Heterogeneous catalytic oxidation of styrene by an oxo bridged divanadium(V) complex of an acetohydrazide-Schiff base Dipali Sadhukhan a , Monami Maiti a , Ennio Zangrando b , Soyeb Pathan c , Samiran Mitra a,⇑ , Anjali Patel c,⇑ a Department of Chemistry, Jadavpur University, Raja S. C. Mullick Road, Kolkata 700032, West Bengal, India b Department of Chemical and Pharmaceutical Sciences, Via Licio Giorgieri 1, 34127 Trieste, Italy c Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, India article info Article history: Received 20 June 2013 Accepted 6 November 2013 Available online 15 November 2013 Keywords: l-Oxido divanadium compound Crystal structure Catalytic oxidation of styrene Al 2 O 3 support Catalyst recycling abstract We have synthesized a l-oxido divanadium compound [(VOL) 2 (l-O)] with an aliphatic hydrazone ligand LH 2 = (E)-N 0 -(1-(2-hydroxyphenyl)ethylidene)acetohydrazide. The complex was characterized by ele- mental analysis, IR and UV–Vis spectroscopy and the molecular structure was established by single crys- tal X-ray diffraction technique. The complex has been infused over alumina to prepare a heterogeneous catalyst which was characterized by IR spectroscopy, thermogravimetric and powder XRD analyses. The catalyst has been studied for the oxidation of styrene in presence of H 2 O 2 and appears to be easily recy- clable. A conversion of 99.7% and selectivity of 88.1% for benzaldehyde formation and a turnover number of 1354 was detected under the most favorable reaction conditions. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Vanadium is a versatile bio-essential element. Its presence in vanadate-dependent haloperoxidases [1,2] and vanadium-nitro- genases [3], its therapeutic application as insulin-enhancing agent in treatment of diabetes [4–10] and anticancer activity [11–14] have stimulated a considerable amount of research. Possible phar- macological activities of vanadate compounds are supported by the coordinating ligands. Chelation improves absorption, transport- ability and uptake of the metal ions into cells, thus reducing the necessary dose of efficacy as well as the metal ion toxicity [15]. Vanadium ion in coordination complexes is capable of existing in a range of oxidation states from 3+ to 5+ and possesses unusual re- dox ability. In 4+ and 5+ oxidation states, vanadium is highly redox active and acts as strong Lewis acid due to low radius to charge ra- tio, which can stimulate various organic transformation reactions [16]. The role of vanadium-based compounds as efficient catalysts attests the applicability of this metal ion in bulk industrial produc- tions, viz., the use of oxovanadium complexes in asymmetric syn- thesis [17–21], in C–C bond formation as well as C–C, C–O and C–H bond cleavages [22–26], involvement as an intermediate in hydro- sulfurization of crude oils [27], oxidative halogenation and selec- tive epoxidation of unsaturated hydrocarbons and allyl alcohols [22,28]. Acid hydrazides R–CO–NH–NH 2 and their corresponding hydra- zones with aromatic carbonyl compounds R–CO–NH–N = CR 1 R 2 are the N/O donor ligands that can act as neutral as well as anionic chelator towards metal ions and have remarkable biological impor- tance [29]. Additionally, hydrazone metal complexes have also at- tracted research interest for their possible pharmacological applications [30] antimicrobial, antitumor activities [31] and also in catalysis [32]. The usefulness of both vanadium and hydrazone Schiff bases prompted us to investigate the structure–activity stud- ies of vanadium-hydrazone chelate complexes. There are few re- ports on the structures and solution state properties of vanadium complexes of hydrazone Schiff bases derived by the condensation of an aroyl hydrazide with an aromatic carbonyl compound [33]. A very few have been investigated for their catalytic potential [34]. In this paper we are concerned about the structural investiga- tion and catalytic activity of a vanadium complex chelated by a hydrazone ligand, which is the condensation product of a less investigated alkyl hydrazide with an aromatic ketone. In search of catalysts for organic transformations we have direc- ted our research on immobilization of complexes on various solid supports. As benefits of this process, in many cases these catalysts (or catalyst precursors) are easily recyclable and maintain their activity after several catalytic cycles. Although the solid support can constitute a protective environment around the metal com- plex, the physical restrictions imposed by the matrix can make the access of reactants to the metal centre more difficult compared to homogeneous system affecting the conversion and sometimes selectivity [35]. 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.11.007 ⇑ Corresponding authors. Tel.: +91 9836883638; fax: +91 033 2414 6414. E-mail addresses: smitra_2002@yahoo.com (S. Mitra), aupatel_chem@yahoo. com (A. Patel). Polyhedron 69 (2014) 1–9 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly