1,3,5-triiodo-2,4,6-trimethylbenzene at 293 K

Article abstract of DOI:10.1107/S0108270101010113

In the structure of triiodomesitylene (1,3,5-triiodo-2,4,6-trimethylbenzene), C₉H₉I₃, at 293 K, the benzene ring is found to be significantly distorted from ideal D_6h symmetry; the average endocyclic angles faci

Full text of DOI:10.1107/S0108270101010113

organic compounds
120ꢁ in equivalent positions (Eveno & Meinnel, 1966).
Furthermore, TCM and TBM can be considered as prototype
systems for studying the rotational tunnelling behaviour
(Prager & Heidemann, 1997) of methyl groups. Indeed, the
three methyl groups of each molecule display different
tunnelling excitations, re¯ecting different hindering potentials
(Meinnel et al., 1992). As the tunnelling behaviour of 1,3,5-
triiodo-2,4,6-trimethylbenzene (triiodomesitylene, TIM) is
similar to that of TBM but with one methyl group being still
less hindered, high quality structural data concerning TIM are
required. We report herein a structural study of TIM, (I), at
293 K, in order to complete and enlighten the previously
published work concerning the two aforementioned isomor-
phous compounds, i.e. TCM (Tazi et al., 1995) and TBM
(Meinnel et al., 2000).
Acta Crystallographica Section C
Crystal Structure
Communications
ISSN 0108-2701
1,3,5-Triiodo-2,4,6-trimethylbenzene
at 293 K
Ali Boudjada,a Olivier Hernandez,b* Jean Meinnel,c
Mohammed Manid and Werner Paulusb
a
Â
Â
Laboratoire de Cristallographie, Departement de Physique, Universite Mentouri
b
Â
Constantine, 25000 Constantine, Algeria, LCSIM UMR 6511 CNRS±Universite de
Ã
Rennes 1, Institut de Chimie de Rennes, Batiment 10B, Campus de Beaulieu, Avenue
c
Â
Â
Â
du General Leclerc, 35042 Rennes, France, GMCM UMR 6626 CNRS±Universite
Ã
 Â
de Rennes 1, Batiment 11A, Campus de Beaulieu, Avenue du General Leclerc,
d
Â
Â
35042 Rennes, France, and Faculte des Sciences, Universite Chouaib-Doukkali,
El Jadida, Morocco
Correspondence e-mail: olivier.hernandez@univ-rennes1.fr
Received 20 February 2001
Accepted 19 June 2001
The conformation of the molecule of (I) (Fig. 1) is char-
acterized mainly by a signi®cant distortion of the endocyclic
angles of the benzene ring from ideal D6h symmetry; 123.8 (3)ꢁ
on average for atoms facing the I atoms versus 116.2 (3)ꢁ on
average for atoms facing the methyl groups. The widest angle
is found for the most electronegative substituent, corrobor-
ating the general trends established by Domenicano et al.
(1975). It is also for the same reason that the difference
between the mean endocyclic angle for atoms facing the
halogens and that for atoms facing the methyl groups
decreases progressively from TCM [124.4 (2) and 115.6 (2)ꢁ,
respectively] to TBM [124.1 (1) and 115.9 (1)ꢁ, respectively]
and then to (I). The average intramolecular bond lengths
In the structure of triiodomesitylene (1,3,5-triiodo-2,4,6-
trimethylbenzene), C9H9I3, at 293 K, the benzene ring is
found to be signi®cantly distorted from ideal D6h symmetry;
the average endocyclic angles facing the I atoms and the
methyl groups are 123.8 (3) and 116.2 (3)ꢁ, respectively. The
angle between the normal to the molecular plane and the
normal to the (100) plane is 5.1ꢁ. No disorder was detected at
293 K. The thermal motion was investigated by a rigid-body
motion tensor analysis. Intra- and intermolecular contacts are
described and topological differences compared with the
isomorphous compounds trichloromesitylene and tribromo-
mesitylene are discussed.
Ê
[CarÐI 2.117 (3), CarÐCar 1.403 (5) and CarÐCMe 1.506 (5) A]
are in agreement with the distances reported in the literature.
We have computed the least-squares best plane (unit
weights) through all the non-H atoms using the MOLAX
routine in CRYSTALS (Watkin, Prout, Carruthers & Better-
idge, 1999). The angle between the normal to this molecular
plane and the normal to the (100) plane is found to be 5.1ꢁ.
While deviations from this plane are negligible for C atoms
Comment
At room temperature, almost all hexasubstituted halogeno-
methylbenzenes with different substituents crystallize in the
monoclinic system, with two molecules per unit cell located on
a centre of symmetry, even if the isolated molecules are
asymmetric. This apparent contradiction is understood if one
takes into account the occurrence of dynamical orientational
disorder, whereby molecules jump by 60ꢁ around their centre
of gravity in the ring plane [see Tazi et al. (1995) for a
comprehensive review]. In some cases, when the isolated
molecule displays a threefold symmetry axis, other molecular
stacking modes are encountered, e.g. the hexagonal system,
space group P63/m, for 1,3,5-trichloro-2,4,6-tri¯uorobenzene
(Chaplot et al., 1981), and the triclinic system, space group P1,
for 1,3,5-trichloro-2,4,6-trimethylbenzene (trichloromesityl-
ene, TCM; Tazi et al., 1995) and 1,3,5-tribromo-2,4,6-tri-
methylbenzene (tribromomesitylene, TBM; Meinnel et al.,
2000). Even though no disorder is observed in TCM and TBM
at 293 K, the molecules nonetheless jump in their plane by
Ê
belonging to the ring [between 0.003 (4) and 0.010 (4) A in
absolute values], it appears that atom I5 is signi®cantly more
Ê
out of the plane than atoms I1 and I3 [ 0.052 (1) A versus
Ê
0.013 (1) and 0.016 (1) A, respectively]; the same conclusion
can be drawn for atom C21 of the methyl group facing I5
Ê
Ê
[ 0.046 (5) A versus 0.023 (5) and 0.031 (5) A for C41 and
C61, respectively]. The re®nement of the occupancies of the
substituted groups, i.e. I atoms and methyl groups, reveals no
deviation (within the s.u.'s) from the site-symmetry multi-
plicity. We can thus conclude that no disorder can be detected
in (I) at 293 K.
In compound (I), eight molecules are located on the four
unit-cell edges parallel to the a axis and they are related to
ꢀ
1106 # 2001 International Union of Crystallography
Printed in Great Britain ± all rights reserved
Acta Cryst. (2001). C57, 1106±1108

organic compounds
each other by an inversion centre at (12, 0, 0), leading to Z = 2
(Fig. 2). This description has been chosen in order to make it
easier to compare the structure with that of the isomorphous
compound TCM (Tazi et al., 1995).
body group leads to an overall R factor for Uij of 0.064, indi-
cating that the latter assumption is reasonably relevant.
Nevertheless, once the methyl groups or the I atoms are
removed from the above-mentioned set of atoms used for the
TLS calculation, the R factor for Uij decreases to 0.036 or to
0.035, respectively, while the main TLS features remain
unchanged. The diagonal values of the translational T and
screw S thermal tensors, with respect to the principal axes of
the librational L tensor, are negligible, while those of the L
The structure of (I) can be described as a stacking of
1
3
molecular layers (at x/a ' and ) along the a axis forming
4
4
antiferroelectric columns along this axis, i.e. an I atom is more
Ê
or less directly below (a-projected shift ' 1.04 A) the methyl
groups of the two molecules generated by an inversion centre
and belonging to adjacent layers. This shift between molecules
when looking along the a axis (Fig. 2) is explained by the fact
that the centre of gravity of the molecules of (I) is not located
2
tensor are L11 = 5.4, L22 = 10.3 and L33 = 14.3ꢁ . The centre of
libration is close to the centre of gravity of the molecule. Axis
Ê
on the unit-cell edge parallel to the a axis, but at ' 0.5236 A in
the (100) plane. Within each layer, one molecule of (I) is
surrounded by six neighbours in such a way that each I atom
or methyl group is opposite another, with two of the I atoms or
methyl groups belonging to nearby molecules, the three
corresponding contact distances being very similar. The
shortest intermolecular distances are IÁ Á ÁI 3.8875 (4), CMeÁ Á ÁI
Ê
4.095 (4) and CMeÁ Á ÁCMe 4.740 (6) A. The shortest intra-
Ê
molecular distances are 6.0479 (4), 3.229 (4) and 5.077 (5) A,
respectively. For molecules in different layers, it turns out that
the minimum intermolecular distances, compared with those
between molecules within the same layer, are signi®cantly
Ê
shortened for the CMeÁ Á ÁCMe contacts [4.000 (7) A], almost
Ê
unchanged for the CMeÁ Á ÁI contacts [4.007 (5) A] and signi®-
Ê
cantly increased for the IÁ Á ÁI contacts [4.2843 (6) A].
Figure 2
The packing diagram for (I) along the a axis, with labels shown for the
substituted atoms of one molecule.
The C atoms of the ring display Ueq parameters signi®cantly
2
lower (mean 0.0324 A ) than those of the substituted atoms
Ê
2
(means of 0.0477 and 0.0479 A for the I and methyl-C atoms,
Ê
2 is roughly parallel to the C6ÐC61 bond, axis 1 is almost
perpendicular to the molecular plane and axis 3 bisects the
C2ÐC21 and axis 1 directions.
respectively). In order to clarify this point, we performed a
conventional TLS analysis using the CRYSTALS program
(Watkin, Prout, Carruthers & Betteridge, 1999). The overall
rigid-body motion tensors T, L and S (Schomaker & True-
blood, 1968) were least-squares ®tted to the individual
anisotropic displacement parameters. Considering the whole
molecule of (I), with the exception of the H atoms, as a rigid-
Let us compare the main crystallographic features of (I),
TCM (Tazi et al., 1995) and TBM (Meinnel et al., 2000). In the
latter two compounds, the molecular layers are also anti-
1
ferroelectrically stacked along the a axis at x/a ' and 34, but in
4
TCM, the centre of gravity of one given molecule is located at
(0.7494, 0.0016, 0.0007), i.e. almost exactly on the unit-cell
edge parallel to the a axis. Consequently, and with respect to
the [100] direction, a Cl atom is directly below and perfectly
aligned with the methyl groups of the two molecules generated
by an inversion centre and belonging to adjacent layers. On
the contrary, following the example of (I), the centre of gravity
of the TBM molecules is not located on the unit-cell edge
Ê
parallel to the a axis, but at ' 0.4256 A in the (100) plane,
leading to a zigzag antiferroelectric stacking of the molecules
along the latter axis. On the other hand, the minimum inter-
molecular CMeÁ Á ÁCMe contact distance within one layer is
Ê
drastically lower for TCM and TBM (3.887 and 4.168 A,
respectively) than for (I) and increases slightly between layers
Ê
(4.037 and 4.070 A, respectively). We note that in (I), as well
as in TCM, the largest bond lengths, CarÐI(Cl) or CarÐCMe
,
coincide with the largest endocyclic angles. Lastly, it turns out
that at room temperature and for the three isomorphous
compounds under discussion, the two extracyclic angles on
both sides of each substituted group are very similar.
Figure 1
The molecular view of (I) showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level and H
atoms have been omitted for clarity.
ꢀ
Acta Cryst. (2001). C57, 1106±1108
Ali Boudjada et al. C9H9I3 1107

organic compounds
Table 1
Selected geometric parameters (A, ).
Experimental
ꢁ
Ê
Compound (I) was synthesized at 343 K as follows: a solution
containing nitric acid (22 ml) and sulfuric acid (18 ml) diluted in
acetic acid (400 ml) was introduced drop by drop into a balloon ¯ask
containing mesitylene (14 ml), pulverized iodine (36 g) and acetic
acid (200 ml). After 5 h of reaction, periodically monitored by gas
chromatography, the mixture was cooled by adding distilled water.
Compound (I) precipitated with the excess of iodine and the crude
material was vacuum ®ltered under a ¯ux of dimethyl ether in order
to eliminate the residual iodine. A subsequent recrystallization by
sublimation yielded pure colourless crystals of (I) suitable for X-ray
analysis.
I1ÐC1
I3ÐC3
I5ÐC5
C1ÐC2
C1ÐC6
C2ÐC3
C3ÐC4
C4ÐC5
C5ÐC6
2.117 (3)
2.120 (3)
2.115 (3)
1.404 (5)
1.402 (5)
1.400 (5)
1.400 (5)
1.401 (5)
1.410 (5)
C2ÐC21
C4ÐC41
C6ÐC61
I1ÐC21
I1ÐC61
I3ÐC21
I3ÐC41
I5ÐC41
I5ÐC61
1.507 (5)
1.515 (5)
1.496 (5)
3.229 (4)
3.262 (4)
3.270 (4)
3.240 (4)
3.259 (4)
3.240 (4)
I1ÐC1ÐC2
I1ÐC1ÐC6
C2ÐC1ÐC6
C1ÐC2ÐC3
C1ÐC2ÐC21
C3ÐC2ÐC21
I3ÐC3ÐC2
I3ÐC3ÐC4
C2ÐC3ÐC4
117.6 (2)
118.4 (2)
124.1 (3)
116.2 (3)
121.7 (3)
122.2 (3)
118.6 (2)
117.7 (2)
123.7 (3)
C3ÐC4ÐC5
C3ÐC4ÐC41
C5ÐC4ÐC41
I5ÐC5ÐC4
I5ÐC5ÐC6
C4ÐC5ÐC6
C1ÐC6ÐC5
C1ÐC6ÐC61
C5ÐC6ÐC61
116.6 (3)
121.9 (3)
121.5 (3)
118.8 (2)
117.7 (2)
123.5 (3)
115.9 (3)
122.3 (3)
121.8 (3)
Crystal data
C9H9I3
Z = 2
Dx = 2.818 Mg m
Mo Kꢀ radiation
3
Mr = 497.88
Triclinic, P1
a = 8.0486 (1) A
Ê
Ê
Cell parameters from 6527
re¯ections
b = 9.6105 (1) A
c = 9.6204 (1) A
ꢃ = 1.0±40.3ꢁ
ꢄ = 7.94 mm
T = 293 (1) K
Ê
1
ꢀ = 60.1766 (6)ꢁ
ꢁ = 66.7586 (7)ꢁ
ꢂ = 85.3542 (7)ꢁ
Â
We would like to thank the Centre de Diffractometrie de
l'Universite de Rennes 1 for the opportunity to collect data on
Prism, colourless
0.26 Â 0.18 Â 0.16 mm
3
Ê
Â
V = 586.97 (2) A
the Nonius KappaCCD X-ray diffractometer. In particular,
the useful advice of Dr T. Roisnel is gratefully acknowledged.
Data collection
Nonius KappaCCD diffractometer
CCD scans
6822 independent re¯ections
3123 re¯ections with I > 3ꢅ(I)
Rint = 0.069
Absorption correction: multi-scan
from symmetry-related measure-
ments (SORTAV; Blessing, 1995)
Tmin = 0.365, Tmax = 0.577
31 221 measured re¯ections
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: AV1075). Services for accessing these data are
described at the back of the journal.
ꢃ
max = 40.3ꢁ
h = 13 ! 13
k = 17 ! 17
l = 17 ! 17
Re®nement
References
Re®nement on F
R = 0.030
wR = 0.030
Weighting scheme: Chebychev
polynomial with 5 parameters
(Carruthers & Watkin, 1979)
3.59, 1.92, 4.50, 0.767, 1.48
(Á/ꢅ)max = 0.004
Altomare, A., Cascarano, G., Giacovazzo, C., Guagliardi, A., Moliterni,
A. G. G., Burla, M. C., Polidori, G., Camalli, M. & Spagna, R. (1997). SIR97.
University of Bari, Italy.
S = 1.12
3123 re¯ections
140 parameters
All H-atom parameters re®ned
Blessing, R. H. (1995). Acta Cryst. A51, 33±38.
3
Ê
Áꢆmax = 1.29 e A
Carruthers, J. R. & Watkin, D. J. (1979). Acta Cryst. A35, 698±699.
Chaplot, S. L., McIntyre, G. J., Mierzejewski, A. & Pawley, G. S. (1981). Acta
Cryst. B37, 1896±1900.
Domenicano, A., Vaciago, A. & Coulson, C. A. (1975). Acta Cryst. B31, 221±
234.
3
Ê
1.26 e A
Áꢆmin
=
Extinction correction: equation 22
in Larson (1970)
Extinction coef®cient: 77 (3)
Eveno, M. & Meinnel, J. (1966). J. Chim. Phys. 63, 108±113.
Larson, A. C. (1970). Crystallographic Computing, edited by F. R. Ahmed, S. R.
Hall & C. P. Huber, pp. 291±294. Copenhagen: Munksgaard.
The H atoms of the methyl groups were generated geometrically
and their positions were further re®ned along with the other atoms
using geometric (distance, angle and planarity) soft restraints
È
Meinnel, J., Hausler, W., Mani, M., Tazi, M., Nusimovici, M., Sanquer, M.,
Wyncke, B., Heidemann, A., Carlile, C. J., Tomkinson, J. & Hennion, B.
(1992). Physica B, 180/181, 711±713.
Meinnel, J., Mani, M., Cousson, A., Boudjada, F., Paulus, W. & Johnson, M.
(2000). Chem. Phys. 261, 165±187.
Nonius (1999). COLLECT. Nonius BV, Delft, The Netherlands.
Otwinowski, Z. & Minor, W. (1997). Methods Enzymol. 276, 307±326.
Prager, M. & Heidemann, A. (1997). Chem. Rev. 97, 2933±2966.
Schomaker, V. & Trueblood, K. N. (1968). Acta Cryst. B24, 63±76.
Tazi, M., Meinnel, J., Sanquer, M., Nusimovici, M., Tonnard, F. & Carrie, R.
(1995). Acta Cryst. B51, 838±847.
Watkin, D. J., Prout, C. K., Carruthers, J. R. & Betteridge, P. W. (1999).
CRYSTALS. Issue 11. Chemical Crystallography Laboratory, Oxford,
England.
Ê
[CÐH = 1.04 (3)±1.05 (3) A]. The isotropic displacement parameters
of the H atoms of a given methyl group were set to a single least-
squares parameter.
Data collection: COLLECT (Nonius, 1999); cell re®nement:
SCALEPACK (Otwinowski
DENZO (Otwinowski
&
Minor, 1997); data reduction:
Minor, 1997) and SCALEPACK;
&
program(s) used to solve structure: SIR97 (Altomare et al., 1997);
program(s) used to re®ne structure: CRYSTALS (Watkin, Prout,
Carruthers & Betteridge, 1999); molecular graphics: CAMERON
(Watkin, Prout & Pearce, 1999); software used to prepare material for
publication: CRYSTALS.
Watkin, D. J., Prout, C. K. & Pearce, L. J. (1999). CAMERON. Chemical
Crystallography Laboratory, Oxford, England.
ꢀ
1108 Ali Boudjada et al.
C9H9I3
Acta Cryst. (2001). C57, 1106±1108