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-Pr-i]LnCl(2)(THF)(2) (Ln = Sc (1); Y (2); Dy (3); Ho (4); Tm (5); Lu (6)) were synthesized via transmetalation between [(S,S)-Phebox-Pr-i]Li and LnCl(3) in THF solvent. Interestingly, treatment of LaCl3 by the same method generated tris(ligand) lanthanum complex [(S,S)-Phebox-Pr-i](3)La (7). Molecular structures of complexes 1, 2, 3, and 7 were established by single-crystal X-ray diffraction study. Pincer ligand (S,S)-Phebox-Pr-i adopts a kappa C:kappa N:kappa N' tridentate coordination mode to the central metal ion. Upon activation with [PhNHMe2][B(C6F5)(4)] and (AlBu3)-Bu-i, 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 (AlBu3)-Bu-i for isoprene polymerization, the resultant polymer has a high cis-1,4-selectivity up to 99.5%. The reaction temperature had little effect on the regioselectivity, and high cis-1,4-selectivity almost remained even at 80 degrees C.