Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270109032715/fg3100sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270109032715/fg3100Isup2.hkl |
CCDC reference: 755981
Compound (I) was synthesized at 353 K as follows. A mixture of sulfuric acid (20 ml), nitric acid (30 ml) and acetic acid (150 ml) was added dropwise into a balloon flask containing dibromomesitylene (0.07 mol), iodine (0.04 mol) and acetic acid (200 ml). After 1 h of reaction, a powder of dibromoiodomesitylene was obtained. Subsequent recrystallization of a saturated solution in chloroform yielded pure colourless crystals of (I) suitable for X-ray analysis. High-resolution mass spectrometry (electron energy 70 eV, temperature source 418 K) indicated m/z = 401.8114 versus a theoretical value of 401.81157. NMR decoupled spectra for protons were recorded for solutions of (I) and reveal that only one material is present. Solid-state C13 NMR spectra carried out with a rotation at the magic angle (speed 6000 Hz) demonstrate that there is only one component at each molecular site within the entire crystal.
The triclinic unit-cell setting, although not conventional, was chosen for the sake of comparison with the previously published isomorphous compounds TCM, TBM and TIM (Table 1). For the sake of stability, the refinement was undertaken by initially fixing the occupancies of the Br and I atoms to 0.667 and 1/3, respectively. These values are in agreement with the compound stoichiometry and with the above-mentioned dynamic disorder, and they were checked once convergence was reached (see below). Due to the disorder, 54 geometric soft restraints were applied in order to maintain a chemically reasonable molecule, according to the distances and angles calculated for the isolated molecule by means of density functional theory quantum chemistry calculations (MPW1PW91 function, LanL2DZ basis set; Software and reference?): Cr—Br = 1.920 (5), Cr—I = 2.120 (5), Cr—Cr = 1.40 (1) and Cr—CMe = 1.50 (1) Å, Cr—Cr—Br(I) = 117.0 (3), Cr—Cr—Cr = 124.0 (1) or 116.0 (1)° in front of a halogen site or methyl group, respectively, and Cr—Cr—CMe = 122.0 (4)°, with planarity = 0.02 Å for all non-H atoms. The final refinement cycles included the atomic coordinates and the anisotropic displacement parameters for all non-H atoms. Methyl H atoms, not detectable in the difference Fourier map, were generated geometrically, with their positions riding on their parent C atoms and their isotropic displacement parameters set to one least-squares parameter. Starting from the final structure model, refinement of the Br and I occupancies for each halogen site (plus the scale factor) led to the following values: 0.671 (3)/0.3351 (16), 0.663 (3)/0.3309 (17) and 0.666 (3)/0.3327 (16) for sites 1, 3 and 5, respectively, without changing the agreement factors at all (i.e. not significantly different from the reported model with 2/3:1/3 disorder).
Data collection: COLLECT (Nonius, 2001); cell refinement: DIRAX (Duisenberg et al., 2003); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003); molecular graphics: CAMERON (Watkin et al., 1996); software used to prepare material for publication: CRYSTALS (Betteridge et al., 2003).
C9H9Br2I | Z = 2 |
Mr = 403.84 | F(000) = 371.964 |
Triclinic, P1 | Dx = 2.459 Mg m−3 |
a = 7.895 (1) Å | Mo Kα radiation, λ = 0.71073 Å |
b = 9.3180 (9) Å | Cell parameters from 10298 reflections |
c = 9.3130 (11) Å | θ = 2.9–30.0° |
α = 60.187 (8)° | µ = 10.21 mm−1 |
β = 72.998 (9)° | T = 293 K |
γ = 95.10 (1)° | Plate, colourless |
V = 545.33 (13) Å3 | 0.29 × 0.28 × 0.18 mm |
Nonius KappaCCD area-detector diffractometer | 2995 independent reflections |
Radiation source: X-ray tube | 1629 reflections with I > 3σ(I) |
Horizontally mounted graphite crystal monochromator | Rint = 0.045 |
ω/2θ scans | θmax = 30.0°, θmin = 3.6° |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | h = −10→9 |
Tmin = 0.06, Tmax = 0.16 | k = −11→13 |
5479 measured reflections | l = −13→13 |
Refinement on F | H-atom parameters constrained |
Least-squares matrix: full | Method, part 1, Chebychev polynomial (Watkin, 1994, Prince, 1982)
[weight] = 1.0/[A0*T0(x) + A1*T1(x) ··· + An-1]*Tn-1(x)],
where Ai are the Chebychev coefficients listed below and x = F /Fmax. Method = Robust Weighting (Prince, 1982) W = [weight] * [1-(deltaF/6*sigmaF)2]2 Ai are: 1.38 0.469 1.07 |
R[F2 > 2σ(F2)] = 0.040 | (Δ/σ)max = 0.000206 |
wR(F2) = 0.044 | Δρmax = 1.21 e Å−3 |
S = 1.18 | Δρmin = −1.01 e Å−3 |
1629 reflections | Extinction correction: Larson (1970), Eq. 22 |
138 parameters | Extinction coefficient: 12 (3) |
54 restraints |
C9H9Br2I | γ = 95.10 (1)° |
Mr = 403.84 | V = 545.33 (13) Å3 |
Triclinic, P1 | Z = 2 |
a = 7.895 (1) Å | Mo Kα radiation |
b = 9.3180 (9) Å | µ = 10.21 mm−1 |
c = 9.3130 (11) Å | T = 293 K |
α = 60.187 (8)° | 0.29 × 0.28 × 0.18 mm |
β = 72.998 (9)° |
Nonius KappaCCD area-detector diffractometer | 2995 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | 1629 reflections with I > 3σ(I) |
Tmin = 0.06, Tmax = 0.16 | Rint = 0.045 |
5479 measured reflections |
R[F2 > 2σ(F2)] = 0.040 | 54 restraints |
wR(F2) = 0.044 | H-atom parameters constrained |
S = 1.18 | Δρmax = 1.21 e Å−3 |
1629 reflections | Δρmin = −1.01 e Å−3 |
138 parameters |
Experimental. The structure was successfully solved using direct methods (SIR97 program, Altomare et al., 1999) anticipating the rotational 120°-jumps of the whole molecule within its plane. A pseudo-atom of niobium (41 electrons) was used for mimicing each disordered halogen site constituted by 67% of bromine and 33% of iodine. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Br1 | 0.7753 (12) | 0.4473 (5) | −0.1329 (7) | 0.0590 | 0.6670 |
I1 | 0.7722 (13) | 0.4701 (6) | −0.1390 (7) | 0.0466 | 0.3330 |
Br3 | 0.7668 (15) | −0.0906 (7) | −0.2533 (7) | 0.0565 | 0.6670 |
I3 | 0.7631 (19) | −0.1002 (8) | −0.2681 (9) | 0.0511 | 0.3330 |
Br5 | 0.7121 (13) | −0.2268 (6) | 0.4226 (6) | 0.0564 | 0.6670 |
I5 | 0.7119 (15) | −0.2428 (6) | 0.4473 (7) | 0.0454 | 0.3330 |
C12 | 0.7765 (11) | 0.2802 (8) | −0.3524 (8) | 0.0559 | |
C14 | 0.7322 (12) | −0.3195 (8) | 0.1431 (9) | 0.0599 | |
C16 | 0.7395 (12) | 0.1641 (9) | 0.2504 (9) | 0.0616 | |
C1 | 0.7589 (8) | 0.2097 (4) | −0.0471 (5) | 0.0392 | |
C2 | 0.7632 (8) | 0.1578 (5) | −0.1642 (4) | 0.0399 | |
C3 | 0.7569 (8) | −0.0131 (5) | −0.0980 (5) | 0.0407 | |
C4 | 0.7405 (8) | −0.1324 (4) | 0.0760 (5) | 0.0388 | |
C5 | 0.7371 (8) | −0.0693 (4) | 0.1823 (4) | 0.0382 | |
C6 | 0.7465 (9) | 0.1014 (5) | 0.1267 (5) | 0.0394 | |
H121 | 0.7789 | 0.3967 | −0.3735 | 0.21 (3)* | |
H122 | 0.8923 | 0.2885 | −0.4404 | 0.21 (3)* | |
H123 | 0.6673 | 0.2369 | −0.3683 | 0.21 (3)* | |
H141 | 0.7342 | −0.3394 | 0.0466 | 0.21 (3)* | |
H142 | 0.8417 | −0.3448 | 0.1741 | 0.21 (3)* | |
H143 | 0.6164 | −0.3971 | 0.2522 | 0.21 (3)* | |
H161 | 0.7304 | 0.0664 | 0.3693 | 0.21 (3)* | |
H162 | 0.8543 | 0.2588 | 0.1956 | 0.21 (3)* | |
H163 | 0.6293 | 0.2083 | 0.2677 | 0.21 (3)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Br1 | 0.094 (2) | 0.0204 (9) | 0.0652 (15) | 0.0239 (9) | −0.0396 (14) | −0.0192 (8) |
I1 | 0.072 (2) | 0.0275 (15) | 0.0497 (17) | 0.0201 (16) | −0.0293 (15) | −0.0239 (14) |
Br3 | 0.0792 (19) | 0.0605 (14) | 0.0499 (14) | 0.0163 (12) | −0.0248 (13) | −0.0445 (12) |
I3 | 0.074 (2) | 0.0557 (17) | 0.0447 (16) | 0.0202 (16) | −0.0276 (15) | −0.0393 (13) |
Br5 | 0.0742 (17) | 0.0561 (16) | 0.0270 (11) | 0.0144 (14) | −0.0272 (12) | −0.0116 (10) |
I5 | 0.070 (2) | 0.0328 (11) | 0.0270 (14) | 0.0140 (11) | −0.0253 (15) | −0.0085 (10) |
C12 | 0.070 (5) | 0.050 (4) | 0.042 (4) | 0.019 (4) | −0.025 (4) | −0.019 (3) |
C14 | 0.079 (6) | 0.037 (4) | 0.060 (5) | 0.013 (4) | −0.028 (4) | −0.025 (3) |
C16 | 0.082 (6) | 0.067 (5) | 0.052 (4) | 0.027 (4) | −0.031 (4) | −0.040 (4) |
C1 | 0.038 (3) | 0.037 (3) | 0.045 (3) | 0.006 (3) | −0.017 (3) | −0.024 (3) |
C2 | 0.031 (3) | 0.041 (3) | 0.038 (3) | 0.009 (3) | −0.012 (3) | −0.016 (3) |
C3 | 0.037 (3) | 0.044 (3) | 0.039 (3) | 0.007 (3) | −0.010 (3) | −0.025 (3) |
C4 | 0.033 (3) | 0.035 (3) | 0.040 (3) | 0.008 (3) | −0.010 (2) | −0.018 (3) |
C5 | 0.038 (3) | 0.041 (3) | 0.035 (3) | 0.008 (3) | −0.013 (3) | −0.022 (3) |
C6 | 0.041 (3) | 0.042 (3) | 0.039 (3) | 0.013 (3) | −0.017 (3) | −0.023 (3) |
Br1—C1 | 1.920 (4) | C3—C4 | 1.392 (5) |
I1—C1 | 2.114 (5) | C4—C5 | 1.376 (5) |
Br3—C3 | 1.898 (4) | C5—C6 | 1.393 (5) |
I3—C3 | 2.100 (5) | C12—H121 | 1.000 |
Br5—C5 | 1.904 (4) | C12—H122 | 1.000 |
I5—C5 | 2.105 (5) | C12—H123 | 1.000 |
C12—C2 | 1.505 (6) | C14—H141 | 1.000 |
C14—C4 | 1.519 (6) | C14—H142 | 1.000 |
C16—C6 | 1.520 (7) | C14—H143 | 1.000 |
C1—C2 | 1.389 (5) | C16—H161 | 1.000 |
C1—C6 | 1.384 (5) | C16—H162 | 1.000 |
C2—C3 | 1.382 (5) | C16—H163 | 1.000 |
Br1—C1—C2 | 118.04 (19) | C16—C6—C5 | 122.1 (2) |
I1—C1—C2 | 118.28 (18) | C16—C6—C1 | 122.0 (2) |
Br1—C1—C6 | 117.90 (19) | C5—C6—C1 | 115.87 (9) |
I1—C1—C6 | 117.67 (18) | C2—C12—H121 | 109.3 |
C2—C1—C6 | 124.05 (9) | C2—C12—H122 | 109.5 |
C12—C2—C1 | 122.3 (2) | H121—C12—H122 | 109.5 |
C12—C2—C3 | 121.9 (2) | C2—C12—H123 | 109.6 |
C1—C2—C3 | 115.82 (9) | H121—C12—H123 | 109.5 |
Br3—C3—C2 | 117.8 (2) | H122—C12—H123 | 109.5 |
C2—C3—I3 | 117.8 (2) | C4—C14—H141 | 109.4 |
Br3—C3—C4 | 118.0 (2) | C4—C14—H142 | 109.1 |
C2—C3—C4 | 124.19 (9) | H141—C14—H142 | 109.5 |
I3—C3—C4 | 118.0 (2) | C4—C14—H143 | 109.9 |
C14—C4—C3 | 122.3 (2) | H141—C14—H143 | 109.5 |
C14—C4—C5 | 121.9 (2) | H142—C14—H143 | 109.5 |
C3—C4—C5 | 115.85 (9) | C6—C16—H161 | 109.4 |
Br5—C5—C4 | 117.82 (18) | C6—C16—H162 | 109.4 |
I5—C5—C4 | 117.87 (18) | H161—C16—H162 | 109.5 |
Br5—C5—C6 | 117.99 (19) | C6—C16—H163 | 109.6 |
I5—C5—C6 | 117.94 (18) | H161—C16—H163 | 109.5 |
C4—C5—C6 | 124.17 (9) | H162—C16—H163 | 109.5 |
Experimental details
Crystal data | |
Chemical formula | C9H9Br2I |
Mr | 403.84 |
Crystal system, space group | Triclinic, P1 |
Temperature (K) | 293 |
a, b, c (Å) | 7.895 (1), 9.3180 (9), 9.3130 (11) |
α, β, γ (°) | 60.187 (8), 72.998 (9), 95.10 (1) |
V (Å3) | 545.33 (13) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 10.21 |
Crystal size (mm) | 0.29 × 0.28 × 0.18 |
Data collection | |
Diffractometer | Nonius KappaCCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.06, 0.16 |
No. of measured, independent and observed [I > 3σ(I)] reflections | 5479, 2995, 1629 |
Rint | 0.045 |
(sin θ/λ)max (Å−1) | 0.703 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.040, 0.044, 1.18 |
No. of reflections | 1629 |
No. of parameters | 138 |
No. of restraints | 54 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 1.21, −1.01 |
Computer programs: COLLECT (Nonius, 2001), DIRAX (Duisenberg et al., 2003), EVALCCD (Duisenberg et al., 2003), SIR97 (Altomare et al., 1999), CRYSTALS (Betteridge et al., 2003), CAMERON (Watkin et al., 1996).
Formula | C9Cl3H9 | C9Br3H9 | C9I3H9 |
Chemical name | 1,3,5-Trichloro-2,4,6-trimethylbenzene | 1,3,5-Tribromo-2,4,6-trimethylbenzene | 1,3,5-Triiodo-2,4,6-trimethylbenzene |
Common name | Trichloromesitylene | Tribromomesitylene | Triiodomesitylene |
Usual shortened name | TCM | TBM | TIM |
Reference | Tazi et al. (1995) | Meinnel et al. (2000) | Boudjada et al. (2001) |
IUCr CIF code | pa0298b | av1075 | |
T (K) | 297 | 295 | 293 |
Space group | P1 | P1 | P1 |
Z | 2 | 2 | 2 |
a (Å) | 7.738 (6) | 7.808 (4) | 8.0486 (1) |
b (Å) | 8.842 (4) | 9.123 (4) | 9.6105 (1) |
c (Å) | 8.880 (3) | 9.143 (4) | 9.6204 (1) |
α (°) | 59.74 (3) | 59.77 (3) | 60.1766 (6) |
β (°) | 66.51 (5) | 67.97 (4) | 66.7586 (7) |
γ (°) | 73.06 (4) | 73.18 (4) | 85.3542 (7) |
V (Å3) | 478.1 | 517.5 | 586.97 (2) |
Most benzene compounds hexasubstituted by halogens, methyl groups or other small radicals crystallize at room temperature in the monoclinic space group P21/n, Z = 2, according to the hexachlorobenzene structure type (Brown & Strydom, 1974; Reddy et al., 2006). Partial compilations of previous contributions can be found in Kitaigorodsky (1973), Tazi et al. (1995), Brock & Fu (1997) and Reddy et al. (2006). The packing of these compounds is dominated by intermolecular π–π interactions and the molecules are arranged in stacks (at distances typically ranging from 3.8 to 4.3 Å) parallel to the monoclinic unique axis. In addition, they form corrugated (110) molecular planes, with the halogen–halogen interactions within these planes turning out to be weaker than the π–π inter-stack forces. Another crystal packing is observed, too, at room temperature, especially if the molecules display a threefold axis of symmetry: they are stacked into a triclinic unit cell, space group P1, Z = 2, deriving from the hexamethylbenzene structure type (Hamilton et al., 1969; Le Maguères et al., 2001). This is the case for the trihalogenomesitylene series (1,3,5-trihalogeno-2,4,6-trimethylbenzene; Table 1), of which only trichloromesitylene (TCM) (Tazi et al., 1995; Hernandez et al., 2006), tribromomesitylene (TBM) (Meinnel et al., 2000; Bosch & Barnes, 2002) and triiodomesitylene (TIM) (Boudjada et al., 2001, 2002; Bosch & Barnes, 2002) have been characterized so far. The methyl groups in these compounds experience relatively small hindering potentials and interesting tunnelling properties have been reported (Meinnel et al., 1992, 1995, 2000; Boudjada et al., 2002). What happens if one of the halogen atoms is replaced by another atom, breaking the idealized D3h molecular symmetry of these molecules? If it is an H atom, as encountered for dibromomesitylene (DBM) (Hernandez et al., 2003), the ordered low-temperature phase (the phase is disordered above ~297 K) is monoclinic, space group P21/n, Z = 4. Unexpectedly, however, the methyl group located between the two Br atoms is a quasi-free rotor with H atoms highly delocalized in a sixfold potential, whereas the rotation of the other two methyl groups is quasi-forbidden (Meinnel et al., 1995; Plazanet et al., 2002). In order to establish for the trihalogenomesitylene series the impact of small changes on the molecular symmetry, we report here the crystal structure of 1,3-dibromo-5-iodo-2,4,6-trimethylbenzene, (I) (dibromoiodomesitylene, DBIM), obtained by single-crystal X-ray diffraction at 293 K. This compound can indeed be viewed as a TBM with one Br atom substituted by I, or alternatively as a TIM with two I atoms substituted by Br, the idealized symmetry of the isolated molecule (ignoring H atoms) decreasing in both cases from D3h (-6m2) to C2v (mm2).
The best refinement with 54 geometric soft restraints (see Refinement) led to the molecular structure shown in Fig. 1. The crystallographically inequivalent halogen sites are identically constituted by 67% Br and 33% I (see Refinement), the other atoms apparently not being affected by any kind of disorder. NMR measurements at room temperature showed a long time ago that in this family of compounds - at least for the hydrogenated materials - there is evidence for a dynamic reorientation of the molecules by jumps of 2mπ/6 within their plane [Please define m], at a frequency in the MHz range (Eveno & Meinnel, 1966). The disorder in (I) therefore corresponds to fast 2π/3 stochastic in-plane reorientations of the whole molecule between three discernable locations, inducing the apparent overlap of Br and I atoms, according to a 2/3:1/3 ratio. This novel feature has been characterized by diffractometry thanks to the idealized (instantaneous) C2v molecular symmetry of the isolated molecule (ignoring H atoms), which is time-averaged towards an apparent idealized D3h symmetry by means of the diffraction probe. The same kind of rotational disorder, although suspected at room temperature in TCM (Tazi et al., 1995; Hernandez et al., 2006), TBM and TIM, has not been directly proved so far, due to the ternary symmetry of these latter molecules.
In (I), significant deformation of the angles of the benzene ring from a regular D6h (6/mmm) hexagon is observed. The average endocyclic angle is 124.14 (6)° around the C atoms linked to Br/I sites, and an average value of 115.85 (2)° is observed for the C atoms bonded to the methyl groups, making the aromatic ring a distorted hexagon fulfilling approximately the D3 h symmetry. This deformation is close to that found in TCM, TBM and TIM compounds at the same temperature and is in agreement with systematic trends establishing that the endocyclic angle facing the C atom bearing the most electronegative substituent is enlarged (Domenicano et al., 1975). After the restrained refinement, the mean values for the Car—Br and Car—I distances are 1.907 (9) and 2.106 (6) Å, respectively.
The crystal packing of (I) can be described as a stacking of (100) molecular layers at x/a ~1/4 and 3/4, forming zigzag molecular columns propagating along the a axis (Figs. 2 and 3). The angle between the normal to the aromatic plane and the normal to the (100) plane is 4.1°. Within each column, the arrangement is approximately `antiferroelectric': a given molecule is sandwiched between two molecules at ~4.06 Å generated by inversion centres and belonging to adjacent layers. A Br/I site is more or less directly below the methyl groups and vice versa. The symmetry centres are located on the a axis, but the centres of mass of the molecules are shifted away from the latter axis by 0.46 Å (Fig. 3), which explains the slightly zigzag shape of the molecular rows. Within each (100) layer, one probe molecule is surrounded by six neighbours in such a way that characteristic triangular halogen···halogen and Me···Me intermolecular contacts are formed, the three corresponding contact distances being very similar (e.g. mean value of 3.61 Å for I···I). For molecules in different layers, the shortest intermolecular contact distances, compared with those between molecules within the same layer, are significantly shortened for CMe···CMe contacts, almost unchanged for CMe···halogen contacts and significantly increased for halogen···halogen contacts. The same structural topology is encountered at room temperature in TBM (Meinnel et al., 2000) and TIM (Boudjada et al., 2001), but also in TCM below 160 K (lowest temperature phase, or phase IV; Hernandez et al., 2006).
The C atoms of the ring display Ueq displacement parameters significantly lower (mean 0.039 Å2) than those of the substituted atoms (mean values of 0.057, 0.048 and 0.059 Å2 for Br, I and CMe, respectively). In order to clarify this point, we have performed a conventional TLS analysis using the CRYSTALS program (Betteridge et al., 2003). The overall rigid-body motion tensors T, L and S (Schomaker & Trueblood, 1968) were least-squares fitted to the individual anisotropic displacement parameters. At first, we included only the C atoms in the rigid-body treatment, and the reliability factor for the U values was R = 0.054, revealing the rigid character of the body constituted by the aromatic ring and the methyl groups. Clearly, the translation and screw tensors are negligible, the thermal motion of the aforementioned rigid body being essentially of librational origin. The eigenvalues of the libration tensor are L11 = 9°2 (in-plane libration), and L22 = 19°2 and L33 = 22°2 (both corresponding to off-plane librations). In the second step of the TLS analysis, we added the Br and I atoms, i.e. we took into account all the non-H atoms. The reliability factor for the U values increased substantially to R = 0.131, indicating that, relative to the aromatic ring and methyl groups, the halogen substituents do not behave rigidly.