Synthesis, characterisation and catalytic activity of dithiocarbazate Schiff base complexes in oxidation of cyclohexane
Introduction
The reaction of an amine with an aldehyde or a ketone in suitable conditions produces Schiff base compounds. Basically, Schiff base can be synthesised by the replacement of a carbonyl group with an imine or azomethine in the nitrogen analogue of an aldehyde or ketone. Schiff base can interact with most of the metal ions to produce various complexes [1], [2], [3]. Schiff base compounds have several unique physical and chemical properties. Most Schiff base ligands have an important role in many reactions due to their high catalytic activity that increase the yield and selectivity. In general, the catalytic application of Schiff bases as metal complexes depends on the method that has been used for synthesis and thermal stability of these ligands [4]. Schiff base complexes of transition metal ions work as active catalysts in homogeneous and heterogeneous reactions and their activity differs according to the metal ions, kind of ligands and coordination sites [5], [6]. The metal complexes of Schiff bases have been used as catalysts during the last decades in many reactions such as; thionyl chloride reduction polymerization reaction, ketones reduction, reaction of aldol, Henry reaction, alkenes epoxidation, ketones hydrosilylation, reaction of Dielss Alder and oxidation of organic compound that has attracted a huge attention in the field of academic and industrial study [7]. There are a lot of hydrocarbons in the constituents of the natural gas and oil, thus, the usage of the hydrocarbons in reactions was the way for several economic industries. Selective oxidation is the most significant functionalization of hydrocarbons. In the liquid phase, different oxidants such as; hydrogen peroxide, tert-butyl hydroperoxide (TBHP) and others have been used for oxidation of cycloaliphatic and aliphatic compounds by using oxygen to produce suitable alcohol and ketones under tough conditions like high temperature and pressure due to the difficulty of making the activation of CH bonds [8], [9].
Annually, more than one billion tons of cyclohexanone and cyclohexanol are formed widely around the world, for the industrialization of the polymers Nylon-6 and Nylon-6,6, that have been used in the manufacture of adipic acid, polyamide-6, acidulant in baking powder and caprolactam [10], [11], [12]. The oxidation reaction of cyclohexane to cyclohexanone and cyclohexanol (also known as KA oil) has a significant importance in the industry field. In the last few decades, the oxidation of cyclohexane using Schiff base metal complexes has attracted scientists to conduct research in this area. Schuchardt et al. discussed an alternate method on how to develop the industrial method of the oxidation of cyclohexane, that focused on the improvement of new catalysts to oxidize cyclohexane under a suitable condition [13]. The use of metal complexes as catalysts to activate hydrocarbons was improved by applying Schiff base transition metal complexes as catalysts because of their cheap and easy synthesis and their chemical and thermal stability [14], [15], [16], [17]. The use of Schiff base metal complexes in the cyclohexane oxidation has a great interest due to their potential catalytic activity [18], [19], [20].
In the present work, we have synthesised new metal complexes containing a bis-dithiocarbazate ligand. The synthesised complexes were investigated as catalysts for cyclohexane oxidation and tert-butyl hydroperoxide was used as an oxidant. The prepared metal complexes as catalysts were cobalt(II), copper(II), iron(II) manganese(II), nickel(II), cobalt(II) and zinc complexes derived from the Schiff base (2E,2′E)-dibenzyl 2,2′-(1,3-phenylenebis(ethan-1-yl-1-ylidene))bis (hydrazine carbo dithioate) [SBdiAP]. The catalytic activity of the prepared metal complexes was screened for cyclohexane oxidation. These catalysts showed high yield and good products quality under mild conditions.
Section snippets
Materials
The chemicals and solvents used in the present work were used as received without any additional purification. Manganese(II) chloride tetrahydrate (Merk), ferrous chloride tetrahydrate (Aldrich), cobalt(II) chloride hexahydrate (Merk), nickel(II) chloride hexahydrate (Merk), copper(II) chloride dehydrate (Merk), zinc chloride (Merk), tert-butyl hydroperoxide (70%w/v, Aldrich), cyclohexane (99.7%, Merck), 2,6-diacetylpyridine (Fluka), dichloromethane (Analar), ethanol (99.8%, Scharlau),
Physico-chemical data
The physical characteristics of the Schiff base and all the metal complexes are as shown in Table 1. All compounds were characterised according to yields, melting point, and colour. In general, the metal complexes have different colours compared to Schiff base indicating an initial successful reaction between the Schiff base and metal salt. The melting points of the prepared metal complexes were higher than the melting point of the of Schiff base.
CHNS elemental analysis
CHNS analysis was used to calculate the
Conclusions
The catalytic activities of the synthesised metal complexes of Schiff base were investigated for the oxidation of cyclohexane using TBHP as an oxidant under mild conditions. The main products of the oxidation were cyclohexanone and cyclohexanol and no conversion of the cyclohexane in the absence of the catalyst was obtained. The selectivity of cyclohexanone was found to be greater. The catalytic activity of iron complex (FeSBdiAP) that gave the highest conversion in the screening study of the
Acknowledgement
This research was funded by Ministry of Education Malaysia under the Fundamental Research Grant Scheme (FRGS No. 01-02-13-1342FR) and Universiti Putra Malaysia (UPM) under the Putra Grant (IPS No. 9462800). Ali Ahmed Alshaheri would like to thank Albaida University for the study financial support.
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