Concomitant but disappearing: Two polymorphs of 1,4-bis-(tribromomethyl) benzene

Article abstract of DOI:10.1107/S0108270111037930

The title compound, C₈H₄Br6, (I), initially crystallized from deuterochloro-form as the comcomitant polymorphs (Ia) (prisms, space group P21/n, Z = 2) and (Ib) (hexa-gonal plates, space group C2/c, Z = 4). The molecules in both forms

Full text of DOI:10.1107/S0108270111037930

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
colourless prisms, which when examined with polarized light
proved to be twinned lengthwise. Larger crystals (up to 2 mm
in length) were difficult to cut and, even ignoring the problems
of twinning and absorption, tended to be of low quality, but
eventually we succeeded in cutting a small crystal lengthwise
to provide a single-crystalline fragment of usable quality. This
ISSN 0108-2701
Concomitant but disappearing: two
polymorphs of 1,4-bis(tribromo-
methyl)benzene
is polymorph (Ia) (space group P2 /n). A few thin hexagonal
1
plates were also observed. One of these was investigated and
proved to be a second polymorph, (Ib) (space group C2/c).
The crystals used for X-ray measurements are shown in Fig. 1.
a
b
b
Peter G. Jones, * Henning Hopf, Anamaria Silaghi and
c
Christian N a¨ ther
a
Institut f u¨ r Anorganische und Analytische Chemie, Technische Universit a¨ t
b
Braunschweig, Postfach 3329, 38023 Braunschweig, Germany, Institut f u¨ r
Organische Chemie, Technische Universit a¨ t Braunschweig, Postfach 3329, 38023
c
Braunschweig, Germany, and Institut f u¨ r Anorganische Chemie, Christian-
Albrechts-Universit a¨ t zu Kiel, Otto-Hahn-Platz 6/7, 24098 Kiel, Germany
Correspondence e-mail: p.jones@tu-bs.de
Received 14 September 2011
Accepted 16 September 2011
Online 28 September 2011
The molecules in both polymorphs crystallize with imposed
inversion symmetry (Figs. 2 and 3). The main difference is in
The title compound, C H Br , (I), initially crystallized from
6
the orientation of the CBr group; in (Ia), one Br atom (Br3)
3
8
4
deuterochloroform as the comcomitant polymorphs (Ia)
prisms, space group P2 /n, Z = 2) and (Ib) (hexagonal plates,
lies approximately in the ring plane, whereas for (Ib) this is not
the case (Tables 1 and 3).
(
1
space group C2/c, Z = 4). The molecules in both forms display
crystallographic inversion symmetry. All further attempts to
crystallize the compound led exclusively to (Ib), so that (Ia)
may be regarded as a ‘disappearing polymorph’. Surprisingly,
however, the density of (Ia) is greater than that of (Ib). The
only significant difference between the molecular structures is
Clearly, the major structural interest centres on the mol-
ecular packing. Not surprisingly for a compound for which
60% of the terminal atoms are bromine, Brꢀ ꢀ ꢀBr interactions
(Tables 2 and 4) dominate, at least numerically. Polymorph
˚
(Ia) has eight independent interactions of this type <3.99 A,
˚
with the next longest at 4.14 A, while (Ib) has seven (entries 4
˚
and 5 are symmetry-equivalent) <4.08 A, with the next longest
the orientation of the CBr groups. The molecular packing of
3
˚
at 4.23 A. Additionally, (Ia) has one ‘weak’ hydrogen bond
both structures is largely determined by Brꢀ ꢀ ꢀBr interactions,
although (Ia) also displays a C—Hꢀ ꢀ ꢀBr hydrogen bond and
both polymorphs display one Brꢀ ꢀ ꢀꢀ contact. For (Ia), six of
the eight contacts combine to form a tube-like substructure
parallel to the a axis. For (Ib), the two shortest Brꢀ ꢀ ꢀBr
i
˚
[H3ꢀ ꢀ ꢀBr3 = 3.00 A; symmetry code: (i) x ꢁ 1, y, z] and a
ii
˚
Brꢀ ꢀ ꢀꢀ interaction Br3ꢀ ꢀ ꢀCg = 3.518 A [Cg is the centroid of
ii
the aromatic ring; individual Brꢀ ꢀ ꢀC distances = 3.738 (2)–
ii
ꢂ
˚
3.779 (2) A and C—Brꢀ ꢀ ꢀCg = 115 ; symmetry code: (ii)
3
1
1
contacts link ‘half’ molecules consisting of C—CBr groups to
ꢁx + , y + , ꢁz + ]. Polymorph (Ib) has a Brꢀ ꢀ ꢀꢀ interaction
˚
3
2
2
2
1
3
iii
iii
form double layers parallel to (001) in the regions z ’ , .
Br1ꢀ ꢀ ꢀCg = 3.503 A [individual Brꢀ ꢀ ꢀC distances =
4
4
iii
ꢂ
˚
3
.473 (4)–3.702 (5) A and C—Brꢀ ꢀ ꢀCg = 153 ; symmetry
1 1
code: (iii) x ꢁ , y + , z].
2
2
Comment
It is not a trivial problem to decide when a Brꢀ ꢀ ꢀBr contact
We are interested in secondary interactions in brominated
aromatic hydrocarbons [see, for example, our studies of all ten
isomers of di(bromomethyl)naphthalenes; Jones & Kus´
corresponds to a significant interaction. The shortest such
˚
contacts are ca 3.1–3.2 A and tend to be observed in charge-
+
assisted systems such as [Ph PSBr] [AuBr ] [3.151 (1) A;
ꢁ
˚
3
4
(2010), and references therein]. Such interactions may include
‘
ꢀ
weak’ C—Hꢀ ꢀ ꢀBr hydrogen bonds, Brꢀ ꢀ ꢀBr halogen bonds,
–ꢀ stacking, and Hꢀ ꢀ ꢀꢀ and Brꢀ ꢀ ꢀꢀ contacts. We are currently
preparing a study of several benzene derivatives multiply
substituted with bromo, methyl and bromomethyl groups
(
Jones & Ku s´ , 2011). The title compound, 1,4-bis(tribromo-
methyl)benzene, (I), as a tribromomethyl derivative, is loosely
related to these.
Single crystals of compound (I) were originally obtained
when a deuterochloroform solution of (I) in an NMR tube was
allowed to evaporate. The sample consisted mostly of
Figure 1
The measured crystals of polymorphs (Ia) (left) and (Ib) (right),
compared with a human hair (centre; diameter ca 0.05 mm).
Acta Cryst. (2011). C67, o405–o408
doi:10.1107/S0108270111037930
# 2011 International Union of Crystallography o405

organic compounds
Figure 2
The molecule of polymorph (Ia), showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level. Only the
asymmetric unit is numbered; unlabelled atoms are related by the
symmetry operation (ꢁx + 1, ꢁy, ꢁz).
Figure 3
Figure 5
The molecule of polymorph (Ib), showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level. Only the
asymmetric unit is numbered; unlabelled atoms are related by the
symmetry operation (ꢁx + 1, ꢁy + 1, ꢁz + 1).
A packing diagram for polymorph (Ia), viewed parallel to the a axis. H
atoms have been omitted. Brꢀ ꢀ ꢀBr interactions indicated by thin dashed
lines correspond to those shown in Fig. 4, while those indicated by thick
dashed lines and numbered (1 and 6) do not appear in Fig. 4. One
representative Brꢀ ꢀ ꢀꢀ interaction is represented by ‘ꢀ’. Labelled Br
atoms belong to the asymmetric unit and the labels refer to atoms nearer
the viewer.
which are significantly longer than twice the van der Waals
˚
radius (3.7 A; Bondi, 1964) but may lead to striking patterns
that are at the very least useful in describing molecular
aggregates. We have described such long contacts as ‘tertiary
interactions’ (du Mont et al., 2008). Pedireddi et al. (1994)
defined two categories of halogen–halogen contact in terms of
the two C—Halꢀ ꢀ ꢀHal angles ꢁ; type II contacts tend to have
ꢂ
ꢂ
ꢁ
’ 90 and ꢁ ’ 180 (or vice versa), whereas for type I
1
2
contacts ꢁ ’ ꢁ . The former type may correspond better to
1
2
significant interactions, consistent with the theoretical model
of a region of positive charge in the extension of the C—Hal
vector, whereas the latter type may correspond better to
‘chance’ contacts not indicating significant interactions.
However, any inspection of systems with halogen–halogen
contacts will reveal many cases not entirely consistent with the
two standard types (e.g. short contacts with approximately
equal angles; cf. Tables 2 and 4).
Figure 4
A packing diagram for polymorph (Ia), viewed perpendicular to (011). H
atoms have been omitted. Brꢀ ꢀ ꢀBr interactions are indicated by dashed
lines and are numbered according to Table 2; contact 4 lies behind contact
˚
Polymorph (Ia) has only one Brꢀ ꢀ ꢀBr contact <3.7 A (entry
1 in Table 2), and despite its shortness this is a type I inter-
action with ꢁ = ꢁ by symmetry; all seven other contacts lie in
7
. One representative C(—H)ꢀ ꢀ ꢀBr interaction is represented by a dotted
1
2
line (top right). Labelled Br atoms belong to the asymmetric unit and Br1
lies behind Br2.
˚
the range 3.8–4.0 A. Six of the eight contacts combine to form
a tube-like substructure (Fig. 4) parallel to the a axis. Aview of
the structure parallel to the a axis (Fig. 5) shows that the tubes
are connected by the two contacts Br1ꢀ ꢀ ꢀBr1(ꢁx + 1, ꢁy,
ꢁz + 1) and Br2ꢀ ꢀ ꢀBr2(ꢁx + 1, ꢁy + 1, ꢁz) (entries 1 and 6 in
Table 2), which are topologically closely similar but represent
the shortest and longest contacts, respectively. This is a further
reminder that the lengths of secondary or tertiary interactions
Taouss & Jones, 2011]. ‘Spoke’ structures such as Ph PBr ,
3
2
originally interpreted by the authors (Bricklebank et al., 1992)
˚
as involving a long covalent Br—Br bond of 3.123 (2) A, may
also perhaps be interpreted as at least partly ionic
[
+
Ph PBr] Br . At the other extreme are contacts of ca 4 A,
ꢁ
˚
3
ꢃ
o406 Jones et al. Two polymorphs of C
H Br
8 4 6
Acta Cryst. (2011). C67, o405–o408

organic compounds
the layers are linked by contact No. 2 (almost perpendicular to
the paper). Including all the interactions gives an impression
of their density, but the resulting diagram is otherwise too
complicated to interpret.
It seemed worthwhile to examine the relationship and
possible interconversions between the two polymorphs. The
original sample was no longer available and all further
attempts at recrystallization led only to polymorph (Ib), as
shown by powder diffractometry; measured and calculated
[
for (Ib)] powder patterns were essentially identical. Differ-
ential scanning calorimetry (DSC) measurements gave only a
peak at the melting point (462.2 K). Melted and resolidified
samples were amorphous, presumably because of decom-
position (supported by DSC measurements, which showed a
very broad melting peak for resolidifed samples). Rapid
evaporation from dichloromethane, slow evaporation from
chloroform or stirring the solid with a saturated chloroform
solution for a week all resulted in samples consisting only of
Figure 6
A packing diagram for polymorph (Ib), viewed parallel to the b axis. H
˚
atoms have been omitted. Brꢀ ꢀ ꢀBr interactions <4 A are indicated by thin
dashed lines. One representative Brꢀ ꢀ ꢀꢀ interaction is represented by ‘ꢀ’.
Labelled Br atoms belong to the asymmetric unit.
may not be closely correlated with their (subjective) structural
relevance.
(
Ib). It seems, therefore, that (Ib) is the thermodynamically
stable form at room temperature and that form (Ia) is a
disappearing polymorph’ (Dunitz & Bernstein, 1995).
The overall packing diagram of polymorph (Ib) is shown in
Fig. 6. The Brꢀ ꢀ ꢀBr interactions can be seen in the regions
‘
ꢁ3
1
4
3
4
˚
Curiously, the crystallographic density of (Ia) (3.165 Mg m )
ꢁ3
z ’ , . Two Brꢀ ꢀ ꢀBr contacts <3.7 A (Table 4, entries 1 and 2)
is greater than that of (Ib) (3.080 Mg m ), which would not
be expected if the predominance of (Ib) were attributable to
more efficient packing.
are appreciably shorter than all the others, and both corre-
spond reasonably well to the type II criteria. To display the
1
4
region at z ’ more clearly, it is convenient to use only ‘half’
molecules consisting of C—CBr groups, which are linked to
3
form double layers, and to display only the two shortest
Brꢀ ꢀ ꢀBr contacts (Fig. 7). Each individual layer is formed via
contact No. 1 (these contacts run diagonally in the figure) and
Experimental
1,4-Bis(dibromomethylene)cyclohexane (0.50 g, 1.179 mmol) (Neid-
lein & Winter, 1998; cf. Hopf et al., 2002) was dissolved in carbon
tetrachloride (25 ml) under nitrogen. N-Bromosuccinimide (0.84 g,
4
.716 mmol, 4 equivalents) and azobis(isobutyronitrile) (AIBN;
.01 g; each 0.1 mol NBS requires 0.2 g AIBN) were added to the
0
solution. The mixture was stirred for 1.5 h under reflux then allowed
to cool to room temperature. The progress of the reaction was
monitored by thin-layer chromatography (silica gel) with pentane.
Purification by flash chromatography with pentane gave the pure
product (yield 0.49 g, 72%; colourless crystals, m.p. 461–462 K).
1
13
H NMR (400 MHz, CDCl
100 MHz, CDCl ): ꢂ 33.7 (s, CBr
arom. C); IR (film, ꢃ, cm ): 3087 (w), 2923 (w), 2778 (w), 1490 (w),
398 (m), 1181 (m), 1013 (w), 841 (w), 809 (s), 715 (vs), 682 (m), 647
3
): ꢂ 8.0 (s, 4H, arom.); C NMR
(
3
3
), 126.3 (d, arom. CH), 148.0 (s,
ꢁ
1
1
8
1
79
3
+
3
] ,
(
vs, br); EI–MS (m/z; relative intensity, %): 579.4 (2) [M Br
Br
81
79
2
+
81 79
+
4
98.5 (14) [M Br
Br
] , 259.8 (65) [M Br Br] , 178.9 (22) [M Br] , 100.0
32), 74.0 (32), 50.0 (26). Elemental analysis calculated for C Br
3 2 2
] , 419.6 (100) [M Br Br ] , 338.7 (10)
81
79
+
81 79
+
79
+
[
M Br Br
2
(
8
H
4
6
:
C 16.58, H 0.70, Br 82.72%; found: C 16.63, H 0.57, Br 82.22%. For an
alternative preparation of the hexabromide from p-xylene, see
Mataka et al. (1994).
Polymorph (Ia)
Crystal data
˚
V = 608.19 (5) A
3
C
H
8 4
Br
6
Figure 7
A packing diagram for polymorph (Ib), viewed perpendicular to (001), in
M
r
= 579.57
Z = 2
Mo Kꢅ radiation
ꢆ = 19.76 mm
T = 100 K
Monoclinic, P2 =n
a = 6.3150 (3) A
b = 9.8178 (4) A
˚
c = 9.8398 (4) A
1
˚
˚
1
ꢁ1
the region z ’ . H atoms have been omitted and the molecules are
4
halved’ to C—CBr
indicated by thick dashed lines and numbered according to the sequence
of entries in Table 4. Labelled Br atoms belong to the asymmetric unit.
‘
3
groups. The two shortest Brꢀ ꢀ ꢀBr interactions are
0.08 ꢄ 0.06 ꢄ 0.03 mm
ꢂ
ꢄ = 94.486 (4)
ꢃ
Acta Cryst. (2011). C67, o405–o408
Jones et al.
Two polymorphs of C H Br
8 4 6
o407

organic compounds
Table 1
Selected torsion angles ( ) for polymorph (Ia).
Table 4
Brꢀ ꢀ ꢀBr contacts in polymorph (Ib) (A, ).
ꢂ
˚
ꢂ
j
j
j
j
C2—C1—C4—Br3
C3—C1—C4—Br3
C2—C1—C4—Br1
ꢁ8.3 (2)
172.88 (14)
ꢁ129.69 (17)
C3—C1—C4—Br1
C2—C1—C4—Br2
C3—C1—C4—Br2
51.5 (2)
111.73 (18)
ꢁ67.09 (19)
C—Brꢀ ꢀ ꢀBr —C
Brꢀ ꢀ ꢀBr
C—Brꢀ ꢀ ꢀBr angles
Symmetry operator j
system
1
1
1
1
2
3
C4—Br1ꢀ ꢀ ꢀBr2—C4 3.6912 (7) 87.83 (12), 163.67 (13) ꢁx + , y + , ꢁz +
1 1
2 2
2
2
2
C4—Br1ꢀ ꢀ ꢀBr3—C4 3.5799 (8) 94.47 (14), 154.20 (12) x ꢁ , y ꢁ , z
1
C4—Br2ꢀ ꢀ ꢀBr2—C4 4.0094 (10) 101.61 (12), 101.61 (12) ꢁx + 1, y, ꢁz +
2
1
1
1
Table 2
4 C4—Br2ꢀ ꢀ ꢀBr2—C4 3.8353 (6) 146.53 (13), 84.08 (14) ꢁx + , y ꢁ , ꢁz +
1 1 1
2 2 2
2
2
2
˚
ꢂ
5 C4—Br2ꢀ ꢀ ꢀBr2—C4 3.8353 (6) 84.08 (14), 146.53 (13) ꢁx + , y + , ꢁz +
1 1
6
2 2
Brꢀ ꢀ ꢀBr contacts in polymorph (Ia) (A, ).
C4—Br2ꢀ ꢀ ꢀBr3—C4 3.8068 (7) 88.14 (12), 148.31 (13) x ꢁ , y ꢁ , z
1 1
2 2
1
2
j
j
j
j
7 C4—Br2ꢀ ꢀ ꢀBr3—C3 4.0730 (8) 138.09 (12), 77.51 (13) ꢁx + , y ꢁ , ꢁz +
1
8
C4—Br3ꢀ ꢀ ꢀBr3—C4 3.9507 (10) 107.85 (12), 107.85 (12) ꢁx + 1, y, ꢁz + 2
C—Brꢀ ꢀ ꢀBr —C
Brꢀ ꢀ ꢀBr
C—Brꢀ ꢀ ꢀBr angles
Symmetry operator j
system
1
2
3
4
5
6
7
8
C4—Br1ꢀ ꢀ ꢀBr1—C4 3.6979 (4) 139.63 (6), 139.63 (6) ꢁx + 1, ꢁy, ꢁz + 1
1 1 1
2 2 2
C4—Br1ꢀ ꢀ ꢀBr2—C4 3.8427 (3) 115.18 (6), 158.59 (6) ꢁx + , y ꢁ , ꢁz +
1
1
1
C4—Br1ꢀ ꢀ ꢀBr2—C4 3.8354 (3) 107.96 (6), 117.67 (6) x + , ꢁy + , z +
2
2
2
Refinement
2
C4—Br1ꢀ ꢀ ꢀBr3—C4 3.9750 (3) 92.15 (6), 152.00 (6) x ꢁ 1, y, z
1
1
1
2
2
C4—Br1ꢀ ꢀ ꢀBr3—C4 3.8573 (3) 145.15 (6), 94.69 (6) x ꢁ , ꢁy + , z +
R[F > 2ꢇ(F )] = 0.037
wR(F ) = 0.103
64 parameters
H-atom parameters constrained
2
2
2
C4—Br2ꢀ ꢀ ꢀBr2—C4 3.9819 (5) 130.68 (6), 130.68 (6) ꢁx + 1, ꢁy + 1, ꢁz
C4—Br2ꢀ ꢀ ꢀBr3—C4 3.9288 (3) 93.53 (6), 153.70 (6) x ꢁ 1, y, z
˚
ꢁ3
Áꢈmax = 1.02 e A
S = 1.05
1271 reflections
3
1
1
2
ꢁ3
C4—Br2ꢀ ꢀ ꢀBr3—C4 3.8815 (3) 129.46 (6), 93.57 (6) ꢁx + , y + , ꢁz +
Áꢈmin = ꢁ0.93 e A˚
2
2
For polymorph (Ib), 34 reflections at high angle with F
o
<< F
c
were
Table 3
Selected torsion angles ( ) for polymorph (Ib).
ꢂ
omitted from the refinement. It is not clear whether these errors were
caused by hardware or software problems, or by any imperfection of
the crystal. H atoms were introduced at calculated positions and
C3—C1—C4—Br2
C2—C1—C4—Br2
C3—C1—C4—Br3
ꢁ35.3 (5)
150.3 (4)
ꢁ156.5 (4)
C2—C1—C4—Br3
C3—C1—C4—Br1
C2—C1—C4—Br1
29.1 (5)
84.6 (5)
ꢁ89.8 (4)
˚
refined using a riding model, with C—H = 0.95 A and Uiso(H) =
1
.2Ueq(C).
For both polymorphs, data collection: CrysAlis PRO (Oxford
Diffraction, 2009); cell refinement: CrysAlis PRO; data reduction:
CrysAlis PRO; program(s) used to solve structure: SHELXS97
Data collection
(Sheldrick, 2008); program(s) used to refine structure: SHELXL97
Oxford Xcalibur Eos diffractometer
Absorption correction: multi-scan
15682 measured reflections
1795 independent reflections
1449 reflections with I > 2ꢇ(I)
(Sheldrick, 2008); molecular graphics: XP (Siemens, 1994); software
used to prepare material for publication: SHELXL97.
(
CrysAlis PRO; Oxford
Diffraction, 2009)
min = 0.554, Tmax = 1.000
R
int = 0.033
T
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: FA3260). Services for accessing these data are
described at the back of the journal.
Refinement
2
2
R[F > 2ꢇ(F )] = 0.015
wR(F ) = 0.026
64 parameters
H-atom parameters constrained
2
˚
ꢁ3
S = 0.86
1795 reflections
Áꢈmax = 0.53 e A
References
Áꢈmin = ꢁ0.53 e A˚
ꢁ3
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Polymorph (Ib)
Crystal data
˚
V = 1250.1 (2) A
3
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Diffraction Ltd, Yarnton, Oxfordshire, England.
C
M
8
H
4
Br
6
r
= 579.57
Z = 4
Cu Kꢅ radiation
Monoclinic, C2=c
˚
a = 10.1640 (9) A
ꢁ1
ꢆ = 22.89 mm
T = 103 K
˚
b = 6.5048 (7) A
˚
c = 19.3410 (17) A
0.04 ꢄ 0.04 ꢄ 0.02 mm
ꢂ
ꢄ = 102.152 (9)
Data collection
Oxford Xcalibur Nova Atlas
diffractometer
Absorption correction: multi-scan
CrysAlis PRO (Oxford
Diffraction, 2009)
7801 measured reflections
1271 independent reflections
1163 reflections with I > 2ꢇ(I)
Pedireddi, V. R., Reddy, D. S., Goud, B. S., Craig, D. C., Rae, A. D. & Desiraju,
G. R. (1994). J. Chem. Soc. Perkin Trans. 2, pp. 2353–2360.
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Siemens (1994). XP. Siemens Analytical X-ray Instruments Inc., Madison,
Wisconsin, USA.
Taouss, C. & Jones, P. G. (2011). Dalton Trans. 40. In the press.
R
int = 0.076
T
min = 0.439, Tmax = 1.000
ꢃ
o408 Jones et al. Two polymorphs of C
H Br
8 4 6
Acta Cryst. (2011). C67, o405–o408