3-(3-Chloro-2-hydroxyphenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde

In the title compound, C16H11ClN2O2, the pyrazole ring makes dihedral angles of 11.88 (13) and 22.33 (13)° with the 3-chloro-2-hydroxybenzene group and phenyl rings, respectively. The phenolic hydroxy group forms an intramolecular O—H⋯N hydrogen bond with the imine N atom of the pyrazole unit. The formyl group is virtually coplanar with the pyrazole ring [dihedral angle = 4.5 (19)°] and acts as an acceptor in an intramolecular C—H⋯O hydrogen bond closing seven-membered ring. In the crystal, adjacent molecules are linked through C—H⋯O hydrogen bonds into infinite chains along the b axis.

In the title compound, C 16 H 11 ClN 2 O 2 , the pyrazole ring makes dihedral angles of 11.88 (13) and 22.33 (13) with the 3-chloro-2-hydroxybenzene group and phenyl rings, respectively. The phenolic hydroxy group forms an intramolecular O-HÁ Á ÁN hydrogen bond with the imine N atom of the pyrazole unit. The formyl group is virtually coplanar with the pyrazole ring [dihedral angle = 4.5 (19) ] and acts as an acceptor in an intramolecular C-HÁ Á ÁO hydrogen bond closing sevenmembered ring. In the crystal, adjacent molecules are linked through C-HÁ Á ÁO hydrogen bonds into infinite chains along the b axis.
Financial support from the University of Malaya is highly appreciated (PPP grant PS359/2009 C).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: GK2410). The title compound was synthesized through the action of Vilsmeier-Haack reagent (DMF/POCl 3 ) on 3-chloro-2-hydroxyacetophenone phenylhydrazone. The compound contains three aromatic rings, the dihedral angles between them being 11.88 (13)° (pyrazole and phenol), 22.33 (13)° (pyrazole and phenyl) and 31.29 (12)° (phenyl and phenol). The phenol hydroxyl is hydrogen bonded to the pyrazole nitrogen, N2, and the formyl oxygen atom is directed towards the phenol ring to make an intramolecular C-H···O hydrogen bond with C5-H5. In contrary, in the crystal structures of the related compounds (Jeyakanthan et al., 2001;Shanmuga Sundara Raj et al., 1999) the formyl oxygen atoms are directed away from the phenol rings, being involved in intermolecular C-H···O hydrogen bonding. The crystal packing of the present compound exhibits infinite chains along the b axis formed by intermoleculoar C-H···O hydrogen bonds (Table 1).

Experimental
A mixture of equivalent amounts (24 mmol) of 3-chloro-2-hydroxyacetophenone and phenyl hydrazine in methanol (40 ml) was refluxed for 2 h. The reaction mixture was then cooled to room temperature whereupon the condensation product, 3-chloro-2-hydroxy acetophenone phenylhydrazone, was seperated out with 92% yield. The hydrazone (2.6 g, 0.01 mol) was dissolved in DMF (15 ml) and then POCl 3 (0.03 mol) was added dropwise at 0 o C. After the addition was complete, the reaction mixture was warmed to 60-70 o C and stirred for 2.5 h. The mixture was then poured onto crushed ice and neutralized by aqueous NaOH solution (10%). The precipitate was filtered, strongly washed with water and recrystallized from ethanol, yielding 85% of the pyrazole product (m.p. = 422-423 K). The needle shaped crystals of the compound were grown in a DMF solution at room temperature.

Refinement
Hydrogen atoms were all located in a difference Fourier map and their positions refined with U iso (H) set to 1.2U eq (C) or 1.2U eq (O).