Intramolecular Benzylation of an Imino Group of Tridentate 2,5-Bis(N-aryliminomethyl)pyrrolyl Ligands Bound to Zirconium and Hafnium Gives Amido-Pyrrolyl Complexes That Catalyze Ethylene Polymerization

Amido-pyrrolyl complexes of zirconium ( 3a − d ) and hafnium ( 4a − f ) were prepared by the reaction of tetrabenzyl-zirconium and -hafnium with 2,5-bis( N -aryliminomethyl)pyrrole ligands ( 2a − e ), respectively. During the course of the reaction, one of two imino moieties of the ligand was selectively benzylated to give unique dianionic tridentate ligands, which stabilized dibenzyl complexes of zirconium and hafnium. The coordinative unsaturation around the metal center was compensated by not only the donation of the imino moiety but also by the η 2 -coordination of one of the two benzyl ligands, as confirmed by spectral data together with X-ray analysis of 3b and 3c . The zirconium complexes 3b and 3c bearing bulky substituents at the nitrogen atoms of the ligand exhibited high catalytic activities ( 3b, 131 (kg PE)(mol cat) -1 h -1 at 60 °C; 3c, 458 (kg PE)(mol cat) -1 h -1 at 75 °C) upon combining with 1000 equiv of MMAO. Lewis-base-free cationic alkyl complexes 5b, 5c, 6b, and 6c were prepared by alkyl abstruction from the corresponding dibenzyl complexes of zirconium 3b, c and hafnium 4b, c, and the resulting cationic complexes 5c and 6c were found to catalyze the ethylene polymerization without MMAO.