Binuclear Rare-Earth-Metal Alkyl Complexes Ligated by Phenylene-Bridged β-Diketiminate Ligands: Synthesis, Characterization, and Catalysis toward Isoprene Polymerization
作者:Lei Li, Chunji Wu, Dongtao Liu, Shihui Li and Dongmei Cui
关键字:rare-earth-metal; complexes; catalytic; precursors; precursors

Deprotonation of m-phenylene-bridged bis(β-diketiminate) ligands (PBDIiPr-H2 = [2,6-iPr2C6H3NHC(Me)C(H)C(Me)N]2-(m-phenylene); PBDIEt-H2 = [2,6-Et2C6H3NHC(Me)C(H)C(Me)N]2-(m-phenylene); PBDIMe-H2 = [2,6-Me2C6H3NHC(Me)C(H)C(Me)N]2-(m-phenylene)) by rare-earth-metal tris(alkyls) Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu, Sc) gave a series of rare-earth-metal bis(alkyl) complexes: PBDIiPr-[Y(CH2SiMe3)2]2(THF)2 (1), PBDIEt-[Ln(CH2SiMe3)2]2(THF)n (2a, Ln = Y, n = 2; 2b, Ln = Lu, n = 2; 2c, Ln = Sc, n = 1), and PBDIMe-[Y(CH2SiMe3)2]2(THF)2 (3). All these complexes were fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analyses, adopting binuclear structures with the two rare-earth-metal ions taking trans positions versus the phenyl ring. Complexes 12a,b, and 3 coordinate two solvated THF molecules, while the scandium complex 2c incorporates only one THF molecule, owing to the steric crowding. Upon activation with 2 equiv of organoborate, the yttrium systems showed higher catalytic activity toward isoprene polymerization in comparison to those based on lutetium, and the scandium system was less active. Addition of aluminum alkyls to the above binary systems accelerated dramatically the polymerization rate irrespective of the central metal type through scavenging impurities in the systems and abstracting the solvated THF molecules in the precursors. The resultant polyisoprene had higher 3,4-regularity (20% vs 5%) as well as higher molecular weights in comparison with the mononuclear systems, which might be attributed to the steric bulky effect of the binuclear systems.