Achiral Lanthanide Alkyl Complexes Bearing N,O Multidentate Ligands. Synthesis and Catalysis of Highly Heteroselective Ring-Opening Polymerization of rac-Lactide
writer：Xinli Liu, Xiaomin Shang, Tao Tang, Ninghai Hu, Fengkui Pei, Dongmei Cui, Xuesi Chen and Xiabin Jing
keywords：alkane elimination; polylactide; stereoselectivity; coordination?insertion
source：期刊
specific source：http://pubs.acs.org/doi/pdf/10.1021/om0700359
Issue time：2007年
Alkane
elimination reactions of amino-amino-bis(phenols) H2L1-4,
Salan H2L5, and methoxy-β-diimines HL6,7 with
lanthanide tris(alkyl)s, Ln(CH2SiMe3)3(THF)2 (Ln
= Y, Lu), respectively, afforded a series of lanthanide alkyl complexes 1?8 with
the release of tetramethylsilane. Complexes 1?6 are
THF-solvated mono(alkyl)s stabilized by O,N,N,O-tetradentate
ligands. Complexes 1?3 and 5 adopt
twisted octahedral geometry, whereas 4 contains a tetragonal
bipyramidal core. Bearing a monoanionic moiety L6 (L7),
complex 7 (8) is a THF-free bis(alkyl). In
complex 7, the O,N,N-tridentate ligand combined with
two alkyl species forms a tetrahedral coordination core. Complexes 1, 2,
and 3 displayed modest activity but high stereoselectivity for
the polymerization of rac-lactide to give heterotactic polylactide
with the racemic enchainment of monomer units Pr ranging
from 0.95 to 0.99, the highest value reached to date. Complex 5exhibited
almost the same level of activity albeit with relatively low selectivity. In
contrast, dramatic decreases in activity and stereoselectivity were found for
complex 4. The Salan yttrium alkyl complex 6 was
active but nonselective. Bis(alkyl) complexes 7 and 8 were
more active than 1?3toward polymerization of rac-LA,
however, to afford atactic polylactides due to di-active sites. The ligand
framework, especially the “bridge” between the two nitrogen atoms, played a
significant role in governing the selectivity of the corresponding complexes
via changing the geometry of the metal center. The NMR spectrum of the active
species of the rac-lactide oligomer attached to complex 1 demonstrated
a coordination?insertion mechanism. In addition it also confirmed that the
geometry of the metal center of complex 1 in the solid state
was retained in solution (THF) during the polymerization, which contributed
significantly to the high selectivity of the complex.
