Synthesis and structure of 4-{[(E)-(7-methoxy-1,3-benzodioxol-5-yl)methylidene]amino}-1,5-dimethyl-2-phenyl-2,3-dihydro-1H-pyrazol-3-one

The reaction of 4-amino-1,5-dimethyl-2-phenyl-1,2-dihydropyrazol-3-one with 4-methoxybenzo[1,3]dioxole-5-carbaldehyde resulted in the title compound, which crystallizes in space group C2/c. Two solvent-accessible voids, each of 397 Å3, were found to be evident in the crystal structure.


Chemical context
Compounds such as 4-aminoantipyrine (4-amino-1,5-dimethyl-2-phenylpyrazole) and its Schiff base analogues are chemically attractive because of the various biological properties they possess, their synthetic flexibility and their selectivity and sensitivity towards metal ions (Keskiog lu et al., 2008). Pyrazol-3-one Schiff bases can be obtained from the condensation of 4-aminophenazone or 4-aminoantipyrine (4-AAP) and the corresponding carbonyl compound (Sakthivel et al., 2020). Schiff bases can find applications in analytical chemistry, material sciences and in various biological fields. In analytical chemistry, Schiff bases obtained from 4-AAP and 2-hydroxy-1,2-diphenylethenone have been used as a colorimetric sensor for Fe III and as a fluorescent sensor for Al III (Soufeena & Aravindakshan, 2019). Some other 4aminophenazone analogues have been applied in the separation and determination of pentachlorophenol in treated softwoods and preservative solutions (Williams, 1971). In material sciences, the corrosion inhibition tendency of 4-AAP and its derivatives has also been discussed (Junaedi et al., 2013). Other derivatives have also been used to improve solar cell efficiency (Ismail et al., 2020). Various 4-AAP derivatives have several biological applications and 4-AAP Schiff bases from the condensation with para-methoxycinnamaldehyde display antimicrobial activity against a large spectrum of microorganisms (Obasi et al., 2016). Still more 4-AAP derivatives show DNA binding and cleavage activity has also been reported (Rosenberg et al., 1969). Several other biological applications include antioxidant, anti-inflammatory (Deng et al., 2019), analgesic and antipyretic (Murtaza et al., 2017) among others. Platinum(II) complexes of Schiff bases have been reported as potential anti-cancer agents. Some of these complexes have a better toxicity than that of Cisplatin (Li et al., 2013).
As part of our studies in this area, the title compound, C 20 H 19 N 3 O 4 , was obtained from 4-AAP and myristicin aldehyde and its crystal structure determined.

Structural commentary
The title compound (I) crystallizes in the monoclinic centrosymmetric space group C2/c, and the asymmetric unit consists of one non-planar independent molecule. The phenyl ring (C15-C20) is twisted away from the plane of the pyrazole ring moiety (N2/N3/C10-C12) by 50.95 (8) , most likely because of steric hindrance of the phenyl ring and the methyl substituents on the pyrazole ring. Puckering analysis (Cremer & Pople, 1975) carried out in PLATON (Spek, 2020) showed that the methylene carbon atom (C8) on the benzodioxolyl ring (consisting of atoms O3/C4/C5/O4/C8) can be described as the flap of an envelope with a puckering amplitude Q of 0.162 (2) Å and angle of 323.1 (8) . A Mogul (Bruno et al., 2004) geometry check as performed in Mercury (Macrae et al., 2020) did not yield any significant unusual geometrical parameters within the structure. An intramolecular C9-H9Á Á ÁO2 hydrogen bond (Fig. 1, Table 1) generates an S(6) ring.
Interestingly, after completing the structural refinement the structural checks suggested that the structure contains two solvent-accessible voids, each of 397 Å 3 . The PLATON SQUEEZE (Spek, 2015) algorithm was applied to the refinement to explain this structural feature and assign the electron density accordingly. Since the material was synthesized in ethanol, it is likely that the voids were created by the solvent and once the crystals were extracted from the reaction mixture and the solvent evaporated, voids were formed in this way. The voids can be seen in the packing arrangement (Fig. 2).

Supramolecular features
Analysis of the crystal packing of I clearly shows the channels of void space, especially when viewed down the c-axis direction (Fig. 2). The molecules tend to stack on top of one The molecular structure of I, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. Dashed red lines indicate hydrogen-bonding interactions. Table 1 Hydrogen-bond geometry (Å , ).
Cg2 and Cg4 are the centroids of the pyrazole (N2/N3/C10-C12) and phenyl (C15-C20) rings, respectively.   Table 1), which help to consolidate the packing. This is particularly evident in Fig. 3 where the hydrogen bonds can be seen to be connecting layers of molecules together. The hydrogenbonding network (three-dimensional in nature) showing the four most prominent hydrogen-bonding interactions (one being an intramolecular interaction) can be seen in Fig. 4. It may be noted that atom O2 accepts all the hydrogen bonds (one intramolecular and three intermolecular). The second graph-set that is clearly visible in Fig. 4 is a ring motif with graph-set descriptor R 1 2 (7). It is these intermolecular interactions that connect the molecules between layers, as shown in Fig. 3. Two weak C-HÁ Á Á interactions are also present (Table 1).

Database survey
A search for the exact structure of the title compound in the Cambridge Structural Database (CSD Version 2020.2.0; Groom et al., 2016) yielded no hits. In order to determine if the structures of other similar compounds had been published, we expanded the structure search to only include the 2,3-Diihydro-1H-pyrazole moiety as the backbone for other possible structures. A search was carried out in the CSD with no filters applied and this yielded 322 compounds. Of these, 92 of the compounds were coordinated to metals or were co-crystals and classified as 'organometallic? under the CSD search filter. The remaining 230 compounds are then classified as 'organic? under the CSD search filter. Thus, the title compound falls into this latter category.

Synthesis and crystallization
The title compound was prepared by reflux of a solution containing 4-amino-1,5-dimethyl-2-phenyl-1,2-dihydropyrazol-3-one (0.244 g, 1.20 mmol) in 5 ml of ethanol and a solution of 4-methoxybenzo[1,3]dioxole-5-carbaldehyde (0.179 g, 1.20 mmol) in 5 ml of ethanol. The reaction mixture was stirred for 24 h under reflux. Crystals of the title compound were obtained from ethanol solution by slow evaporation. A suitable crystal was selected from the mother liquor for the single-crystal X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-bound H atoms were placed in geometrically idealized positions, with C-H = 0.93-0.99 Å , and were constrained to ride on their parent atoms with relative isotropic displacement coefficients, with U iso (H) = 1.2U eq (C) for aromatic and methylene H atoms, and U iso (H) = 1.5U eq (C) for methyl H atoms. The methyl H atoms were initially located in a different-Fourier map and they were placed in idealized positions as described above and refined as rotating groups. The structure contained two solvent accessible voids of 397 Å 3 each, thereby giving a total void volume of 794 Å 3 . No substantial electron density peaks were found in the solvent-accessible voids and the residual electron density peaks could not arranged in an interpretable pattern. The cif and fcf files were thus corrected for using reverse Fourier transform methods using the SQUEEZE routine (Spek, 2015) as implemented in the program PLATON (Spek, 2020). The resultant files were used in the further refinement. The SQUEEZE procedure corrected for 28 electrons within the two solvent-accessible voids.

Acknowledgements
The Research Centre for Synthesis and Catalysis is acknowledged for providing funding for the characterization of the compounds discussed in this paper. The University of Johannesburg X-ray Diffraction Unit is acknowledged for infrastructure to collect the data of the title compound.

Computing details
Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). 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.