Synthesis, catalytic oxidation and antimicrobial activity of copper(II) Schiff base complex

Abstract

A new polymer supported Cu(II) Schiff base complex was synthesized. The solid complex was characterized by Fourier transform infrared spectroscopy (FT-IR), UV–vis diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). Its homogeneous analogue was also prepared. The catalytic performances of the copper complex in oxidation reactions were evaluated for both homogeneous and heterogeneous systems. The copper(II) complex was found to be efficient catalyst for the oxidation of alkenes, alkanes and aromatic alcohols in the presence of hydrogen peroxide as oxidant at room temperature. The catalytic investigation revealed that the solid complex performs better than the homogeneous one as an oxidation catalyst. The solids containing the immobilized complex can be recovered from the reaction medium and reused almost five times, maintaining good catalytic activity. Furthermore, the in vitro toxicity of the ligand and complex was tested against the growth of bacterial species, viz., Staphylococcus aureus and Escherichia coli.

Introduction

The oxidation of organic compounds with an eco friendly oxidant, aqueous hydrogen peroxide, is a challenging goal of catalytic chemistry [1], [2], [3]. In recent years, a considerable amount of research was dedicated to the preparation of various solid heterogeneous copper catalysts and their application for oxidations of various organic compounds [4], [5], [6], [7], [8].

Olefin oxidation is especially interesting because of the industrial importance of this type of reaction. Oxidation of alkenes to give oxygen containing value added products like alcohols, aldehydes, ketones, acids, epoxides, etc. is an extremely important and useful reaction in both chemical and pharmaceutical industries [9]. Activation of the carbon–hydrogen bonds of an alkane is considerably more difficult due to its stability. The energy required to overcome this stability leads to deep oxidation rather than selective oxidation [10], [11]. On the other hand, the oxidation of primary and secondary alcohols to carbonyl compounds (corresponding aldehydes and ketones) is one of the simplest and most useful transformations in organic chemistry. In particular, the oxidation of benzyl alcohol to benzaldehyde is an important organic transformation which has applications in the industry of perfumery, pharmaceutical, dyestuff and agrochemicals [12], [13].

In the recent past, there has been an increasing interest in developing environmental friendly greener processes which are also economically viable. However, some problems such as excessive use of oxidants, corrosion, difficulty in recovery and separation of the catalyst from reaction mixtures are associated with homogeneous catalysts and make such systems environmentally unsuitable [14]. In recent years, the design and synthesis of catalytically active supported metal complexes have received considerable interest. Many effective and recyclable heterogeneous catalysts have been studied for the liquid phase oxidation [15], [16]. Various approaches have been focused on the incorporation of metal-based catalysts onto or into inert supports by different methods, such as alumina [17], amorphous silicates [18], polymers [19], zeolites [20] and MCM-41 [21]. Application of polymer supported catalysts in oxidation reactions has been received attention in recent years due to their potential advantages over the homogeneous one [22], [23], [24]. The activity of polymer supported Schiff base complexes of transition metal ions varies with the type of Schiff base ligands, coordination sites and metal ions used in their formation.

In this work, we have prepared homogeneous and its immobilized polymer anchored Cu(II) Schiff base complex. The catalytic efficiency of the neat and polymer supported metal Schiff base complexes was tested in the oxidation of alkenes, alkanes and aromatic alcohols using H₂O₂ as oxygen source. Polymer supported copper catalyst showed excellent catalytic activity and selectivity. Comparison of homogeneous and supported catalyst for oxidation reaction was also done. The catalytic activities were also tested with the recycled catalyst. Further, the antibacterial activity of the Schiff base and its copper complex was examined.