(Z)-1-(2-Chlorophenyl)-3-methyl-4-[2-(4-nitrophenyl)hydrazin-1-ylidene]-1H-pyrazol-5(4H)-one

There are two independent molecules, A and B, in the asymmetric unit of the title compound, C16H12ClN5O3. The relative orientations of the chlorophenyl ring with respect to the pyrazole ring in the two crystallographically independent molecules are different, and their corresponding dihedral angles are −53.3 (2) and 114.09 (18)° in molecules A and B, respectively. There are two strong intramolecular N—H⋯O hydrogen bonds, and two weak intramolecular C—H⋯O and C—H⋯Cl hydrogen bonds. The crystal packing is constructed by weak C—H⋯O and N—H⋯O interactions, and two π–π stacking interactions [centroid–centroid distances = 3.7894 (9) and 3.5719 (10) Å], forming a molecular ladder along the a axis.


Maria Teresa Garland Comment
In previous papers, we have informed preliminary results about the synthesis of a large library of pyrazoles by reaction of β-diketohydrazones with substituted arylhydrazines (Bustos et al., 2009) and three molecular structures of this type of compounds have been reported (Bustos et al., 2006(Bustos et al., , 2007(Bustos et al., , 2012. As a consequence of these studies we have found that α-hydrazo-β-ketoesters yield pyrazolones by reaction with substituted arylhydrazines. Now, we present the synthesis and the molecular structure of the title compound, prepared by reaction of (E)-ethyl 2-(2-(4-nitrophenyl)hydrazinylidene)-3oxobutanoate with (2-chlorophenyl)hydrazine.
The title compound shown in Fig. 1, crystallizes with two independent molecules in the asymmetric unit, where the chlorophenyl ring display a different orientation with respect to the pyrazole ring in molecules A and B. The corresponding dihedral angles between the chlorophenyl and pyrazole rings, in molecule A and molecule B are -53.3 (2)° and 114.09 (18)° respectively. In the crystal, two strong intramolecular hydrogen bonds (N4-H7···O1 and N9-H8···O4) are observed (Fig. 1, Table 1). As shown in Fig. 1 and Table 1, there are three additional weak intramolecular contacts that link molecules A and B (N9-H8···O2, C15-H15···Cl2 and C28-H28···O2). The atoms in the C1-C6 ring are slightly disordered due to thermal motion.

Experimental
In a 100 ml round-bottomed flask were added 2.34 g (8.38 mmole) of (E)-ethyl 2-(2-(4-nitrophenyl)hydrazinylidene)-3oxobutanoate, 1.60 g (8.94 mmole) 2-(2-chlorophenyl)hydrazine hydrochloride, 5 ml of glacial acetic acid and 50 ml of ethanol. The reaction mixture was magnetically stirred and heated under reflux during 36 h. Then, after cooling at room temperature, the precipitate was filtrated by suction and dried in a vacuum oven at 45°C during 24 h. Yield 78% of crude product. Single crystals suitable for X-ray studies were obtained by recrystallization from tetrahydrofuran.

Refinement
All H atoms were found in difference Fourier maps. The H atoms attached to the N9 and N4 atoms were refined freely against the difraction data, but all other H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with aromatic C-H = 0.95 Å, methyl C-H = 0.98 Å and U iso (H) = 1.2U eq (aromatic C) or U iso (H) = 1.5U eq (methyl C).    Part of the crystal packing showing the formation of a dimer via C10-H10B···O3 iii weak contacts (dashed lines). In addition, there are two π-π stacking interactions (Cg···Cg i and Cg···Cg iii ) that form a molecular ladder along the a direction, where Cg is the centroid of the C11-C16 ring. [Symmetry codes: (i) -x, -y + 1, -z + 1; (iii) -x + 1, -y + 1, -z + 1].

(Z)-1-(2-Chlorophenyl)-3-methyl-4-[2-(4-nitrophenyl)hydrazin-1-ylidene]-1H-pyrazol-5(4H)-one
Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses 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 observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs 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.