Polynuclear oxomolybdenum(VI) complexes of dihydroxybenzoic acids: Synthesis, spectroscopic and structure characterization of a tetranuclear catecholato-type coordinated 2,3-dihydroxybenzoate and a novel tridentate salicylato-type coordinated 2,5-dihydroxybenzoate trinuclear complex Charalambos Litos a , Aris Terzis b , Catherine Raptopoulou b , Aliki Rontoyianni c , Alexandra Karaliota a, * a Department of Inorganic Chemistry, University of Athens, 15771 Panepistimiopolis, Athens, Greece b Institute of Materials Science, NCSR ÔDemokritosÕ, 15310 Aghia Paraskevi, Athens, Greece c Institute of Physical Chemistry, NCSR ÔDemokritosÕ, 15310 Aghia Paraskevi, Athens, Greece Received 25 August 2005; accepted 20 September 2005 Available online 7 November 2005 Abstract In order to study the reaction of molybdenum(VI) with dihydroxybenzoic acids (DHBAs), compounds (PPh 4 ) 2 Æ [Mo 4 O 6 (l-O) 4 - (l 3 -OCH 3 ) 2 (2,3-DHBA) 2 ](1) and (PPh 4 ) 2 Æ [Mo 3 O 6 (l-O) 2 (2,5-DHBA) 2 ] Æ CH 3 CN (2) were isolated and determined by X-ray crystallog- raphy. The structure of complex 1 consists of four distorted MoO 6 octahedra with two cis-MoO 2 2þ units and two molybdenum atoms having each ligand coordinated and one terminal oxygen atom, while two triple bridged methanol molecules are present. The ligand 2,3- dihydroxybenzoic acid (2,3-DHBA) prefers the catecholate-type coordination with strong hydrogen bonding between the carboxylic hydrogen and the ortho-phenolic oxygen. The structure of complex 2 consists of three distorted MoO 6 octahedra. The 2,5-dihydroxyben- zoic acid (2,5-DHBA) acts as a tridentate ligand using both carboxyl oxygen atoms, giving thus an unusual trinuclear molybdenum com- plex with a unique coordination mode of the ligand. The compounds were further investigated using IR, UV–Vis, ESR and NMR spectroscopy, elemental and thermogravimetric analysis, as well as cyclic voltammetry. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Dihydroxybenzoic acids; Oxomolybdenum; Crystal structure 1. Introduction The importance of molybdenum chemistry is relevant to its biological capacity and its participation in certain enzy- matic processes [1]. The multiple oxidative centers catalyz- ing oxygen atom transfer are characteristic of enzymes containing molybdenum atoms [2]. Especially, the cis- MoO 2 2þ unit role in these enzymes has led to studies of complexes with ligands containing nitrogen, sulphur or oxygen donors, analyzing their chemical, spectroscopic and structural properties [3]. Molybdate anions have also demonstrated insulin-mimetic behavior [4]. Molybdenum is found in a huge range of complexes from monomeric to oligomeric and polymeric cores [5]. The polyoxomolyb- denum anions have received a lot of interest with their po- tential applications in catalysis, solid-state technology and medicine, including antitumor and anti-virus (HIV) activity [6]. These anions are analogues of metal oxide surfaces and are used as models for heterogeneous catalysis of organic molecules [1,7–11]. The co-existence of molybdenum and 0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.09.025 * Corresponding author. Tel.: +30210 7274456; fax: +30210 7274782. E-mail address: akaraliota@chem.uoa.gr (A. Karaliota). www.elsevier.com/locate/poly Polyhedron 25 (2006) 1337–1347