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-Me4CpC6H4CH2(R)­NTiCl2 (R = iPr (1), Cy
(2), nPr (3),
4-MePh (4)), have been synthesized from the chlorotrimethylsilane
elimination reactions of TiCl4 with the double-trimethylsilyl-substituted
preligands 2-Me4(TMS)­CpC6H4CH2(R)­N­(TMS) (R = iPr (TMS2L1), Cy (TMS2L2), nPr (TMS2L3), 4-MePh (TMS2L4)). The free ligands H2L1–H2L4 were synthesized by reduction
of the corresponding imine compounds 2-Me4CpHC6H4CHNR with LiAlH4, 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 1H NMR spectroscopy, and the titanium complexes
were characterized by 1H and 13C 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 AliBu3 and Ph3CB­(C6F5)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.