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A new polymorph, (Ib), of the title compound, C8H8Br2, crystallizes in the space group P21/n, the same as the known polymorph (Ia) but with Z = 2 (imposed inversion symmetry) rather than Z = 4. The mol­ecular structures are closely similar because the mol­ecule has no degrees of torsional freedom except for methyl groups, but the packing arrangements are completely different. Polymorph (Ia) is characterized by linked trapezia of Br...Br inter­actions, whereas polymorph (Ib) features H...Br and Br...π inter­actions.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270111008742/gd3380sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270111008742/gd3380Isup2.hkl
Contains datablock I

CCDC reference: 824043

Comment top

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, ππ stacking, and H···π and Br···π contacts. We are currently preparing a study of several benzene derivatives multiply substituted with bromo, methyl and bromomethyl groups. The title compound, (I), is being published separately because it is a known compound and its structure, as crystallised from ethanol, has already been determined in space group P21/n with Z = 4 [Reiter et al. (2005), hereinafter polymorph (Ia); refcode JAQJAN in the Cambridge Structural Database (Allen, 2002)]. However, in our hands the compound crystallized from acetone as a new polymorph in P21/n with Z = 2 and thus with imposed inversion symmetry, polymorph (Ib). We describe here the packing of both polymorphs, which are totally different from each other. The previous study described the packing of (Ia) in only general terms, but clearly recognized the presence of Br···Br contacts; the compound was a starting material (for the synthesis of p-xylylene-1,4-diphosphines) and thus the structure was only of peripheral interest.

The molecule of polymorph (Ib) is shown in Fig. 1. It has the same general features as the previous polymorph, (Ia), such as coplanarity of all non-H atoms (r.m.s. deviation = 0.002 Å) and deviations of endocyclic angles from the ideal 120° (slightly wider at atom C1 and slightly narrower at C2; Table 1).

The molecular packing of polymorph (Ib) is surprising. There are no interactions of the types H···π or ππ, and the shortest Br1···Br1 contact is 4.1761 (3) Å via symmetry operators (x + 1/2, -y + 1/2, z + 1/2) or (x - 1/2, -y + 1/2, z - 1/2); this distance would usually be considered too long for any significant interaction. There is one weak hydrogen bond of the type C—H···Br1 (Table 2) involving an aromatic H atom; the methyl H atoms play no significant role in the aggregation. The weakness of C—H···Br—C interactions has been commented on by Brammer et al. (2001). A search for contacts to the centre of gravity (Cg) of the ring reveals a Br1···Cg contact of 3.57 Å via the symmetry operator (x + 1/2, -y + 1/2, z + 1/2), but closer inspection shows that the interaction is better represented by Br1···(mid-point of C1—C2), with a distance of 3.37 Å and an angle at Br1 of 167°. The interaction is almost perpendicular (80°) to the ring plane. The Br1···Br1 contact mentioned above is therefore best seen as a consequence of the Br1···π interaction; if this is short, then the two Br atoms related by the same operator must also approach each other.

The combination of these two contacts with the inversion symmetry of the molecule leads to a three-dimensional packing, but the general features are easily recognisable, especially when depth-cued (Fig. 2).

The packing of the previously known polymorph (Ia) is conceptually much simpler. Apart from some rather nonlinear weak hydrogen bonds (C—Hmethyl···Br = 3.08 Å and angle = 136°; C—Har···Cg = 2.92 Å and angle = 133°; C—Hmethyl···Cg = 2.88 Å and angle = 134°), the main features are Br···Br interactions. The Br atoms occupy the regions z 0, 1/2, 1, etc., and the contacts link the molecules via chains of inversion-symmetric Br12Br22 trapezia parallel to the a axis, themselves connected by Br1···Br1 contacts (Fig. 3 and Table 3). The classification of Pedireddi et al. (1994) suggests that the two shorter contacts are type I in nature (approximately equal C—Br···Br angles), whereas the longest contact does not fit either of the two main types (type II, usually considered to represent stronger interactions, has one angle ca 90° and one ca 180°).

We have previously commented (Jones & Kuś, 2010) that it is difficult to predict or rationalize which type(s) of contacts will appear in the packing of any given brominated aromatic hydrocarbon. Our findings here lend weight to this view. The packing of the previously known polymorph (Ia) of compound (I) was largely determined by Br···Br interactions (as one might expect), but the new polymorph (Ib) features H···Br and Br···π contacts. The packing efficiency as judged by the density is slightly greater for polymorph (Ib) [2.100 Mg m-3 at 133 K, compared with 2.087 Mg m-3 at 143 K for polymorph (Ia)].

Related literature top

For related literature, see: Allen (2002); Brammer et al. (2001); Jones & Kuś (2010); Pedireddi et al. (1994); Reiter et al. (2005).

Experimental top

The title compound was synthesized by bromination of p-xylene and recrystallized by slow evaporation from acetone.

Refinement top

Methyl H atoms were identified in difference syntheses and idealized (C—H = 0.98 Å and H—C—H = 109.5 °). The methyl group was refined as a rigid group allowed to rotate but not tip. Aromatic H atoms were introduced at calculated positions and refined using a riding model, with C—H = 0.95 Å. The Uiso(H) values were set equal to mUeq(C), with m = 1.2 for aromatic and 1.5 for methyl H atoms.

Computing details top

Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT (Bruker, 1998); 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).

Figures top
[Figure 1] Fig. 1. The molecule of polymorph (Ib) in the crystal structure. Displacement ellipsoids are drawn at the 50% probability level. Only the asymmetric unit is numbered.
[Figure 2] Fig. 2. A packing diagram for polymorph (Ib), viewed perpendicular to (001). The molecules are depth-cued; molecules with thicker bonds are nearer the viewer. C—H···Br hydrogen bonds are represented by thin dashed lines and Br···π interactions by thick dashed lines. For the sake of clarity, the methyl H atoms have been omitted.
[Figure 3] Fig. 3. A packing diagram for polymorph (Ia), viewed parallel to the b axis. Br···Br interactions are indicated by dashed lines, while thin lines represent the longest contact. [Symmetry codes: (i) -x + 1/2, y - 1/2, -z + 1/2; (ii) x - 1/2, -y + 3/2, z + 1/2; (iii) -x, -y + 1, -z + 1; (iv) x + 1/2, -y + 3/2, z + 1/2.]
1,4-dibromo-2,5-dimethylbenzene top
Crystal data top
C8H8Br2F(000) = 252
Mr = 263.96Dx = 2.100 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ynCell parameters from 5832 reflections
a = 6.2597 (6) Åθ = 3.2–30.5°
b = 10.4820 (11) ŵ = 9.63 mm1
c = 6.4281 (6) ÅT = 133 K
β = 98.134 (4)°Irregular, colourless
V = 417.53 (7) Å30.20 × 0.15 × 0.12 mm
Z = 2
Data collection top
Bruker SMART 1000 CCD area-detector
diffractometer
1269 independent reflections
Radiation source: fine-focus sealed tube1214 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.021
Detector resolution: 8.192 pixels mm-1θmax = 30.5°, θmin = 3.8°
ϕ and ω scansh = 88
Absorption correction: multi-scan
(SADABS; Bruker, 1998)
k = 1414
Tmin = 0.292, Tmax = 0.391l = 99
7909 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.019Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.051H-atom parameters constrained
S = 1.13 w = 1/[σ2(Fo2) + (0.0241P)2 + 0.4037P]
where P = (Fo2 + 2Fc2)/3
1269 reflections(Δ/σ)max = 0.001
47 parametersΔρmax = 0.63 e Å3
0 restraintsΔρmin = 0.46 e Å3
Crystal data top
C8H8Br2V = 417.53 (7) Å3
Mr = 263.96Z = 2
Monoclinic, P21/nMo Kα radiation
a = 6.2597 (6) ŵ = 9.63 mm1
b = 10.4820 (11) ÅT = 133 K
c = 6.4281 (6) Å0.20 × 0.15 × 0.12 mm
β = 98.134 (4)°
Data collection top
Bruker SMART 1000 CCD area-detector
diffractometer
1269 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 1998)
1214 reflections with I > 2σ(I)
Tmin = 0.292, Tmax = 0.391Rint = 0.021
7909 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0190 restraints
wR(F2) = 0.051H-atom parameters constrained
S = 1.13Δρmax = 0.63 e Å3
1269 reflectionsΔρmin = 0.46 e Å3
47 parameters
Special details top

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.

Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane)

2.5962 (0.0041) x + 6.9385 (0.0034) y + 3.5958 (0.0046) z = 4.7762 (0.0019)

* -0.0015 (0.0007) Br1 * 0.0011 (0.0011) C1 * 0.0036 (0.0014) C2 * -0.0026 (0.0008) C3 * -0.0006 (0.0005) C4

Rms deviation of fitted atoms = 0.0022

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Br10.67598 (3)0.269096 (18)0.32055 (3)0.02107 (7)
C10.5701 (3)0.40421 (15)0.1370 (2)0.0147 (3)
C20.3763 (3)0.46429 (16)0.1617 (2)0.0147 (3)
C30.3091 (3)0.56219 (15)0.0196 (3)0.0152 (3)
H30.17870.60640.03040.018*
C40.2409 (3)0.42605 (17)0.3321 (3)0.0175 (3)
H4A0.11810.48440.32880.021*
H4B0.33000.43060.47010.021*
H4C0.18780.33870.30650.021*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Br10.02050 (10)0.02010 (11)0.02220 (11)0.00190 (6)0.00156 (7)0.00761 (6)
C10.0157 (7)0.0131 (7)0.0148 (7)0.0005 (5)0.0002 (5)0.0014 (5)
C20.0157 (7)0.0146 (7)0.0136 (6)0.0019 (6)0.0019 (5)0.0013 (5)
C30.0142 (7)0.0148 (7)0.0166 (7)0.0001 (5)0.0025 (5)0.0015 (5)
C40.0142 (7)0.0216 (8)0.0174 (7)0.0014 (6)0.0046 (5)0.0025 (6)
Geometric parameters (Å, º) top
Br1—C11.9016 (16)C3—H30.9500
C1—C3i1.387 (2)C4—H4A0.9800
C1—C21.396 (2)C4—H4B0.9800
C2—C31.399 (2)C4—H4C0.9800
C2—C41.530 (2)
C3i—C1—C2122.68 (15)C2—C3—H3119.5
C3i—C1—Br1117.48 (12)C2—C4—H4A109.5
C2—C1—Br1119.84 (12)C2—C4—H4B109.5
C1—C2—C3116.27 (14)H4A—C4—H4B109.5
C1—C2—C4122.80 (14)C2—C4—H4C109.5
C3—C2—C4120.92 (15)H4A—C4—H4C109.5
C1i—C3—C2121.05 (15)H4B—C4—H4C109.5
C1i—C3—H3119.5
C3i—C1—C2—C30.4 (3)Br1—C1—C2—C40.2 (2)
Br1—C1—C2—C3179.64 (12)C1—C2—C3—C1i0.4 (3)
C3i—C1—C2—C4179.01 (15)C4—C2—C3—C1i179.03 (15)
Symmetry code: (i) x+1, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C3—H3···Br1ii0.953.063.9829 (16)165
Symmetry code: (ii) x+1/2, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaC8H8Br2
Mr263.96
Crystal system, space groupMonoclinic, P21/n
Temperature (K)133
a, b, c (Å)6.2597 (6), 10.4820 (11), 6.4281 (6)
β (°) 98.134 (4)
V3)417.53 (7)
Z2
Radiation typeMo Kα
µ (mm1)9.63
Crystal size (mm)0.20 × 0.15 × 0.12
Data collection
DiffractometerBruker SMART 1000 CCD area-detector
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker, 1998)
Tmin, Tmax0.292, 0.391
No. of measured, independent and
observed [I > 2σ(I)] reflections
7909, 1269, 1214
Rint0.021
(sin θ/λ)max1)0.714
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.019, 0.051, 1.13
No. of reflections1269
No. of parameters47
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.63, 0.46

Computer programs: SMART (Bruker, 1998), SAINT (Bruker, 1998), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), XP (Siemens, 1994).

Selected bond angles (º) top
C3i—C1—C2122.68 (15)C1i—C3—C2121.05 (15)
C1—C2—C3116.27 (14)
Symmetry code: (i) x+1, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C3—H3···Br1ii0.953.063.9829 (16)164.7
Symmetry code: (ii) x+1/2, y+1/2, z+1/2.
Br···Br contacts (Å, °) in polymorph (Ia) top
C—Br···Br—C systemBr···Br (Å)C—Br···Br angles (°)Symmetry code
C1—Br1···Br1—C13.719151 and 151-x, -y + 2, -z
C4—Br2···Br1—C13.93974 and 124-x + 1/2, y - 1/2, -z + 1/2
C4—Br2···Br1—C13.672151 and 136x - 1/2, -y + 3/2, z + 1/2
 

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