Synthesis, crystal structure and Hirshfeld surface analysis of dimethyl 3-(3-bromophenyl)-6-methyl-7-oxo-3,5,6,7-tetrahydropyrazolo[1,2-a]pyrazole-1,2-dicarboxylate

In the title compound, one of the fused pyrazole rings adopts an envelope conformation while the other displays a twisted conformation. In the crystal, the molecules are linked by C—H⋯O hydrogen bonds and aromatic π–π interactions.

As part of our studies in this area, the title compound was synthesized and its molecular and crystal structure and Hirshfeld surface analysis are reported herein.

Structural commentary
The molecular structure of the the title compound is shown in Fig. 1. There are two stereogenic centres at C2 and C5: in the arbitrarily chosen asymmetric molecule, they have configurations of S and R, respectively, but a racemic mixture in the crystal is generated in the centrosymmetric P1 space group. The structure is characterized by a disorder of the Br atom over two adjacent sites [BrÁ Á ÁBr = 0.32 (2) Å ]. The dihedral angle between the fused pyrazole rings (all atoms) is 32.91 (10) . The C1-C3/N1/N2 oxo-pyrazole ring displays an envelope conformation on C3 whereas the C5-C7/N1/N2 pyrazole ring is twisted on N2-C5, as indicated by the following respective puckering parameters: Q(2) = 0.2339 (19) Å , '(2) = 257.9 (4) and Q(2) = 0.2127 (16) Å , '(2) = 50.5 (4) . Moreover, the mean plane passing through the oxo-pyrazole ring subtends a dihedral angle of 61.15 (10) with the C12-C17 bromophenyl ring, which is practically perpendicular to the other pyrazole ring as indicated by the dihedral angle of 88.95 (9) . The non-H atoms of the ester groups are virtually coplanar, the maximum deviations from the mean planes being 0.017 (2) Å at C10 for the O2/O3/C10/ C11 grouping and 0.013 (1) Å at O5 for the O4/O5/C8/C9 grouping. The dihedral angle between these two planes is 62.15 (12) .

Supramolecular features
In the crystal, the molecules are linked by C-HÁ Á ÁO hydrogen bonds: O1 accepts two such bonds and O2 and O3 accept one each (Table 1 and Fig. 2). The bromophenyl rings of adjacent molecules are linked by an aromatic stackinginteraction with an inter-centroid distance of 3.8369 (10) Å .

Figure 2
Crystal packing for the title compound showing hydrogen bonds as dashed blue lines.

Figure 1
The molecular structure of the title compound showing displacement ellipsoids drawn at the 50% probability level. rings, the difference of which does not exceed one degree, i.e. 34.13 in RICFUF and 32.91 (10) in the title compound. It may be noted that the phenyl substituent is linked to the oxopyrazole ring and the two carboxylate groups to the other pyrazole ring in RICFUF, while in the title compound the phenyl and both carboxylate groups are linked to the same pyrazole ring.

Computational chemistry
Hirshfeld surface analysis The Hirshfeld surface (HS) analyses ) and two-dimensional fingerprint plots (McKinnon et al., 2007) generated using CrystalExplorer17.5 (Turner et al., 2017) show the various intermolecular interactions in the crystal structure. The three-dimensional d norm surface of the title compound using a standard surface resolution with a fixed colour scale of À0.21 to 1.38 a.u is shown in Fig. 3a,b. The intense red spots on the surface are due to the C-HÁ Á ÁO hydrogen bonds and C-HÁ Á ÁBr contacts. The bright-red spots in Fig. 3c indicate atoms with the potential to be hydrogenbond acceptors (negative electrostatic potential), while blue regions indicate atoms with positive electrostatic potential (hydrogen-bond donors) (Spackman et al., 2008).
Interaction energy calculations The intermolecular interaction energies between molecules in the title compound computed using a B3LYP/6-31G (d, p)     energy model available in Crystal Explorer 17.5 (Turner et al., 2017), where a cluster of molecules was generated within a radius of 3.8 Å by default. The total intermolecular energy (E tot ) is the sum of electrostatic (E ele ), polarization (E pol ), dispersion (E dis ), and exchange-repulsion (E rep ) energies. The energy frameworks, which provide a view of the supramolecular architecture of crystals, are represented by cylinders joining the centroids of molecular pairs using red, green and, blue colour codes for the E ele , E dis , and E tot energy components, respectively, with a cut-off value of 5 kJ mol À1 and a scale factor of 80 to all energy components (Fig. 5). The benchmarked energies E ele , E pol , E dis and E rep were scaled as 1.057, 0.740, 0.871 and 0.618, respectively (Mackenzie et al., 2017). The nature and strength of the energies for the key identified intermolecular interactions are summarized in Table 3. The computed interaction energies for electrostatic, polarization, dispersion and exchange repulsion are À107.7 kJ mol À1 , À33.9 kJ mol À1 , À299.7 kJ mol À1 and 185.2 kJ mol À1 , respectively. These data reveal that the dispersive component makes the major contribution to the intermolecular interactions in the crystal. The calculations showed that the C3-H3BÁ Á ÁO2 hydrogen bond has the greatest energy among all close contacts present in the crystal with its energy (-52.1 kJ mol À1 ) having a major electrostatic contribution (-21.9 kJ mol À1 ). The next most significant contribution, with a total energy of À34.9 kJ mol À1 , arises from the C11-H11BÁ Á ÁO2 hydrogen bond. Lower energies, compared to the above interactions, are calculated for the Br1AÁ Á ÁO4, C9-H9AÁ Á ÁO1 and C14-H14Á Á ÁO1 contacts.

Frontier molecular orbital (FMO) calculations
The optimized structure of the title compound was established in the gas phase using density functional theory (DFT) using the B3LYP exchange correlation functional and basis-set calculations (Becke, 1993) as implanted in GAUSSIAN 09 (Frisch et al., 2009). The differences between calculated and experimental bond lengths and angles are within a few Å ngstroms and degrees, respectively, when compared to the experimental parameters, which indicate that our calculations are acceptable (see supplementary Tables 1 and 2). The HOMO-LUMO gap of the molecule is calculated to be about 4.16 eV.

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.