Crystal structure and the DFT and MEP study of 4-benzyl-2-[2-(4-fluorophenyl)-2-oxoethyl]-6-phenylpyridazin-3(2H)-one

The title pyridazin-3(2H)-one derivative, crystallizes with two independent molecules in the asymmetric unit. The two molecules differ essentially in the orientation of the benzyl ring with respect to the central pyridazine ring; this dihedral angle being 3.70 (9) ° in one molecule and 10.47 (8) ° in the other.

The title pyridazin-3(2H)-one derivative, C 25 H 19 FN 2 O 2 , crystallizes with two independent molecules (A and B) in the asymmetric unit. In molecule A, the 4-fluorophenyl ring, the benzyl ring and the phenyl ring are inclined to the central pyridazine ring by 86.54 (11), 3.70 (9) and 84.857 (13) , respectively. In molecule B, the corresponding dihedral angles are 86.80 (9), 10.47 (8) and 82.01 (10) , respectively. In the crystal, the A molecules are linked by pairs of C-HÁ Á ÁF hydrogen bonds, forming inversion dimers with an R 2 2 (28) ring motif. The dimers are linked by C-HÁ Á ÁO hydrogen bonds and a C-HÁ Á Á interaction, forming columns stacking along the a-axis direction. The B molecules are linked to each other in a similar manner and form columns separating the columns of A molecules.

Chemical context
Pyridazin-3(2H)-ones are pyridazine derivatives, being constructed about a six-membered ring that contains two adjacent nitrogen atoms, at positions one and two, and with a carbonyl group at position three. The interest in these nitrogen-rich heterocyclic derivatives arises from the fact that they exhibit a number of promising pharmacological and biological activities. These include anti-oxidant (Khokra et al., 2016), anti-bacterial and anti-fungal (Abiha et al. 2018), anti-cancer (Kamble et al. 2017), analgesic and anti-inflammatory (Ibrahim et al. 2017), anti-depressant (Boukharsa et al. 2016) and anti-ulcer activities (Yamada et al., 1981). In addition, a number of pyridazinone derivatives have been reported to have potential as agrochemicals, for example as insecticides (Nauen & Bretschneider, 2002), acaricides (Igarashi & Sakamoto, 1994) and herbicides (Azaari et al., 2016). The present work is a part of an ongoing structural study of heterocyclic compounds and their utilization as molecular (Faizi et al., 2016) and fluorescence (Mukherjee et al., 2018;Kumar et al., 2017; sensors. Given the interest in this class of compounds and the paucity of structural data, the crystal structure analysis of the title pyridazin-3(2H)-one derivative has been undertaken, along with a DFT study, in order to gain further insight into the molecular structure. ISSN 2056-9890

Structural commentary
The title compound crystallizes with two independent molecules (A and B) in the asymmetric unit (Fig. 1). In each molecule, a central oxopyridazinyl ring is connected to a fluorobenzylacetate group, a phenyl group, and a benzyl residue. The oxopyridazinyl ring (B) is planar in both molecules; r.m.s. deviations are 0.029 Å for molecule A and 0.009 Å for molecule B. In molecule A, the 4-fluorophenyl ring (A; C1A-C6A), the benzyl ring (C; C20A-C25A) and the phenyl ring (D; C13A-C18A) are inclined to the central pyridazine ring (B; N1A/N2A/C9A-C12A) by 86.54 (11), 3.70 (9) and 84.87 (13) , respectively. In molecule B, the corresponding dihedral angles are 86.80 (9), 10.47 (8) and 82.01 (10) , respectively. Hence, the conformation of the two molecules differs essentially in the orientation of the benzyl ring (C) with respect to the central pyridazine ring (B); 3.70 (9) in molecule A compared to 10.47 (8) in molecule B. The two molecules have an r.m.s. deviation of 0.683 Å for the 30 non-hydrogen atoms ( Fig. 2; PLATON; Spek, 2009).

Supramolecular features
In the crystal, the A molecules are linked by pairs of C-HÁ Á ÁF hydrogen bonds, forming inversion dimers with an R 2 2 (28) ring motif (Table 1 and Fig. 3). The dimers are linked by C-HÁ Á ÁO hydrogen bonds and a C-HÁ Á Á interaction (Table 1) A structural overlap view of molecule A (black) on molecule B (red), drawn using PLATON (Spek, 2009). Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
A view along the a axis of the crystal packing of the title compound. The C-HÁ Á ÁF hydrogen bonds are shown as dashed lines (see Table 1).

Figure 1
The molecular structure of the title compound, with the atom labelling and displacement ellipsoids drawn at the 30% probability level.
molecules are linked to each other in a similar manner (Table 1), and also form columns separating the columns of A molecules, as illustrated in Fig. 3.

Frontier molecular orbitals analysis
The highest occupied molecular orbitals (HOMOs) and the lowest-lying unoccupied molecular orbitals (LUMOs) are named as frontier molecular orbitals (FMOs). The FMOs play an important role in the optical and electric properties, as well as in quantum chemistry and UV-vis spectra. As a result of the interaction between the HOMO and LUMO orbitals of a structure, a transition state of the -* type is observed according to molecular orbital theory. The frontier orbital gap helps characterize the chemical reactivity and the kinetic stability of the molecule. A molecule with a small frontier orbital gap is generally associated with a high chemical reactivity, low kinetic stability and is also termed as a soft molecule. The DFT quantum-chemical calculations for the title compound were performed at the B3LYP/6-311 G(d,p) level (Becke, 1993) as implemented in GAUSSIAN09 (Frisch et al., 2009). The DFT structure optimization was performed starting from the X-ray geometry and the experimental values of the bond lengths and bond angles match the theoretical values.
The DFT study shows that the HOMO and LUMO are localized in the plane extending from the whole substituted oxopyridazinyl ring. The electron distribution of the HOMOÀ1, HOMO, LUMO and LUMO+1 energy levels is shown in Fig. 4. The HOMO molecular orbital exhibits both and character, whereas HOMOÀ1 is dominated by -orbital density. The LUMO is mainly composed of -density while LUMO+1 has both and electronic density. The HOMO-LUMO gap is 0.15669 a.u. and the frontier molecular orbital energies, E HOMO and E LUMO are À0.22571 and À0.06902 a.u., respectively.

Molecular electrostatic potential surface analysis
The molecular electrostatic potential (MEP) is a technique of mapping electrostatic potential onto the iso-electron density surface. The MEP surface provides information about the reactive sites. The colour scheme is as follows: red for electron rich, partial negative charge; blue for electron-deficient, partial positive charge; light blue for a slightly electron deficient region; yellow for a slightly electron-rich region; green for neutral (Politzer & Murray, 2002). In addition to these, in the majority of the MEPs, while the maximum positive region, which is the preferred site for nucleophilic attack, is indicated in blue, the maximum negative region, which is the preferred site for electrophilic attack, is indicated in red. The threedimensional plot of the MEP of the title compound is shown in Fig. 5. According to the MEP map results, the negative regions of the whole molecule are located on donor oxygen atoms (red regions). The resulting surface simultaneously displays the molecular size and shape and electrostatic potential values. As can be seen from the MEP map contours, regions having negative potential are over the electronegative atoms (viz. atoms O1A and O2A of molecule A and O1B and O2B of molecule B). The positive regions are over hydrogen atoms, indicating that these sites are the most likely to be involved in nucleophilic processes. Electron distribution of the HOMOÀ1, HOMO, LUMO and the LUMO+1 energy levels for the title compound.

Figure 5
Total electron density mapped over the molecular electrostatic potential surface of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The carbon-bound H atoms were placed in calculated positions (C-H = 0.93-0.97 Å ) and included in the refinement in the riding-model approximation, with U iso (H) = 1.2U eq (C). The image plate disc in the diffractometer used for the data collection was unfortunately distorted at the outer edges, hence the maximum 2 value available was limited to 48.8 .    (Farrugia, 2012), PLATON (Spek, 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2018 (Sheldrick, 2015b), WinGX (Farrugia, 2012) and PLATON (Spek, 2009). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.16 e Å −3 Δρ min = −0.14 e Å −3 Extinction correction: (SHELXL2018; Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0116 (12) 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.