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

Four novel bridged‐amidines H 2 L {1,4‐R 1 [C(=NR 2 )(NHR 2 )] 2 [R 1 =C 6 H 4 , R 2 =2,6‐ i Pr 2 C 6 H 3 (H 2 L 1 ); R 1 =C 6 H 4 , R 2 =2,6‐Me 2 C 6 H 3 (H 2 L 2 ); R 1 =C 6 H 10 , R 2 =2,6‐ i Pr 2 C 6 H 3 (H 2 L 3 ); R 1 =C 6 H 10 , R 2 =2,6‐Me 2 C 6 H 3 (H 2 L 4 )]} 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°C. Alkane elimination reaction of Ln(CH 2 SiMe 3 ) 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)(Me 3 SiCH 2 ) 2 LnL 1 Ln(CH 2 SiMe 3 ) 2 (THF) [Ln=Y ( 1 ), Lu ( 2 )], (THF)(Me 3 SiCH 2 ) 2 LnL 2 Ln‐ (CH 2 SiMe 3 ) 2 (THF) [Ln=Y ( 3 ), Lu ( 4 )], (THF)(Me 3 SiCH 2 ) 2 YL 3 Y(CH 2 SiMe 3 ) 2 (THF) ( 5 ), (THF)(Me 3 SiCH 2 ) 2 YL 4 ‐ Y(CH 2 SiMe 3 ) 2 (THF) ( 6 ) in 72% –80% isolated yields. These neutral complexes showed activity towards L ‐lactide polymerization in toluene at 70°C to give high molecular weight ( M >10 4 ) and narrow molecular weight distribution ( M w / M n ≦1.40) polymers