(E)-3-(4-Chlorophenyl)-3-[3-(4-chlorophenyl)-1H-pyrazol-1-yl]prop-2-enal

In the title compound, C18H12Cl2N2O, the pyrazole ring is almost planar [r.m.s. deviation = 0.002 Å] while the two chlorophenyl rings are twisted out from the plane of the pyrazole ring, making dihedral angles of 5.3 (1) and 65.34 (4)°. In the crystal, centrosymmetric R 2 2(24) dimers are formed about crystallographic inversion centres through a pair of C—H⋯Cl interactions. These dimers are further linked through a C—H⋯O hydrogen bond, forming a C(8) chain extending along the a axis. C—H⋯π interactions are also observed.

In the title compound, C 18 H 12 Cl 2 N 2 O, the pyrazole ring is almost planar [r.m.s. deviation = 0.002 Å ] while the two chlorophenyl rings are twisted out from the plane of the pyrazole ring, making dihedral angles of 5.3 (1) and 65.34 (4) . In the crystal, centrosymmetric R 2 2 (24) dimers are formed about crystallographic inversion centres through a pair of C-HÁ Á ÁCl interactions. These dimers are further linked through a C-HÁ Á ÁO hydrogen bond, forming a C(8) chain extending along the a axis. C-HÁ Á Á interactions are also observed.

Comment
Pyrazole and its derivatives have been successfully tested for their fungicidal (Chen & Li, 1998), antihistaminic (Mishra et al., 1998), anti-inflammatory (Smith et al., 2001), antiarrhythmic and sedative (Bruno et al., 1990), hypoglycemic (Cottineau et al., 2002), antiviral (Baraldi et al., 1998) activities. Pyrazole derivates possess antimicrobial (Velaparthi et al.,2008), anticancer (Magedov et al., 2007) and anti-inflammatory (Rovnyak et al., 1982) properties. They can also be used as biodegradable agrochemicals (Wamhoff et al., 1993) as well as pesticides (Londershausen, 1996). Wide variety of biological effects of these molecules provoked interest for their crystal structure study and accordingly we have synthesized the title compound by multi-component reaction which conforms to principles of green chemistry. The crystal structure of the title compound is reported here.
The title molecule is shown in Fig. 1. The pyrazole ring is planar with the r.m.s. deviation equal to 0.002 Å. The sum of the bond angles at N1 of the pyrazole ring (359.7 (1)°) is in accordance with the sp 2 hybridization of this atom (Beddoes et al., 1986). The C-N bond lengths in the pyrazole ring are 1.370 (2) [C5-N1] and 1.327 (2)Å [C3-N2] long. These distances are shorter than the pertinent single bond length (1.443 Å), however, they are longer than the double bond length (1.269 Å) (Jin et al., 2004). The values of these distances in the title structure indicate electron delocalization. Each of two centrosymmetric R 2 2 (24) dimers (Etter et al., 1990) are formed around the crystallographic inversion centres through a pair of the respective C-H···Cl interactions (Figs. 3 and 4 referring to C13-H13···Cl2 and C36-H36···Cl1, respectively; Tab. 1). Though the latter H···Cl distances are somewhat longer by about 0.5Å than the accepted values for the C-H···Cl hydrogen bonding (Desiraju & Steiner, 1999) these dimers are an important motif in the present structure and therefore they are reported here.
These dimers are linked through a C-H···O bond making a chain C(8) motif (Etter et al., 1990) that extends along the a axis of the unit cell (Fig. 5). Further, one of these primary ring R 2 2 (24) motifs and the chain C(8) motif are combined to form a secondary ring R 4 4 (32) motif (Fig. 6). Also, the C-H···π-electron ring interactions are observed in the structure. The crystallographic inversions link the latter motifs into pairs, forming another ring motif.
The reaction mixture was then irradiated under microwaves for 30 sec using a Biotage Microwave Synthesizer (frequency 2.45 GHz corresponding to the wavelength equal to 12.24 cm). The process of the reaction was monitored by thin layer chromatography using petroleum ether and ethyl acetate (4:1 v/v) as an eluent. The R f value of the product was 0.62. After completion of the reaction, the reaction mixture was poured into crushed ice and extracted with dichloromethane. The organic layer was dried over anhydrous sodium sulfate. The title compound was separated by column chromatography: carrier: silica gel (60-120 mesh), eluent: petroleum ether and ethyl acetate mixture (98:2 v/v). The compound was crystallized from dichloromethane. Colourless crystals of a prismatic habitus with the average size of 0.5 × 0.5 × 0.2 cm were grown within a week.

Refinement
All the H atoms were observable in the difference electron density map. All the hydrogens except the one from the aldehyde group were situated into the idealized positions and refined by the riding model approximation. The used values for the constraints: d(C-H) = 0.93 Å. U iso (H)= 1.2U eq (C). The positional parameters of the aldehyde hydrogen were refined freely while U iso (H)= 1.2U eq (C). Fig. 1. The title molecule with the atom numbering scheme. The displacement ellipsoids are shown at the 50% probability level.

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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.