Crystal structure of a new polymorph of di(thiophen-3-yl) ketone

A new polymorph of di(thiophen-3-yl) ketone differing from the previous structure by the molecular assembly is reported and comparatively discussed.


Chemical context
With reference to the principle of bioisosterism (Lima & Barreiro, 2005), thiophene is an important structural moiety replacing benzene rings in drugs and biomolecules. Moreover, thiophene is a highly polarizable group due to the presence of the -electrons and the sulfur atom available in the ring, making it a structural unit worthy of investigation related to crystal engineering (Desiraju et al., 2012). This involves potential -stacking (Tiekink & Zukerman-Schpector, 2012) and C-HÁ Á Á (Nishio et al., 2009) interactions, as well as other contacts including a chalcogen atom such as sulfur (Gleiter et al., 2003). From this point of view, the title compound is likely to be an interesting study object. However, searching in the literature shows its crystal structure being already described twice (Sheldrick et al., 1978;Benassi et al., 1989). On the other hand, a polymorph resulted from our work, the structure of which is reported here and comparatively discussed in connection with the previous findings, bearing in mind the attention currently attracted by the field of polymorphism in molecular crystals (Bernstein, 2002;Cabri et al., 2007;Braga et al., 2009).

Structural commentary
The title compound crystallizes in the space group Pbcn with one half of the molecule in the asymmetric unit, i.e. the molecule is located on the twofold symmetry axis. A perspective ISSN 2056-9890 view of the molecular structure of the title compound is presented in Fig. 1. The bond distances within the molecule agree with those found in the reported crystal structures of the polymorphs of this compound (Sheldrick et al., 1978;Benassi et al., 1989). Taking into account experimental error, the thiophene rings are perfectly planar. The heteroatom of the ring is always on the opposite side with respect to C O, showing the molecule to be in an S,O-trans/S,O-trans conformation, as was predicted to be the more stable conformation for the compound (Benassi et al., 1989). The torsion angle along the atomic sequence O1-C5-C3-C4 is À155.2 (3) and corresponds to an interplanar angle of 42.3 (1) between the thiophene rings, being ascribed to steric hindrance between the H atoms on C4 and C4 0 .

Supramolecular features
The crystal structure is composed of molecular layers extending parallel to the ab plane (Table 1, Fig. 2). Within a given layer the molecules are connected via C-HÁ Á ÁO hydrogen bonds (Desiraju & Steiner, 1999) in which the oxygen atom acts as a bifurcated acceptor. Moreover, the layer structure featuresstacking (Tiekink & Zukerman-Schpector, 2012) with a centroidÁ Á Ácentroid distance of 3.946 (2) Å and a slippage of 1.473 Å between the interacting thiophene rings. No directed non-covalent bonding is observed between the molecules of consecutive layers, so that the crystal structure appears to be stabilized only by van der Waals forces in the stacking direction of the molecular layers.  (Sheldrick et al., 1978) and DTHKET01 (Benassi et al., 1989)]. In these polymorphs (space group: P2 1 /c, P2 1 /n, Z = 4) the molecules show slight conformational differences and one of their thiophene rings is disordered over two positions. It is research communications

Figure 2
Packing diagram of the title compound viewed down the a axis. Dashed lines represent hydrogen-bonding interactions.

Figure 3
Packing excerpt of the previously reported polymorph (Benassi et al., 1989). Hydrogen-bonding interactions are shown as dashed lines.

Figure 1
Perspective view of the molecular structure of the title compound. Anisotropic displacement ellipsoids are drawn at the 30% probability level.
obvious that crystallization from different solvents may have a fundamental influence on the molecular assembly in the solidstate structure, thus giving rise to polymorphism (Bernstein, 2002;Cabri et al., 2007;Braga et al., 2009). Unfortunately, the previous reports do not include information about the solvent used for crystallization of the compound and thus it is not possible to engage in a more qualified discussion of the facts.
In the structures of the reported polymorphs, C-HÁ Á ÁO hydrogen bonds connect the molecules into undulating sheets, in which the oxygen atom acts as a bifurcated acceptor (Fig. 3). Intersheet association is accomplished by C-HÁ Á Á contacts, resulting in a three-dimensional supramolecular architecture. In summary, the structures of the two polymorphs differ basically in the molecular assembly.

Synthesis and crystallization
The synthesis of the title compound has been reported by different groups and following different procedures (Gronowitz & Erickson, 1963;Pittman & Hanes, 1977;Lucas et al., 2000). We used the method of Lucas et al., reacting thiophen-3yl lithium (prepared from 3-bromothiophene and n-BuLi in dry diethyl ether/n-hexane at 195 K under argon) with N,Ndimethylcarbamoyl chloride. Column chromatography on SiO 2 with n-hexane/ethyl acetate (10:1) followed by recrys-tallization from methanol yielded the title compound as colourless crystals, m.p. 353 K. Previous values for the m.p. are 345-346 K (Gronowitz & Erickson, 1963) and 346 K (Lucas et al., 2000) pointing to polymorphic structures of the previously and presently isolated crystals.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms were positioned geometrically and refined isotropically using the riding model with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).

Funding information
We acknowledge the financial support within the Cluster  Computer programs: APEX2 (and SAINT (Bruker, 2008), SHELXS97 and SHELXTL (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and ORTEP-3 for Windows (Farrugia, 2012). Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). 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.