New Titanium Complexes Having Two Pyrrolide−Imine Chelate Ligands: Syntheses, Structures, and Ethylene Polymerization Behavior

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Abstract

New titanium complexes 1−4 having two nonsymmetric bidentate pyrrolide−imine chelate ligands, [2-(RNCH)C4H3N]2TiCl2 (1, R = Ph; 2, R = Et; 3, R = n-hexyl; 4, R = cyclohexyl), are prepared in good yields from the lithium salt of the corresponding ligands and TiCl4. Complex 1 is suggested by DFT calculations to adopt a distorted-octahedral structure in which the two pyrrolide-N atoms are situated in trans positions, while the two imine-N atoms and the two Cl atoms are located cis to one another. The spatial disposition elucidated by X-ray crystallographic analysis of complex 1 is consistent with the preferred structure predicted by DFT calculations. The molecular structures of complexes 2 and 4 established by X-ray analyses are very similar to that of complex 1. DFT calculations suggest that an active species derived from complex 1, for ethylene polymerization, possesses cis-located active sites trans to the imine-N atoms. These complexes were investigated as ethylene polymerization catalysts. Using methylalumoxane (MAO) as a cocatalyst, these complexes display very high activities and produce high-molecular-weight polyethylenes. Among them, complex 4 exhibits the highest activity (14 100 kg of polymer/((mol of Ti) h) comparable to that of Cp2TiCl2 with a very high molecular weight (Mv) value of 2 601 000. Alternatively, using Ph3CB(C6F5)4/i-Bu3Al as a cocatalyst, these complexes produce ultrahigh-molecular-weight polyethylenes (Mv > 4 000 000) with high activities (1500−2000 kg of polymer/((mol of Ti)/h).

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