Bis(arylimino)pyridine derivatives of Group 4 metals: preparation, characterization and activity in ethylenepolymerization

Abstract

Titanium tetrachloride reacts with 2,6-bis[(1-phenylimino)ethyl]pyridine, 1, and 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine, 2, giving the adducts of general formulae [Ti(1)Cl₃]Cl, 3, and [Ti(2)Cl₃]Cl, 6, the latter through the intermediacy of the covalently bonded [Ti(2)Cl₄], 4. Heating 6 leads to reduction to the titanium(III) derivative [Ti(2)Cl₃], 12, the latter characterized by X-ray diffraction methods. The reaction of [Ti(1)Cl₃]Cl with a toluene solution of MAO proceeds with methylation at the ortho-position of the pyridine ring to give the titanium(IV) derivative [Ti(C₂₂H₂₁N₃)Cl₃], 8. The reaction of [Ti(2)Cl₃]Cl with MAO gives a mixture of products containing [Ti(2)Cl₂(OAlCl₃)], 9. Compound 9, which has been prepared independently by reacting 6 with AlOCl, is a rare case of a compound containing the –OAlCl₃ moiety, as shown by a single-crystal X-ray diffraction study. From the tetrachlorides of zirconium and hafnium with 1 or 2, the corresponding adducts [M(L)Cl₄] have been obtained in high yields. These derivatives of Group 4 metals act as ethylene polymerization catalytic precursors: the substitution of the phenyl ring of the imino fragment strongly influences the catalytic activity which is 5,544 kg_PE mol_Ti⁻¹ h⁻¹ in the case of 3 and 267 kg_PE mol_Ti⁻¹ h⁻¹ with 6. Catalytic activity has been observed for zirconium and hafnium too, the activity decreasing from zirconium to hafnium, under comparable conditions.