Achiral Lanthanide Alkyl Complexes Bearing N,O Multidentate Ligands. Synthesis and Catalysis of Highly Heteroselective Ring-Opening Polymerization of rac-Lactide

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Abstract

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 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.

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