Crystal structure of 4,5-dibromophenanthrene

The molecule is positioned on a twofold rotation axis and the asymmetric unit consists of half a molecule with the other half being generated by symmetry. The presence of two large bromine atoms in the bay region significantly distorts the molecule from planarity. The molecules pack in layers in the crystal with slippage in the stacking arrangement. While all of the molecules within each layer are oriented in the same direction, those in adjacent layers are oriented in the opposite direction, leading to anti-parallel stacks.


Chemical context
In the course of our research into non-planar polycyclic hydrocarbons, we became interested in the preparation of helical phenanthrene systems bearing bulky substituents in the 4-and 5-positions. Towards that end, we undertook the synthesis of 4,5-dibromophenanthrene (2) from the known dialdehyde 1 (Suzuki et al., 2009) using a recently published procedure (Xia et al., 2012), as shown in Fig. 1. Although there is one reference to the title compound 2 in the literature (Cosmo et al., 1987a), neither the procedure for its synthesis nor its X-ray crystal structure has previously been reported.

Structural commentary
The asymmetric unit consists of half a molecule with the other half generated by symmetry as the molecule is positioned on a ISSN 2056-9890 twofold rotation axis that bisects the central ring. The crystal structure shows a deformed phenanthrene framework (Fig. 2) in which the planes of the two terminal rings are twisted away from each other by 28.51 (14) and the torsion angle between the two C-Br bonds (Br1-C4-C4 0 -Br1 0 ) is 74.70 (14) . The C4-C5-C5 0 -C4 0 torsion angle is 32.8 (6) , and the distance between the two bromine atoms is 3.277 (13) Å , a value consistent with a previous report (Cosmo et al., 1987a). A comparison of the key structural features of the title compound 2 to those of other known 4,5-dihalophenanthrenes (Cosmo et al., 1987b;Bock et al., 1998) is presented in Table 1 with reference to the general structure shown in Fig. 3. The distance between the two halogen atoms, and the torsion angle between the two carbon-halogen bonds (X-C4-C5-X), increase as expected with the increasing size of the halogen atom. Interestingly, however, the distortion of the phenanthrene framework, as measured by either the angle between the mean planes of the terminal rings A and C, or the C4-C4 0 -C5 0 -C5 torsion angle (see Fig. 3), is the largest for the dichloro derivative 4 (Table 1), larger than for the dibromo and diodo compounds. A combination of both size and electronegativity may account for compound 4 showing the largest twist of the phenanthrene system in the series of 4,5-dihalophenathrene compounds. Crystal structure of 2 with displacement ellipsoids shown at the 50% probability level. H atoms omitted for clarity. [Symmetry code: (') 1 À x, + y, 3 2 À z].

Figure 3
The 4,5-dihalo derivatives of phenanthrene shown with conventional chemical numbering. This figure is used as a reference for the data in Table 1.

Table 1
A comparison of selected structural parameters (Å , ) in a series of known 4,5-dihalophenanthrene derivatives.
Refer to Fig. 3 for parameters used in this table.
Compound angle between rings A and C in adjacent layers (marked in blue in Fig. 4, see Fig. 3 for ring numbering), separated by a distance of 4.0287 (10) Å . These (blue) centroids are shifted by 2.266 (6) Å relative to each other, indicating a slippage in the stacking arrangement. This ring slippage is also evidenced by the centroid of the B ring being at a shorter distance of 3.7533 (19) Å to the A ring centroid (shown in orange in Fig. 4) of the closest phenanthrene unit in an adjacent layer. In addition, short contacts of 3.328 (5) Å are found between C6 (or C6 0 ; refer to Fig. 2. for atom numbering) and an equivalent carbon atom in an adjacent layer. These atoms, which are in terminal rings offset from each other, are shown in green in Fig. 4. A view along the a axis (Fig. 5) shows the opposing orientation of the molecules in going from one layer to the next, leading to anti-parallel stacks.
The reaction system was degassed with argon and the resulting solution was stirred at 363 K for 1 h, producing a deep brownred color after 20 min. The mixture was cooled to room temperature and the crude product was purified by silica gel column chromatography to give 2, as colorless crystals (24

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Crystal packing of 2 when viewed along the b axis. The separation between the centroids of the middle rings (blue spheres) is slightly longer than that between the centroids of the middle and terminal rings (blue and orange spheres) in adjacent layers. Close contacts are also observed between equivalent carbon atoms in the terminal rings (shown in green) that are offset from each other. All lengths are in Å .

4,5-Dibromophenanthrene
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )