A novel tridentate Schiff base dioxo-molybdenum(VI) complex: Synthesis, crystal structure and catalytic performance in green oxidation of sulfides by urea hydrogen peroxide Iran Sheikhshoaie a, * , Abdolreza Rezaeifard b, * , Niaz Monadi a , Samira Kaafi b a Department of Chemistry, Faculty of Science, Shahid Bahonar University of Kerman, Kerman 76175-133, Iran b Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand, Birjand 97179-414, Iran article info Article history: Received 3 October 2008 Accepted 17 December 2008 Available online 3 February 2009 Keywords: Dioxo-molybdenum(VI) Urea hydrogen peroxide Sulfoxide Catalyst Schiff base X-ray structure abstract A new dioxo-molybdenum(VI) complex [MoO 2 (L)(CH 3 OH)] has been synthesized, using 2-[(2-hydroxy- propylimino)methyl]phenol as tridentate ONO donor Schiff base ligand (H 2 L) and MoO 2 (acac) 2 . A mono- clinic space group P2 1 /c was determined by X-ray crystallography from single-crystal data of this complex. The high catalytic activity of this new Schiff base complex has been observed in the oxidation of various sulfides by urea hydrogen peroxide in ethanol affording sulfoxides and sulfones using different molar ratio of oxidant/sulfide in high/excellent yields and selectivity under mild and eco-friendly condi- tions. The relative high stability and desired turnover numbers have been observed for this Mo-catalyst in the oxidation reactions. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The significant enzymatic role of molybdenum in biochemical reactions [1–3] specially in the oxidation of aldehydes, purines and sulfides [4] induced the chemists to use the molybdenum com- plexes as biomimetic catalysts in the oxygenation of organic com- pounds [5,6]. In this light, molybdenum(VI) dioxo-complexes have been extremely investigated [7–9] particularly with respect to the catalytic role of transferase enzymes like nitrate reductase in which their active sites consist of a cis molybdenum dioxo moiety [10,11]. The ability of molybdenum to formation of stable com- plexes with oxygen-, nitrogen-, and sulfur-containing ligands led to development of molybdenum Schiff base complexes which are efficient catalysts both in homogeneous and heterogeneous reac- tions [12–15]. The activity of these complexes varied markedly with the type of ligands and coordination sites [16]. Thus, we decided to synthesize a new dioxo-molybdenum(VI) Schiff base complex con- taining an asymmetric ONO tridentate ligand (Scheme 1), and eval- uate its catalytic activity in the oxidation of sulfides to sulfoxides (Scheme 1A) and sulfones (Scheme 1B) both of which are useful precursors for biologically and chemically important compounds [17,18]. Sulfoxides have been utilized as ligands in asymmetric catalysis [19] and as oxotransfer reagents [20] and sulfones have been employed for stabilizing a-radicals [21], a-anions [22], and acting as cationic synthons [23]. The sulfoxides and sulfones were secured in high/excellent yields and selectivity under the influence of this molybdenum catalyst using urea hydrogen peroxide (UHP) as oxidant in ethanol under mild conditions. We also report the crystal structure of this molybdenum complex determined by sin- gle-crystal X-ray diffraction. 2. Experimental 2.1. General remarks All reagents were used as received. Solvents were purified by standard methods and dried if necessary. Methanol was distilled from magnesium methoxide. Acetylacetonate and ammoniumhep- ta molybdate were purchased from Fluka Company and used as received. All reactions and workups were carried out under air. Dioxo-molybdenum acetylacetonate was prepared as reported [24]. The synthesis and characterization of 2-[(2-hydroxypropyli- mino)methyl]phenol as tridentate ONO donor Schiff base ligand (H 2 L) and dioxo-molybdenum complex are given in the Supple- mentary data. 2.2. Instrumentation X-ray data collections were recorded on a Bruker SMART CCD diffractometer. Purity determinations of the products were 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.12.044 * Corresponding authors. Tel./fax: +98 341 3222033. E-mail addresses: i_shoaie@yahoo.com (I. Sheikhshoaie), rrezaeifard@birjand.ac. ir (A. Rezaeifard). Polyhedron 28 (2009) 733–738 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly