Synthesis, structure and properties of eight novel molybdenum(VI) complexes of the types: [MoO 2 LD] and [{MoO 2 L} 2 D] (L = thiosemicarbazonato ligand, D = N-donor molecule) Višnja Vrdoljak, Ivica Ðilovic ´, Marina Cindric ´ * , Dubravka Matkovic ´-C ˇ alogovic ´ , Neven Strukan, Astrid Gojmerac-Ivšic ´, Predrag Novak Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10001 Zagreb, Croatia article info Article history: Received 3 November 2008 Accepted 26 December 2008 Available online 14 February 2009 Keywords: Thiosemicarbazones Mononuclear and dinuclear molybdenum(VI) complexes abstract Eight new molybdenum(VI) complexes with 4-(diethylamino)salicylaldehyde and 2-hydroxy- 3-methoxybenzaldehyde thiosemicarbazones have been prepared. They were characterized as mononuclear [MoO 2 LD] or dinuclear [{MoO 2 L} 2 D] complexes. In all the compounds the MoO 2 2+ core is coordinated by a tridentate ONS thiosemicarbazonato ligand and by the N-donor molecule (imidazole, pyridine or c-picoline). All the complexes were characterized by chemical analysis, IR spectroscopy and thermogravimetry. Three of the mononuclear complexes, dioxo[(2-hydroxy-3-methoxybenzalde- hyde thiosemicarbazonato)(pyridine)]molybdenum(VI), dioxo[(2-hydroxy-3-methoxybenzaldehyde thiosemicarbazonato)(c-picoline)]molybdenum(VI) and dioxo[(2-hydroxy-3-methoxybenzaldehyde thiosemicarbazonato)(imidazole)]molybdenum(VI) were also characterized by single-crystal X-ray structural analysis. A spectrophotometric method for the determination of molybdenum based on extrac- tion of ion-pairs formed by the cationic surfactant and the [MoO(SCN) 4 ]  anion is described. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Molybdenum-containing enzymes hold key positions both in biochemical cycles of carbon, nitrogen and sulfur [1] and in the metabolism of every organism. EXAFS studies of xanthine oxidase have confirmed the presence of nitrogen, sulfur and oxygen donors in its active site [2,3]. On the basis of these observations, we are interested in the syntheses and characterization of molybdenum complexes with ligands containing such donor atoms. Thiosemi- carbazones as ONS donor molecules are a large group of biological active organic molecules due to their ability to form tridentate chelates with metal ions. Recently, we have published the syntheses, as well as structural and spectral properties of several molybdenum(VI) and molybdenum(V) complexes with thiosemi- carbazones [4,5]. In a continuation of our research, in this paper we describe a study of eight new molybdenum(VI) complexes with the molybdenum atoms coordinated by 4-(diethylamino)salicylal- dehyde and 3-methoxysalicylaldehyde thiosemicarbazones (Scheme 1). The complexes [MoO 2 L(py)] (2a), [MoO 2 L(c-pic)] (2b) and [MoO 2 L(im)] (2c) (where L = 2-hydroxy-3-methoxybenzaldehyde thiosemicarbazonato ligand) were also characterized by X-ray crystallography. In all the [MoO 2 LD] complexes, the tridentate thi- osemicarbazonato ligand (L 2 ) is meridionally bound to the MoO 2 2+ core and a N-donor molecule (imidazole, pyridine or c-pic- oline) completes the distorted octahedral coordination around the metal atom. We also describe a spectrophotometric method for the determi- nation of molybdenum based on extraction of ion-pairs formed by the cationic surfactant and the [MoO(SCN) 4 ]  anion. 2. Experimental 2.1. General remarks The starting complexes [MoO 2 (C 12 H 16 N 4 OS)] n (1) (C 12 H 18 N 4 - OS = 4-(diethylamino)salicylaldhehyde thiosemicarbazonato ligand) and [MoO 2 (C 9 H 11 N 3 O 2 S)] n (2) (C 9 H 9 N 3 O 2 S = 2-hydroxy- 3-methoxybenzaldehyde thiosemicarbazonato ligand) were prepared as described in the literature [5]. All the aldehydes, thio- semicarbazide, 4,4 0 -bipyridine, pyridine, c-picoline and imidazole were of reagent grade and were used as purchased. Methanol was dried using magnesium shavings and iodine and then distilled. C, H, N and S analyses were provided by the Analytical Services Laboratory of Rudjer Boškovic ´ Institute, Zagreb. 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.12.058 * Corresponding author. Tel.: +385 1 460 6351; fax: + 385 1 460 6340. E-mail address: marina@chem.pmf.hr (M. Cindric ´). Polyhedron 28 (2009) 959–965 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly