Crystal structures of (2E)-1-(3-bromothiophen-2-yl)-3-(2-methoxyphenyl)prop-2-en-1-one and (2E)-1-(3-bromothiophen-2-yl)-3-(3,4-dimethoxyphenyl)prop-2-en-1-one

Two closely related related nearly coplanar molecules of 1-(3-bromothiophen-2-yl)-3-(methoxyphenyl)prop-2-en-1-ones exhibit different patterns of weak inter- or intramolecular interactions and crystallize in different space groups.


Structural commentary
The structure of C 14 H 11 BrO 2 S, (I), has triclinic (P1) symmetry, while in (II), C 15 H 13 BrO 3 S, it crystallizes in the monoclinic, I2/ a space group. In (I), four independent molecules (A, B, C, D) crystallize in the asymmetric unit (Z 0 = 8) (Fig. 1), while only one molecule (Z 0 = 8) is present in (II) (Fig. 2). A search for possible additional crystallographic symmetry or pseudosymmetry in compound (II) (Spek, 2009) produced none, while in compound (I) there was indication of the possibility of either P1 symmetry with the a-axis halved or the presence of C2/c symmetry. Structural solution of the structure in the C2/c space group after transforming the axes in PLATON gave a negative result, confirming the (P1) symmetry assignment. Refinement of the structure with two independent molecules in the asymmetry unit rather than four also gave a negative result, even though the coordinates for the A/B and C/D pairs of molecules are related by translation of 0.5 along the a axis, displaying pseudo symmetry which gave B alerts in checkCIF even after many cycles of refinement. The molecular structure of title compound (I), C 14 H 11 BrO 2 S, showing the atom-labelling scheme with 30% probability displacement ellipsoids.

Figure 2
The molecular structure of title compound (II), C 15 H 13 BrO 3 S, showing the atom-labelling scheme with 30% probability displacement ellipsoids.
In the molecular structures of both compounds, (I) and (II), the non-H atoms are almost coplanar, as shown by their relevant torsional and dihedral angles (Table 1). In (I), the mean plane of the keto group is twisted slightly out of plane with that of the thiophene ring in the range of 3-4 and with torsion angles in the range of 174-176 in each of the four molecules (Table 1). The dihedral angle between the mean planes of the phenyl and thiophene rings are in the range of 10-11 . In (II), the mean plane of the keto group is twisted slightly out of plane with that of the thiophene ring by 0.9 (9) , with a torsion angle of À178.2 (6) , and a dihedral angle between the mean planes of the phenyl and thiophene rings of 8.4 (2) . In both compounds, bond lengths and angles are in normal ranges (Allen et al., 2002).
Dihedral 1 represents the dihedral angle between the mean planes of the phenyl and thiophene rings, Dihedral 2 represents the dihedral angle between the mean planes of the thiophene ring and the keto unit, and Dihedral 3 represents the dihedral angle between the mean planes of the phenyl ring and the keto unit.
A comparison of the supramolecular features of the title compounds (Table 4) suggests that the presence or absence of direction-specific weak intermolecular interactions plays a role in their influence on the small differences in planarity observed and supported by similar types of interactions in closely related compounds. No classical hydrogen bonds are observed in any of the five compounds. All five compounds do display a similar weak C-HÁ Á ÁBr intramolecular interaction. In (I) and (III) only weak C-HÁ Á Á intermolecular interactions are observed, while in (IV) only weak C-HÁ Á ÁO intermolecular interactions are present.

Figure 6
Compounds (III), (IV) and (V). methanol (20 ml) was mixed with 2-methoxybenzaldehyde (1.36 g, 0.01 mol) for crystal (I) and 3,4-dimethoxybenzaldehyde (1.66 g, 0.01 mol) for crystal (II) in methanol (20 ml) in the presence of NaOH (5 ml, 30%) at 283 K. After stirring for four h, the contents of the flask were poured into ice-cold water (250 ml). The resulting crude solid was collected by filtration and dried in a hot-air oven at 323 K. A supersaturated solution was obtained by dissolving the sample in acetone at ambient temperature. The prepared solution was filtered, warmed slightly and allowed to evaporate slowly at room temperature. After several days X-ray quality crystals were obtained by the slow the evaporation technique, m.p.: 367 K for (I) and 405 K for (II).

Refinement
Crystal data, data collection and structure refinement details are summarized in   software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).