New Titanium and Hafnium Complexes Bearing [−NNN–] Pyrrolylpyridylamido Ligands as Olefin Polymerization Catalysts

Four new complexes bearing either unsubstituted or substituted pyrrolylpyridylanilido ligands, with the general formula [ − NNN – ]M(NMe 2 ) 2 (M = Ti(IV), Hf(IV)), were synthesized and fully characterized by NMR spectroscopy and elemental analyses. The solid-state structures of one Ti and one Hf complex (both bearing the 2-isopropylphenyl substituent on the carbon bridging the pyridine and the aniline moieties) were obtained by single-crystal X-ray diffraction analyses, showing in both cases slightly distorted square pyramidal geometries, with the [ − NNN – ] ligand and one Me 2 N forming the base plane and the second Me 2 N as the apex. The new complexes and, for comparison, their zirconium analogues were tested as precatalysts for olefin polymerization, after activation with Al i Bu 2 H/methylalumoxane. Polymerization of ethylene under mild conditions (25 °C and 1 atm) was promoted by all catalysts with high activities, resulting in highly linear polyethylenes with ultrahigh molecular weights ( M w up to 2.7 MDa). Polymerization of propylene (at 25 °C and 6 atm) was promoted by all complexes, with activities decreasing in the order Ti > Zr > Hf for the C s -symmetric complexes and in the order Zr ≫ Ti > Hf for the C 1 -symmetric complexes. Concerning stereoselectivity, catalysts based on Zr and Hf complexes behave similarly, with the C 1 -symmetric species of both metals affording much more isotactic polymers than the C s -symmetric species ([ mmmm ] 95% vs 73–79%) and performing better also in terms of activities and production of high-molecular-weight polypropylenes ( M w up to 1.8 MDa for Hf). The Ti-based catalysts are less stereospecific and produce polypropylenes having very high molecular weights and rather broad molecular weight distributions. Solvent extraction of the raw samples showed the presence, in both cases, of an isotactic and an atactic fraction, also having different regioerrors, suggesting the formation of at least two active species.