Hydroxylation of Phenol Catalyzed by Oxovanadium(IV) of Salen-Type Schiff Base Complexes with Hydrogen Peroxide
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Hydroxylation of Phenol Catalyzed by Oxovanadium(IV) of Salen-Type Schiff Base Complexes with Hydrogen Peroxide Tahseen A. Alsalim • Jabbar S. Hadi Einas A. Al-Nasir • Hanna S. Abbo • Salam J. J. Titinchi
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Received: 6 January 2010 / Accepted: 8 March 2010 / Published online: 26 March 2010 Ó Springer Science+Business Media, LLC 2010
Abstract Active and highly selective catalytic systems of oxovanadium(IV) salen-type have been prepared and characterized by various physico-chemical techniques. Substituted salen-type Schiff base ligands were prepared from 3-ethoxy salicylaldehyde with 1,2-diaminobenzene and 1,8-diaminonaphthalene abbreviated (EtOsalphen) and (EtOsalnaph), respectively. The catalytic activity of the complexes for hydroxylation of phenol to catechol and hydroquinone using H2O2 as an oxidant has been studied. The best suited reaction conditions were obtained by considering the effect of solvent, concentration of substrate, reaction time, concentration of catalyst and temperature. Under the optimized reaction conditions, VO-(EtOsalphen) catalyst shows high conversion (71%) at a short reaction time (2 h) with selectivity of 92.5% towards catechol, while VO-(EtOsalnaph) complex also shows higher conversion (76.6%) after longer reaction time (6 h) with almost similar selectivity to catechol (94.2%). Keywords Phenol hydroxylation Oxovanadium(IV) complexes Salen-type Schiff base Catalytic activity Selectivity
T. A. Alsalim J. S. Hadi E. A. Al-Nasir Department of Chemistry, College of Education, University of Basrah, Basrah, Iraq H. S. Abbo S. J. J. Titinchi (&) Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville, Cape Town 7535, South Africa e-mail: [email protected]
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1 Introduction Condensation reactions between carbonyl compounds and primary amines have provided one of the most important and widely studied classes of chelating ligand. A wide variety of ligand may be obtained via Schiff base condensation reaction which shows variations in flexibility and electronic properties. The family of Schiff bases derived from aromatic or aliphatic diamine and phenolic aldehydes which are known as salen has proved to be the source of tetradentate ligands for many transition metals including vanadium(IV) [1–5]. Salen-type complexes show a great potential as catalysts for various reactions [6–16]. The selective and one-step hydroxylation of phenol by hydrogen peroxide is an attractive and challenging subject from economical and environmental point of view. The dihydroxybenzenes viz. catechol (CAT) and hydroquinone (HQ), are important intermediates for manufacturing of petrochemicals, agrochemicals and plastics [1]. They are widely used as photographic chemicals, antioxidants, pesticides, flavoring agents, and medicine. Catechol was also used as an organic sensitizer in a photoelectrochemical cell [4]. Vanadium salen-type complexes have attracted much interest as catalysts for various types of reactions. These vanadium complexes have attracted attention due to its inte
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