Bimetallic Rare Earth Alkyl Complexes Bearing Bridged Amidinate Ligands: Synthesis and Activity for L-Lactide Polymerization

Four novel bridged-amidines H2L {1,4-R-1[C(=NR2)(NHR2)}(2) [R-1 =C6H4, R-2=2,6-(Pr2C6H3)-Pr-i (H2L1); =C6H4, R-2=2,6-Me2C6H3 (H2L2); R-I =C6H10, R-2=2,6-(Pr2C6H3)-Pr-i (H2L3); R-i =C6H10, R-2= 2,6-Me2C6H3 (H2L4)]} were synthesized in 65%-78% isolated yields by the condensation reaction of dicarboxylic acid with four equimolar amounts of amines in the presence of PPSE at 180 degrees C. Alkane elimination reaction of Ln(CH2SiMe3)(3)(THF)(2) (Ln=Y. Lu) with 0.5 equiv. of amidine in THF at room temperature afforded the corresponding bimetallic rare earth alkyl complexes (THF)(Me3SiCH2)(2)LnL(1)Ln(CH2SiMe3)(2)(THF) [Lit = Y (1), Lu (2)1, (THF)(Me3SiCH2),LnL(2)Ln(CH2SiMe3)(2)(THF) [Ln=Y (3), Lu (4)], (THF)(Me3SiCH2)(2)(YLY)-Y-3(CH2S1Me3)(2)(THF) (5), (THF)(Me3SiCH2)(2)YL4-Y(CH2SiMe3)(2)(THF) (6) in 72%-80% isolated yields. These neutral complexes showed activity towards L-lactide polymerization in toluene at 70 degrees C to give high molecular weight (M>10(4)) and narrow molecular weight distribution 1.40) polymers