1-(5-Hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)ethanone: a new monoclinic polymorph

The title compound, C12H12N2O2, crystallized in the monolinic space group P21/n, with two independent molecules (A and B) in the asymmetric unit. This is in contrast to the first monoclinic polymorph reported [Cingolani et al. (2002 ▶). Inorg. Chem. 41, 1151–116], which crystallized in the space group C2/c with one independent molecule per asymmetric unit. The dihedral angles between the two rings differ slightly; in molecule A it is 4.90 (11)° and in molecule B it is 16.05 (13)°. In both molecules, there is an intramolecular O—H⋯O hydrogen bond involving the hydroxyl substituent and the carbonyl O atom of the adjacent acetyl group. In the crystal structure, molecules A and B are linked via a C—H⋯N interaction. There are also some weak C—H⋯π interactions involving the phenyl ring of molecule A and H atoms of the acetyl groups of both molecules.

The title compound, C 12 H 12 N 2 O 2 , crystallized in the monolinic space group P2 1 /n, with two independent molecules (A and B) in the asymmetric unit. This is in contrast to the first monoclinic polymorph reported [Cingolani et al. (2002). Inorg. Chem. 41, 1151-116], which crystallized in the space group C2/ c with one independent molecule per asymmetric unit. The dihedral angles between the two rings differ slightly; in molecule A it is 4.90 (11) and in molecule B it is 16.05 (13) . In both molecules, there is an intramolecular O-HÁ Á ÁO hydrogen bond involving the hydroxyl substituent and the carbonyl O atom of the adjacent acetyl group. In the crystal structure, molecules A and B are linked via a C-HÁ Á ÁN interaction. There are also some weak C-HÁ Á Á interactions involving the phenyl ring of molecule A and H atoms of the acetyl groups of both molecules.

Related literature
For early literature on pyrazoles, see: Knorr (1883). For information on the pharamceutical properties of pyrazoles, see: Grimmett (1970). For the monoclinic C2/c polymorph of the title compound, see: Cingolani et al. (2002).

Experimental
Crystal data C 12 H 12 N 2 O 2 M r = 216.24 Monoclinic, P2 1 =n a = 13.8735 (7) Å b = 9.2037 (4) Å c = 18.3702 (8) Table 1 Hydrogen-bond geometry (Å , ).  The history of pyrazoles began already in the late nineteenth century (Knorr, 1883). Pyrazole is isomeric with the biologically important imidazole ring system but, unlike imidazole, has fewer natural derivatives. The ring system is very stable and inert, and interest in such compounds stemmed from their applications as drugs, dyes and as anesthetics. They are also used as antioxidants in fuels but their major applications have been in the pharmaceutical (Grimmett, 1970) and agricultural industries. In view of the importance of pyrazole derivatives we have planned a systematic study of such compounds, and describe here the crystal structure of a new polymorph of the title compound.

D-HÁ
It crystallized in the monoclinic space group P2 1 /n, with two independent molecules (A and B) per asymmetric unit ( Fig.   1). This is in contrast to an earlier reported monoclinic polymorph, (Cingolani et al., 2002), which crystallized in the space group C2/c with one independent molecule per asymmetric unit. The bond distances and angles in both polymorphs are very similar. The dihedral angles between the two rings differ slightly; in molecule A it is 4.90 (11)° and in molecule B it is 16.05 (13)°. The corresponding value in the other polymorph is 5.33 (10)°.

In both molecules (A and B), there is an intramolecular O-H···O hydrogen bond involving the hydroxyl substituent
and the carbonyl O atom of the adjacent acetyl group (Table 1); this feature is also present in the C2/c polymorph. In the crystal structure, molecules A and B are linked via a C-H···N interaction ( Fig. 2 and Table 1). There are also some weak C-H···π interactions involving the phenyl ring (centroid Cg2) of molecule A and some H atoms of the acetyl groups of both molecules (Table 1).
Experimental 1-Phenyl-3-methyl-5-pyrazolone (7.5 g) was dissolved by heating in tetrahydrofuran (80 ml). Calcium hydroxide (12 g) was added and acetyl chloride (4 ml) was then added dropwise over a period of 1 min. The temperature increased during the first few minutes and the reaction mixture became a thick paste. This mixture was then refluxed for 30 min. The calcium complex of the title compound that had formed in the flask was decomposed by pouring the mixture into a dilute solution of HCl (100 ml). A dark brownish-red organic layer was obtained which was extracted using dichloromethane. The solvent was then removed by vacuum distillation and the solid obtained was washed with a little water and THF. Crystals of the title compound, suitable for X-ray analysis, were obtained by recrystallization from methanol/water (1:1, v:v).

Refinement
The H atoms were included in calculated positions and treated as riding atoms: O-H = 0.83 Å, C-H = 0.93 -0.96 Å with U iso (H) = kU eq (parent atom), where k = 1.2 for aromatic H and 1.5 for all other H atoms. Methyl group C34 undergoes considerable thermal motion but splitting the atom did not improve the situation.