Rare earth metal bis(silylamide) complexes bearing pyridyl-functionalized indenyl ligand: Synthesis, structure and performance in the living polymerization of l-lactide and rac-lactide

Abstract

Amine elimination of rare earth tris(silylamide) complexes Ln[N(SiHMe₂)₂]₃(THF)₂ (Ln = La, Sm, Er, Lu) with 1 equiv. of the pyridyl-functionalized indenyl ligand C₉H₇CMe₂CH₂C₅H₄N-α afforded a series of neutral mono-indenyl-ligated rare earth metal bis(silylamide) complexes (C₉H₆CMe₂CH₂C₅H₄N-α)Ln[N(SiHMe₂)₂]₂ (Ln = La (1), Sm (2), Er (3), Lu (4)) in 83–87% isolated yields. Reaction of La[N(SiHMe₂)₂]₃(THF)₂ with 2 equivalents of C₉H₇CMe₂CH₂C₅H₄N-α provided the neutral bis(indenyl) lanthanum mono(silylamide) complex (C₉H₆CMe₂CH₂C₅H₄N-α)₂LaN(SiHMe₂)₂ (5). These complexes were characterized by elemental analysis, FT-IR and NMR (except for 3 for the strong paramagnetic property of the central metal). X-ray single crystal structural diffraction showed that 1–4 are isostructural and the central metals are four-coordinated by one indenyl ring, one nitrogen atom from the pendant pyridyl group, and two amide groups to form a distorted tetrahedral geometry; while the central metal in 5 is five-coordinated by two indenyl rings, two nitrogen atoms from the pendant pyridyl groups, and one amide group to adopt a distorted pyramidal geometry, if the indenyl ring is regarded as occupying an independent vertex. The monoanionic pyridyl-functionalized indenyl ligand is bonded to the central metal in η⁵/κ¹ constrained geometry configuration (CGC) mode. 1–4 are highly active for the ring-opening polymerization of L-lactide and rac-lactide. In the presence of 2 equivalents of benzyl alcohol, 1 shows high activity toward L-lactide and rac-lactide in a living fashion.