Received 23 February 2011
A new polymorph of 1,4-dibromo-2,5-dimethylbenzene: HBr and Br versus BrBr interactions
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 BrBr interactions, whereas polymorph (Ib) features HBr and Br interactions.
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, 2010, and related references therein]. Such interactions may include `weak' C-HBr hydrogen bonds, BrBr halogen bonds, - stacking, and H and Br 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 crystallized from ethanol, has already been determined in the 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, hereafter 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 BrBr 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 value of 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 or -, and the shortest Br1Br1 contact is 4.1761 (3) Å between the molecule at (x, y, z) and those at (x + , -y + , z + ) or (x - , -y + , z - ); this distance would usually be considered too long for any significant interaction. There is one weak intermolecular hydrogen bond of the type C-HBr1 (Table 2) involving an aromatic H atom; the methyl H atoms play no significant role in the aggregation. The weakness of C-HBr-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 Br1Cg contact of 3.57 Å between the molecule at (x, y, z) and that at (x + , -y + , z + ), 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 to C1-Br1 of 167°. The interaction is almost perpendicular (80°) to the ring plane. The Br1Br1 contact mentioned above is therefore best seen as a consequence of the Br1 interaction; if this is short, then the two Br atoms related by the same symmetry 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 intermolecular hydrogen bonds (C-HmethylBr = 3.08 Å and angle = 136°; C-HarCg = 2.92 Å and angle = 133°; C-HmethylCg = 2.88 Å and angle = 134°; distances are HA), the main features are BrBr interactions. The Br atoms occupy the regions z 0, , 1, etc., and the contacts link the molecules via chains of inversion-symmetric Br12Br22 trapezia parallel to the a axis, themselves connected by Br1Br1 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-BrBr 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 & Kus, 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 BrBr interactions (as one might expect), but the new polymorph (Ib) features HBr and Br 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)].
| || Figure 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; unlabelled atoms are related by the symmetry operator (-x+1, -y+1, -z).
| || Figure 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-HBr hydrogen bonds are represented by thin dashed lines and Br interactions by thick dashed lines. For the sake of clarity, methyl H atoms have been omitted.
| || Figure 3 |
A packing diagram for polymorph (Ia), viewed parallel to the b axis. BrBr interactions are indicated by dashed lines, while thin lines represent the longest contact. [Symmetry codes: (i) -x + , y - , -z + ; (ii) x - , -y + , z + ; (iii) -x, -y + 1, -z + 1; (iv) x + , -y + , z + .]
The title compound was synthesized by bromination of p-xylene and was recrystallized by slow evaporation from acetone.
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.
Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT; 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.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: GD3380 ). Services for accessing these data are described at the back of the journal.
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