21 Molecular and Cellular Biochemistry 253: 21–29, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. Spectroscopic, structural and antibacterial properties of copper(II) complexes with bio- relevant 5-methyl-3-formylpyrazole N(4)-benzyl- N(4)-methylthiosemicarbazone Dhiman Kumar Sau, 1 Ray J. Butcher, 2 Siddhartha Chaudhuri 3 and Nityananda Saha 1 1 Department of Chemistry, University of Calcutta, Kolkata, India; 2 Department of Chemistry, Howard University, Washington, DC, USA; 3 RSIC, Bose Institute, Kolkata, India Abstract The coordination behaviour of the title ligand, 5-methyl-3-formylpyrazole N(4)-benzyl-N(4)-methylthiosemicarbazone (HMPz4BM), is reported with solid state isolation of copper(II) complexes, [Cu(HMPz4BM)X 2 ] (X = Cl, Br, NO 3 , ClO 4 and BF 4 ) which have been spectroscopically and structurally characterised. I.r. data for the free ligand and its Cu(II) complexes indicate that HMPz4BM exhibits a neutral NNS tridentate function via the pyrazolyl nitrogen(tertiary), azomethine nitrogen and thione sulphur. Electronic spectral data are suggestive of a square pyramidal environment for the seemingly pentacoordinated Cu(II) species. E.s.r parameters (RT and LNT) of the reported copper(II) complexes are indicative of a d x2–y2 ground state for the reported species. Cyclic voltammograms of Cu(II) complexes show a quasireversible Cu II /Cu III couple and also an irrevers- ible Cu II /Cu I couple. X-ray crystallography of a representative species, [Cu(HMPz4BM)(NO 3 ) 2 ] (C2/c, monoclinic), has unam- biguously documented the conjectural findings from i.r. data that coordinating sites of the title ligand are pyrazolyl (tertiary) nitrogen, azomethine nitrogen and the thione sulphur (NNS); and the oxygen of one of the nitrate ions has occupied the basal plane; the fifth coordination position has been occupied by the oxygen of another nitrate ion in a square pyramidal geometry. The antibac- terial properties of the ligand and its copper(II) complexes studied on microorganism, Staphylococcus aureus have pointed out that most of the complexes have higher activities than that of the free ligand. (Mol Cell Biochem 253: 21–29, 2003) Key words: pyrazole, thiosemicarbazone, copper(II) complexes, spectroscopy, X-ray crystallography in DNA strands [4]. Moreover, recent studies have indicated that for these ligands the coordination by the tridentate thio- semicarbazone (tsc) moiety involving the heterocyclic atom as well as the substitution on the terminal N(4) atom are cru- cial for their antifungal activity [4a]. The antimicrobial activi- ties of the copper(II) complexes of pyridyl dithiocarbazates have been studied by Ali et al. [4b] who have found that the parent Schiff base ligand is inactive against various bacteria, but its square pyramidal copper(II) complexes of the type Cu(NNS)X 2, are remarkably active. Introduction Heterocyclic thiosemicarbazones and their transition metal complexes have received considerable attention from the co- ordination chemists, because of their well-documented bio- logical activities like antitumor [1, 1a], antimalarial [2] and antiviral [3] properties. Copper(II) complexes of heterocyclic N(4)-substituted thiosemicarbazones are known to exercise bio-activity via a mechanism involving either inhibition of the enzyme ribonucleotide reductase, or creation of lesions Address for offprints: N. Saha, University of Kalyani, Kalyani 741235, India (E-mail: n.saha@cucc.ernet.in; nsaha2@yahoo.com)