Rare-earth metal bis(alkyl) complexes bearing pyrrolidinyl-functionalized cyclopentadienyl, indenyl and fluorenyl ligands: synthesis, characterization and the ligand effect on isoprene polymerization

Abstract

A series of rare-earth metal bis(alkyl) complexes bearing pyrrolidinyl-functionalized cyclopentadienyl, indenyl and fluorenyl ligands were prepared and characterized. Alkane elimination of Ln(CH₂SiMe₃)₃(THF)₂ with 1 equiv. of the corresponding ancillary ligand precursors in THF at room temperature afforded the rare-earth metal bis(alkyl) complexes LLn(CH₂SiMe₃)₂(THF)₂ (L = C₅Me₄SiMe₂NC₄H₈, Ln = Y (1), Lu (2); L = C₉H₆SiMe₂NC₄H₈, Lu (4)). Intramolecular C–H activation occurred when Sc(CH₂SiMe₃)₃(THF)₂ was treated with 1 equiv. of C₅Me₄HSiMe₂NC₄H₈, leading to the generation of the scandium mono(alkyl) complex (C₅Me₄SiMe₂NC₄H₇)Sc(CH₂SiMe₃)(THF) (3). One-pot salt metathesis reaction of LnCl₃ with 1 equivalent of fluorenyl lithium C₁₃H₈SiMe₂NC₄H₈Li, followed by the addition of 2 equivalents of LiCH₂SiMe₃ in THF at room temperature, gave the mono-fluorenyl-ligated rare-earth metal bis(alkyl) complexes (C₁₃H₈SiMe₂NC₄H₈)Ln(CH₂SiMe₃)₂(THF) (Ln = Y (5), Lu (6)). All these complexes were characterized by elemental analysis and NMR spectroscopy. 3, 5 and 6 were subjected to X-ray single crystal determination. Upon activation with one equivalent of [Ph₃C][B(C₆F₅)₄] and excess AlⁱBu₃, the rare-earth metal bis(alkyl) complexes 1, 2, 4–6 were active for cis-1,4-selective polymerization of isoprene in toluene at room temperature. The ligand effect on the polymerization activity was observed, with a sterically more bulky ligand providing higher activity (fluorenyl > indenyl > Cp). Moreover, employing 6/[Ph₃C][B(C₆F₅)₄]/AlⁱBu₃ as the catalyst system, polymerization proceeded in a controllable fashion.