Mono(amidinate) rare earth metal bis(alkyl) complexes: Synthesis, structure and their activity for l-lactide polymerization

Alkane elimination reaction between Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu) with one equivalent of the amidines with different steric demanding HL ([CyC(N-2,6-iPr2C6H3)2]H (HL1), [CyC(N-2,6-Me2C6H3)2]H (HL2), [PhC(N-2,6-Me2C6H3)2]H (HL3)) in THF afforded a series of mono(amidinate) rare earth metal bis(alkyl) complexes [CyC(N-2,6-iPr2C6H3)2]Ln(CH2SiMe3)2(THF) (Ln = Y (1), Lu (3)), [CyC(N-2,6-Me2C6H3)2]Ln(CH2SiMe3)2(THF)2 (Ln = Y (4), Lu (6)), and [PhC(N-2,6-Me2C6H3)2]Y(CH2SiMe3)2(THF)2 (7) in 75–89% isolated yields. For the early lanthanide metal Nd, THF slurry of NdCl3 was stirred with three equiv of LiCH2SiMe3 in THF, followed by addition of one equiv of the amidines HL1 or HL2 gave an “ate” complex [CyC(N-2,6-iPr2C6H3)2]Nd(CH2SiMe3)2(μ-Cl)Li(THF)3(2) in 48% yield and a neutral [CyC(N-2,6-Me2C6H3)2]Nd(CH2SiMe3)2(THF)2(5) in 52% yield, respectively. They were characterized by elemental analysis, FT-IR, NMR spectroscopy (except for 2 and 5 for their strong paramagnetic property). Complexes 2, 3, 4 and 5 were subjected to X-ray single crystal structure determination. These neutral mono(amidinate) rare earth metal bis(alkyl) complexes showed activity towards l-lactide polymerization to give high molecular weight and narrow molecular weight distribution polymers.