Achiral Lanthanide Alkyl Complexes Bearing N,O Multidentate Ligands. Synthesis and Catalysis of Highly Heteroselective Ring-Opening Polymerization of rac-Lactide
作者：Xinli Liu, Xiaomin Shang, Tao Tang, Ninghai Hu, Fengkui Pei, Dongmei Cui, Xuesi Chen and Xiabin Jing
关键字：alkane elimination; polylactide; stereoselectivity; coordination?insertion
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.