Synthesis of Schiff Base Ligand Based on 2-Aminobenzothiazole and 4-Hydroxybenzaldehyde and Their Nickel Complexes: Characterization and Antibacterial Studies

A Schiff base, (E)-4-((benzo[d]thiazol-2-ylimino)methyl)phenol (L1), was synthesized via the condensation of 2-aminobenzothiazole (L2) with 4-hydroxybenzaldehyde (L3), along with three nickel complexes: [Ni(L1)2Cl2]·5H2O, [Ni(L2)6]Cl2·3H2O, and [Ni(L3)6]Cl2·4H2O. The compounds were characterized using NMR, AAS, TGA, FTIR, UV-vis, PXRD, molar conductivity, and magnetic susceptibility analyses. Results showed that L1 acts as a bidentate ligand through its imine and thiazole groups, whereas L2 and L3 coordinate via amino and carbonyl groups, respectively. FTIR spectra confirmed Ni–N and Ni–O coordination, while magnetic and electronic spectra indicated octahedral geometries with paramagnetic properties (μeff = 2.89–2.98 B.M.). PXRD revealed nanocrystalline phases with high crystallinity (96–99%) and particle sizes of ~44 nm. Conductivity measurements classified the Schiff base nickel complex as a non-electrolyte, whereas the other two were electrolytes. Antibacterial assays demonstrated that [Ni(L1)2Cl2]·5H2O exhibited significantly higher inhibitory activity against Escherichia coli and Pseudomonas aeruginosa compared to Ni2+, the free ligands, and the other nickel complexes. These findings highlight Schiff base nickel complexes, particularly [Ni(L1)2Cl2]·5H2O, as promising candidates for the development of new antibacterial agents, offering potential alternatives for combating antibiotic-resistant bacterial infections.