940000-60-2Relevant academic research and scientific papers
Achiral lanthanide alkyl complexes bearing N,O multidentate ligands. Synthesis and catalysis of highly heteroselective ring-opening polymerization of rac-lactide
Liu, Xinli,Shang, Xiaomin,Tang, Tao,Hu, Ninghai,Pei, Fengkui,Cui, Dongmei,Chen, Xuesi,Jing, Xiabin
, p. 2747 - 2757 (2007)
Alkane elimination reactions of amino-amino-bis(phenols) H 2L1-4, 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 5 exhibited 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 1-3 toward 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.
