Crystal structure of 7,8-dichloro-4-oxo-4H-chromene-3-carbaldehyde

In the crystal of this dichlorinated 3-formylchromone derivative, molecules are linked through stacking interactions, C—H⋯O hydrogen bonds and short C⋯O contacts. Halogen bonds between the formyl O and Cl atoms and type II halogen–halogen contacts between the Cl atoms are also formed.


Chemical context
Halogen bonding and halogen-halogen interactions have recently attracted much attention in medicinal chemistry, chemical biology, supramolecular chemistry and crystal engineering (Auffinger et al., 2004;Metrangolo et al., 2005;Wilcken et al., 2013;Sirimulla et al., 2013;Persch et al., 2015). Halogen bonding is defined as a net attractive interaction between an electrophilic region of a halogen atom in a molecule and a nucleophilic region of an atom in a molecule, and is characterized by a short contact between the two atoms. Halogen-halogen interactions are generally classified into two categories, type I ( 1 = 2 ) and type II ( 1 = 180, 2 = 90) where 1 and 2 are the two C-ClÁ Á ÁCl angles. The type I contact is considered to be van der Waals, and the type II is halogen bonding, i.e., an electrostatic interaction (Mukherjee et al., 2014;Metrangolo et al., 2014).

Structure commentary
The molecular structure of the title compound is shown in Fig. 2. The fused-ring system is slightly puckered [dihedral angle between the benzene and pyran rings = 3.66 (10) ]. The dihedral angle between the pyran ring and the formyl plane is 8.64 (7) .

Supramolecular features
In the crystal, the molecules are linked throughstacking interactions between molecules related by translationsymmetry i [centroid-centroid distance between the benzene and pyran rings of the 4H-chromene units = 3.727 (2) Å ; symmetry code: (i) x, y + 1, z], and through C-HÁ Á ÁO hydrogen bonds that involve C1/O2 and C4/O3 atoms, as shown in Fig. 3 and listed in Table 1.

Figure 2
The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as small spheres of arbitrary radius.
positions. Thus, it is suggested that the chlorine atoms should make their -holes larger, and their electropositive regions contact the electronegative regions of the oxygen and chlorine atoms.
In addition to the C-HÁ Á ÁO hydrogen bonds, halogen bonds and type II halogen-halogen contacts, an unusually short contact is revealed between the ,-unsaturated carbonyl O2 and the C-H group of C1 iv [2.838 (4) Å ; symmetry code (iv) -x + 1 2 , y, z + 1 2 ; Fig. 1d]. This interesting feature is possibly caused by a dipole-dipole interaction between the O atom and the C-H group that is enhanced by the polarizing effect of the two chlorine atoms at the 7-and 8positions of the chromone ring. These observations should be helpful in understanding interactions of halogenated ligands with proteins, and thus valuable for rational drug design.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 (Rigaku, 2010). Special details Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 σ(F 2 ) is used only for calculating R-factor (gt).