C₃-Symmetric Chiral Organolanthanide Complexes:  Synthesis, Characterization, and Stereospecific Polymerization of α-Olefins^‖

The trialkyl complexes [M(iPr-trisox)(CH₂SiMe₂R)₃] (R = Me, M = Y, (1), R = Ph, M = Lu (2a), R = Me, M = Lu (2b), Tm (3), Er (4), Ho (5), and Dy (6)) were prepared from 1,1,1-tris[(S)-4-isopropyloxazolinyl]ethane (iPr-trisox) and the corresponding trialkyl precursors [M(CH₂SiMe₂R)₃(THF)_n]. Their molecular structures all display a highly distorted octahedral geometry, with the angles subtended at the metal center significantly deviating from the ideal 90°, which is attributed to the steric demands imposed by the large CH₂SiMe₂R ligands, both with each other and with the isopropyl groups of the iPr-trisox ligand. Active catalysts for the polymerization of α-alkenes (n-hexene, n-heptene, and n-octene) were generated in situ by reaction of the trialkyl precatalyst with 2 equiv of trityl tetrakis(pentafluorophenyl)borate. In all cases polyolefins with M_w/M_n values of between 1.58 and 2.08 and isotacticities of 80−95% were obtained. The polymerization activity increases from lutetium to thulium and then subsequently decreases with increasing ionic radius of the metal due to a combination of activation with increasing ionic radius and decreasing catalyst stability.