5-Amino-1-phenyl-1H-pyrazole-4-carboxylic acid

In the molecule of the title compound, C10H9N3O2, the pyrazole ring is approximately coplanar with the amino and carboxyl groups. The phenyl group is twisted by 48.13 (3)° relative to this plane. An intramolecular N—H⋯O hydrogen bond stabilizes the planar conformation of the molecule. The molecules are linked into two-dimensional sheets by two strong intermolecular N—H⋯N and O—H⋯O hydrogen bonds. The latter forms the classic carboxylic acid dimer motif.

In the molecule of the title compound, C 10 H 9 N 3 O 2 , the pyrazole ring is approximately coplanar with the amino and carboxyl groups. The phenyl group is twisted by 48.13 (3) relative to this plane. An intramolecular N-HÁ Á ÁO hydrogen bond stabilizes the planar conformation of the molecule. The molecules are linked into two-dimensional sheets by two strong intermolecular N-HÁ Á ÁN and O-HÁ Á ÁO hydrogen bonds. The latter forms the classic carboxylic acid dimer motif.
As part of our ongoing research on the synthesis and biological evaluation of heterocyclic compounds (Zia-ur-Rehman et al., 2005, 2006Siddiqui et al., 2007), the crystal structure of the title compound, (I), was determined. In (I), the pyrazole ring is approximately co-planar with the amino and carboxylic acid groups. The C-N bond lengths in the pyrazole ring are 1.3146 (18) and 1.3530 (16) Å, which are shorter than a typical C-N single bond length of 1.443 Å, but longer than a typical C-N bond length of 1.269 Å (Jin et al., 2004), indicating electron delocalization. Most of the bond lengths and angles in N-phenylpyrazole group are in consistent with those in similar molecules (Li et al., 2006;Zhong et al., 2006).
Each molecule exhibits an intramolecular N-H···O hydrogen bond which stabilizes the planar conformation and is linked to an adjacent one through head-to-tail pairs of O-H···O intermolecular interactions giving rise to dimeric motifs typical for carboxylic acids. Neighbouring dimers are further arranged into two-dimensional sheets in the (101) plane through N-H···N interactions (Fig.2).

Experimental
A mixture of 5-amino-1-phenyl-1H-pyrazole-4-carboxylic acid, ethyl ester (2.312 g; 10.0 mmoles), potassium hydroxide (1.12 g; 20 mmoles) and ethanol (25 ml) was refluxed for two hours. The reaction mixture was poured into ice cooled water and acidified with dilute hydrochloric acid to Congo Red. The precipitated solids were collected by filtration, washed and dried. Crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of solution of the title compound in a mixture of ethanol and water (85:15); m.p. 460 K; yield: 68%.

Refinement
H atoms bound to C were placed in geometric positions (C-H distance = 0.95 Å) using a riding model. H atoms on N and O had coordinates freely refined. U iso values were set to 1.2U eq (1.5U eq for OH). Fig. 1. The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

5-Amino-1-phenyl-1H-pyrazole-4-carboxylic acid
Crystal data 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.