4-[2-(4-Chlorophenyl)hydrazinylidene]-3-methyl-1H-pyrazol-5(4H)-one

In the title compound, C10H9ClN4O, the pyrazole ring [maximum deviation = 0.014 (2) Å] forms a dihedral angle of 7.06 (14)° with the chlorobenzene ring. The molecular conformation is stabilized by an intramolecular N—H⋯O hydrogen bond, which generates an S(6) ring motif. In the crystal, inversion dimers linked by pairs of C—H⋯O hydrogen bonds generate R 2 2(16) ring motifs. The dimers are further connected by N—H⋯N hydrogen bonds, thereby forming layers lying parallel to the bc plane.

In the title compound, C 10 H 9 ClN 4 O, the pyrazole ring [maximum deviation = 0.014 (2) Å ] forms a dihedral angle of 7.06 (14) with the chlorobenzene ring. The molecular conformation is stabilized by an intramolecular N-HÁ Á ÁO hydrogen bond, which generates an S(6) ring motif. In the crystal, inversion dimers linked by pairs of C-HÁ Á ÁO hydrogen bonds generate R 2 2 (16) ring motifs. The dimers are further connected by N-HÁ Á ÁN hydrogen bonds, thereby forming layers lying parallel to the bc plane.

Comment
Pyrazole are nitrogen-containing heterocyclic compounds and various procedures have been developed for their synthesis (Rai & Kalluraya, 2006). The chemistry of pyrazole derivatives has been the subject of much interest due to their various applications and widespread potential and proven biological and pharmacological activities (Rai et al., 2008). Steroids containing a pyrazole moiety are of interest as psychopharmacological agents. Some alkyl-and aryl-substituted pyrazoles have a sharply pronounced sedative action on the central nervous system. Furthermore, certain alkyl pyrazoles show significant bacteriostatic, bacteriocidal, fungicidal, analgesic and anti-pyretic activities (Sridhar & Perumal, 2003).

Experimental
To a solution of ethyl-(2-[2-(4-chlorophenyl)hydrazinylidene]-3-oxobutanoate (0.01 mol) dissolved in glacial acetic acid (20 ml), a solution of hydrazine hydrate (0.02 mol) in glacial acetic acid (25 ml) was added and the mixture was refluxed for 4 h. It is cooled and allowed to stand overnight. The solid product that separated was filtered and dried. It was then recrystallized from ethanol. Yellow needles were obtained from 1:2 mixtures of DMF and ethanol by slow evaporation.
The remaining H atoms were positioned geometrically and refined using a riding model with C-H = 0.95 or 0.98 Å and U iso (H) = 1.2 or 1.5 U eq (C). A rotating-group model was applied for the methyl group.    (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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 > 2sigma(F 2 ) is used only for calculating R-factors(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.