3,5-Dimethyl-1H-pyrazole–2-hydroxy-5-(phenyldiazenyl)benzoic acid (1/1)

There are two independent 3,5-dimethylpyrazole and two independent 2-hydroxy-5-(phenyldiazenyl)benzoic acid molecules [in which intramolecular O—H⋯O bonds form S(6) graph-set motifs] in the asymmetric unit of the title compound, C5H8N2·C13H10N2O3. In the crystal, the components are linked by intermolecular O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonds, forming four-component clusters. Further stabilization is provided by weak C—H⋯π interactions.

We gratefully acknowledge financial support by the Education Office Foundation of ZheJiang Province (project No. Y201017321) and the innovation project of ZheJiang A & F University.
3,5-Dimethyl-1H-pyrazole-2-hydroxy-5-(phenyldiazenyl)benzoic acid (1/1) Y. Xu, S. Jin, J. Zhu, Y.-J. Liu and C.-C. Shi Comment Intermolecular interactions are responsible for crystal packing and gaining an understanding of these interactions allows the comprehension of the collective properties and permits the design of new crystals with specific physical and chemical properties (Lam & Mak, 2000). Hydrogen bonding is one of the most important noncovalent interactions that determines and controls the assembly of molecules and ions (Desiraju, 2002, Liu et al., 2008, Biswas et al., 2009. As an extension of our study concentrating on hydrogen bonded assembly of organic acids and organic bases (Jin et al., 2010), herein we report the crystal structure of the 1:1 adduct of 3,5-dimethyl pyrazole and 2-hydroxy-5-(phenyldiazenyl)benzoic acid.
The asymmetric unit of the compound consists of two independent 3,5-dimethyl pyrazole and two independent 2-hydroxy-5-(phenyldiazenyl)benzoic acid molecules (Fig. 1). Intramolecular hydrogen bonds between the phenol O-H groups and the carbonyl groups form S(6) graph motifs (Bernstein et al., 1995). The two independent carboxylic acid molecules form a dimer through O-H···O hydrogen bonds in which the phenol group is the donor and the carbonyl O atom acts as the acceptor. The two pyrazole molecules are linked to the carboxylic dimer through the N-H···O, and O-H···N hydrogen bonds to form a four component adduct containing R 2 2 (12) and R 3 3 (9) ring motifs (Fig. 2). Further stabilization is provided by weak C-H···π interactions.

Experimental
A solution of 3,5-dimethyl pyrazole (19.2 mg, 0.2 mmol) in 5 ml of MeOH was added to a MeOH solution (3 ml) containing 2-hydroxy-5-(phenyldiazenyl)benzoic acid (48.4 mg, 0.2 mmol) under continuous stirring. The solution was stirred for about 1 h at room temperature, then the solution was filtered into a test tube. The solution was left standing at room temperature for several days, red block-shaped crystals were isolated after slow evaporation of the solution in air at ambient temperature.

Refinement
All H atoms were placed in calculated positions with C-H = 0.93-0.96 Å, N-H = 0.86Å and O-H = 0.82Å and were included in the refinement with U iso (H)=1.2U eq (C,N) or U iso (H)=1.2U eq (C methyl ,O).   as those based on F, and R-factors based on ALL data will be even larger.