Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270111037930/fa3260sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270111037930/fa3260Iasup2.hkl | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270111037930/fa3260Ibsup3.hkl | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270111037930/fa3260Iasup4.cml | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270111037930/fa3260Ibsup5.cml |
CCDC references: 851746; 851747
1,4-Bis(dibromomethylene)cyclohexane (0.50 g, 1.179 mmol) (Neidlein & 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; 0.01 g; for each 0.1 mol NBS, 0.2 g AIBN is needed) were added to the 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). Spectroscopic analysis: 1H NMR (400 MHz, CDCl3, δ, p.p.m.): 8.0 (s, 4H, arom.); 13C NMR (100 MHz, CDCl3, δ, p.p.m.): 33.7 (s, CBr3), 126.3 (d, arom. CH), 148.0 (s, arom. C); IR (film, ν, cm-1): 3087 (w), 2923 (w), 2778 (w), 1490 (w), 1398 (m), 1181 (m), 1013 (w), 841 (w), 809 (s), 715 (vs), 682 (m), 647 (vs, br); EI–MS (m/z, relative intensity, %): 579.4 (2) [M81Br379Br3]+, 498.5 (14) [M81Br279Br3]+, 419.6 (100) [M81Br279Br2]+, 338.7 (10) [M81Br79Br2]+, 259.8 (65) [M81Br79Br]+, 178.9 (22), [M79Br]+, 100.0 (32), 74.0 (32), 50.0 (26); elemental analysis, calculated for C8H4Br6: 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).
H atoms were introduced at the calculated positions and refined using a riding model, with C—H = 0.95 Å, and with Uiso(H) = 1.2Ueq(C).
For both compounds, data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO (Oxford Diffraction, 2009); data reduction: CrysAlis PRO (Oxford Diffraction, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP (Siemens, 1994); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).
C8H4Br6 | F(000) = 524 |
Mr = 579.57 | Dx = 3.165 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
a = 6.3150 (3) Å | Cell parameters from 7251 reflections |
b = 9.8178 (4) Å | θ = 2.9–30.7° |
c = 9.8398 (4) Å | µ = 19.76 mm−1 |
β = 94.486 (4)° | T = 100 K |
V = 608.19 (5) Å3 | Prism, colourless |
Z = 2 | 0.08 × 0.06 × 0.03 mm |
Oxford Xcalibur Eos diffractometer | 1795 independent reflections |
Radiation source: Enhance (Mo) X-ray Source | 1449 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.033 |
Detector resolution: 16.1419 pixels mm-1 | θmax = 30.8°, θmin = 2.9° |
ω scans | h = −8→8 |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2009) | k = −14→13 |
Tmin = 0.554, Tmax = 1.000 | l = −13→13 |
15682 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.015 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.026 | H-atom parameters constrained |
S = 0.86 | w = 1/[σ2(Fo2) + (0.0111P)2] where P = (Fo2 + 2Fc2)/3 |
1795 reflections | (Δ/σ)max = 0.001 |
64 parameters | Δρmax = 0.53 e Å−3 |
0 restraints | Δρmin = −0.53 e Å−3 |
C8H4Br6 | V = 608.19 (5) Å3 |
Mr = 579.57 | Z = 2 |
Monoclinic, P21/n | Mo Kα radiation |
a = 6.3150 (3) Å | µ = 19.76 mm−1 |
b = 9.8178 (4) Å | T = 100 K |
c = 9.8398 (4) Å | 0.08 × 0.06 × 0.03 mm |
β = 94.486 (4)° |
Oxford Xcalibur Eos diffractometer | 1795 independent reflections |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2009) | 1449 reflections with I > 2σ(I) |
Tmin = 0.554, Tmax = 1.000 | Rint = 0.033 |
15682 measured reflections |
R[F2 > 2σ(F2)] = 0.015 | 0 restraints |
wR(F2) = 0.026 | H-atom parameters constrained |
S = 0.86 | Δρmax = 0.53 e Å−3 |
1795 reflections | Δρmin = −0.53 e Å−3 |
64 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
Br1 | 0.48497 (3) | 0.14673 (2) | 0.38185 (2) | 0.01413 (5) | |
Br2 | 0.45320 (3) | 0.37459 (2) | 0.15381 (2) | 0.01700 (6) | |
Br3 | 0.89449 (3) | 0.24735 (2) | 0.25034 (2) | 0.01286 (5) | |
C1 | 0.5516 (3) | 0.09760 (18) | 0.10138 (19) | 0.0067 (4) | |
C2 | 0.7060 (3) | 0.04754 (19) | 0.0232 (2) | 0.0095 (4) | |
H2 | 0.8478 | 0.0797 | 0.0381 | 0.011* | |
C3 | 0.3437 (3) | 0.04956 (19) | 0.0774 (2) | 0.0102 (4) | |
H3 | 0.2354 | 0.0835 | 0.1301 | 0.012* | |
C4 | 0.5959 (3) | 0.20434 (19) | 0.2105 (2) | 0.0092 (4) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Br1 | 0.01701 (11) | 0.01814 (12) | 0.00751 (10) | −0.00361 (9) | 0.00278 (8) | −0.00222 (8) |
Br2 | 0.02362 (12) | 0.00858 (10) | 0.01762 (11) | 0.00541 (9) | −0.00595 (9) | −0.00269 (8) |
Br3 | 0.01056 (10) | 0.01386 (11) | 0.01406 (11) | −0.00266 (8) | 0.00036 (8) | −0.00617 (8) |
C1 | 0.0100 (10) | 0.0040 (9) | 0.0060 (10) | 0.0008 (7) | 0.0000 (8) | 0.0006 (7) |
C2 | 0.0073 (9) | 0.0109 (10) | 0.0103 (10) | −0.0018 (8) | 0.0003 (8) | −0.0001 (8) |
C3 | 0.0127 (10) | 0.0097 (10) | 0.0088 (10) | 0.0022 (8) | 0.0035 (8) | −0.0009 (8) |
C4 | 0.0087 (10) | 0.0088 (10) | 0.0100 (10) | −0.0001 (8) | 0.0004 (8) | 0.0002 (8) |
Br1—C4 | 1.959 (2) | C1—C4 | 1.511 (3) |
Br2—C4 | 1.9590 (19) | C2—C3i | 1.392 (3) |
Br3—C4 | 1.9424 (19) | C3—C2i | 1.392 (3) |
C1—C2 | 1.379 (3) | C2—H2 | 0.9500 |
C1—C3 | 1.398 (3) | C3—H3 | 0.9500 |
C2—C1—C3 | 118.76 (17) | C1—C4—Br2 | 109.51 (13) |
C2—C1—C4 | 122.99 (17) | Br3—C4—Br2 | 106.83 (9) |
C3—C1—C4 | 118.24 (17) | Br1—C4—Br2 | 107.70 (9) |
C1—C2—C3i | 120.73 (17) | C1—C2—H2 | 119.6 |
C2i—C3—C1 | 120.51 (18) | C3i—C2—H2 | 119.6 |
C1—C4—Br3 | 114.63 (13) | C2i—C3—H3 | 119.7 |
C1—C4—Br1 | 110.66 (12) | C1—C3—H3 | 119.7 |
Br3—C4—Br1 | 107.21 (9) | ||
C3—C1—C2—C3i | −0.4 (3) | C3—C1—C4—Br3 | 172.88 (14) |
C4—C1—C2—C3i | −179.16 (18) | C2—C1—C4—Br1 | −129.69 (17) |
C2—C1—C3—C2i | 0.4 (3) | C3—C1—C4—Br1 | 51.5 (2) |
C4—C1—C3—C2i | 179.22 (17) | C2—C1—C4—Br2 | 111.73 (18) |
C2—C1—C4—Br3 | −8.3 (2) | C3—C1—C4—Br2 | −67.09 (19) |
Symmetry code: (i) −x+1, −y, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
C3—H3···Br3ii | 0.95 | 3.00 | 3.933 (2) | 166 |
Symmetry code: (ii) x−1, y, z. |
C8H4Br6 | F(000) = 1048 |
Mr = 579.57 | Dx = 3.080 Mg m−3 |
Monoclinic, C2/c | Cu Kα radiation, λ = 1.54184 Å |
a = 10.1640 (9) Å | Cell parameters from 5599 reflections |
b = 6.5048 (7) Å | θ = 4.7–75.7° |
c = 19.3410 (17) Å | µ = 22.89 mm−1 |
β = 102.152 (9)° | T = 103 K |
V = 1250.1 (2) Å3 | Hexagonal plate, colourless |
Z = 4 | 0.04 × 0.04 × 0.02 mm |
Oxford Xcalibur Nova Atlas diffractometer | 1271 independent reflections |
Radiation source: Nova (Cu) X-ray Source | 1163 reflections with I > 2σ(I) |
Mirror monochromator | Rint = 0.076 |
Detector resolution: 10.3543 pixels mm-1 | θmax = 75.9°, θmin = 4.7° |
ω scans | h = −12→12 |
Absorption correction: multi-scan CrysAlis PRO (Oxford Diffraction, 2009) | k = −7→8 |
Tmin = 0.439, Tmax = 1.000 | l = −24→24 |
7801 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.037 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.103 | H-atom parameters constrained |
S = 1.05 | w = 1/[σ2(Fo2) + (0.0769P)2 + 1.4614P] where P = (Fo2 + 2Fc2)/3 |
1271 reflections | (Δ/σ)max = 0.001 |
64 parameters | Δρmax = 1.02 e Å−3 |
0 restraints | Δρmin = −0.93 e Å−3 |
C8H4Br6 | V = 1250.1 (2) Å3 |
Mr = 579.57 | Z = 4 |
Monoclinic, C2/c | Cu Kα radiation |
a = 10.1640 (9) Å | µ = 22.89 mm−1 |
b = 6.5048 (7) Å | T = 103 K |
c = 19.3410 (17) Å | 0.04 × 0.04 × 0.02 mm |
β = 102.152 (9)° |
Oxford Xcalibur Nova Atlas diffractometer | 1271 independent reflections |
Absorption correction: multi-scan CrysAlis PRO (Oxford Diffraction, 2009) | 1163 reflections with I > 2σ(I) |
Tmin = 0.439, Tmax = 1.000 | Rint = 0.076 |
7801 measured reflections |
R[F2 > 2σ(F2)] = 0.037 | 0 restraints |
wR(F2) = 0.103 | H-atom parameters constrained |
S = 1.05 | Δρmax = 1.02 e Å−3 |
1271 reflections | Δρmin = −0.93 e Å−3 |
64 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
Br1 | 0.23064 (4) | 0.88693 (8) | 0.39391 (2) | 0.0225 (2) | |
Br2 | 0.33314 (5) | 0.57825 (9) | 0.28981 (2) | 0.0243 (2) | |
Br3 | 0.51252 (4) | 0.95164 (9) | 0.35332 (2) | 0.0236 (2) | |
C1 | 0.4507 (4) | 0.6150 (8) | 0.4383 (2) | 0.0187 (9) | |
C2 | 0.5490 (4) | 0.6959 (8) | 0.4929 (2) | 0.0206 (9) | |
H2 | 0.5824 | 0.8307 | 0.4887 | 0.025* | |
C3 | 0.4012 (5) | 0.4194 (8) | 0.4462 (3) | 0.0222 (10) | |
H3 | 0.3329 | 0.3634 | 0.4099 | 0.027* | |
C4 | 0.3895 (4) | 0.7433 (7) | 0.3746 (2) | 0.0174 (8) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Br1 | 0.0120 (3) | 0.0271 (3) | 0.0290 (3) | 0.00241 (17) | 0.0058 (2) | 0.00151 (19) |
Br2 | 0.0223 (3) | 0.0296 (3) | 0.0202 (3) | −0.00253 (19) | 0.0024 (2) | −0.00249 (18) |
Br3 | 0.0143 (3) | 0.0309 (4) | 0.0259 (3) | −0.00473 (18) | 0.0046 (2) | 0.00412 (19) |
C1 | 0.0088 (18) | 0.024 (3) | 0.024 (2) | 0.0000 (17) | 0.0050 (16) | −0.0023 (18) |
C2 | 0.0134 (19) | 0.023 (3) | 0.025 (2) | −0.0056 (17) | 0.0039 (15) | 0.0001 (18) |
C3 | 0.015 (2) | 0.026 (3) | 0.025 (2) | −0.0015 (18) | 0.0027 (16) | −0.0033 (19) |
C4 | 0.0104 (18) | 0.021 (2) | 0.0217 (19) | 0.0002 (15) | 0.0041 (14) | −0.0007 (17) |
Br1—C4 | 1.968 (4) | C1—C4 | 1.510 (6) |
Br2—C4 | 1.943 (4) | C3—C2i | 1.398 (7) |
Br3—C4 | 1.946 (4) | C2—C3i | 1.398 (7) |
C1—C3 | 1.389 (7) | C2—H2 | 0.9500 |
C1—C2 | 1.394 (6) | C3—H3 | 0.9500 |
C3—C1—C2 | 118.7 (5) | C1—C4—Br1 | 108.7 (3) |
C3—C1—C4 | 120.0 (4) | Br2—C4—Br1 | 108.3 (2) |
C2—C1—C4 | 121.0 (4) | Br3—C4—Br1 | 107.4 (2) |
C1—C2—C3i | 120.6 (5) | C1—C2—H2 | 119.7 |
C1—C3—C2i | 120.6 (5) | C3i—C2—H2 | 119.7 |
C1—C4—Br2 | 112.4 (3) | C1—C3—H3 | 119.7 |
C1—C4—Br3 | 112.6 (3) | C2i—C3—H3 | 119.7 |
Br2—C4—Br3 | 107.14 (18) | ||
C3—C1—C2—C3i | 1.4 (7) | C2—C1—C4—Br2 | 150.3 (4) |
C4—C1—C2—C3i | 175.8 (4) | C3—C1—C4—Br3 | −156.5 (4) |
C2—C1—C3—C2i | −1.4 (8) | C2—C1—C4—Br3 | 29.1 (5) |
C4—C1—C3—C2i | −175.9 (4) | C3—C1—C4—Br1 | 84.6 (5) |
C3—C1—C4—Br2 | −35.3 (5) | C2—C1—C4—Br1 | −89.8 (4) |
Symmetry code: (i) −x+1, −y+1, −z+1. |
Experimental details
(Ia) | (Ib) | |
Crystal data | ||
Chemical formula | C8H4Br6 | C8H4Br6 |
Mr | 579.57 | 579.57 |
Crystal system, space group | Monoclinic, P21/n | Monoclinic, C2/c |
Temperature (K) | 100 | 103 |
a, b, c (Å) | 6.3150 (3), 9.8178 (4), 9.8398 (4) | 10.1640 (9), 6.5048 (7), 19.3410 (17) |
β (°) | 94.486 (4) | 102.152 (9) |
V (Å3) | 608.19 (5) | 1250.1 (2) |
Z | 2 | 4 |
Radiation type | Mo Kα | Cu Kα |
µ (mm−1) | 19.76 | 22.89 |
Crystal size (mm) | 0.08 × 0.06 × 0.03 | 0.04 × 0.04 × 0.02 |
Data collection | ||
Diffractometer | Oxford Xcalibur Eos diffractometer | Oxford Xcalibur Nova Atlas diffractometer |
Absorption correction | Multi-scan (CrysAlis PRO; Oxford Diffraction, 2009) | Multi-scan CrysAlis PRO (Oxford Diffraction, 2009) |
Tmin, Tmax | 0.554, 1.000 | 0.439, 1.000 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 15682, 1795, 1449 | 7801, 1271, 1163 |
Rint | 0.033 | 0.076 |
(sin θ/λ)max (Å−1) | 0.720 | 0.629 |
Refinement | ||
R[F2 > 2σ(F2)], wR(F2), S | 0.015, 0.026, 0.86 | 0.037, 0.103, 1.05 |
No. of reflections | 1795 | 1271 |
No. of parameters | 64 | 64 |
H-atom treatment | H-atom parameters constrained | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.53, −0.53 | 1.02, −0.93 |
Computer programs: CrysAlis PRO (Oxford Diffraction, 2009), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), XP (Siemens, 1994).
C2—C1—C4—Br3 | −8.3 (2) | C3—C1—C4—Br1 | 51.5 (2) |
C3—C1—C4—Br3 | 172.88 (14) | C2—C1—C4—Br2 | 111.73 (18) |
C2—C1—C4—Br1 | −129.69 (17) | C3—C1—C4—Br2 | −67.09 (19) |
C3—C1—C4—Br2 | −35.3 (5) | C2—C1—C4—Br3 | 29.1 (5) |
C2—C1—C4—Br2 | 150.3 (4) | C3—C1—C4—Br1 | 84.6 (5) |
C3—C1—C4—Br3 | −156.5 (4) | C2—C1—C4—Br1 | −89.8 (4) |
C—Br···Br—C system | Br···Br (Å) | C—Br···Br angles (°) | Operator | |
1 | C4—Br1···Br1—C4 | 3.6979 (4) | 139.63 (6), 139.63 (6) | -x+1, -y, -z+1 |
2 | C4—Br1···Br2—C4 | 3.8427 (3) | 115.18 (6), 158.59 (6) | -x+1/2, y-1/2, -z+1/2 |
3 | C4—Br1···Br2—C4 | 3.8354 (3) | 107.96 (6), 117.67 (6) | x+1/2, -y+1/2, z+1/2 |
4 | C4—Br1···Br3—C4 | 3.9750 (3) | 92.15 (6), 152.00 (6) | x-1, y, z |
5 | C4—Br1···Br3—C4 | 3.8573 (3) | 145.15 (6), 94.69 (6) | x-1/2, -y+1/2, z+1/2 |
6 | C4—Br2···Br2—C4 | 3.9819 (5) | 130.68 (6), 130.68 (6) | -x+1, -y+1, -z |
7 | C4—Br2···Br3—C4 | 3.9288 (3) | 93.53 (6), 153.70 (6) | x-1, y, z |
8 | C4—Br2···Br3—C4 | 3.8815 (3) | 129.46 (6), 93.57 (6) | -x+3/2, y+1/2, -z+1/2 |
C—Br···Br—C system | Br···Br (Å) | C—Br···Br angles (°) | operator | |
1 | C4—Br1···Br2—C4 | 3.6912 (7) | 87.83 (12), 163.67 (13) | -x+1/2, y+1/2, -z+1/2 |
2 | C4—Br1···Br3—C4 | 3.5799 (8) | 94.47 (14), 154.20 (12) | x-1/2, y-1/2, z |
3 | C4—Br2···Br2—C4 | 4.0094 (10) | 101.61 (12), 101.61 (12) | -x+1, y, -z+1/2 |
4 | C4—Br2···Br2—C4 | 3.8353 (6) | 146.53 (13), 84.08 (14) | -x+1/2, y-1/2, -z+1/2 |
5 | C4—Br2···Br2—C4 | 3.8353 (6) | 84.08 (14), 146.53 (13) | -x+1/2, y+1/2, -z+1/2 |
6 | C4—Br2···Br3—C4 | 3.8068 (7) | 88.14 (12), 148.31 (13) | x-1/2, y-1/2, z |
7 | C4—Br2···Br3—C3 | 4.0730 (8) | 138.09 (12), 77.51 (13) | -x+1/2, y-1/2, -z+1/2 |
8 | C4—Br3···Br3—C4 | 3.9507 (10) | 107.85 (12), 107.85 (12) | -x+1, y, -z+1/2 |
We are interested in secondary interactions in brominated aromatic hydrocarbons [see, for example, our studies of all ten isomers of di(bromomethyl)naphthalenes; Jones & Kuś, 2010, and related references therein]. Such interactions may include `weak' C—H···Br hydrogen bonds, Br···Br halogen bonds, π–π stacking, and H···π and Br···π contacts. We are currently preparing (Jones & Kuś, 2012) a study of several benzene derivatives multiply substituted with bromo, methyl and bromomethyl groups. The title compound, 1,4-bis(tribromomethyl)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 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 is polymorph (Ia), space group P21/n. A few thin hexagonal 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.
Both polymorphs crystallize with imposed inversion symmetry (Figs. 2 and 3). The main difference is in the orientation of the CBr3 group; in (Ia), one Br atom (Br3) lies approximately in the ring plane, whereas for (Ib) this is not the case (Tables 1 and 2).
Clearly, the major structural interest centres on the molecular packing. Not surprisingly for a compound for which 60% of the terminal atoms are bromine, Br···Br interactions (Tables 3 and 4) dominate, at least numerically. Polymorph (Ia) has eight such interactions <3.99 Å, with the next longest at 4.14 Å, while (Ib) has seven (Nos. 4 and 5 are equivalent) <4.08 Å, with the next longest at 4.23 Å. Additionally, (Ia) has one `weak' hydrogen bond (H3···Br3 = 3.00 Å) and a Br···π interaction Br3···Cg = 3.518 Å [Cg is the centroid of the aromatic ring; individual Br···C distances = 3.738 (2)–3.779 (2) Å and C—Br···Cg = 115°; symmetry code: -x + 3/2, y + 1/2, -z + 1/2]. Polymorph (Ib) has a Br···π interaction Br1···Cg = 3.503 Å [individual Br···C distances = 3.473 (4)–3.702 (5) Å and C—Br···Cg = 153°; symmetry code: x - 1/2, y + 1/2, z].
It is not a trivial problem to decide when a Br···Br contact corresponds to a significant interaction. The shortest such contacts are ca 3.1–3.2 Å and tend to be observed in charge-assisted systems such as [Ph3PSBr]+[AuBr4]- [3.151 (1) Å; Taouss & Jones, 2011]. `Spoke' structures such as Ph3PBr2, originally interpreted by the authors (Bricklebank et al., 1992) as involving a long covalent Br—Br bond of 3.123 (2) Å, may also perhaps be interpreted as at least partly ionic [Ph3PBr]+Br-. At the other extreme are contacts of ca 4 Å, which are significantly longer than the double van der Waals radius of 3.7 Å (Reference?) 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—Hal···Hal angles θ; type II contacts tend to have θ1 ≈ 90° and θ2 ≈ 180° (or vice versa), whereas for type I contacts θ1 ≈ θ2. The former type may correspond better to 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 3 and 4).
Polymorph (Ia) has only one Br···Br contact <3.7 Å (No. 1 in Table 3), and despite its shortness this is a type I interaction with θ1 = θ2 by symmetry; all seven other contacts lie in the range 3.8–4.0 Å. Six of the eight contacts combine to form a tube-like substructure (Fig. 4) parallel to the a axis. A view of the structure parallel to the a axis (Fig. 5) shows that the tubes are connected by the two contacts, Br1···Br1 and Br2···Br2 (No. 6; for operators see Table 3), 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 may not be closely correlated with their (subjective) structural relevance.
The overall packing diagram of polymorph (Ib) is shown in Fig. 6. The Br···Br interactions can be seen in the regions z ≈ 1/4, 3/4. Two Br···Br contacts <3.7 Å (Table 4, Nos. 1 and 2) are appreciably shorter than all the others, and both correspond reasonably well to the type II criteria. To display the region at z ≈ 1/4 more clearly, it is convenient to use only `half' molecules consisting of C—CBr3 groups, which are linked to form double layers, and to display only the two shortest Br···Br contacts (Fig. 7). Each individual layer is formed via contact No. 1 (these contacts run diagonally in the figure) and 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. Differential scanning calorimetry (DSC) measurements gave only a peak at the melting point (462.2 K). Melted and resolidified samples were amorphous, presumably because of decomposition (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 (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). Curiously, the crystallographic density of (Ia) (3.165 Mg m-3) is higher than that of (Ib) (3.080 Mg m-3), which would not be expected if the predominance of (Ib) were attributable to more efficient packing.