Ni(II) and Cu(II) Complexation Enhances Reactivity and Bioactivity of a New 3‐(4‐Methoxyphenyl)‐1‐(4‐[(2‐Hydroxy‐1‐Naphthyl)Diazenyl]Phenyl)Prop‐2‐en‐1‐One‐Based Schiff Base Ligand: Synthesis, Characterization, and Antimicrobial Evaluation

The novel bidentate N, O-donor Schiff base ligand RB, identified as 3-(4-methoxyphenyl)-1-(4-[(2-hydroxy-1-naphthyl)diazenyl]phenyl)prop-2-en-1-one, was synthesized. FT-IR analysis revealed characteristic bands for phenolic ─OH at 3115 cm^-1 and carbonyl C═O at 1669 cm^-1, whereas the ¹H nuclear magnetic resonance (NMR) spectrum confirmed the phenolic proton at δ 9.87 ppm. Complexation with Ni(II) and Cu(II) led to the formation of neutral octahedral [M(RB)₂(H₂O)₂] complexes, as supported by multiple analytical techniques. The low molar conductivities indicate their non-electrolytic nature. Upon coordination, the IR spectra exhibit a shift of ν(OH) to higher values (3420-3423 cm^-1), consistent with phenolic deprotonation, and a significant decrease in ν(C═N) to 1511-1514 cm^-1, indicative of azomethine nitrogen coordination. Magnetic measurements show a high-spin d⁸ configuration for NiRB (3.11 B.M.) and a typical d⁹ value for CuRB (1.73 B.M.), aligning with observed d-d transitions at 525 nm (NiRB) and 540 nm (CuRB). Mass spectra confirm molecular ions at m/z 911.105 (NiRB) and 915.882 (CuRB). Density functional theory (DFT) calculations confirm the octahedral structures and reveal reduced highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps upon complexation (CuRB: 2.15 eV; NiRB: 2.24 eV; RB: 3.04 eV), reflecting increased reactivity. The complexes also display enhanced softness (CuRB: 0.47 eV^-1 > NiRB: 0.45 eV^-1 > RB: 0.33 eV^-1), correlating with improved biological activity. Antimicrobial assays demonstrate that CuRB exhibits the highest efficacy, showing inhibition zones up to 20 mm against Escherichia coli and 85% activity against Klebsiella pneumoniae, followed by NiRB and RB. Molecular docking studies with bacterial tyrosyl-tRNA synthetase (TyrRS, PDB ID: 1KZN) show that CuRB has the strongest binding affinity (-8.80 kcal/mol), forming key hydrogen bonds with ARG76 and ILE90, and electrostatic interaction with GLU50. These interactions directly align with the observed bioactivity trends, establishing the order CuRB > NiRB > RB.