Bis(oxazolinyl)phenyl-Ligated Rare-Earth-Metal Complexes: Highly Regioselective Catalysts for cis-1,4-Polymerization of Isoprene

NCN-pincer (S,S)-2,6-bis(4′-isopropyl-2′-oxazolinyl)phenyl-ligated rare-earth-metal dichlorides [(S,S)-Phebox-iPr]LnCl2(THF)2 (Ln = Sc (1); Y (2); Dy (3); Ho (4); Tm (5); Lu (6)) were synthesized via transmetalation between [(S,S)-Phebox-iPr]Li and LnCl3 in THF solvent. Interestingly, treatment of LaCl3 by the same method generated tris(ligand) lanthanum complex [(S,S)-Phebox-iPr]3La (7). Molecular structures of complexes 1, 2, 3, and 7 were established by single-crystal X-ray diffraction study. Pincer ligand (S,S)-Phebox-iPr adopts a κC:κN:κN′ tridentate coordination mode to the central metal ion. Upon activation with [PhNHMe2][B(C6F5)4] and AliBu3, complexes 2–5 exhibited highly catalytic activities and more than 98% cis-1,4-selectivity for isoprene polymerization while complexes 1 and 6 were inactive for this reaction. When use of the catalyst system consisted of complex 2, [PhNHMe2][B(C6F5)4], and AliBu3 for isoprene polymerization, the resultant polymer has a high cis-1,4-selectivity up to 99.5%. The...