In this work, a novel series of pyridazinone derivatives (3–17) were synthesized and characterized by NMR (¹H and ¹³C), FT-IR spectroscopies, and ESI-MS methods. All synthesized compounds were screened for their antibacterial activities against Staphylococcus aureus (Methicillin-resistant), Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa, and Acinetobacter baumannii. Among the series, compounds 7 and 13 were found to be active against S. aureus (MRSA), P. aeruginosa, and A. baumannii with the lowest MIC value range of 3.74–8.92 µM. Afterwards, DFT calculations of B3LYP/6-31++G(d,p) level were carried out to investigate geometry structures, frontier molecular orbital, molecular electrostatic potential maps, and gap energies of the synthesized compounds. In addition, the activities of these compounds against various bacterial proteins were compared with molecular-docking calculations. Finally, ADMET studies were performed to investigate the possibility of using of the target compounds as drugs.

The title pyridazinone derivative, C₁₉H₁₄Cl₂N₂O, an important pharmacophore with a wide variety of biological applications is not planar, the chlorophenyl and pyridazinone rings being almost perpendicular, subtending a dihedral angle of 85.73 (11)°. The phenyl ring of the styryl group is coplanar with the pyridazinone ring [1.47 (12)°]. In the crystal, N—H...O hydrogen bonds form inversion dimers with an R ₂ ²(8) ring motif and C—H...Cl hydrogen bonds also occur. The roles of the intermolecular interactions in the crystal packing were clarified using Hirshfeld surface analysis, and two-dimensional fingerprint plots indicate that the most important contributions to the crystal packing are from H...H (37.9%), C...H/H...C (18.7%), Cl...H/ H...Cl (16.4%) and Cl...C/C...Cl (6.7%) contacts.

Two polymorphs of the title compound, C₁₉H₁₆N₂O₃, were obtained from ethanolic (polymorph I) and methanolic solutions (polymorph II), respectively. Both polymorphs crystallize in the monoclinic system with four formula units per cell and a complete molecule in the asymmetric unit. The main difference between the molecules of (I) and (II) is the reversed position of the hydroxy group of the carboxylic function. All other conformational features are found to be similar in the two molecules. The different orientation of the OH group results in different hydrogen-bonding schemes in the crystal structures of (I) and (II). Whereas in (I) intermolecular O—H...O hydrogen bonds with the pyridazinone carbonyl O atom as acceptor generate chains with a C(7) motif extending parallel to the b-axis direction, in the crystal of (II) pairs of inversion-related O—H...O hydrogen bonds with an R ₂ ²(8) ring motif between two carboxylic functions are found. The intermolecular interactions in both crystal structures were analysed using Hirshfeld surface analysis and two-dimensional fingerprint plots.