Metal imido-catalyzed ethylene and 1-hexene dimerization

This thesis investigates the development of well-defined ethylene and 1-hexene dimerization pro-initiators related to the industrially important SASOL α-olefin dimerization system, which consists of WCl6/RNH2/Et3N activated by EtAlCl2. The role of Et3N in the SASOL system is to remove HCl generated from the reaction of the amine with WCl6, something that is suggestive of the possible in situ formation of tungsten imido complexes. To probe the potential role of tungsten imido species, here a range of ethylene and 1-hexene dimerization experiments are described using discrete tungsten, niobium, and tantalum imido complexes in combination with an alkyl aluminum activator (usually EtAlCl2). In order to better understand the possible dimerization mechanisms, selected reactions of the aforementioned imido complexes with alkyl aluminum reagents are also explored. Chapter 1: the industrial importance of olefins and the olefin oligomerization processes are introduced with emphasis on the SASOL α-olefin dimerization system. Mechanistic aspects of the olefin oligomerization process are outlined giving examples of previously reported ethylene and 1-hexene dimerization systems. The aims of this PhD thesis are summarized. Chapter 2: the synthesis and structural characterization of a range of tungsten bis(imido) complexes W(NR)(NR’)Cl2(DME) (R, R’ = alkyl or aryl) is presented. The activity of such complexes as pro-initiators in the dimerization of ethylene and 1-hexene is detailed and the effects of ethylene pressure, temperature, activator, pro-initiator concentration, pro-initiator:activator ratios, and use of additives explored; pro-initiator structure-catalytic activity correlations are established. Preliminary mechanistic studies of tungsten bis(imido)-mediated dimerization are described. Chapter 3: tungsten mono(imido) complexes, W(NR)Cl4(THF) and [W(NR)Cl4]2 (R = alkyl or aryl), are tested in the dimerization of ethylene and 1-hexene. Studies similar to those in Chapter 2 are described, demonstrating that such mono(imido) complexes are reasonably active and selective ethylene and 1-hexene dimerization pro-initiators. Reactions between the tungsten mono(imido) complexes and alkyl aluminum reagents are detailed, providing insight into the role of the aluminum activator in the dimerization process; these suggest that reduction of the tungsten centre is an important part of the initiation process. Hence, discrete W(IV) and W(V) mono(imido) complexes were tested for olefin dimerization, with the latter being active. The beneficial effect of additives (chloride anions, amines) in catalytic dimerization reaction is demonstrated and their mode of action probed. Chapter 4: the use of niobium and tantalum imido complexes, M(NR)Cl3(DME), as pro-initiators is explored. In contrast to their tungsten counterparts these group V imido complexes were able to both polymerize and/or dimerize ethylene, but were inactive towards1-hexene. Chapter 5: full experimental details are presented. Chapter 6: supplementary information including - reactivity of the alkyl aluminum reagents employed for the activation of the pro-initiators; definitions used in the analysis of catalysis test data (TON, etc); and development and validation of methods for the analysis of olefin dimerization products

Synthesis and Characterisation of Phosphenium Ions with Aromatic Amido Substituents

σ2λ3-Phosphenium cations A are valence isoelectronic and isolobal with silenes B and hence, carbenes C. Notably, the chemistry of these divalent phosphorus cations mirrors that of their carbon-based analogues (e.g. cycloaddition hemistry, C-H insertions, etc.), making them versatile reagents in a variety of transformations and reactions. Just as for carbenes (e.g. amino carbenes D and E), a combination of kinetic and electronic stabilisation has been used to prepare and isolate stable phosphenium ions such as F, with amido substituents. This approach is also followed in this work. In particular, the synthesis, characterisation, and properties of a range of phosphenium ions (4) with alkyl or aryl amido substituents (R2N, R = alkyl or aryl) is described. The desired phosphenium ions 4 are synthesized in three steps by reacting the secondary amines 1 with nBuLi, which results in the production of the appropriate lithium amide salts (2) followed by subsequent reaction with PCl3, which yields the chlorophosphines 3. Reaction of 3 with a halogen abstracting reagent such as AlCl3 (or TMSOTf) results in the formation of phosphenium ions (4), which were studied by NMR spectroscopy and X-ray diffraction