Synthesis, characterization, crystal structures and antibacterial activity of copper(II), nickel(II) and zinc(II) complexes with a chloro-substituted salicylaldimine Schiff base

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A chloro-substituted salicylaldimine Schiff base, 2-[(cyclopentylimino)methyl]-6-chlorophenol (HL), was synthesized by condensation of 3-chlorosalicylaldehyde with cyclopentylamine in methanol. Reactions of HL with copper(II), nickel(II) and zinc(II) acetates afforded [CuL₂] (1), [NiL₂] (2) and [ZnL₂] (3), respectively, while the reaction of HL with Zn(OAc)₂ in the presence of NH₄SCN produced the mixed-ligand complex [ZnL(HL)(NCS)] (4). The ligand and complexes were characterized by elemental analysis, IR and UV–Vis spectroscopy, and the ligand was additionally characterized by ¹H NMR spectroscopy. Single crystal X-ray diffraction revealed that complexes 1 and 2 possess essentially square-planar CuN₂O₂ and NiN₂O₂ coordination environments, whereas complexes 3 and 4 exhibit distorted tetrahedral ZnN₂O₂ coordination geometries. The antibacterial activities of HL and complexes 1–4 were evaluated against three Gram-positive and three Gram-negative bacterial strains. Complexes 3 and 4 showed the lowest MIC values against B. subtilis (0.39 μg mL^–1) and S. aureus (1.56 μg mL^–1), while complex 4 exhibited a MIC of 12.5 μg mL^–1 against E. coli. The reported MIC endpoints suggest compound- and strain-dependent activity, but replicate variability is not documented and a causal relationship between solid-state coordination geometry and antibacterial activity is not established.