Zinc complexes of Schiff base ligands exhibit interesting properties which could be exploited for potential applications as catalysts and models for biochemical systems. In this study, we report of the synthesis of a new zinc Schiff base complex, bis(N'-(pyridine-3-carboxaldehyde) isonicotinoylhydrazone) zinc(II), which has been characterized by physico-chemical methods. The X-ray single crystal structure of the complex has also been determined. The Schiff base ligand is bonded to the metal in a monodentate manner, coordinating through the pyridine N-atom, as shown from spectral studies. The crystal structure shows the zinc atom bonded to two Schiff bases and four aqua ligands, with each Schiff base ligand coordinating through its pyridine N-atom forming a distorted octahedral ZnN 2 O 4 complex. Two water molecules and two nitrate ions found in the crystal structure link the different monomers through hydrogen bonding to form a layered supramolecular structure.

Two tetradentate Schiff base ligands, (N,N '' -bis((2-hydroxy-1-phenyl)methylidene)-4-nitrophenyl-1,2-diamine (H 2 L 1 ) and N,N'-bis((2-hydroxy-1-naphthyl)methylidene)-4-nitrophenyl-1,2-diamine (H 2 L 2 )) 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 UVvis., FT-IR, 1 H NMR, 13 C 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.