Synthesis of five new molybdenum(VI) thiosemicarbazonato complexes. Crystal structures of salicylaldehyde and 3-methoxy-salicylaldehyde 4-methylthiosemicarbazones and their molybdenum(VI) complexes Vis ˇnja Vrdoljak a , Marina Cindric ´ a, * , Dalibor Milic ´ a , Dubravka Matkovic ´-C ˇ alogovic ´ a , Predrag Novak b , Boris Kamenar a a Laboratory of General and Inorganic Chemistry, Chemistry Department, Faculty of Science, University of Zagreb, Ulica kralja Zvonimira 8, 10000 Zagreb, Croatia b Laboratory of Analytical Chemistry, Chemistry Department, Faculty of Science, University of Zagreb, Strossmyerov trg 14, 10000 Zagreb, Croatia Received 24 February 2005; accepted 11 May 2005 Available online 20 June 2005 Abstract Polynuclear [MoO 2 {SalTSC-4-Me}] n (I), [MoO 2 {VanTSC-4-Me}] n (II) and mononuclear [MoO 2 {SalTSC-4-Me}(CH 3 OH)] (1a), [MoO 2 {VanTSC-4-Me}(CH 3 OH)] (2a) and [MoO 2 {VanTSC-4-Me}(H 2 O)] Æ CH 3 CN (2b) complexes have been prepared by the reaction of the [MoO 2 (acac) 2 ] (acetylacetonate ligand, acac  ,C 5 H 7 O 2  ) with 1 (salicylaldehyde 4-methylthiosemicarbazone ligand, C 6 H 4 (OH)CH:NNC(SH)NHCH 3 H 2 SalTSC-4-Me) or 2 (3-methoxy-salicylaldehyde 4-methylthiosemicarbazone, CH 3 OC 6 H 3 (OH)- CH:NNC(SH)NHCH 3 ,H 2 VanTSC-4-Me) in acetonitrile or in dry methanol. All complexes have been characterized by means of chemical analyses, thermogravimetric analyses, IR spectroscopy and some of them by 1 H, 13 C NMR spectroscopy and X-ray crys- tallography. In all mononuclear complexes molybdenum atom is coordinated by two terminal oxo-oxygen atoms, by ONS atoms from the ligand molecule and by an oxygen atom from methanol (in 1a and 2a) or water (in 2b) molecules. Ligands 1 and 2 have been characterized by means of 1 H, 13 C NMR spectroscopy and X-ray crystallography. Molecules of both ligands are found to be in the thioketo tautomeric form. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Molybdenum(VI) complexes; Thiosemicarbazones; Mononuclear and polynuclear complexes; Tridentate ligands; Crystal structures 1. Introduction In this paper we present a study of five new molybde- num(VI) complexes in which molybdenum atoms are coordinated by salicylaldehyde and 3-methoxysalicylal- dehyde 4-methylthiosemicarbazones as ONS ligands. The structure of both ligands as well as of three com- plexes have been characterized by X-ray crystallography. Our interest in studies of molybdenum(VI) com- plexes containing ligands with oxygen, nitrogen and sulfur donor atoms arises from their significant anti- fungal, antiprotozoal, antibacterial and anticancer activity [1,2]. Similarly, the thiosemicarbazone ligands and related metal complexes have experienced long standing applications in biology and medicine [3,4] as well as in the catalysis of chemical and petrochemical processes [5,6]. The thiosemicarbazones and their com- plexes showed lower antibacterial activity than thiose- micarbazides [7]. This may be correlated with the 0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.05.002 * Corresponding author. Tel.: +385 1 460 6653; fax: +385 1 461 1191. E-mail address: marina@chem.pmf.hr (M. Cindric ´). www.elsevier.com/locate/poly Polyhedron 24 (2005) 1717–1726