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

Abstract

Alkane elimination reactions of amino-amino-bis(phenols) H 2 L 1 - 4, Salan H 2 L 5, and methoxy-β-diimines HL 6,7 with lanthanide tris(alkyl)s, Ln(CH 2 SiMe 3 ) 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 L 6 ( L 7 ), 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 P r 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.