Crystal structures and Hirshfeld surface analysis of 5-amino-1-(4-methoxyphenyl)pyrazole-4-carboxylic acid and 5-amino-3-(4-methoxyphenyl)isoxazole

The title compounds feature O—H⋯O carboxylic-acid inversion dimers and N—H⋯N chains in their extended structures.

The title compounds, C 11 H 11 N 3 O 3 , (I), and C 10 H 10 N 2 O 2 , (II), are commercially available and were crystallized from ethyl acetate solution. The dihedral angle between the pyrazole and phenyl rings in (I) is 52.34 (7) and the equivalent angle between the isoxazole and phenyl rings in (II) is 7.30 (13) . In the crystal of (I), the molecules form carboxylic acid inversion dimers with an R(8) ring motif via pairwise O-HÁ Á ÁO hydrogen bonds. In the crystal of (II), the molecules are linked via N-HÁ Á ÁN hydrogen bonds forming chains propagating along [010] with a C(5) motif. A weak N-HÁ Á Á interaction also features in the packing of (II). Hirshfeld surface analysis was used to explore the intermolecular contacts in the crystals of both title compounds: the most important contacts for (I) are HÁ Á ÁH (41.5%) and OÁ Á ÁH/HÁ Á ÁO (22.4%). For (II), the most significant contact percentages are HÁ Á ÁH (36.1%) followed by CÁ Á ÁH/HÁ Á ÁC (31.3%).

Structural commentary
The molecular structure of compound (I) is shown in Fig. 1. The pyrazole ring (r.m.s. deviation = 0.010 Å ) is rotated by 52.34 (7) relative to the phenyl ring (r.m.s. deviation = 0.010 Å ), which is the primary contribution to the general nonplanarity of the molecule. An intramolecular N3-H3AÁ Á ÁO2 hydrogen bond is observed (Table 1 and Fig. 1). This bond forms an S(6) ring motif ( Fig. 1 and Table 1) with an N3Á Á ÁO2 distance of 2.941 (3) Å . This is a common feature in analogous compounds (such as those listed in the Database survey). The C3-N3 distance of 1.353 (2) Å is typical for an amino group bound to an aromatic ring. The carboxylic carbon-oxygen distances are 1.255 (2) and 1.316 (2) for C4-O2 and C4-O1, respectively, indicating that the former bond may be affected by the intramolecular N-HÁ Á ÁO hydrogen bond.
The molecular structure of compound (II) is shown in Fig. 2. The angle between the phenyl and isoxazole rings is 7.30 (13) , resulting in the overall molecule being close to planar with the r.m.s. deviation of all non-hydrogen atoms being 0.054 Å . The N1-O1 distance is 1.434 (4) Å and is consistent with other isoxazoles (see Database survey section). The C3-N2 distance is 1.350 (5) Å and is typical of an amino group bound to an aromatic ring.

Hirshfeld surface analysis
The intermolecular interactions were further investigated by quantitative analysis of the Hirshfeld surface, and visualized with Crystal Explorer 17.5 (Turner et al., 2017;Spackman et al., 2009) and the two-dimensional fingerprint plots (McKinnon et al., 2007). The shorter and longer contacts are indicated as red and blue spots, respectively, on the Hirshfeld surfaces, and contacts with distances approximately equal to the sum of the van der Waals radii are colored white. The function d norm is a The molecular structure of (II) with displacement ellipsoids drawn at the 50% probability level.

Figure 3
A view along the a-axis direction of the crystal packing of (I) with hydrogen bonds shown as red dashed lines. Table 2 Hydrogen-bond geometry (Å , ) for (II).

Figure 4
A view along the a-axis direction of the crystal packing of (II) with hydrogen bonds shown as red dashed lines.

Synthesis and crystallization
Compounds (I) and (II) are commercially available and were purchased from Aldrich. Both were dissolved in ethyl acetate until saturated and these solutions were allowed to evaporate slowly at room temperature, which resulted in X-ray quality crystals.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 3. All carbon-bound H atoms were positioned geometrically and refined as riding, with C-H = 0.95 or 0.98 Å and U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). In order to ensure a chemically meaningful O-H distance in (I), this was restrained to a target value of 0.84 (2) Å and U iso (H) = 1.5U eq (O). In (I), the amino H atoms were located in a difference-Fourier map. In (II), the N-H distances were restrained to a target value of 0.84 (2) Å and U iso (H) = 1.5U eq (N). The absolute structure of (II) was indeterminate based on the present refinement.

Funding information
The authors would like to thank Georgia Southern University, Department of Chemistry and Biochemistry for the financial support of this work. Hirshfeld surface for (II) mapped over d norm .

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

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