Rare‐Earth Amido and Borohydrido Complexes Supported by Tetradentate Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in Polymerization of Cyclic Esters

Abstract

Amido [ButC(NC6H4‐2‐OMe)2]2LnN(SiMe3)2 (Ln = Y (3), Nd (4)) and borohydrido complexes [ButC(NC6H4‐2‐OMe)2]2LnBH4 (Ln = Y (5), Nd (6)) coordinated by new amidinate ligand bearing two pendant anisolyl groups are synthesized. According to X‐ray analysis in complexes 3 and 4 one amidinate ligand is coordinated to the Ln3+ cation in a bidentate fashion (κ2‐NN), while the second one is tetradentate (κ4‐NNOO). At the same time in borohydrido neodymium complex 6 the amidinate ligands demonstrate κ4‐NNOO and κ3‐NNO coordination modes. Complexes 3–6 proved to be efficient initiators for ring‐opening polymerization of rac‐lactide and allow for achieving quantitative conversion of 500 equivalents of monomer into polymer in 0.3–4.5 h at 25 °C. The polymerization of ε‐caprolactone initiated by 3–6 proceeds much faster under similar conditions.