SYNTHESIS AND CHARACTERISATION OF NICKEL AND PALLADIUM BIMETALLIC COMPLEXES AND THEIR CATALYTIC APPLICATION IN OLIGOMERISATION OF 1-HEXENE

Abstract

In this work, typical homobimetallic complexes derived from N,O-salicylaldimine ligands were investigated for their catalytic potential in oligomerisation of 1-hexene. Catalysts of monometallic nature, that is, those with one active metal centre have been used effectively in polymerisation for a long time. However, the few reports available have shown that catalysts of homobimetallic or heterobimetallic nature exhibited superior catalytic activity in comparison with their monometallic analogues. This work thus reports the synthesis of a series of new N,Osalicylaldimine Schiff base ligands from the condensation of substituted salicylaldehyde and primary amines. The ligands (propane-1,x-diylbis(azanylylidene))bis(methanylylidene))bis(y) [x = 3; y = 2-methylphenol, 4-methylphenol, 4-methoxyphenol, 2-bromo-4-chlorophenol, 4- bromophenol, and 4-chlorophenol for corresponding SL1, SL2, SL3, SL4, SL5, and SL6, respectively], (butane-1,x-diylbis(azanylylidene))bis(methanylylidene))bis(y) [x = 4; y = 2- methylphenol, 4-methylphenol, 4-methoxyphenol, 4-bromophenol, and 4-chlorophenol for SL7 – SL11, respectively], (pentane-1,x-diylbis(azanylylidene))bis(methanylylidene))bis(y) [x = 5; y = 2-methylphenol, 4-methylphenol, 4-methoxyphenol, 2-bromo-4-chlorophenol, 4-bromophenol, and 4-chlorophenol corresponding to SL12 – SL17, respectively] were prepared via condensation reaction of a primary diamine; 1,3-diaminopropane, 1,4-diaminobutane or 1,5- diaminopentane with 2 equivalents of an appropriate salicylaldehyde derivative, stirred at reflux for between 1 – 6 h. The ligands were obtained as yellow or bright yellow solids which were stable in both moisture and air and in appreciable yields ranging from 41 to 98 %. They were fully characterised using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), elemental analysis (EA), mass spectroscopy (MS) and for some of them (SL2, SL3, SL6, and SL9) using single crystal X-ray diffraction (XRD). The ligands have their crystal structures reported for the first time. The N,O-salicylaldimine ligands were subsequently used to synthesize a series of new homobimetallic nickel(II) and palladium(II) complexes. To obtain palladium(II) complexes, ligands used included; SL1 – SL6, SL9, SL12 – SL14, whilst nickel(II) complexes were synthesized using SL4, SL7, SL9, SL13, and SL15. The ligands were reacted in a 1:2 mole ratio with PdCl2(COD) and NiCl2(DME) complex precursors stirred at room temperature which obtained light-yellow, yellow, orange or light-green complexes in appreciable yields ranging between 43 – 95 %. All the palladium(II) (C1 – C10) and nickel(II) (C11 – C15) complexes were observed to be both air and moisture stable. These complexes were characterised using FTIR, NMR and EA. They were reacted with 1-hexene to test their catalytic capability to polymerise 1-hexene. It was generally found that the nickel(II) complexes were highly active when activated with the cocatalyst, modified methyl aluminoxane (MMAO) compared with the palladium complexes. All the tested complexes were active for 1- hexene oligomerisation with C14 showing the highest conversion. These catalysts were selective towards C12 and to some extent C18 and C24.