2-Bromo-1,3-bis(bromomethyl)-benzene, with Z′ = 1.5: Whole-molecule disorder of one of the two independent molecules

Article abstract of DOI:10.1107/S0108270106017707

The title compound, C₈H₇r₃, possesses normal geometrical parameters. There are two independent molecules; one shows whole-molecule disorder with respect to an inversion-symmetry-generated partner, while the other is undiso

Full text of DOI:10.1107/S0108270106017707

organic compounds
Acta Crystallographica Section C
Crystal Structure
mean plane of the C11±C16 benzene ring [with displacements
Ê
for Br12 and Br13 of 1.825 (16) and 1.74 (3) A, respectively].
Communications
This situation of one ordered and one disordered molecule
results in the atypical situation of Z0 = 1.5 and Z = 6 for a
monoclinic system.
ISSN 0108-2701
As well as van der Waals forces, the crystal packing is
in¯uenced by ꢀ±ꢀ stacking interactions involving the C1-
containing molecule (Fig. 3) generated by the c-glide
symmetry operation. The CgÁ Á ÁCgi separation [Cg is the
centroid of the C1±C6 ring; symmetry code: (i) x, 32 y, 21 + x] is
2-Bromo-1,3-bis(bromomethyl)-
benzene, with Z0 = 1.5: whole-
molecule disorder of one of the
two independent molecules
i
i
Ê
3.755 (4) A and the C1±C6/C1 ±C6 interplanar separation is
Ê
3.411 A. A PLATON (Spek, 2003) analysis of (I) revealed a
ii
Ê
slightly short Br1Á Á ÁBr11 contact of 3.595 (2) A [symmetry
Peter Kirsop, John M. D. Storey and William T. A.
Harrison*
Ê
code: (ii) 2 x, 1 y, 1 z], some 0.1 A less than the van der
Ê
Waals radius sum of 3.70 A (Spek, 2003). Such BrÁ Á ÁBr
contacts are quite common and their signi®cance ± speci®c
attractive forces (Desiraju & Parthasarathy, 1989) or packing
contacts (Eriksson & Hu, 2001) ± has been debated.
Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen
AB24 3UE, Scotland
Correspondence e-mail: w.harrison@abdn.ac.uk
The packing of (I) is shown in Fig. 4, indicating how the
ordered and disordered molecules segregate into (010) sheets.
Received 10 May 2006
Accepted 12 May 2006
Online 15 June 2006
The title compound, C8H7Br3, possesses normal geometrical
parameters. There are two independent molecules; one shows
whole-molecule disorder with respect to an inversion-
symmetry-generated partner, while the other is undisordered.
This results in the unusual situation of Z0 = 1.5 and Z = 6 for a
monoclinic crystal system. The undisordered molecule inter-
acts with its neighbours by way of ꢀ±ꢀ stacking.
Comment
The title compound, (I), prepared earlier by Newcombe et al.
(1977), was obtained during our ongoing studies to determine
the philicity of aryl radicals by competitive cyclization reac-
tions (Kirsop et al., 2004a,b,c,d).
Figure 1
A view of (I), showing 50% probability displacement spheres and
ellipsoids (H atoms are drawn as spheres of arbitrary radii).
There are two independent molecules of (I) (Fig. 1). Both
appear to possess their expected geometrical parameters,
allowing for the rather low bond precisions obtained in this
study. The C1-containing species is unexceptional. With
respect to the mean plane of the C1±C6 benzene ring, one of
the side-arm terminal Br atoms points `up' [the displacement
Ê
of Br2 is 1.790 (12) A] and one points `down' [the displace-
Ê
ment of Br3 is 1.792 (12) A].
The most interesting feature of the structure is the whole-
molecule disorder displayed by the C11-containing molecule.
1
1
This arises from inversion symmetry at the point (1, , ) and
2
2
symmetry-related locations. The resulting overlapped mol-
ecules (Fig. 2) are constrained by symmetry to have equal
population parameters of 0.5 for all atoms in the molecule. As
with the C1-containing molecule, the two side-arm terminal Br
atoms are displaced in opposite senses with respect to the
Figure 2
A detail of (I), showing the whole-molecule disorder of the C11-
containing molecule (50% probability displacement spheres/ellipsoids; all
H atoms have been omitted for clarity). Atoms with the suf®x A are
generated by the symmetry operation (2 x, 1 y, 1 z).
o376 # 2006 International Union of Crystallography
DOI: 10.1107/S0108270106017707
Acta Cryst. (2006). C62, o376±o378

organic compounds
Because the C11-containing molecules are almost perpendi-
cular to, and are sandwiched between, the ꢀ±ꢀ stacks of C1-
containing molecules, there can be no ꢀ±ꢀ forces involving the
former molecules [the dihedral angle between the C1±C6 and
C11±C16 mean planes is 80.8 (6)ꢀ].
Aside from very simple molecules and fragments, whole-
molecule disorder (WMD) is not particularly common. A
classic example is the 10 ꢀ electron molecule azulene, C10H8,
containing fused, planar, ®ve- and seven-membered rings.
After several con¯icting studies it was concluded (Robertson
et al., 1962) that azulene shows WMD with the 5/7 and 7/5
conformations overlapped at random. More recently,
Ichharam & Boeyens (2001) observed WMD in 2-(2-thienyl)-
1-(2-pyrazinyl)ethene and 2-(2-thienyl)-1-(2-quinoxalinyl)-
ethene. In both cases, the disordered components were related
by pseudo-twofold axes. Cox & Wardell (2003) found WMD
in 4,40-sulfonylbis[N-(4-nitrophenylmethylene)benzenamine],
with no (pseudo)symmetry relating the two slightly displaced
disorder components.
Experimental
2-Bromo-1,3-dimethylbenzene (5.0 g, 0.027 mol), N-bromosuccin-
amide (NBS, 9.6 g, 0.054 mol) and azobisisobutyronitrile (0.88 g,
0.0054 mol) were added to chloroform (100 ml). The mixture was
stirred at re¯ux under a nitrogen atmosphere for 12 h. After cooling,
the mixture was ®ltered and the solvent was removed at reduced
pressure to give a yellow solid. Thin-layer chromatography (hexane)
showed 2-bromo-1,3-bis(bromomethyl)benzene as a sharp spot at
RF = 0.21. The NBS residues were removed by ¯ash column chro-
matography (20:1 hexane±ethyl acetate) and the solvent was
removed. The product was washed with hexane, giving a white solid
(4.9 g, 53%). A sample was recrystallized from hot hexane±ethyl
acetate (20:1) to give clear needles of (I) [m.p. 371±373 K, literature
(Newcombe et al., 1977) 374±376 K]. 1H NMR (CDCl3): ꢁH 4.64 (4H,
s, 2 Â CH2), 7.28 (1H, t, J = 8.1 Hz, Ar±H), 7.41 (2H, d, J = 8.1 Hz, 2 Â
Ar±H); 13C NMR (CDCl3): ꢁC 33.8, 126.6, 128.0, 131.3, 138.5.
Crystal data
C8H7Br3
Mr = 342.87
Z = 6
Dx = 2.385 Mg m
Mo Kꢃ radiation
ꢄ = 12.61 mm
T = 120 (2) K
Blade, colourless
0.60 Â 0.10 Â 0.01 mm
3
Monoclinic, P21=c
1
Ê
a = 9.1114 (4) A
Ê
b = 22.6016 (10) A
Ê
c = 7.5004 (3) A
ꢂ = 111.971 (3)ꢀ
V = 1432.40 (11) A
3
Ê
Data collection
Nonius KappaCCD diffractometer
! and ' scans
Absorption correction: multi-scan
(SORTAV; Blessing, 1995)
Tmin = 0.049, Tmax = 0.940
(expected range = 0.046±0.882)
14563 measured re¯ections
3266 independent re¯ections
2406 re¯ections with I > 2ꢅ(I)
Rint = 0.099
Figure 3
A detail of (I), showing the ꢀ±ꢀ stacking interaction involving the C1-
containing molecule. The molecules containing atoms Br1A and Br1B are
generated by the symmetry operations (x, 32 y, 21 + z) and (x, 23 y, z 21 ),
respectively.
ꢆmax = 27.6ꢀ
Re®nement
Re®nement on F2
R[F2 > 2ꢅ(F2)] = 0.084
wR(F2) = 0.253
S = 1.01
3266 re¯ections
146 parameters
H-atom parameters constrained
w = 1/[ꢅ2(F2o) + (0.1847P)2]
where P = (F2o + 2Fc2)/3
(Á/ꢅ)max < 0.001
3
Ê
Áꢇmax = 1.89 e A
3
Ê
2.79 e A
Áꢇmin
=
The C1-containing molecule was located and re®ned straightfor-
wardly. The C11-containing molecule evidently showed massive
disorder. By careful analysis of difference maps, the disorder could be
resolved into two overlapped symmetry-related molecules of (I) (as
described in the Comment). The C atoms of the disordered molecule
were re®ned isotropically. All H atoms were placed in calculated
Ê
positions (CÐH = 0.95±0.99 A) and re®ned as riding, with Uiso(H)
Ê
values of 1.2Ueq(C). The largest difference peak is 1.04 A from atom
Ê
Br2 and the deepest difference hole is 0.85 A from the same atom.
Attempts to model the crystal in lower-symmetry space groups were
not successful.
Data collection: COLLECT (Nonius, 1998); cell re®nement:
SCALEPACK (Otwinowski & Minor, 1997); data reduction:
DENZO (Otwinowski & Minor, 1997), SCALEPACK and SORTAV
Figure 4
The packing in (I), viewed down [001], with H atoms omitted.
ꢁ
Acta Cryst. (2006). C62, o376±o378
Kirsop et al. C8H7Br3 o377

organic compounds
Eriksson, L. & Hu, J. (2001). Acta Cryst. E57, o930±o932.
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.
Ichharam, V. & Boeyens, J. C. A. (2001). Cryst. Eng. 4, 171±178.
Kirsop, P., Storey, J. M. D. & Harrison, W. T. A. (2004a). Acta Cryst. C60,
o353±o355.
(Blessing, 1995); program(s) used to solve structure: SHELXS97
(Sheldrick, 1997); program(s) used to re®ne structure: SHELXL97
(Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997);
software used to prepare material for publication: SHELXL97.
Kirsop, P., Storey, J. M. D. & Harrison, W. T. A. (2004b). Acta Cryst. E60,
o222±o224.
Kirsop, P., Storey, J. M. D. & Harrison, W. T. A. (2004c). Acta Cryst. E60,
o1147±o1148.
Kirsop, P., Storey, J. M. D. & Harrison, W. T. A. (2004d). Acta Cryst. E60,
o1636±o1638.
The authors thank the EPSRC UK National Crystal-
lography Service for the data collection.
Newcombe, M., Moore, S. S. & Cram, D. J. (1977). J. Am. Chem. Soc. 99, 6405±
6410.
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: SK3024). Services for accessing these data are
described at the back of the journal.
Nonius (1998). COLLECT. Nonius BV, Delft, The Netherlands.
Otwinowski, Z. & Minor, W. (1997). Methods in Enzymology, Vol. 276,
Macromolecular Crystallography, Part A, edited by C. W. Carter Jr & R. M.
Sweet, pp. 307±326. New York: Academic Press.
Robertson, J. M., Shearer, H. M. M., Sim, G. A. & Watson, D. G. (1962). Acta
Cryst. 15, 1±8.
References
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Cox, P. J. & Wardell, J. L. (2003). Acta Cryst. C59, o706±o708.
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Spek, A. L. (2003). J. Appl. Cryst. 36, 7±13.
ꢁ
o378 Kirsop et al. C8H7Br3
Acta Cryst. (2006). C62, o376±o378