Discovery and Development of Novel P,N Ligands for Selective Ethylene Oligomerisation Systems

This thesis documents the design, discovery, and development of a range of P,N-type ligands and their application in selective ethylene oligomerisation processes. Two classes of ligand have been investigated, PNE (Ph2P(CH2)2NC4H8E, E = NMe, O, CH2) ligands, and iminophosphine ligands (R(PR’2)C=NAr). The coordination chemistry of both these series of compounds has been investigated with various chromium species, and the behaviour of these molecules in ethylene oligomerisation systems probed. Chapter 2 explores the application of PNE ligands to ethylene oligomerisation. The coordination chemistry of the PNE ligands with a selection of group VI starting materials has been investigated. It has been shown that both the PNO and PNC ligands form bidentate complexes upon reaction with CrCl3(THF)3 (e.g. CrCl3(THF)(PNO), 2.1), while attempts to synthesise the PNN analogue proved unsuccessful. The coordination of PNE ligands with Cr(0) and Mo(0) has been studied, with the ligands exhibiting both mono- (e.g. Cr(CO)5(PNN) (2.4) and bi-dentate coordination (e.g. Mo(CO)4(PNN) (2.7)), dependant on the combination of metal and ligand used. Conversion of monodentate Cr(CO)5(κ1-PNN) (2.4) to bidentate Cr(CO)4(κ2-PNN) (2.10) is achieved on reaction with trimethylamine N-oxide. The application of PNE ligands to ethylene oligomerisation systems has been carried out. All of the catalytic systems tested gave primarily polymer products, with no selective ethylene oligomerisation occurring. Chapter 3 describes the development of a modular synthetic route to novel iminophosphine compounds via reaction of imidoyl chlorides with trimethylsilyl phosphines. Using this synthetic process, a library of iminophosphines has been synthesised (3.1-3.26) with varying steric and electronic characteristics. The donor properties of the iminophosphines have been analysed by measurement of ǀ1JSe-Pǀ couplings of the derivative phosphine selenides, with all the compounds demonstrating good phosphine donor properties. Finally, the E/Z isomerisation behaviour of the PCN compounds has been studied, revealing that P-aryl substituted iminophosphines exist as a mixture of the E and Z isomers in equilibrium in solution. Chapter 4 details investigations into the coordination chemistry of iminophosphine ligands with chromium. A selection of ligands have been reacted with CrCl3(THF)3, forming a range of bidentate CrIII(κ2-PCN) complexes (e.g. CrCl3(THF)(Ph(PPh2)C=NPh), 4.1). The coordination of iminophosphine ligands with Cr(0) has shown that the ligands react with Cr(CO)6 to yield both mono- and bi-dentate products (e.g. Cr(CO)4(Ph(PiPr2)C=N(2,6-iPr2C6H3)) (4.7) and Cr(CO)5(Ph(PiPr2)C=N(2,6-iPr2C6H3)) (4.8), respectively). The denticity of these Cr(CO)(6-n)(κn-PCN) complexes can be controlled through the removal and addition of CO in the atmosphere. Chapter 5 recounts the use of the novel iminophosphines as ligands in selective ethylene oligomerisation systems. A wide range of catalytic conditions were tested, including varying the chromium source, solvent, reaction temperature and pressure, the use of polymer reducing additives, and the dosing of O2 into the reaction. It has been shown that a catalytic process using Cr(2-EH)3, 1 equivalent of iminophosphine 3.9, 49 bar C2H4, in methylcyclohexane at 60 °C, with 0.66 ppm O2 and 100 equivalents of ZnEt2 yields a highly active and selective ethylene tri-/tetramerisation process.