Synthesis, stability assessment, thermodynamic function analysis, and biological activity of novel Schiff base complexes with manganese (II) and cadmium (II) ions

In this study, two novel complexes were synthesized via the reaction of a Schiff base ligand ( L ) with manganese (II) and cadmium (II) ions. The free ligand and its metal complexes were systematically characterized using FT-IR, UV–Vis spectrophotometry, molar conductance, magnetic susceptibility measurements, and melting point determination. Job's method of continuous variation confirmed a 1 : 2 (metal-to-ligand) stoichiometric ratio for both complexes. Molar conductance values in ethanol ( 13.5 muS · cm − 1 for Mn − L and 16.6 muS · cm − 1 for Cd − L ) verified their non-electrolytic nature. FT-IR spectra demonstrated coordination through the azomethine nitrogen atom, as evidenced by the shift of ν C = N to 1630 cm − 1 and the appearance of new ν M − N bands at 546 cm − 1 . UV–Vis electronic spectra and magnetic susceptibility measurements supported an octahedral geometry around both metal centers. Thermodynamic studies based on the Van ‘t Hoff approach revealed high stability constants ( log K f ), which decreased with increasing temperature ( 288 − 308 K ), confirming the exothermic nature ( Δ H ∘ < 0 ) of the complexation process. Negative Gibbs free energy changes ( Δ G ∘ < 0 ) across all temperatures indicated the thermodynamic feasibility and spontaneity of the reactions. Furthermore, positive Δ S ∘ values for Mn − L pointed to an entropy-driven pathway, whereas negative Δ S ∘ values for Cd − L reflected structural rigidity upon complexation. Cd(II) coordination enhances the complex's electronic distribution, steric properties, and lipophilicity, boosting its bacterial target binding. The Cd–L complex shows higher activity than the free ligand and Mn–L. Molecular docking reveals ligand and complexes interact with active site amino acids via hydrogen bonding, polar, hydrophobic, and π interactions, consistent with bioactivity.