Active O,N_py,N-Titanium(IV) Fluoride Precatalysts for Ethylene Polymerization: Exploring “Fluoride Effects” on Polymer Properties and Catalytic Performance

Abstract

Three examples of discrete six-coordinate O,N py,N-bearing titanium(IV) trifluoride complexes, [{2-(C 5 H 4 N)-6-(CMe 2 O)C 5 H 3 N}TiF 3 ] ( 1 ), [{2-(C 6 H 4 -2′-O),6-(CMe═N(2,6- i -Pr 2 C 6 H 3 ))C 5 H 3 N}TiF 3 ] ( 2a ), and [{2-(3-C 12 H 8 -2-O)-6-(CMe═N(2,6- i -Pr 2 C 6 H 3 ))C 5 H 3 N}TiF 3 ] ( 3a ), were prepared using a one-step HF elimination protocol from cis -[(THF) 2 TiF 4 ] and the corresponding pincer ligand precursor (H L1, H L2 H, and H L2 Ph ). A mer configuration of the three fluoride ligands is adopted in their solid-state structures, which is mirrored in solution, as shown by mutual two-bond F–F coupling in their 19 F{ 1 H} NMR spectra. For purposes of comparison, the chloride counterparts of 2a and 3a, [{2-(C 6 H 4 -2′-O),6-(CMe═N(2,6- i -Pr 2 C 6 H 3 )C 5 H 3 N}TiCl 3 ] ( 2b ) and [{2-(3-C 12 H 8 -2-O)-6-(CMe═N(2,6- i -Pr 2 C 6 H 3 ))C 5 H 3 N}TiCl 3 ] ( 3b ), are also reported. On treatment with excess MAO, 1 is scarcely active in ethylene polymerization, 2a is more active, and the most sterically protected system, 3a, represents the most active nonmetallocene metal-fluoride precatalyst reported to date (340 g mmol –1 h –1 bar –1 ) and produces ultra-high-molecular-weight polyethylene (UHMWPE). Similar structure–activity correlations are displayed for chloride-containing 2b and 3b, but, in general, they are significantly more active than their fluoride counterparts, with 3b being the most productive of the series (990 g mmol –1 h –1 bar –1 ). Using fluoride rather than chloride in the precatalyst has a marked effect on not only the molecular weight but also the molecular weight distribution, with broad dispersities ( Đ ) being a feature of the polymers obtained from 2a and 3a, whereas those for chloride-containing 2b and 3b are appreciably narrower. Single-crystal X-ray structures are reported for 1, 2a, 2b, and 3a .