Schiff Base Ligand and its Metal Complexes: Synthesis, Structural Insights, Thermal Behavior, and Antimicrobial Activity

Abstract

A Schiff base ligand, 1-{[(3,5-dimethylphenyl)imino]methyl}naphthalen-2-ol (NL₁), was synthesized via condensation of 3,5-dimethylaniline and 2-hydroxy-1-naphthaldehyde in methanol (1:1 molar ratio), affording a yellow solid in 93% yield (m.p. 138°C–140°C). Spectroscopic studies (FT–IR, ¹H/¹³C NMR, EI–MS) confirmed N, O-bidentate coordination via azomethine nitrogen (ν(C═N) = 1614 cm⁻¹) and phenolic oxygen, with shifts to 1595–1599 cm⁻¹ and metal–ligand vibrations at 424–574 cm⁻¹. The ligand showed a molecular ion peak at m/z 275.1 (C₁₉H₁₇NO). Thermal analysis indicated enhanced stability upon complexation, with decomposition onset at ∼373°C (Ni), ∼296°C (Co), and ∼422°C (Zn), compared to 230–362°C (94.9% mass loss) for NL₁. Antimicrobial assays revealed moderate activity for NL₁ (11.0–14.5 mm), while metal complexes showed improved efficacy. The Zn(II) complex exhibited the highest activity, with inhibition zones of 15.0–17.0 mm against Escherichia coli, Proteus mirabilis, and Staphylococcus aureus, and 15.5 ± 1.6 mm against Candida albicans. Enhanced activity is attributed to chelation-induced lipophilicity, improved membrane permeability, Zn⁺²-mediated disruption, and ROS generation, highlighting [Zn(L₁)₂] as a promising antimicrobial candidate.