Cobalt (II), Nickel (II) and Copper (II) complexes of Tetradentate Schiff base ligands derived from 4-Nitro-O-phenylenediamine: Synthesis, Characterization, cyclic and studies

Two tetradentate Schiff base ligands, (N,N''-bis((2-hydroxy-1-phenyl) methylidene)-4-nitrophenyl-1,2-diamine (H2L1) and N,N'-bis((2-hydroxy-1-naphthyl)methylidene)-4-nitrophenyl-1,2-diamine (H2L2)) have been synthesized by the condensation of 4-nitro-orthophenylenediamine with 2-hydroxy-1-naphthaldehyde and salicylaldehyde respectively in absolute ethanol and reacted with Co(II), Ni(II) and Cu(II) salts to form the corresponding complexes. The compounds were characterized by UV-vis., FT-IR, H-1 NMR, C-13 NMR, ESI-mass spectra, elemental analysis, and molar conductance measurements. The ligands were found to be tetradentate, coordinating to the metal ions through the azomethine nitrogen and oxygen atoms of phenol and naphthol and the sulphur atoms of thiophenol groups. Conductance measurements shows that these compounds are molecular. Cyclic voltammetry studies shows that the Co(III)/Co(II) and Ni(III)/Ni(II) redox systems are quasi-reversible involving a mono electronic transfer while Cu(III)/Cu(II) is irreversible. In vitro antimicrobial screening against five bacterial strains (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Enterococcus faecalis, Proteus mirabilis and five fungal strains (Candida albicans, Candida glabrata, Candida tropicalis, Candida krusei, Candida parapsilosis), shows that some of the compounds exhibited moderate antibacterial and antifungal activities compared to the reference drugs. Antioxidant studies reveal that the complexes are more potent than the ligands to eliminate free radicals.