Titanium and zirconium complexes bearing new tridentate [OSO] bisphenolato-based ligands: synthesis, characterization and catalytic properties for alkene polymerization

Abstract

A number of new sulfur-bridged tridentate [OSO] bisphenolato-based ligand precursors S(2-CH₂-4-^tBu-6-R-C₆H₂OH)₂ [R = CMe₃ (H₂L1), CMe₂Ph (H₂L2), CMePh₂ (H₂L3), CPh₃ (H₂L4), and C(p-Tol)₃ (H₂L5)] were synthesized by reactions of Na₂S·9H₂O with 2 eq. of the corresponding 2-(bromomethyl)-4-(tert-butyl)-6-R-phenol. Their neutral titanium complexes [S(2-CH₂-4-^tBu-6-R-C₆H₂O)₂]TiCl₂ [R = CMe₃ (1), CMe₂Ph (2), CMePh₂ (3), CPh₃ (4), and C(p-Tol)₃ (5)] were synthesized in high yields by direct HCl-elimination reactions of TiCl₄ with the corresponding ligand precursors in toluene. Ionic titanium complexes [NHEt₃][S(2-CH₂-4-^tBu-6-R-C₆H₂O)₂TiCl₃] [R = CMe₃ (6), CMePh₂ (7)] and [NH₂Et₂][S(2-CH₂-4-^tBu-6-R-C₆H₂O)₂TiCl₃] [R = CMe₃ (8) and CMePh₂ (9)] were obtained in high yields from the reactions of TiCl₄ with the corresponding ligand precursors in the presence of 2 eq. of triethylamine or diethylamine. Neutral zirconium complexes [S(2-CH₂-4-^tBu-6-R-C₆H₂O)₂]ZrCl₂(THF) [R = CMe₂Ph (10·THF), and CMePh₂ (11·THF)] were synthesized by reactions of ZrCl₄ with 1 eq. of the dilithium salt of the corresponding ligand precursors Li₂L in THF. The new titanium and zirconium complexes were characterized by ¹H and ¹³C NMR, IR and elemental analyses. The molecular structures of complexes 4, 6 and 10·THF were determined by single-crystal X-ray diffraction analysis. The X-ray crystallography analysis reveals that titanium complex 4 has a five-coordinating environment surrounding the central metal atom, while the titanium complex 6 and the THF-solvated zirconium complex 10·THF possess a six-coordinating pseudo-octahedral environment around the central metal atom. Upon activation with MAO or AlⁱBu₃/Ph₃CB(C₆F₅)₄, all these titanium and zirconium complexes exhibit moderate to high catalytic activities for ethylene polymerization and ethylene/1-hexene copolymerization with moderate to high comonomer incorporation, and the ionic titanium complexes 6, 7, 8 and 9 show lower catalytic activity than their corresponding neutral complexes under similar conditions.