Titanium and zirconium complexes with non-salicylaldimine-type imine-phenoxy chelate ligands: Syntheses, structures, and ethylene- polymerization behavior

New Ti and Zr complexes that bear imine-phenoxy chelate ligands, [{2,4-di-tBu-6-(RCH = N)C₆H₄O}₂MCl₂] (1: M = Ti, R = Ph; 2: M = Ti, R = C₆F₅; 3: M = Zr, R = Ph; 4: M = Zr, R = C₆F₅), were synthesized and investigated as precatalysts for ethylene polymerization. ¹H NMR spectroscopy suggests that these complexes exist as mixtures of structural isomers. X-ray crystallographic analysis of the adduct 1·HCl reveals that it exists as a zwitterionic complex in which H and Cl are situated in close proximity to one of the imine nitrogen atoms and the central metal, respectively. The X-ray molecular structure also indicates that one imine phenoxy group with the syn C=N configuration functions as a bidentate ligand, whereas the other, of the anti C=N form, acts as a monodentate phenoxy ligand. Although Zr complexes 3 and 4 with methylaluminoxane (MAO) or [Ph ₃C]⁺[B(C₆F₅)₄] ^-/AliBu3 displayed moderate activity, the Ti congeners 1 and 2, in association with an appropriate activator, catalyzed ethylene polymerization with high efficiency. Upon activation with MAO at 25°C, 2 displayed a very high activity of 19900 (kg PE)(mol Ti)^-1 h^-1, which is comparable to that for [Cp₂TiCl₂] and [Cp ₂ZrCl₂], although increasing the polymerization temperature did result in a marked decrease in activity. Complex 2 contains a C₆F₅ group on the imine nitrogen atom and mediated nonliving-type polymerization, unlike the corresponding salicylaldimine-type complex. Conversely, with [Ph₃C]⁺[B(C₆F ₅)₄]^-/AliBu3 activation, 1 exhibited enhanced activity as the temperature was increased (25-75 °C) and maintained very high activity for 60 min at 75 °C (18740 (kg PE)(mol Ti)^-1 h ^-1). ¹H NMR spectroscopic studies of the reaction suggest that this thermally robust catalyst system generates an amine-phenoxy complex as the catalytically active species. The combinations 1/[Ph₃C] ⁺[B(C₆F₅)₄]-/AliBu₃ and 2/MAO also worked as high-activity catalysts for the copolymerization of ethylene and propylene.