Heteroleptic lanthanide amide complexes bearing carbon-bridged bis(phenolate) ligands: synthesis, structure and their application in the polymerization of ε-caprolactone

Abstract

A convenient method for the synthesis of anionic lanthanide amide complexes bearing carbon-bridged bis(phenolate) ligands H₂L [L = (o-CH₃)PhCH(C₆H₂-3-^tBu-5-R-2-O)₂; R = Me, L = L₁ and R = ^tBu, L = L₂] is described. The bis(phenolato)lanthanide complexes LLnN(SiMe₃) [L = L₁, Ln = La (1), Ln = Gd (2) and L = L₂, Ln = La (3), Ln = Gd (4)] were synthesized in nearly quantitative yields by the reaction of Ln[N(SiMe₃)₂]₃ (Ln = La or Gd) with H₂L in a 1 : 1 molar ratio in THF at 60 °C. The bis(phenolato)lanthanide pyrazolato complexes LLnPzMe₂(THF)₃ [L = L₁, Ln = La (5), Ln = Gd (6) and L = L₂, Ln = La (7), Ln = Gd (8)] were obtained in high yields by further reacting the obtained LLnN(SiMe₃) with another 1 equiv. of 3,5-dimethylpyrazole (Me₂PzH). Meanwhile, the complexes LLnPzMe₂ can also be synthesized by the direct reaction of Ln[N(SiMe₃)₂]₃ with H₂L and Me₂PzH in 1 : 1 : 1 molar ratio in situ in THF. Complexes 5–8 have been characterized by X-ray crystal structural analysis. The central lanthanide metal atom is seven-coordinated by one bis(phenolate) ligand, one pyrazole ligand and three THF molecules in a distorted pentagonal bipyramid. The catalytic activity of compounds 1–8 in the ring-opening polymerization of ε-caprolactone was studied. The catalytic mechanism was studied and discussed as well.