(4Z)-4-[(2-Chloroanilino)(phenyl)methylidene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

The title compound, C23H18ClN3O, exists in an enamine–keto form with the amino group involved in an intramolecular N—H⋯O hydrogen bond. The five-membered ring is nearly planar, the largest deviation being 0.0004 (7) Å, and makes dihedral angles of 16.62 (6), 41.89 (5) and 71.27 (4)° with the phenyl rings. In the crystal, weak C—H⋯O hydrogen bonds link the molecules into supramolecular chains along the b axis.

The molecular structure of the title compound is shown in Fig.1. Atoms O1, C10, C9 and C11 of the PMBP moiety and atom N3 of the o-chloroaniline group are coplanar, the largest deviation being 0.038 (11) Å for atom C10. The dihedral angle between this mean plane and the pyrazole ring of PMBP is 5.76 (3)°. The C9-C11 bond length of 1.3887 (15) Å, between usual C-C and C=C bond, indicates the delocalization of the electrons because of the addition of a proton to N3 is more favorable than to O2. The atom O2 of the PMBP moiety and the N3 atom of the o-chloroaniline group are on the same side of the C9-C11 bond, which are available for coordination with metal cations. A strong intramolecular hydrogen bond N3-H3A···O1 (Table 1) is also indicative of the enamine-keto form. In the crystal structure, the intramolecular hydrogen bond C6-H6···O1 and intermolecular hydrogen bond C16-H16···O1 are observed, the latter links the molecules into supramolecular chains along the b axis. All bond lengths and angles are normal and comparable with those found in related compounds (Zhang et al., 2007;Li et al., 2009;Chi et al., 2010).

Experimental
A mixture of a 10 ml PMBP (2 mmol, 0.5566 g) anhydrous ethanol solution, and a 0.21 ml of an o-chloroaniline (2 mmol, 0.2545 g) solution was refluxed for ca 5 h, with addition of a few drops of glacial acetic acid as a catalyst. The ethanol was removed by evaporation and the resulting green precipitate formed was filtered off, washed with cold anhydrous ethanol and dried in air. Yellow block single crystals suitable for analysis were obtained by slow evaporation of a solution in anhydrous ethanol at room temperature for a few days.

Refinement
The H3A atom bonded to N3 was located in a difference Fourier map and refined freely. The other H atoms were placed in calculated positions, with C-H = 0.93 Å for phenyl, 0.96 Å for methyl H atoms, and refined as riding, with U iso (H) = 1.2U eq (C) for phenyl H, and 1.5U eq (C) for methyl H.

Figure 1
The molecular structure of the title compound (thermal ellipsoids are shown at the 30% probability level).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.86725 (13