Synthesis, Characterization, and Antimicrobial Activity of a Schiff Base Derived from 2-Pyridinecarboxaldehyde and 5-Amino-2-Methoxypyridine and Its Mn(II), Co(II), and Ni(II) Complexes

A new pyridyl Schiff base ligand, L (C ₁₂ H ₁₁ N ₃ O), was synthesized by condensation of 2-pyridinecarboxaldehyde with 5-amino-2-methoxypyridine, and its Mn(II), Co(II), and Ni(II) complexes of the type [M(L) ₂ Cl ₂ ].xH ₂ O were prepared. The ligand and its complexes were characterized by melting/decomposition temperature, elemental analysis, solubility, molar conductance, magnetic susceptibility, FTIR and UV-Vis spectroscopy. L is a coffee-brown solid with a melting point of 90 ˚C and forms dark brown/black non-electrolytic complexes with Mn(II), Co(II) and Ni(II) in good yields (63–77%), all soluble in DMSO and DMF. FTIR spectra show a ⱱ(C = N) band at 1622 cm ^− 1 in L, shifting to 1568–1626 cm ^− 1 in the complexes, together with new bands assigned to M-N and coordinated water, consistent with coordination via the azomethine nitrogen and pyridyl nitrogen. Electronic spectra and effective magnetic moments (Mn: 5.66 BM; Co: 3.94 BM; Ni: 2.78 BM) support octahedral geometries around the metal centers. Antimicrobial activity of L and its complexes was evaluated by the agar well diffusion method against Gram-positive bacteria ( Streptococcus mutans , and Staphylococcus aureus ), Gram-negative bacteria ( Escherichia coli ) and fungal strains ( Candida albicans, Trichophyton rubrum, Trichophyton sp. Associated with Tinea pedis ). All compounds exhibited concentration-dependent activity (250–2000 µg/mL). Notably, the free Schiff base ligand generally displayed higher antimicrobial activity than its corresponding metal complexes, with inhibition zones reaching up to 18 mm against Streptococcus mutans and Trichophyton rubrum at 2000 µg/mL. These results demonstrate that metal coordination does not universally enhance antimicrobial efficacy and highlight the critical influence of ligand structure and metal ion identity on biological activity.