Synthesis, characterization and reactivity of molybdenum and tungsten imido complexes of relevance to alkene dimerization

Abstract

This thesis describes a range of investigations undertaken with the aim of increasing insight into alkene dimerization systems formed via reaction of WCI(_6) and an aniline. Chapter 1 introduces alkene dimerization systems based on reaction of WCI(_6) with an amine and an Et(_x)AICI(_2-x) co-initiator. As imido complexes are postulated to form in situ in such WCI(_6) -based dimerization systems, the imido chemistry of group VI has briefly been reviewed. Finally the aims of this work are outlined. Chapter 2 outlines a comparative investigation, examining the capacity of a range of discrete molybdenum and tungsten imido pre-catalysts to initiate ethylene dimerization. This has enabled for the first time direct comparison as to the relative activities of each type of imido complex. With the aim of establishing the nature of the initiator complex formed in situ, a range of discrete imido complexes were reacted with the co-initiators EtAICI(_2) and Et(_3)AI(_2)CI(_3) in the absence of any alkenes. Furthermore, the relative capacities of EtAICI(_2), Et(_3)AI(_2)CI(_3) and EtMgCI to activate mono(imido) complexes for propylene and ethylene dimerization were assessed. Lastly, WCI(_6) was reacted with an aniline using similar conditions to those employed in the preparation of WCI(_6)-based initiator solutions. Chapter 3 outlines the reactions of discrete bis(imido) chloride complexes with Me(_x)AICI(_3-x) reagents. This study was undertaken with the aim of clarifying the mode by which bis(imido) complexes are activated for ethylene dimerization by the co- initiator EtAICI(_2). During this investigation a new class of complexes, with the general formula M(NAIMe(_(x-1))CI(_(3-x)))(NAr)Me(_2) (M = Mo or W) were discovered. These dimethyl compounds result from coordination of a Me(_x)AICI(_3-x) (x ≥ 1) fragment to an imido ligand. Furthermore, the reactivity of these new complexes with both Lewis bases and Lewis acids was examined. Finally, a number of ethylene dimerization systems were identified based on bis(imido) complexes. Chapter 4 examines the reactivity of discrete mono(imido) chloride complexes with Me(_x)AICI(_3-x) reagents. It was determined that in contrast to the bis(imido) complexes examined in Chapter 3, coordination of Me(_x)AICI(_3-x) groups to mono(imido) ligands was disfavoured. Instead, a range of adducts were formed via coordination of Me(_x)AICI(_3-x) fragments to tungsten chloride ligands. Next, attention turned to assessing the capacity of Me(_x)AICI(_3-x) reagents to activate mono(imido) complexes for ethylene dimerization. All the experimental details and supporting information and data for this thesis are presented in Chapter 5. In Chapter 6 further possible investigations are outlined.