Bioactive Cr(III), Co(II), and Mn(II) Complexes with N′((3-Hydroxynaphthalen-2-yl)methylene)picolinohydrazide: Structural, Computational, and Biological Studies

The present study focuses on the synthesis and characterization of novel bioactive complexes of Cr-(III), Co-(II), and Mn-(II) derived from N'-((3-hydroxynaphthalen-2-yl)-methylene)-picolinohydrazide (H₂L), a Schiff-base ligand featuring multifunctional donor sites. The primary objective was to investigate the coordination behavior, stability, and biological efficacy of these metal chelates. The complexes were synthesized via direct metal-ligand reactions and characterized using elemental analysis, FT-IR spectroscopy, UV-Vis, PXRD, ¹H NMR spectroscopy, MS, magnetic susceptibility measurements, and thermogravimetric analysis. Spectroscopic evidence supported tetrahedral geometries for the Co-(II) and Mn-(II) complexes, while the Cr-(III) complex exhibited an octahedral arrangement. Thermogravimetric and kinetic studies yielded positive activation free energy (Δ<i>G</i>*) values, indicating nonspontaneous decomposition pathways and high thermal stability. Quantum Theory of Atoms in Molecules (QTAIM) and reduced density gradient (RDG) analyses were performed to elucidate the nature of ligand-receptor interactions. Biological assessments revealed promising results, as DNA degradation assays demonstrated notable nuclease-like activity, particularly for the Co-(II) complex. Antibacterial potency was evaluated via minimum inhibitory concentration (MIC), where the Mn-(II) complex exhibited the strongest activity (0.313 mg/mL), followed by Co-(II) and Cr-(III) (0.625 mg/mL each), and the free ligand (1.250 mg/mL). Cytotoxicity testing against HeLa, HCT-116, and MCF-7 cancer cell lines showed high anticancer efficacy for the [CoL]·2H₂O complex with IC₅₀ values of 7.76 ± 0.4, 10.23 ± 0.8, and 6.88 ± 0.4 μM, respectively. Molecular docking studies using an induced fit protocol highlighted the strong noncovalent interactions of the complexes with DNA targets relevant to each cell line.