β-Polymorph of phenazepam: a powder study

The title compound [systematic name: 7-bromo-5-(2-chlorophenyl)-1H-1,4-benzodiazepin-2(3H)-one] (β-polymorph), C15H10BrClN2O, has been obtained via cryomodification of the known α-polymorph of phenazepam [Karapetyan et al. (1979 ▶). Bioorg. Khim. 5, 1684–1690]. In both polymorphs, the molecules, which differ only in the dihedral angles between the aromatic rings [75.4 (2)° and 86.2 (3)° in the α- and β-polymorphs, respectively], are linked into centrosymmetric dimers via N—H⋯O hydrogen bonds. In the crystal structure of the β-polymorph, weak intermolecular C—H⋯O hydrogen bonds further link these dimers into layers parallel to bc plane.

This work was supported in part by the RFBR project 09-03-13557.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: LH5126).

Comment
Phenazepam is a benzodiazepine drug produced in Russia, which is used in the treatment of neurological disorders such as epilepsy, alcohol withdrawal syndrome and insomnia. The crystal structure of its α-polymorph has been reported by Karapetyan et al. (1979). Herewith we present the crystal structure of β-polymorph of phenazepam, which was obtained from the α-polymorph via cryomodification, i.e. through the preparation of metastable solid-phase from the vapor phase at low temperature (Sergeev & Komarov, 2006).

Experimental
The title β-polymorph of phenazepam has been obtained via cryomodification of α-polymorph of phenazepam. Cryomodification was realized by vapor deposition on a cold surface in vacuo at temperatures varying from 77 to 273 K following the known procedure (Sergeev & Komarov, 2006).

Refinement
During the exposure, the specimen was spun in its plane to improve particle statistics. The triclinic unit-cell dimensions were determined with the indexing program TREOR (Werner et al., 1985), M 20 =37, using the first 35 peak positions. A number of weak unindexed lines (d-spacings of most significant ones were 8.54, 8.31, 6.90, 5.25 and 5.04 Å) demonstrated that the sample contained a small amount of α-polymorph. The crystal structure of β-polymorph was solved by simulated annealing procedure (Zhukov et al., 2001) and refined following the methodology described in (Ryabova et al., 2005). All non-H atoms were isotropically refined. H atoms were placed in geometrically calculated positions and not refined. The diffraction profiles and the differences between the measured and calculated profiles after the final two-phases Rietveld refinement are shown in Fig. 2. On the results of two-phases Rietveld refinement the ratio of β-and α-polymorphs in the sample was estimated as 1.000 (2) to 0.045 (2), respectively. For the α-polymorph, the atomic coordinates and displacement parameters were fixed to literature values (Karapetyan et al., 1979), so only scale factor and profile parameters were refined.

Special details
Geometry.