Unexpected formation of a co-crystal containing the chalcone (E)-1-(5-chlorothiophen-2-yl)-3-(3-methylthiophen-2-yl)prop-2-en-1-one and the keto–enol tautomer (Z)-1-(5-chlorothiophen-2-yl)-3-(3-methylthiophen-2-yl)prop-1-en-1-ol

The crystallization of (E)-1-(5-chlorothiophen-2-yl)-3-(3-methylthiophen-2-yl)prop-2-en-1-one furnished a superimposed co-crystal consisting of the expected chalcone and a minor component identified as (Z)-1-(5-chlorothiophen-2-yl)-3-(3-methylthiophen-2-yl)prop-1-en-1-ol tautomer


Structural commentary
The crystal structure ( Fig. 1) exhibits two superimposed molecules with occupancies of 93% and 7%: this was surprising since the formation of the minor (enol) component was quite unexpected. A possible mechanism for the formation of this component is shown in Fig. 2. Equilibria between keto and enol isomers are regularly observed in solution but not in crystals. This issue needs a thorough exploration, which is beyond the scope of this report.
The molecular structures show similar conformations but differ in bond lengths and the carbon-atom geometry (hybridization), which we will describe for the major component in more detail. The molecular structure ( Fig. 1) is composed of two substituted thiophene rings, 5-chlorothiophen-2-yl and 3methylthiophen-2-yl, which are linked by the central -CO-CH CH-spacer. The configuration about the C C bond [1.344 (3) Å ] is E and the carbonyl group is syn with respect to the C C bond. The molecule is effectively planar as indicated by the torsion angles O1-C1-C10-C14 = 175.0 (3), C2- (19) and C2-C3-C4-C8 = À177.5 (2) . The hydrogen atoms of the propenone unit are trans configured and each is involved in an intramolecular short contact that forms an S(5) motif ( Fig. 1, Table 1). The bond lengths and angles are consistent with those in related structures (Vu Quoc et al., 2019;Yesilyurt et al., 2018;Sreenatha et al., 2018). The S atoms of the terminal 5-chlorothiophen-2-yl (S11/C10/C12-C14) and 3-methylthiophen-2-yl (S5/C4/C6-C8) rings are anti and the rings are inclined slightly to each other [dihedral angle = 6.92 (13) ].

Supramolecular features
The extended structure exhibits several hydrogen-bonding contacts ( Table 1). The hydrogen bonds involve a carbonyl O atom serving as a double-acceptor with H atoms from the chlorothiophenyl unit, and a methyl group from the methylthiophenyl unit of a neighbouring molecule. Additional C-HÁ Á ÁS contacts are also present ( Possible mechanism for the formation of the minor enol component. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; y þ 1; z; (ii) Àx þ 1; Ày; Àz þ 1; (iii) x; y; z À 1.

Figure 1
(a) The molecular structure of the title co-crystal showing the superposition of the two components, whose occupancies are 93% (black bonds) and 7% (white bonds), (b) the molecular structure with the atomlabelling scheme of the major component and (c) the minor component. Displacement ellipsoids are drawn at the 50% probability level.

Synthesis and crystallization
The synthesis was carried out using a reported method (Al-Maqtari et al., 2015). Crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation, at room temperature, of a solution in ethanol.

Figure 3
Overall packing of the major component with all intermolecular interactions (dotted and dashed lines) shown.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were included in calculated positions (C-H = 0.95-0.98 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl). Methyl groups were allowed to rotate about the bond to their next atom to fit the electron density.
The crystal structure was refined as a superposition of two molecular structures with formulae C 12 H 9 ClOS 2 (93% occupancy component) and C 12 H 11 ClOS 2 (7% occupancy component), respectively. Restraints were necessary during the refinement of geometric and anisotropic displacement parameters.  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. The crystal structure is an overlay of two molecular structure with ratio 93:7.