Zirconium complexes as catalysts in the oligomerisation of ethylene

This thesis describes the development of a model and subsequent new catalyst systems for zirconium catalysed ethylene oligomerisation. The developed model predicts that conditions which favour transition states for ˜í‚â§-hydrogen elimination, (i.e. coordinatively unsaturated metal centre, a low lying metal orbital and high Lewis acidity), would lead to better oligomerisation catalysts. This led to the in situ catalytic testing of hemi-labile, bidentate, chelate ligands containing atoms of varying donor ability, i.e. thioacetylacetones, Schiffs bases, picolinic acid and ˜í‚â§-aminoketones. In situ catalytic tests using ˜í‚â§-aminoketones (I), the primary ligands in this study, have shown them to be particularly effective in promoting oligomerisation and catalytic testing has shown for the first time the ability to control reactivity and oligomer distribution in zirconium based catalyst systems by variation of ligand substituents. Bulky substituents, R¬¨‚⧠& R¬¨‚â•, reduce catalytic activities. Electron withdrawing groups on the amine (imine in complexes) or ligand backbone increase reactivity. The use of bis-˜í‚â§-aminoketone zirconium complexes (II) resulted in catalytic systems with the highest activity and average oligomer weight. Activity variations between the in situ catalytic testing of free ˜í‚â§-aminoketones and preformed complexes have been explained through complex ligand-cocatalyst interactions. It is proposed that a decreased amine basicity reduces the tendency for amine coordination which increases metal centre Lewis acidity thereby resulting in lower average oligomer weight. Synthetic work has led to the isolation and characterisation of a number of new co- ordination, organometallic and cationic zirconium complexes containing ˜í‚â§-aminoketones. Alkyl-zirconium complexes were synthesised by the reaction of free ˜í‚â§-aminoketones with the homoleptic tetraalkyl, ZrBz‚Äövávë. Cationic complexes were synthesised by partial protolysis of the alkyl complexes using PhNMe‚ÄöváváH+BPh‚Äövávë-. This work introduces a new area of ligand stabilised, highly Lewis acidic, alkyl zirconium chemistry. VT-NMR studies have led to the identification of stability regimes for alkylated, bis-ligand complexes and indicated that ˜í‚â§-aminoketones function as bidentate chelate ligands in most complexes. Electron withdrawing groups on or near the imine led to hemi-labile systems with nitrogen dissociation. Cis-benzyl ligands are found to have an average bonding environment somewhat between a true sigma or dihapto bond with a stronger dihapto character at lower temperatures. Benzyl bonding reflects variations in metal centre Lewis acidity. Cationic, bis-˜í‚â§-aminoketone zirconium complexes contain ˜í‚à묨‚â§-bound benzyl. In situ NMR catalysis experiments have shown complex cocatalyst/ligand interactions which depend on the relative basicity of ˜í‚â§-aminoketone protons. ˜í‚â§-Aminoketones remain bonded to zirconium via oxygen during catalysis and ethylene insertion occurs at zirconium. The new catalytic systems oligomerise ethylene under very mild conditions, RT and 1 atm ethylene. Interestingly this work has clearly demonstrated that many of the features and catalytic controls currently being developed in zirconium metallocene (and related systems) based catalysts may be achieved more simply from zirconium coordination complexes and their organometallic derivatives.