Effects of electrochemical properties of ferrocenylpyrazolylnickel(ii) and palladium(ii) compounds on their catalytic activities in ethylene oligomerisation reactions.

Collins Obuah,James Darkwa,Michael K Ainooson

doi:10.1039/c7ra13588b

Collins Obuah, James Darkwa + Show 1 more

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https://doi.org/10.1039/c7ra13588b

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Abstract

Palladium complexes of ferrocenylpyrazolylpyridine and ferrocenylpyrazolylamine were synthesised and screened as pre-catalysts (1–4) for olefin polymerisation. The pre-catalysts 1–4 on activation with EtAlCl2 in the presence of ethylene with chlorobenzene or hexane as solvent were highly active with 1 being the most active, with an activity of 360 kg mol Pd−1 h−1. The major product from the reaction was 1-butene and high carbon content oligomers. The molecular weight (m/z) of the high carbon content oligomers is as high as 623.0. When toluene is used as solvent, the products obtained were ethyltoluene and butyltoluene and 1-butene. The electronic properties of the ligands (L1–L7) and complexes (1–10) were determined by cyclic voltammetry (CV) and molecular modelling. The CV results show that the ferrocenyl is easily oxidized upon the introduction of pyrazolyl derivatives, the process is quasi-reversible. However, complexation of the ligands with palladium or nickel results in difficulty in oxidizing the ferrocenyl moiety. This is an indication of the electrophilic nature of both the palladium and nickel centres. The mechanism of the oxidation was observed to be diffusion-controlled and is independent of scan rate. Molecular modelling experiments show that nickel and palladium complexes have lower HOMO–LUMO gaps and high global descriptors, an indication of a highly electrophilic metal centre. A plot of the electrophilicity indices of the pre-catalysts against yield of the oligomers show a linear correlation, an indication that the electrophilicity of the metal centre plays an important role in the activity of these pre-catalysts.

Highlights

Ethylene is the cheapest and most widely produced organic compound in the world,[1,2] and is an abundant feedstock for the manufacturing of value-added products including linear aole ns (LAOs), a class of commodity chemicals that are used in industries such as synthetic polymers, detergents, plasticizers, and lubricants, and have a global demand of over four million tons per year.[3]
The synthesis of functionalised ferrocene is of great importance in the eld of organometallic chemistry,[4] this can be attributed to the unique properties of ferrocene and how aDepartment of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, Johannesburg, South Africa bDepartment of Chemistry, University of Ghana, Legon, Accra, Ghana
The palladium complexes were used as pre-catalysts for the oligomerisation of ethylene upon activation with EtAlCl2 to 1butene and higher carbon content oligomers, typically with 20 to 52 carbon atoms in the oligomer

Summary

Introduction

Ethylene is the cheapest and most widely produced organic compound in the world,[1,2] and is an abundant feedstock for the manufacturing of value-added products including linear aole ns (LAOs), a class of commodity chemicals that are used in industries such as synthetic polymers, detergents, plasticizers, and lubricants, and have a global demand of over four million tons per year.[3]. The more established N^N catalysts have been the subjects of numerous mechanistic studies based on experimental and theoretical techniques.[12] The catalytic activity of the transition metal complex is relies more on the electronic con guration of the catalyst in the ground state, especially the charge density. Guo et al have shown that metallocene catalyst's activities increase with the reducing charge density[14] while the catalytic activities of a-diimine nickel(II) complexes increase with increase charge density.[15] Other report using Brookhart's type catalysts have demonstrated through theoretical mechanistic studies using quantum mechanics the importance of the conformation of the substituted on the catalysts and the electrophilicity of the central metal toward the activity of the catalysts.[16] A part from theoretical and experimental evidence, other analytical methods to determine the electronic properties of catalysts have not been extensively investigated

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