Zirconium complexes as catalysts for the oligomerisation of ethylene: the role of chelate ligands and the Lewis acid cocatalyst in the generation of the active species

Highly effective catalysts for the conversion of ethylene into linear α-olefins may be generated in situ from zirconium tetrachloride, a β- aminoketone or β-aminothioketone ligand and an alkylaluminium chloride Lewis acid cocatalyst. Catalysts may also be generated from ZrCl4 · 2HL adducts (where HL = monodentate, oxygen-bound β-aminoketones), and from bis-ligand complexes of the type ZrCl2L2 (where L = monovalent, bidentate β- aminoketones or β-aminothioketones) on treatment with an alkylaluminium chloride cocatalyst. Product distribution and catalyst activity can be adjusted by ligand substituent variation and/or by the method of catalyst formation. Catalyst systems generated from preformed complexes, ZrCl2L2, were in general significantly more active than those formed in situ or from the bis-ligand adducts, ZrCl4 · 2HL. Activities of up to 60,000 turnovers/h were obtained with selected complexes. However, in situ mixtures and bis- ligand adducts generally gave a much narrower oligomer distribution, with up to 95% of the oligomers occurring in the C4-C10 range for a number of the systems tested. In situ NMR (nuclear magnetic resonance) tests indicate that complex ligand/cocatalyst interactions are present and that these new oligomerisation systems are active even under very mild conditions, 30°C and 1 atm ethylene.