5-Bromo-1-( 4-fluorophenyl )-1 , 3-dihydro-isobenzofuran

The title compound, C14H10BrFO, possesses normal geometrical parameters. The dihedral angle between the two ring systems is 71.50 (9)°. An unusually short inter­molecular Br⋯Br contact of 3.4311 (5) A occurs.


Comment
The title compound, (I), is an intermediate in the synthesis of the antidepressant drug citalopram (Liechti et al., 2000). More generally, phthalans show distinctive redox chemistry (Azzena et al., 1996). We have previously deposited (CSD-260624;Cambridge Structural Database;Allen, 2002) data for a poor quality structure from a twinned crystal of (I).
The geometrical parameters for (I) are normal. Each molecule of (I) is chiral (the arbitrarily chosen asymmetric unit has an S conformation at C7), but crystal symmetry generates a racemic mixture of the two enantiomers. The ninemembered isobenzofuran ring system (C7-C14/O1) is almost planar [r.m.s. deviation from the mean plane = 0.018 Å ; maximum = 0.038 (3) Å for C14] and the dihedral angle between the two ring systems (C7-C14/O1 and C1-C6) is 71.50 (9) .
A PLATON (Spek, 2003) analysis of (I) identified two possible C-HÁ Á ÁF interactions ( Table 1) that may help to stabilize the crystal packing (Fig. 2). There are no significant stacking interactions in (I).
Inversion symmetry generates a short intermolecular Br1Á Á ÁBr1 i [symmetry code: (i) 2 À x, Ày, 1 À z] separation of 3.4311 (5) Å which is significantly less than the van der Waals contact distance of 3.70 Å for two Br atoms (Bondi, 1964). Some workers have ascribed specific attractive forces to such short intermolecular halogen-halogen contacts (Desiraju & Parthasarathy, 1989). A database survey of such contacts by Eriksson & Hu (2001) shows that the present separation lies at the lower end of the observed range of intermolecular BrÁ Á ÁBr distances. However, these workers are less certain of the nature of such contacts, and suggest that they may be the consequence -rather than the cause -of the crystal packing.
In the related 1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile [i.e. where a cyanide group replaces the Br atom in (I)], there are two molecules in the asymmetric unit with distinctly different degrees of twist between their ring systems (Yathirajan et al., 2004).
We thank the EPSRC National Crystallography Service for data collection. HGA thanks the University of Mysore for provision of research facilities. View of (I), showing 50% displacement ellipsoids and arbitrary spheres for the H atoms. 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.