627 J. Indian Chem. Soc., Vol. 93, June 2016, pp. 627-634 Synthesis, spectroscopic characterization and antimicrobial studies of oxomolybdenum(V) and dioxomolybdenum(VI) complexes with NNS donor tridentate Schiff base ligands Saikat Sarkar* a† , Sanat Kumar Nag b , Asoke Prasun Chattopadhyay* b and Kamalendu Dey b a Department of Chemistry, Chakdaha College, Chakdaha-741 222, West Bengal, India E-mail : saikat_s@rediffmail.com b Department of Chemistry, University of Kalyani, Kalyani-741 235, West Bengal, India E-mail : asoke.chattopadhyay@gmail.com Manuscript received online 14 January 2016, accepted 24 February 2016 Abstract : The reactions of diacetylmonoxime with thiosemicarbazide and morpholine N-thiohydrazide produce two different tridentate Schiff base ligands. Using these ligands some dioxomolybdenum( VI) and oxomolybdenum(V) complexes have been synthesized. The newly isolated complexes have been characterized by elemental analyses, molecular weight measurements by Rast’s method, magnetic moments, molar conductance and other usual spectroscopic data. The ligands and their dioxomolybdenum(VI) and oxomolybdenum(V) complexes have also been screened for in vitro antimicrobial activity against bacteria ( Escherichia coli and Staphylococcus aureus ) and fungi ( Aspergillus niger and Pencillium chrysogenum). The complexes were found to be more potent as compared to the ligands and the oxo-complex are also found to more active than dioxo-species. Keywords : Schiff base, molybdenum, complex, spectroscopy, antibacterial, antifungal. Introduction Molybdenum having stable and variable oxidation states has been found to play a number of important roles in the biological world 1,2 . Different enzymes like aldehyde oxi- dase, xanthine oxidase, xanthine dehydrogenase, nitrate reductase etc. contain molybdenum in their active site and often called as oxomolybdenum enzymes 3,4 . In recent years much attention has been focused on the molybdenum che- lates, since many molybdenum(V) and molybdenum(VI) complexes have been found to act as simple models for biochemical reactions 5 and in nitrogenase enzymes 6 . Mo- lybdenum complexes are reported to show efficient cata- lytic activity in the reactions like epoxidation of olefins, ammoxidation and olefin metathesis 7–10 . In continuation of our earlier work on the transition metal complexes of hydrazides, hydrazones, thiohydrazides and thio- hydrazones 11 , we have studied the complexation reactions of H 2 L and H 2 L with MoO 2 (acac) 2 , Na 2 MoO 4 .2H 2 O, † Previous address : Department of Chemistry, Santipur College, Santipur-741 404, West Bengal, India. (NH 4 ) 2 [MoOCl 5 ] and (PyH) 2 [Mo(SCN) 6 ] leading to the syntheses of many new dioxomolybdenum( VI ) and oxomolybdenum(V) complexes. Results and discussion Syntheses : The reactions of diacetylmonoximethiosemicarbazone (H 2 L) and diacetylmonoximemorpholine-N-thiohydrazone (H 2 L) with MoO 2 (acac) 2 , Na 2 MoO 4 .2H 2 O, (NH 4 ) 2 – [MoOCl 5 ] and (PyH) 2 [Mo(SCN) 6 ] under varied reaction conditions yielded eight new dioxomolybdenum(VI) and oxomolybdenum(V) complexes (1)-(8) (Scheme 1). The characterization data of the complexes are summarized in Table 1 which supports the formulation of the complexes. Unfortunately, no single crystals of the complexes could be grown inspite of our best efforts. Molecular weight, molar conductance and magnetic moments : The molecular weights (Table 1) of some of the com-