A twinned triclinic polymorph of dibromidotetra-kis(tetra-hydro-furan- κO)magnesium(II)

Article abstract of DOI:10.1107/S0108270107010694

The title compound, [MgBr2(C4H8O)4], forms crystals which appear to be monoclinic but are actually twinned triclinic. The current form is a new triclinic polymorph, with Z′= 2, in addition to the already known tetra-gonal polymorph. Although the geometric

Full text of DOI:10.1107/S0108270107010694

metal-organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
but there are two structure determinations at room tempera-
ture (Metzler et al., 1994; Heeg et al., 1998). Since the quality
of the structure of Heeg et al. [hereinafter (Ia)] is the higher, it
will be used in the following comparison with (I).
ISSN 0108-2701
The most important bond lengths (Table 1) of (Ia) (MgÐBr =
Ê
Ê
2.652 A and MgÐO = 2.111 A) agree well with those of (I)
(Table 2). The packing pattern of the two polymorphs,
however, is completely different. Whereas (Ia) shows planes
A twinned triclinic polymorph
of dibromidotetrakis(tetrahydro-
furan-jO)magnesium(II)
of molecules at z = 0 and z = being rotated by 90ꢀ and
translated by (12, , 0) (Fig. 3), the molecules of (I) are almost
1
2
1
2
located in the same plane (Table 2), but the BrÐBr vectors of
two adjacent molecules are rotated by just 22.0ꢀ (Fig. 2).
It is interesting to note that for dichloridotetrakis(tetra-
hydrofuran)magnesium, two different polymorphs have also
been found. One form (Handlir et al., 1985) is isostructural
with (Ia). The second polymorph, on the other hand, is
Andreas Lorbach, Hans-Wolfram Lerner and Michael
Bolte*
Institut fur Anorganische Chemie, J. W. Goethe-Universitat Frankfurt,
È
È
Max-von-Laue-Strasse 7, 60438 Frankfurt/Main, Germany
Correspondence e-mail: bolte@chemie.uni-frankfurt.de
Received 28 February 2007
Accepted 6 March 2007
Online 31 March 2007
The title compound, [MgBr2(C4H8O)4], forms crystals which
appear to be monoclinic but are actually twinned triclinic. The
current form is a new triclinic polymorph, with Z0= 2, in
addition to the already known tetragonal polymorph.
Although the geometric parameters of the two polymorphs
agree well, their packing patterns are completely different.
Comment
Salt metathesis reactions of Grignard compounds and
organohalides are common preparation routes. However, the
disadvantage of this method is the separation of the organic
compounds from MgX2, since most of these magnesium salts
are soluble in organic solvents such as tetrahydrofuran. We
report here the X-ray crystal structure analysis of the title
adduct, (I), of MgBr2 with four tetrahydrofuran molecules,
which was obtained by the reaction of 1,2-dibromobenzene
with Mg in the presence of Me3SnCl, as shown in the scheme.
Figure 1
A perspective view of the title compound, showing the atom-numbering
scheme. Displacement ellipsoids are drawn at the 50% probability level
and H atoms are shown as small spheres of arbitrary radii.
A perspective view of compound (I) is shown in Fig. 1.
Geometric parameters are in the normal ranges [Cambridge
Structural Database, Version 5.28, November 2006, updated
January 2007 (Allen, 2002); MOGUL, Version 1.1 (Bruno et
al., 2004)].
Compound (I) is a new polymorph of dibromidotetra-
kis(tetrahydrofuran)magnesium, for which a tetragonal poly-
morph (space group P42212) has already been described.
Schroder & Spandau (1966) did not publish any coordinates,
Figure 2
A packing diagram for (I), viewed in the bc plane. C and H atoms have
been omitted for clarity. Key: Mg atoms are shaded, Br atoms are dotted
and O atoms are cross-hatched.
m174 # 2007 International Union of Crystallography
DOI: 10.1107/S0108270107010694
Acta Cryst. (2007). C63, m174±m176

metal-organic compounds
Table 1
Selected bond lengths (A).
monoclinic (space group P21/c). Its structure has been deter-
mined at room temperature (Huang et al., 1987), at 173 K
(Bolte et al., 2002) and at 150 K (Parsons et al., 2004).
Ê
Mg1ÐO31
Mg1ÐO41
Mg1ÐO21
Mg1ÐO11
Mg1ÐBr1
Mg1ÐBr2
2.099 (8)
Mg2ÐO51
Mg2ÐO71
Mg2ÐO81
Mg2ÐO61
Mg2ÐBr4
Mg2ÐBr3
2.087 (9)
2.096 (8)
2.128 (8)
2.138 (8)
2.562 (4)
2.605 (4)
2.105 (9)
2.105 (8)
2.106 (8)
2.569 (3)
2.592 (3)
Table 2
Coordinates of the Mg atoms in the unit cell of (I).
Atom
x
y
z
Mg1
Mg2
Mg1i
Mg2i
0.5332 (4)
0.5228 (4)
0.4668 (4)
0.4772 (4)
0.2517 (2)
0.2559 (2)
0.7483 (2)
0.7441 (2)
0.2364 (2)
0.7201 (2)
0.7636 (2)
0.2799 (2)
Symmetry code: (i) 1 x; 1 y; 1 z.
The frames collected during data collection did not show split
re¯ections and there was no warning sign that the structure deter-
mination would be problematic. The cell parameters of (I) indicate a
Ê
Ê
Ê
monoclinic C-centred cell (a = 23.945 A, b = 20.271 A, c = 8.662 A, ꢀ =
3
ꢀ
ꢀ
ꢀ
Ê
90.05 , ꢁ = 96.35 , ꢂ = 89.97 and V = 4178.64 A ) and the Rint value
for the monoclinic crystal system is 0.096 (compared with 0.080 for
the triclinic crystal system). However, there are no systematic
extinctions and the structure cannot be solved in any of the mono-
clinic space groups. Thus, the structure was solved in the triclinic
space group P1 by locating the heaviest atoms in a Patterson map and
using successive Fourier syntheses to ®nd the remaining atoms. After
encountering severe problems during structure solution, anisotropic
re®nement remained stalled at R1 = 0.278 and wR2 = 0.685, with
several atoms going nonpositive de®nite. It was therefore assumed
that the crystal was twinned. The warning signs for twinning are,
®rstly, that E2 1 is just 0.768, which is a little bit too small for a
centrosymmetric structure and, secondly, that Rint for the higher-
symmetry Laue group is only slightly higher than for the lower-
symmetry Laue group. Nevertheless, the twin law is not a symmetry
operation of the higher-symmetry Laue group, but a twofold rotation
axis along the face diagonal of the bc plane interchanging b and c,
which are almost equal in length, i.e. (100/001/010). Applying this
twin law ultimately provided success and R1 dropped below 0.1. All H
atoms could now be located by difference Fourier synthesis. They
were re®ned using a riding model with ®xed individual displacement
Figure 3
Packing diagram of the tetragonal polymorph of dibromidotetrakis(tetra-
hydrofuran)magnesium, (Ia), viewed in the ab plane. C and H atoms have
been omitted for clarity. Key: Mg atoms are shaded, Br atoms are dotted
and O atoms are cross-hatched.
Experimental
A solution of Me3SnCl (26.08 g, 130.9 mmol) and 1,2-dibromo-
benzene (7.8 ml, 64 mmol) in tetrahydrofuran (THF) (100 ml) was
added dropwise to a mixture of magnesium (7.45 g 306 mmol) and
CH3MgCl (0.6 mmol) in THF (100 ml). The reaction mixture was
heated under re¯ux for 90 min. After cooling to ambient tempera-
ture, colourless crystals of Mg(THF)4Br2 suitable for X-ray crystal-
lographic analysis were obtained (yield 23%).
Crystal data
[MgBr2(C4H8O)4]
Mr = 472.55
Triclinic, P1
a = 8.6620 (9) A
b = 15.6820 (12) A
Ê
c = 15.6910 (13) A
ꢀ = 80.501 (9)ꢀ
ꢁ = 85.131 (11)ꢀ
ꢂ = 85.190 (11)ꢀ
3
Ê
V = 2089.3 (3) A
Z = 4
Ê
Mo Kꢀ radiation
Ê
parameters [Uiso(H) = 1.2Ueq(C)] and with CÐH = 0.99 A. Finally,
1
Ê
ꢃ = 3.93 mm
T = 173 (2) K
the ratio of the twin components re®ned to 0.534 (3):0.466 (3).
Data collection: X-AREA (Stoe & Cie, 2001); cell re®nement:
X-AREA; data reduction: X-AREA; program(s) used to solve
structure: SHELXS97 (Sheldrick, 1997); program(s) used to re®ne
structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in
SHELXTL-Plus (Sheldrick, 1991); software used to prepare material
for publication: SHELXL97.
0.27 Â 0.26 Â 0.24 mm
Data collection
Stoe IPDSII two-circle
diffractometer
Absorption correction: multi-scan
(MULABS; Spek, 2003; Blessing,
1995)
22055 measured re¯ections
7347 independent re¯ections
4969 re¯ections with I > 2ꢄ(I)
Rint = 0.080
Tmin = 0.342, Tmax = 0.420
(expected range = 0.318±0.390)
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: SK3104). Services for accessing these data are
described at the back of the journal.
Re®nement
R[F2 > 2ꢄ(F2)] = 0.087
wR(F2) = 0.232
S = 1.44
416 parameters
H-atom parameters constrained
References
3
Ê
Áꢅmax = 0.97 e A
3
Ê
1.11 e A
7347 re¯ections
Áꢅmin
=
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ꢁ
Acta Cryst. (2007). C63, m174±m176
Lorbach et al. [MgBr2(C4H8O)4] m175

metal-organic compounds
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Acta Cryst. (2007). C63, m174±m176