2T diffractometer	1488 independent reflections
Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus	1212 reflections with I > 2σ(I)
Plane graphite monochromator	Rint = 0.073
Detector resolution: 6.67 pixels mm^-1	θmax = 35.0°, θmin = 2.4°
rotation method scans	h = −11→11
Absorption correction: integration Coppens (1970)	k = −13→13
Tmin = 0.791, Tmax = 0.907	l = −17→17
62835 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.016	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.051	All H-atom parameters refined
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0351P)^2 + 0.0091P] where P = (Fo^2 + 2Fc^2)/3
1482 reflections	(Δ/σ)max = 0.001
50 parameters	Δρmax = 0.18 e Å^−3
0 restraints	Δρmin = −0.24 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cl1	0.23409 (2)	0.877141 (12)	0.098782 (11)	0.01867 (5)
Mg1	0.5000	0.87655 (3)	0.2500	0.01483 (7)
O1	0.5000	0.63243 (6)	0.2500	0.02356 (13)
O2	0.69074 (8)	0.87599 (5)	0.11275 (4)	0.02306 (9)
O3	0.5000	1.12075 (6)	0.2500	0.02312 (13)
H1	0.7108 (19)	0.9575 (15)	0.0681 (11)	0.041 (3)*
H2	0.700 (2)	0.7982 (17)	0.0687 (11)	0.047 (3)*
H3	0.5621 (18)	0.5754 (16)	0.2937 (10)	0.040 (3)*
H4	0.5625 (18)	1.1751 (17)	0.2918 (11)	0.046 (3)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.02546 (7)	0.01555 (7)	0.01500 (7)	−0.00144 (4)	−0.00136 (4)	0.00006 (5)
Mg1	0.02027 (12)	0.01077 (11)	0.01345 (11)	0.000	0.00217 (8)	0.000
O1	0.0300 (3)	0.0118 (2)	0.0289 (3)	0.000	−0.0096 (2)	0.000
O2	0.0326 (2)	0.01935 (19)	0.01726 (17)	−0.00095 (16)	0.00906 (16)	−0.00049 (13)
O3	0.0292 (3)	0.0115 (2)	0.0287 (3)	0.000	−0.0107 (2)	0.000

Geometric parameters (Å, º)

Cl1—Mg1	2.5515 (2)	Mg1—Cl1^i	2.5515 (2)
Mg1—O1	2.0576 (6)	O1—H3	0.817 (13)
Mg1—O2	2.0523 (5)	O2—H1	0.858 (13)
Mg1—O2^i	2.0523 (5)	O2—H2	0.819 (14)
Mg1—O3	2.0583 (6)	O3—H4	0.793 (13)
O2—Mg1—O2^i	179.74 (2)	O2—Mg1—Cl1	91.530 (17)
O2—Mg1—O1	89.869 (12)	O2^i—Mg1—Cl1	88.470 (17)
O2^i—Mg1—O1	89.869 (12)	O1—Mg1—Cl1	90.112 (6)
O2—Mg1—O3	90.131 (12)	O3—Mg1—Cl1	89.888 (6)
O2^i—Mg1—O3	90.131 (12)	Cl1^i—Mg1—Cl1	179.776 (11)
O1—Mg1—O3	180.0	Mg1—O2—H1	122.5 (8)
O2—Mg1—Cl1^i	88.470 (17)	Mg1—O2—H2	119.7 (10)
O2^i—Mg1—Cl1^i	91.530 (17)	H1—O2—H2	106.6 (13)
O1—Mg1—Cl1^i	90.112 (6)	Mg1—O3—H4	125.3 (10)
O3—Mg1—Cl1^i	89.888 (6)	Mg1—O1—H3	126.1 (9)

Symmetry code: (i) −x+1, y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
X—X···X^i	0	0	0	0
O1—H3···Cl1^ii	0.817 (13)	2.400 (13)	3.2097 (4)	171.5 (12)
O2—H1···Cl1^iii	0.858 (13)	2.345 (13)	3.1751 (4)	163.0 (10)
O2—H2···Cl1^iv	0.819 (14)	2.380 (14)	3.1789 (4)	165.2 (12)
O3—H4···Cl1^v	0.793 (13)	2.431 (14)	3.2160 (4)	170.8 (12)

Symmetry codes: (i) −x+1, y, −z+1/2; (ii) x+1/2, y−1/2, −z+1/2; (iii) −x+1, −y+2, −z; (iv) x+1/2, −y+3/2, −z; (v) x+1/2, y+1/2, −z+1/2.