(5Z,7Z,9Z)-5,10-Dibromobenzo[8]annulene

In the structure of the title compound, C12H8Br2, the two bromine substituents are oriented exo to the boat-shaped cyclooctatetraene at the two ring sites that are β to the ring fusion positions. The average Br—C bond distance is 1.919 (2) Å, the average distance for C=C double bonds that are Br substituted is 1.328 (2) Å, while the other two double-bond distances are 1.327 (2) and 1.398 (2) Å for the non-fused and fused bonds, respectively. Each type of ring interatomic distance is within s.u. of the average values for the four known structures, including the title compound, of benzo-fused cycloocatatetraenes that are not coordinated to a metal atom. The crystal structure features short Br⋯Br [3.6620 (3) Å] and C⋯H [2.834 (2) and 2.841 (2) Å] contacts.

In the structure of the title compound, C 12 H 8 Br 2 , the two bromine substituents are oriented exo to the boat-shaped cyclooctatetraene at the two ring sites that are to the ring fusion positions. The average Br-C bond distance is 1.919 (2) Å , the average distance for C C double bonds that are Br substituted is 1.328 (2) Å , while the other two doublebond distances are 1.327 (2) and 1.398 (2) Å for the non-fused and fused bonds, respectively. Each type of ring interatomic distance is within s.u. of the average values for the four known structures, including the title compound, of benzo-fused cycloocatatetraenes that are not coordinated to a metal atom. The crystal structure features short BrÁ Á ÁBr [3.6620 (3) Å ] and CÁ Á ÁH [2.834 (2) and 2.841 (2) Å ] contacts.

Comment
The title compound was prepared as a starting material for synthesis of the corresponding dinitrile derivative, which was of interest in connection with studies of the photochemical conversions of benzocyclooctatetraenes (Bender et al., 1982;1986;1988;1991). In view of the paucity of structures that are crystallographically established for mono-benzofused cyclo-octatetraenes, we decided to undertake a crystallographic study of (I).
Only three prior structures have been reported which contain one benzo-fused cyclooctatetraene ring according to the Cambridge Structural Database (Allen, 2002; WebCSD, August 2013), excluding those with rings coordinated to metals.
The parent hydrocarbon, (5Z,7Z,9Z)-benzo[8]annulene (refcode BUYYUP), has been structurally characterized by X-ray crystallography and investigated by semi-empirical quantum mechanical methods (Li et al., 1983). The only halogensubstituted example is (8-chlorobenzocyclooctatetraen-6-yl)-diphenylphosphine-oxide (refcode: CUDYUV) but this ring bears a large Ph 2 P=O substituent; this structure contains two independent molecules in the asymetric unit (Bohshar et al., 1984). In dimethyl 1,4,6,9-tetramethylbenzocyclooctatetraene-5,10-dicarboxylate (refcode: LEZMAE) the two methyl ester substituents are located where the Br atoms are in (I) but this molecule also has four methyl substituents, two attached to the other ends of the same double bonds that the esters are attached to, and two in the 1 and 4 positions of the benzene rings (Jones et al., 1994). Gratifyingly, all the 1,2 interatomic distances in (I) are found to lie within s.u. of the average values from these three comparison structures and (I).
The C5-Br1 and C10-Br2 distances in BOGWAV are 1.8822 (3) and 1.9309 (3) Å and in FABDOC 1.912 (7) and 1.896 (9). Thus the average C-Br bond distance in this set of three related structures is 1.910 (16) Å, with the distances in (I) found within s.u. of this value.

Experimental
Samples of (I) were prepared from biphenylene via the method of Barton et al. (1964)

Refinement
All the hydrogen atoms were located on a difference map, but for purposes of refinement are treated as riding on their attached aromatic carbon atoms with C-H = 0.95 Å and U iso (H) = 1.2U eq (C).