Molecular Catalysis 439 (2017) 224–232 Contents lists available at ScienceDirect Molecular Catalysis j o ur nal ho me page: www.elsevier.com/locate/mcat Research Paper Microwave-assisted peroxidative oxidation of toluene and 1-phenylethanol with monomeric keto and polymeric enol aroylhydrazone Cu(II) complexes Manas Sutradhar a,∗ , Elisabete C.B.A. Alegria a,b , Tannistha Roy Barman a , Francesco Scorcelletti a , M. Fátima C. Guedes da Silva a , Armando J.L. Pombeiro a,∗ a Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Chemical Engineering Departament, ISEL-Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, 1959-007 Lisboa, Portugal a r t i c l e i n f o Article history: Received 12 May 2017 Received in revised form 29 June 2017 Accepted 4 July 2017 Keywords: Cu(II) complexes Aroylhydrazone X-ray structure Oxidation of toluene Microwave-assisted oxidation a b s t r a c t Cu(II) complexes in two different tautomeric forms (keto and enol) derived from the aroylhydrazone Schiff base 2-hydroxy(2-hydroxybenzylidene)benzohydrazide (H 2 L) have been synthesized and characterized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray crystallography. While the com- pound with the enol form of the ligand exists as the 1D polymer [Cu(1NOO’,2O’,3O”-L)] n (1), that with the keto form of the ligand occurs as the monomer [Cu(NOO’-HL)Cl(CH 3 OH)] (2). Both complexes act as good catalysts for the microwave-assisted peroxidative oxidation of toluene and 1-phenylethanol with tert-butyl hydroperoxide. Complex 2 exhibits the highest activity in both reactions, leading selectively to a maximum product yield of 39 and 92%, respectively. © 2017 Elsevier B.V. All rights reserved. 1. Introduction The use of microwave (MW) irradiation in catalytic reactions has received a great attention in the last decade [1–5]. The grow- ing demand for new environmentally friendly catalytic processes led the researchers in catalysis to explore the advantages of MW heating, one of the simplest and most effective techniques to elicit energy efficiency compared to conventional methods [6–17]. The catalytic activity of a system can be enhanced dramatically by MW irradiation in comparison to the catalysis under conventional heat- ing [1–17]. Our group has also explored the effect of MW irradiation namely in several Cu(II) catalysed oxidation reactions [12–16]. The choice of Cu(II) catalysts for oxidation reactions is supported by its abundance, low cost, non-toxic properties and high catalytic efficiency [18–20]. In this study we have used MW irradiation for the peroxidative oxidation of toluene and 1-phenylethanol with Cu(II) complexes. Benzaldehyde (BzH), the main product of toluene oxidation, is an important starting material in the industry of dyes, perfumes and pharmaceuticals [21]. Recently, catalytic systems involving met- ∗ Corresponding authors. E-mail addresses: manaschem@yahoo.co.in (M. Sutradhar), pombeiro@tecnico.ulisboa.pt (A.J.L. Pombeiro). alloporphyrins [22,23], Au-Pd nanoparticles [24], Cu-Mn oxides [25] and graphite carbon nitride nanocomposites (g-C 3 N 4 ) were successfully tested in solvent-free aerobic oxidation of toluene. However, MW assisted peroxidation of this substrate, as far as we know, has not been attempted. Furthermore, MW assisted oxidation of alcohol is also a highly demandable research topic because selective oxidation of alcohols to carbonyl compounds is a vital step in the synthesis of important intermediates and fine chemicals [26–29]. In our recent studies, the catalytic activity of aroylhydrazone Cu(II) complexes towards the peroxidative oxidation of cyclohex- ane under mild conditions has been reported and the catalytic activities of two tautomeric aroylhydrazone Cu(II) complexes have been compared [30,31]. Several factors, e.g. solvent, pH, the nature of metal ions determine the stability of the tautomeric form of the aroylhydrazone in their metal complexes [30–33]. Comparison of the catalytic activities of different tautomeric aroylhydrazone com- plexes in other oxidation reactions is an aim of the current study. Herein, we report the synthesis of a mononuclear and a 1D poly- mer Cu(II) species with a keto- and an enol-aroylhyrazone ligand, respectively. These two complexes are tested as catalysts in the per- oxidative oxidation of toluene and 1-phenylethanol with tert-butyl hydroperoxide (t-BuOOH). The influence of various parameters, such as, reaction time, type and amount of catalyst, temperature http://dx.doi.org/10.1016/j.mcat.2017.07.006 2468-8231/© 2017 Elsevier B.V. All rights reserved.