New Titanium(IV) Complexes with 2-Cyclopentadienylbenzylamido Ligands: Synthesis, Characterization, and Catalytic Properties for Ethylene Polymerization and Copolymerization with 1-Hexene

A series of new half-sandwich titanium(IV) complexes chelated with 2-(tetramethylcyclopentadienyl)benzylamido ligands, 2-Me 4 CpC 6 H 4 CH 2 (R)NTiCl 2 (R = i Pr ( 1 ), Cy ( 2 ), n Pr ( 3 ), 4-MePh ( 4 )), have been synthesized from the chlorotrimethylsilane elimination reactions of TiCl 4 with the double-trimethylsilyl-substituted preligands 2-Me 4 (TMS)CpC 6 H 4 CH 2 (R)N(TMS) (R = i Pr (TMS 2 L1 ), Cy (TMS 2 L2 ), n Pr (TMS 2 L3 ), 4-MePh (TMS 2 L4 )). The free ligands H 2 L1 –H 2 L4 were synthesized by reduction of the corresponding imine compounds 2-Me 4 CpHC 6 H 4 CH═NR with LiAlH 4, while the imine compounds were formed in situ by a condensation reaction of 2-tetramethylcyclopentadienylbenzaldehyde with the corresponding amine. The free ligands were characterized by LC-MS and 1 H NMR spectroscopy, and the titanium complexes were characterized by 1 H and 13 C NMR, elemental analyses, and single-crystal X-ray crystallography. The X-ray crystallography analysis reveals that these titanium complexes possess a three-legged piano-stool geometry with the amide N atom in a mitered six-membered chelating ring and the two chloride atoms as the legs. The angle between the cyclopentadienyl plane and the attached phenyl plane in these complexes (59.2, 62.7, and 61.9° for complexes 1, 2, and 4, respectively) is much less than 90° in the solid state. Upon activation with Al i Bu 3 and Ph 3 CB(C 6 F 5 ) 4, complexes 1 – 4 exhibit reasonable catalytic activity for ethylene homopolymerization and copolymerization with 1-hexene at 110 °C, producing high-molecular-weight polyethylenes and poly(ethylene- co -1-hexenes) with relatively high comonomer incorporation. Complex 4 was found to show higher catalytic activity for ethylene/1-hexene copolymerization than complexes 1 – 3 under similar conditions, while complexes 1 – 3 produce poly(ethylene- co -1-hexenes) with higher comonomer incorporation.