Binuclear Rare-Earth-Metal Alkyl Complexes Ligated by Phenylene-Bridged β-Diketiminate Ligands: Synthesis, Characterization, and Catalysis toward Isoprene Polymerization
writer：Lei Li, Chunji Wu, Dongtao Liu, Shihui Li and Dongmei Cui
keywords：rare-earth-metal; complexes; catalytic; precursors; precursors
source：期刊
specific source：http://pubs.acs.org/doi/pdf/10.1021/om400105t
Issue time：2013年
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 1, 2a,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.
