1,4-Benzodiazepine derivatives are widely used as daytime sedatives, tranquilizers, sleep inducers, anesthetics, anticonvulsants and muscle relaxants (e.g., [1,2,3,4]). Five-Atom heterocyclic fused benzodiazepine ring systems occupy a prominent place among drugs for the treatment of central nervous system (CNS) disorders ([5,6,7,8], and references therein).

The title compound, (1, Figure 1), C₁₅H₁₃N₂OClS, is structurally similar to clotiazepam, the only difference is the hydrogen atom on the nitrogen atom instead of the methyl group. Of similar compounds, the crystal structures of 6,7-dimethyl-5-phenyl-1H-thieno[2,3-e][1,4]diazepin-2(3H)-one [9] and 5-phenyl-9H-1,3-dioxolo[4,5-h][2,3]benzodiazepin-8(7H)-one [10] as well as of the salt, (E)-5-(2-chlorophenyl)-7-ethyl-2-oxo-2,3-dihydro-1H-thieno[2,3-e][1,4]-diazepin-4-ium 2,4,6-trinitropheno-late [10] have been reported. In view of the importance of heterocyclic fused benzodiazepine ring systems, the paper reports the crystal structure of 5-(2-chlorophenyl)-7-ethyl-1H-thieno[2,3-e][1,4]diazepin-2(3H)-one (1). In the course of the structural studies it turned out that the title compound is an example of the ordering of the methyl group while the temperature drops below ca. 200 K. Also, the changes of the unit cell parameters with the temperature show the interesting abnormality: One of the parameters shortens significantly with the rising temperature, while the unit cell volume—as expected—increases almost monotonically.

The title compound was obtained as a gift sample from R. L. Fine Chem., Bengaluru, India. The crystals appropriate for X-ray diffraction studies were obtained by slow evaporation from toluene solution (m.p. 438–440 K). Sixteen sets of diffraction data were collected for temperatures ranging from 125 to 380 K on Agilent Technologies Xcalibur diffractometer (Eos detector) with graphite-monochromatized MoKα radiation (λ = 0.71073Å). The temperature was controlled by an Oxford Instruments Cryosystems cooling device. The data were corrected for Lorentz-polarization effects as well as for absorption [16]. Accurate unit-cell parameters were determined by a least-squares fit of reflections of highest intensity reflections, chosen from the whole experiment. The calculations were mainly performed within the WinGX program system [17]. The structures were solved with SIR92 [18] and refined with the full-matrix least-squares procedure on F² by SHELXL97 [11]. Scattering factors incorporated in SHELXL97 were used. The function Σw(|F_o|² − |F_c|²)² was minimized, with w⁻¹ = [σ² (F_o)² + (AP)² + BP], where P = [Max(F_o², 0) + 2F_c²]/3. All non-hydrogen atoms were refined anisotropically, in the low temperature structure all hydrogen atoms were found in the difference Fourier maps and isotropically refined, with raising temperature some hydrogen atoms have to be placed in calculated positions (above RT–all), and were refined as “riding” on their parent atoms; the U_iso’s of hydrogen atoms were set as 1.2 (1.5 for methyl groups) times the U_eq value of the appropriate carrier atom. The refinement procedure related to the disordered methyl group is similar to that adopted in our previous paper [19]. For the disordered fragment the s.o.f. (with constraint of adding up to unity) and one common isotropic displacement parameter were initially refined. Then the refined value of s.o.f. was fixed and the individual isotropic and subsequently anisotropic displacement parameters were refined. The soft constraints for the anisotropic parameters have to be applied (ISOR). Relevant crystal data for some chosen temperatures (125 K, 230 K for both ordered and disordered model, and 380 K) are listed in Table 3, together with refinement details. The full data (16 temperatures) are available on request.

Crystallographic data (excluding structure factors) for the structural analyses at 125 K, 185 K (for ordered and disordered models), 300 K and 380 K have been deposited with the Cambridge Crystallographic Data Centre, Nos. CCDC-873580 (125 K), CCDC-881359 and 881360 (185 K), CCDC-873581 (300 K) and CCDC-881361 (380 K). Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: +44(1223)336-033, Email: deposit@ccdc.cam.ac.uk, or www: www.ccdc.cam.ac.uk.

The crystal structure of the close analogue of clotiazepam, 5-(2-chlorophenyl)-7-ethyl-1H-thieno[2,3-e][1,4]diazepin-2(3H)-one, has been studied at 16 different temperatures covering the range 125–380 K. The methyl group, which is disordered at room temperature, undergoes the ordering process and below 200 K it is fully ordered. The hydrogen bonds and weaker interaction of C–H···O and C–H···π type connect molecules into a three dimensional structure.