Magnesium chloride tetra-hydrate, MgCl2·4H2O

Article abstract of DOI:10.1107/S0108270111054709

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 octa-hedra 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 tetra-hydrates of CrCl2, FeCl2, FeBr2 and CoBr2, but they are stacked in a different manner in MgCl₂·4H₂O compared with the transition metal structures.

Full text of DOI:10.1107/S0108270111054709

inorganic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
ISSN 0108-2701
Magnesium chloride tetrahydrate,
MgCl2ꢀ4H2O
Horst Schmidt,* Erik Hennings and Wolfgang Voigt
Institute of Inorganic Chemistry, TU Bergakademie Freiberg, Leipziger Strasse 29,
D-09596 Freiberg, Germany
Correspondence e-mail: horst.schmidt@chemie.tu-freiberg.de
Received 10 October 2011
Accepted 19 December 2011
Online 23 December 2011
Figure 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) ꢂx + 1, y, ꢂz + .]
The title compound, MgCl2ꢀ4H2O, 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—Hꢀ ꢀ ꢀ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 MgCl2ꢀ4H2O compared with the transition
metal structures.
1
2
hydrogen bonds between the water molecules of nearest-
neighbour planes. Instead, all the octahedra are inter-
connected by O—Hꢀ ꢀ ꢀ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(H2O)4 plane by ca 30ꢁ
(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). Each 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
Comment
Recently, Sugimoto et al. (2007) reported a crystal structure of
MgCl2ꢀ4H2O from in situ synchrotron powder diffraction
experiments during dehydration of MgCl2ꢀ6H2O. Under these
conditions, the tetrahydrate represents an intermediate phase
between MgCl2ꢀ6H2O 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 MgCl2ꢀ4H2O, obtained from
single-crystal data under equilibrium conditions.
MgCl2ꢀ4H2O crystallizes as a stable phase between 390 and
454 K, in accordance with the phase diagram of the MgCl2–
¨
H2O system (Fanghanel et al., 1987). The structure is built up
from octahedral MgCl2(H2O)4 units with the Cl atoms in trans
positions, as shown in Fig. 1. The complexes lie on crystal-
lographic twofold 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 90ꢁ between them. However, there are no
Figure 2
A packing diagram, viewed along the b axis, showing the zigzag
arrangement of Cl—Mg—Cl axes along the a axis.
i4 # 2012 International Union of Crystallography
doi:10.1107/S0108270111054709
Acta Cryst. (2012). C68, i4–i6

inorganic compounds
Figure 5
Figure 3
A polyhedral representation of MgCl2ꢀ4H2O (top) and FeBr2ꢀ4H2O
(bottom; data from Waizumi et al., 1992). The horizontal layers of
octahedra are essentially identical in the two structures. Stacking of the
layers in MgCl2ꢀ4H2O can be described as an ABAB sequence, compared
with an AAA sequence in FeBr2ꢀ4H2O.
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) ꢂx + 1, y, ꢂz + 12; (vi) x, ꢂy + 2, z + 21;
1
2
3
2
1
2
1
2
1
2
1
2
1
2
(vii) ꢂx + , ꢂy + , z + ; (viii) ꢂx + , y ꢂ , z; (ix) ꢂx + , y + , z.]
hydrogen bonds, as illustrated in Fig. 4. Three of these are
formed with water molecules from already connected octa-
hedra (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 MgCl2ꢀ4H2O has
also been reported for FeCl2ꢀ4H2O (Meunier-Piret & Van
Meerssche, 1971), CrCl2ꢀ4H2O (Von Schnering & Brand,
1973), and CoBr2ꢀ4H2O and FeBr2ꢀ4H2O (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, MgCl2ꢀ4H2O provides a
structure reference for a cation without any d-electron effects.
FeCl2ꢀ4H2O (space group P21/c), CrCl2ꢀ4H2O (P21/c), CoBr2ꢀ-
4H2O (P21/a) and FeBr2ꢀ4H2O (P21/a) are isomorphous with
each other, although reported with two different cell settings.
In contrast, MgCl2ꢀ4H2O crystallizes in the orthorhombic
space group Pbcn. Comparing, for example, the crystal
structures of MgCl2ꢀ4H2O and FeBr2ꢀ4H2O, as shown in Fig. 5,
identical layers of MX2(H2O)4 octahedra can be recognized.
Figure 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.
ꢃ
Acta Cryst. (2012). C68, i4–i6
Schmidt et al.
MgCl2ꢀ4H2O i5

inorganic compounds
Table 1
Hydrogen-bond geometry (A, ).
However, there is a distinction between the stacking
arrangements of the layers. In FeBr2ꢀ4H2O, they are stacked
by translation along the lattice vector c. This type of stacking is
referred to as AAA. In MgCl2ꢀ4H2O, the layer stacking is of
the type ABAB, with each layer offset laterally by 12 b
compared with the Fe compound. This can also be viewed as a
reflection of adjacent layers, with the mirror plane perpendi-
cular 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 octa-
hedra, and a more symmetric orientation is evident in the case
of MgCl2ꢀ4H2O. 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 halide hydrates. A similar situation was
reported recently by Schau-Magnussen et al. (2011) for
hexaaquamagnesium (space group Pbca) and hexaaqua-
nickel(II) furan-2,5-dicarboxylates (P21/c).
ꢁ
˚
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
O1—H3ꢀ ꢀ ꢀCl1ii
O2—H1ꢀ ꢀ ꢀCl1iii
O2—H2ꢀ ꢀ ꢀCl1iv
O3—H4ꢀ ꢀ ꢀCl1v
0.817 (13)
0.858 (13)
0.819 (14)
0.793 (13)
2.400 (13)
2.345 (13)
2.380 (14)
2.431 (14)
3.2097 (4)
3.1751 (4)
3.1789 (4)
3.2160 (4)
171.5 (12)
163.0 (10)
165.2 (12)
170.8 (12)
1
1
1
2
1
2
3
2
Symmetry codes: (ii) x þ ; y ꢂ ; ꢂz þ ; (iii) ꢂx þ 1; ꢂy þ 2; ꢂz; (iv) x þ ; ꢂy þ ; ꢂz;
2
2
1
1
1
2
(v) x þ ; y þ ; ꢂz þ .
2
2
All H atoms were located in Fourier maps and refined freely with
isotropic displacement parameters.
Data collection: X-AREA (Stoe & Cie, 2001); cell refinement:
X-AREA; 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).
The authors thank one of the referees for comments with
respect to the stacking patterns of the structures.
Experimental
Magnesium chloride hexahydrate (MgCl2ꢀ6H2O, 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 coated with n-paraffin oil.
The crystal selected for data collection was embedded in silicone oil
before being mounted on the diffractometer.
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: BI3028). Services for accessing these data are
described at the back of the journal.
References
Brandenburg, K. (2006). DIAMOND. Crystal Impact GbR, Bonn, Germany.
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¨
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Crystal data
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518.
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3
˚
MgCl2ꢀ4H2O
V = 675.22 (6) A
Z = 4
Mr = 167.27
Orthorhombic, Pbcn
Mo Kꢀ radiation
ꢁ = 0.98 mmꢂ1
T = 200 K
0.5 ꢄ 0.4 ꢄ 0.3 mm
˚
a = 7.2557 (3) A
˚
b = 8.4285 (4) A
˚
c = 11.0412 (6) A
Data collection
Stoe IPDS 2T diffractometer
Absorption correction: integration
(Coppens, 1970)
62835 measured reflections
1488 independent reflections
1212 reflections with I > 2ꢂ(I)
Rint = 0.073
Tmin = 0.791, Tmax = 0.907
Von Schnering, H. G. & Brand, B.-H. (1973). Z. Anorg. Allg. Chem. 402, 159–
168.
Waizumi, K., Masuda, H. & Ohtaki, H. (1992). Inorg. Chim. Acta, 192, 173–
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Bull. Chem. Soc. Jpn, 63, 3426–3433.
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925.
Refinement
R[F2 > 2ꢂ(F2)] = 0.016
wR(F2) = 0.051
S = 1.06
50 parameters
All H-atom parameters refined
ꢂ3
˚
Áꢃmax = 0.18 e A
ꢂ3
˚
1482 reflections
Áꢃmin = ꢂ0.24 e A
ꢃ
i6 Schmidt et al.
MgCl2ꢀ4H2O
Acta Cryst. (2012). C68, i4–i6