The crystal structures of six (2E)-3-aryl-1-(5-halogenothiophen-2-yl)prop-2-en-1-ones

Six closely related (2E)-3-aryl-1-(5-halogenothiophen-2-yl)prop-2-en-1-ones all have nearly planar molecular skeletons. C—H⋯O hydrogen bonds are present in only three of the structures but short Br⋯Br, Br⋯O and Cl⋯Cl contacts are also present in some of the structures.


Chemical context
Chalcones are important constituents of many natural products, and they are abundant in edible plants where they are considered to be precursors of flavonoids and isoflavonoids. They display a wide range of pharmacological properties including antibacterial (Tang et al., 2008;Kumar et al., 2013a), anticancer (Shin et al., 2013), antifungal (Domínguez et al., 2001;Kumar et al., 2013a,b), antimalarial (Li et al., 1995) and antitubercular (Lin et al., 2002) activity. In addition, chalcone derivatives are also important materials in photonic applications because of their excellent blue-light transmittance and good crystallization ability (Goto et al., 1991;Uchida et al.,1998;Indira et al., 2002;Sarojini et al., 2006). In a continuation of our work on chalcones containing a thiophen moiety (Naik et al., 2015), six new chalcones of this type, compounds (I)-(VI) (Figs. 1-6) have now been synthesized and we report herein on their molecular structures and supramolecular assembly. Compounds (I)-(VI) were all prepared using condensation reactions, under basic conditions, between 2-acetyl-5-halogenothiophens and substituted benzaldehydes. The molecular structure of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The molecular structure of compound (II), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 3
The molecular structure of compound (III), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 4
The molecular structure of compound (IV), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 5
The molecular structure of compound (V), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 6
The molecular structure of compound (VI), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.
reported here for orientational disorder of the type commonly observed with otherwise unsubstituted thienyl units; this is presumably a direct consequence of the presence of the halogen substituent.
In each of compounds (III), (IV) and (VI), all of which carry an alkoxy substituent, the atom C37 (Figs. 3, 4 and 6) lies close to the plane of the adjacent aryl ring: the displacements of the atoms C37 from these planes are 0.117 (3), 0.097 (4) and 0.186 (4) Å , respectively. Consistent with these observations, the corresponding pairs of exocyclic C-C-O angles (Table 1) differ significantly, as typically found for alkoxybenzenes with near-planar molecular skeletons (Seip & Seip, 1973;Ferguson et al., 1996). Whereas the whole ethoxy group in each of compounds (III) and (IV) is nearly coplanar with the adjacent aryl ring, this is far from the case for compounds (I) and (II) ( Table 1,  . The bond distances in compounds (I)-(VI) all lie within the usual ranges (Allen et al., 1987).

Supramolecular interactions
There are no direction-specific intermolecular interactions in the structure of compound (I); hydrogen bonds of C-HÁ Á ÁO and C-HÁ Á Á types are absent, as arestacking interactions. Hydrogen bonds andstacking interactions are also absent from the structure of compound (II), but in this structure there is a short intermolecular BrÁ Á ÁBr contact, with parameters Br15Á Á ÁBr15 i = 3.4917 (5) Å and C15-Br15Á Á Á Br15 i = 151.37 (8) [symmetry code: (i) Àx + 1, Ày + 1, Àz + 2]. The Br Á Á ÁBr distance is significantly shorter than the van der Waals contact distance of 3.70 Å (Bondi, 1964;Rowland & Taylor, 1996), while the observed C-BrÁ Á ÁBr angle is consistent with the results of a database analysis of such contacts (Ramasubbu et al., 1986), which found that such angles were, in general, clustered around 165 .

Figure 7
Part of the crystal structure of compound (III), showing the formation of a hydrogen-bonded C(7) chain running parallel to the [001] direction. For the sake of clarity, the H atoms not involved in the motif shown have been omitted. The atoms marked with an asterisk (*) or a hash (#) are at the symmetry positions (x, Ày + 3 2 , z + 1 2 ) and (x, Ày + 3 2 , z + 1 2 ), respectively.
there are no direction-specific interactions between adjacent chains: in particular there are no short intermolecular BrÁ Á ÁBr contacts in the structure of compound (IV), thus differing in this respect from compound (II). There are neither hydrogen bonds norstacking interactions in the structure of compound (V). However, the structure contains a fairly short intermolecular ClÁ Á ÁCl contact, although, rather surprisingly, there are no short contacts of either BrÁ Á ÁBr or BrÁ Á ÁCl types. For the contact C15-Cl15Á Á ÁCl15 ii [symmetry code: (ii) Àx + 1, Ày, Àz + 2], the geometrical parameters are ClÁ Á ÁCl ii = 3.4825 (11) Å and C-ClÁ Á ÁCl ii = 167.83 (10) . The ClÁ Á ÁCl distances is thus just at the van der Waals contact distance 3.48 Å (Rowland & Taylor, 1996) and so this contact cannot be regarded as structurally significant: however, it may be noted that the angle C-ClÁ Á ÁCl ii is entirely consistent with the results of a database analysis (Ramasubbu et al., 1986).
A single C-HÁ Á ÁO hydrogen bond (Table 2) links the molecules of compound (VI) which are related by the 2 1 screw axis along ( 1 2 , y, 3 4 ) into a C(5) chain running parallel to the [010] direction (Fig. 8). Two chains of this type, related to one another by inversion, pass through each unit cell, but there are no direction-specific interactions between adjacent chains. Not only are C-HÁ Á Á hydrogen bonds andstacking interactions absent from the crystal structure of compound (VI), but neither are there any short BrÁ Á ÁBr contacts of the type found in compound (II). There is however a short intermolecular BrÁ Á ÁO contact with parameters Br15Á Á ÁO33 iii = 2.9770 (16) Å and C15-Br15Á Á ÁO33 iii = 167.21 (7) [symmetry code: (iii) x À , y, z + 1].
All of the compounds reported here crystallize either in space group P1 or in P2 1 /c, and there appear to be some interesting connections between the space groups and the nature of the direction-specific intermolecular interactions manifested in the various structures. Thus although all six of the compounds described here contain carbonyl groups, only in compounds (III), (IV) and (VI) do the O atoms of these units participate as acceptors in C-HÁ Á ÁO hydrogen bonds: these happen to be the three examples which crystallize in space group P2 1 /c. Of the three 5-bromothienyl derivatives reported here, a short BrÁ Á ÁBr contact occurs only in compound (II), the only example of this group which crystallizes in space group P1.

Database survey
The structures of a number of (2E)-3-aryl-1-(5-chlorothiophen-2-yl)-prop-2-en-1-one derivatives closely related to compounds (I)-(VI) have been reported recently, usually in the form of brief reports on single structures in which no comparisons with related compounds were made, and sometimes with little or no mention of the supramolecular assembly. It is thus of interest briefly to compare the supramolecular assembly in these compounds with that in compounds (I)-(VI). Compound (VII) (see Scheme below) is isomeric with compound (V), and these two compounds differ only in the exchange of the halogen location. Despite this, they are not isomorphous as compound (VII) crystallizes in space group P2 1 /c (Kavitha et al., 2013), as opposed to P1 for compound (V). There are two C-HÁ Á Á contacts in the structure of compound (VII), but both of these have long HÁ Á ÁD distances and small D-HÁ Á ÁA angles, and so are probably not structurally significant. There is, however, a short intermolecular BrÁ Á ÁCl contact for which the BrÁ Á ÁCl distance of 3.5746 (11) Å (not 3.698 (1) Å as stated in the original report), is larger than the sum, 3.55 Å (Rowland & Taylor (1996), of the van der Waals radii.
For compound (VIII) (Vepuri et al., 2012), which provides a genuine example of Z 0 = 2 in space group Cc (Baur & Kassner, 1992;Marsh, 1997Marsh, , 2004, there are no significant direction interactions in the structure: in particular there are neither C-HÁ Á ÁO hydrogen bonds nor short BrÁ Á ÁBr contacts. Compounds (IX) (Prabhu et al., 2011b) and (X) (Prabhu et al., 2014) are isostructural, and (X) was described as forming chains built from two independent C-HÁ Á ÁO hydrogen bonds. However, one of these contacts involves a methyl C-H bond and the other has a C-HÁ Á ÁO angle of only 130 (cf. Wood et al., 2009), so that neither can be regarded as struc- Part of the crystal structure of compound (VI), showing the formation of a hydrogen-bonded C(5) chain running parallel to the [010] direction. For the sake of clarity, the H atoms not involved in the motif shown have been omitted. The atoms marked with an asterisk (*), a hash (#) or a dollar sign ($) are at the symmetry positions (Àx + 1, y À 1 2 , Àz + 3 2 ), (Àx + 1, y + 1 2 , Àz + 3 2 ) and (x, y À 1, z), respectively. turally significant. On the other hand the structure of (IX) contains a significant aromaticstacking interaction between the phenyl rings of inversion-related molecules, although this was apparently overlooked in the original report. The phenyl rings of the molecules at (x, y, z) and (Àx + 2, Ày + 2, Àz + 2) are strictly parallel with an interplanar spacing of 3.5113 (8) Å : the ring centroid separation is 3.6535 (11) Å , corresponding to a ring-centroid offset of 1.009 (2) Å , so leading to the formation of a centrosymmetric -stacked dimer (Fig. 9). The original report on compound (XI) (Sunitha et al., 2012) provides no analysis or description of the supramolecular assembly. Examination of the original atomic coordinates shows firstly that molecules related by a c-glide plane are linked by a nearly linear C-HÁ Á ÁO hydrogen bond, forming a C(6) chain running parallel to the [001] direction, and secondly that inversion-related pairs of molecules are linked by astacking interaction involving the phenyl rings of the molecules at (x, y, z) and (Àx + 1, Ày + 1, Àz), with interplanar spacing 3.4465 (10) Å , ring-centroid separation 3.749 (3) Å and ring-centroid offset 1.475 (3) Å . The combined effect of these two types of interaction is the formation of a sheet lying parallel to (100); see Fig. 10.
There are two intermolecular C-HÁ Á ÁO contacts in the structure of compound (XII) which were described (Prabhu et al., 2011a) as joining the molecules into chains: however, for these two contacts the HÁ Á ÁO distances, 2.68 and 2.71 Å , both exceed the sum of the van der Waals radii, 2.65 Å (Rowland & Taylor, 1996), so that these contacts certainly cannot be regarded as hydrogen bonds. Simple C(11) chains are formed in the structure of compound (XIII) built from C-HÁ Á ÁO hydrogen bonds (Vepuri et al., 2011), but there are no short BrÁ Á ÁBr contacts in either of (XI) and (XIII).

Synthesis and crystallization
For the synthesis of each compound, an equimolar mixture (0.01 mol of each component) of the appropriate 2-acetyl-5halogenothiophen and the appropriately-substituted benzaldehyde was dissolved in a mixture of methanol (20 ml) and aqueous sodium hydroxide solution (5 ml of 30% w/v solu- Part of the crystal structure of compound (IX), showing the formation of a centrosymmetric -stacked dimer. For the sake of clarity, the H atoms and the unit-cell outline have been omitted. The original atomic coordinates (Prabhu et al., 2011b) have been used and the S atom marked with an asterisk (*) is at the symmetry position (Àx + 2, Ày + 2, Àz + 2).

Figure 10
A stereoview of part of the crystal structure of compound (XI), showing the formation of sheets parallel to (100) built from -stacked hydrogenbonded C(6) chains. The original atomic coordinates (Sunitha et al., 2012) have been used and, for the sake of clarity, the H atoms not involved in the motif shown have been omitted. tion). The mixtures were all stirred at ambient temperature for 4 h, and then poured into ice-cold water (250 ml): the resulting solid products were collected by filtration and dried in air at 323 K. Crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation, at ambient temperature and in the presence of air, of solutions in acetone: melting points: Computer programs: CrysAlis PRO and CrysAlis RED (Agilent, 2012), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and PLATON (Spek, 2008).

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.