Co⁺², Ni⁺², Cu⁺², Zn⁺² metal complexes of 3-mercapto-4-(naphthalen-2-ylmethyleneamino)-5-oxo-1,2,4-triazine Schiff base: synthesis, spectral characterization, and biological applications

3-mercapto-4-(naphthalen-2-ylmethyleneamino)-5-oxo-1,2,4-triazine (<b>HLN</b>), a ligand was prepared by condensing 2-napthaldehyde and 4-amino-3-mercapto-5-oxo-1,2,4-triazine in equimolar ratios at ∼80 °C temperature. Then, synthesized <b>HLN</b> was characterized firstly by elemental analysis and then spectroscopic methods, that is, proton-NMR and IR spectroscopy. Cobalt(II), nickel(II), copper(II), zinc(II) metal complexes of <b>HLN</b> were synthesized and identified with the help of different techniques such as elemental analysis, proton-NMR, IR, electronic, EPR spectroscopy, magnetic moment, and electrochemical measurements. The complexes were found non-electrolytic by conductivity measurements. Thermal studies of metal complexes were also carried out which helped in determining the number of water molecules in coordination spheres. Kinetic variables were calculated by Coats-Redfern plots. The IR, electronic spectroscopic data, and magnetic moment studies proposed octahedral structures for all the complexes except copper complexes. EPR spectra corresponded to square planar geometry of copper complexes. Emission spectral plots showed more fluorescent nature of complexes than <b>HLN</b> ligand. <i>In vitro</i> antibacterial and antifungal activities of complexes were investigated against <i>Staphylococcus aureus</i>, <i>Bacillus subtilis</i> as gram-positive bacteria, <i>Escherichia coli</i>, <i>Pseudomonas aeruginosa</i> as gram-negative bacteria, and <i>Candida albicans</i> as fungi. Metal complexes exhibit noteworthy antipathogenic behavior against both bacterial strains and fungi. The results of antioxidant studies ensure significant utility of these metal complexes as potential antioxidants.