Four 1-aryl-1H-pyrazole-3,4-dicarboxylate derivatives: synthesis, molecular conformation and hydrogen bonding

1-Phenyl-1H-pyrazole-3,4-dicarboxylic acid and 1-(4-methoxyphenyl)-1H-pyrazole-3,4-dicarbohydrazide form complex hydrogen-bonded framework structures each containing multiple hydrogen-bond types, but dimethyl 1-phenyl-1H-pyrazole-3,4-dicarboxylate and dimethyl 1-(4-methylphenyl)-1H-pyrazole-3,4-dicarboxylate form simple cyclic dimers containing only C—H⋯O hydrogen bonds.


Structural commentary
The bond distances in compounds (I)-(IV) show no unexpected values: all are typical of their types (Allen et al., 1987). However, the molecular conformations show some interesting features. In each of (I) and (IV), the two carboxy substituents on the pyrazole ring are nearly coplanar with this ring, as shown by the leading torsional angles (Table 1): this is almost certainly a consequence of the presence on an intramolecular O-HÁ Á ÁO in (I) and an intramolecular N-HÁ Á ÁO hydrogen bond in (IV) ( Table 2). In compounds (I) and (III), where such intramolecular interactions are not possible, the carboxyl groups at C3 are by no means coplanar with the pyrazole ring (Table 1), and in compound (III) the 3-methoxycarbonyl substituent is disordered over two sets of atomic sites having occupancies 0.71 (2) and 0.29 (2) in the crystal selected for The molecular structure of compound (III) showing the atom-labelling scheme. The major disorder component, occupancy 0.71 (2), is drawn using full lines and the minor component, occupancy 0.29 (2), is drawn using dashed lines. Displacement ellipsoids are drawn at the 30% probability level.

Figure 4
The molecular structure of compound (IV) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 1
The molecular structure of compound (I) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The molecular structure of compound (II) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. data collection: the orientations of the two disorder components are related to one another by a rotation about the C3-C31 bond of approximately 23 (Table 1). It may be noted here that the ketonic O atom O31 acts as a hydrogen-bond acceptor in each of (I) and (IV), but not in (II) and (III) ( Table 2), and the disorder in (III) may be associated with this.
In each of (I) and (II), the planes of the aryl and pyrazole rings make much larger dihedral angles than these planes do in (II) and (IV) ( Table 1). This may be associated with the cooperative effect in (III) and (IV) of the C-HÁ Á ÁO hydrogen bonds involving atoms C5 and C12 as donors (Table 2), whereas no such cooperation is found in the structures of (I) and (II).

Supramolecular features
The supramolecular assembly of compound (I) to form a three-dimensional framework structure depends upon four types of hydrogen bonds (Table 2), and the framework formation can readily be analysed in terms of one-dimensional sub-structures (Ferguson et al., 1998a,b;Gregson et al., 2000). A combination of O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, the latter rather weak, links molecules related by translation into a C(6)C(7)[R 2 1 (5)] (Etter, 1990;Etter et al., 1990;Bernstein et al., 1995) chain of rings running parallel to the [010] direction ( Fig. 6). In the second sub-structure, molecules related by the c-glide plane at y = 0.25 are linked by a C-HÁ Á ÁO hydrogen bond to form a simple C(10) chain running parallel to the [001] direction, and the combination of these two chain motifs generates an almost planar sheet lying parallel to (100) in the domain 1 4 < x < 1 2 (Fig. 6). Finally, two weak C-HÁ Á Á(arene) hydrogen bonds link this sheet to the adjacent sheets in the domains 0 < x < 1 4 and 1 2 < x < 3 4 , and in this way all of the (100) sheets are linked to form a three-dimensional framework structure.  Table 1 Selected torsional and dihedral angles ( ).

Figure 6
Part of the crystal structure of compound (I) showing the formation of a hydrogen-bonded sheet parallel to (100). Hydrogen bonds are shown as dashed lines and, for the sake of clarity, the H atoms bonded to C atoms but not involved in the motifs shown have been omitted.
By contrast, the supramolecular assembly in the ester (II) is extremely simple, with inversion-related pairs of molecules linked by C-HÁ Á ÁO hydrogen bonds (Table 2) to form a centrosymmetric R 2 2 (10) dimer (Fig. 7). A similar, but more complex centrosymmetric dimer is formed by the ester (III), where the same R 2 2 (10) motif as found in (II) is present, along with two flanking R 1 2 (7) rings within an outer R 2 2 (16) ring (Fig. 8). In neither (II) nor (III) are there any directionspecific interactions between adjacent dimers.
The supramolecular assembly in the hydrazide (IV) is the most complex of those reported here. A three-dimensional framework structure is built from four types of hydrogen bonds: N-HÁ Á ÁO, N-HÁ Á ÁN, N-HÁ Á Á(arene) and C-HÁ Á ÁO (Table 2). As for (I), the assembly is readily analysed in terms of simpler substructures. The hydrogen bond involving atom H42A links an inversion-related pair of molecules into an R 2 2 (10) dimer centred at ( 1 2 , 1 2 , 0), and this finite, zerodimensional sub-structure can be regarded as the basic building block of the overall structure, which can then be analysed in terms of the ways in which these dimers are linked together. The hydrogen bonds involving the atoms H31 and H32B directly link the reference dimer centred at ( 1 2 , 1 2 , 0) to four similar dimers, centred at (0, 0, À 1 2 ), (0, 1, À 1 2 ), (1, 0, 1 2 ) and (1, 1, 1 2 ), so forming a sheet lying parallel to (101) (Fig. 9), which is reinforced by the N-HÁ Á Á hydrogen bond ( Table 2). The final sub-structure in the assembly of (IV) is one-dimensional: two C-HÁ Á ÁO hydrogen bonds link the basic R 2 2 (10) dimers into a chain of rings running parallel to the [001] direction. Within this chain, two types of centrosymmetric Part of the crystal structure of compound (III) showing the formation of a hydrogen-bonded dimer containing R 1 2 (7), R 2 2 (10) and R 2 2 (16) ring motifs. Hydrogen bonds are shown as dashed lines and, for the sake of clarity, the minor disorder component and the H atoms not involved in the motifs shown have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (Àx, 1 À y, 1 À z).

Figure 7
Part of the crystal structure of compound (II) showing the formation of a hydrogen-bonded R 2 2 (10) dimer. Hydrogen bonds are shown as dashed lines and, for the sake of clarity, the H atoms not involved in the motif shown have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 À x, 1 À y, 1 À z). R 2 2 (10) ring can be identified, one containing N-HÁ Á ÁO hydrogen bonds and the other containing C-HÁ Á ÁO hydrogen bonds, along with R 1 2 (7) rings ( Fig. 10).

Database survey
It is of interest to compare briefly the structures of compounds (I)-(IV) reported here with those of some related compounds.
In each of 1-benzyl-3-phenyl-1H-pyrazole-5-carboxylic acid ( Tang  Part of the crystal structure of 3-phenyl-1H-pyrazole-5-carboxylic acid showing the formation of a sheet of R 2 2 (8) and R 6 6 (28) rings lying parallel to (100): hydrogen bonds are shown as dashed lines. The original atomic coordinates (Zhang et al., 2007) have been used and, for the sake of clarity, the H atoms bonded to C atoms have all been omitted.

Figure 9
Part of the crystal structure of compound (IV) showing the formation of a hydrogen-bonded sheet lying parallel to (101) and built from N-HÁ Á ÁO, N-HÁ Á ÁN and N-HÁ Á Á(arene) hydrogen bonds. Hydrogen bonds are shown as dashed lines and, for the sake of clarity, the H atoms bonded to C atoms have been omitted.

Figure 10
Part of the crystal structure of compound (IV) showing the formation of a hydrogen-bonded chain of rings parallel to [001] and built from N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds. Hydrogen bonds are shown as broken lines and, for the sake of clarity, the H atoms bonded to C atoms but not involved in the motifs shown have been omitted. pairs of molecules are linked by O-HÁ Á ÁO hydrogen bonds to form centrosymmetric R 2 2 (8) dimers. For the simpler analogue 3-phenyl-1H-pyrazole-5-carboxylic acid, the structure was described (Zhang et al., 2007) as consisting of chains built from O-HÁ Á ÁO and N-HÁ Á ÁN hydrogen bonds, which were then linked into sheets by C-HÁ Á ÁO hydrogen bonds. However, scrutiny of the atomic coordinates shows that the structure contains no C-HÁ Á ÁO hydrogen bonds, and that the combination of one O-HÁ Á ÁO hydrogen bond and one N-HÁ Á ÁN hydrogen bond generates sheets lying parallel to (100) and containing alternating R 2 2 (8) and R 6 6 (28) rings (Fig. 11). Finally, we note that structures have been reported for each of the precursor sydnones employed here (Fig. 5), for X = H (Hope, 1978), X = Me (Wang et al., 1984) and X = MeO (Fun et al., 2010) although, when X = H, there are no atomic coordinates deposited in the Cambridge Structural Database (Groom et al., 2016).

Synthesis and crystallization
The precursor sydnones (A) (Fig. 5) were prepared from the corresponding anilines (Greco et al., 1962;Wang et al., 1984;Fun et al., 2010). For the synthesis of the esters (II) and (III), a mixture of the sydnone of type (A) having X = H for (II) or X = CH 3 for (III), (1 mmol) and dimethyl acetylenedicarboxylate (1 mmol) in dry p-xylene (10 ml) was heated under reflux for 1 h. The mixtures were then cooled to ambient temperature, the solvent was removed under reduced pressure and the resulting solid products were recrystallized from ethanol. ( For the synthesis of the acid (I), the ester (II) (1 mmol) and solid sodium hydroxide (2 mmol) were dissolved in a waterethanol mixture (water:ethanol 80:20 v/v, 50 ml). This mixture was heated under reflux for 2h, cooled to ambient temperature and then acidified to pH 2 using dilute aqueous hydrochloric acid. The resulting solid product was collected by filtration,  Computer programs: APEX2, SAINT and XPREP (Bruker, 2004), SHELXS97 (Sheldrick, 2008b), SHELXL2014 (Sheldrick, 2015) and PLATON (Spek, 2009

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. One low-angle reflection, (001) in compound (III), which had been attenuated by the beam stop was removed from the data set. All H atoms were located in difference maps. The H atoms bonded to C atoms were subsequently treated as riding atoms in geometrically idealized position with C-H distances 0.93 Å (aryl and pyrazole) or 0.96 Å (CH 3 ) and with U iso (H) = kU eq (C), where k = 1.5 for the methyl groups, which were permitted to rotate but not to tilt, and 1.2 for all other H atoms bonded to C atoms. For the H atoms bonded to O or N atoms, the atomic coordinates were refined with U iso (H) = 1.5U eq (O) or 1.2U eq (N), leading to the O-H and N-H distances shown in Table 2. It was apparent that one of the ester substituents in compound (III) was disordered over two sets of atomic sites. For the minor disorder component, the bonded distances and the 1,2 nonbonded distances were restrained to be the same as the corresponding distances in the major disorder component, subject to s.u. values of 0.005 and 0.01 Å , respectively. In addition, the anisotropic displacement parameters for the corresponding pairs of atoms in the two disorder components were constrained to be the same, and the two disordered carboxylate fragments were constrained to be planar. Subject to these conditions, the occupancies of the two sets of sites refined to 0.71 (2) and 0.29 (2). For all structures, data collection: APEX2 (Bruker, 2004); cell refinement: APEX2/SAINT (Bruker, 2004); data reduction: SAINT/XPREP (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008b); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015) and PLATON (Spek, 2009). Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0015 (3)

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.