3,6-Dibromophenanthrene

The phenanthrene ring in the title compound, C14H8Br2, is approximately planar [maximum deviation = 0.039 (3) Å]. In contrast, the two bromo atoms are displaced slightly from the phenanthrene plane [maximum deviation = 0.1637 (3) Å]. In the crystal, the molecules adopt a herringbone-like arrangement and form face-to-face slipped π–π stacking interactions along the b axis, with an interplanar distance of 3.544 (3) Å and slippage of 1.81 Å. The crystal studied was a racemic twin with a minor twin fraction of 0.390 (10).


Related literature
For the synthesis of the title compound using the improved photocyclization of 4,4 0 -dibromo-trans-stilbene, see: Talele et al. (2009). For the original synthesis and applications of the title compound, see: Nakamura et al. (1996). Data collection: RAPID-AUTO (Rigaku, 1999); cell refinement: PROCESS-AUTO (Rigaku, 1998); data reduction: PROCESS-AUTO; program(s) used to solve structure: SIR2004 (Burla et al., 2005); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999 Phenanthrene is a polycylic aromatic hydrocarbon (PAH) as well as a potential building block for higher-order πextended PAHs. The title compound, 3,6-dibromophenanthrene, was first prepared by Nakamura et al. (1996). The bromo functional group on the aromatic ring is a suitable substrate for a variety of cross-coupling reaction. Recently, the improved synthesis was reported by Talele et al. (2009). However, the X-ray structure was not reported to date. We report herein the crystal structure of the title compound, (I).

Experimental
The molecular structure of (I) is shown in Fig. 1. The crystal was a racemic twin with a minor twin fraction of 0.390 (10). The molecule is approximately planar except for Br1 and Br2 [the maximum deviation is 0.1637 (3) Å for Br2]. The bonds lengths and angles are in good agreement with the standard values. As shown in Fig. 2, the crystal structure is characterized by a combination of a columnar stacking and a herrinbone-like arrangement. Along the b axis, there are two columns per unit cell in which the molecules form face-to-face slipped π-stacks with an interplanar distance of 3.543 Å. The interplanar tilt angle between the phenanthrene rings in two adjacent columns is 54.21°.

Experimental
The title compound was prepared from 4,4′-dibromo-trans-stilbene according to the literature procedure of Talele et al. (2009). The title compound was dissolved in hot hexane. After cooling of the solution to room temperature, single crystals suitable for X-ray analysis were obtained.

Refinement
All the aromatic H atoms were positioned geometrically and refined using a riding model with C-H = 0.94 Å and U iso (H) = 1.2U eq (C). In final refinement cycles, racemic twinning was taken into account with a TWIN and a BASF instruction of program SHELXL97 (Sheldrick, 2008), giving a minor twin fraction of 0.390 (10).

Special details
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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.