Bimetallic lanthanide amido complexes as highly active initiators for the ring-opening polymerization of lactides

Abstract

A series of neutral bimetallic lanthanide amido complexes supported by rigid phenylene bridged bis(β-diketiminate) ligands were synthesized, and their catalytic behavior for the polymerization of L-lactide and rac-lactide was explored. The amine elimination reaction of Ln[N(TMS)₂]₃(μ-Cl)Li(THF)₃ with PARA-H₂, [PARA-H₂ = 2[2,6-ⁱPr₂C₆H₃NHC(Me)C(H)C(Me)N]-(para-phenylene)] in a 2 : 1 molar ratio in THF at 25 °C afforded the corresponding bimetallic lanthanide amido complexes PARA-{Ln[N(SiMe₃)₂]₂}₂ [Ln = Nd(1), Sm(2), Y(3)] in high isolated yields. Similar reaction of Nd[N(TMS)₂]₃(μ-Cl)Li(THF)₃ with META-H₂, [META-H₂ = 2[2,6-ⁱPr₂C₆H₃NHC(Me)C(H)C(Me)N]-(meta-phenylene)] at 90 °C in toluene for about 48 h gave META-{Nd[N(SiMe₃)₂]₂}₂ (4). Complexes 1–4 were well characterized by elemental analysis, IR spectroscopy, and their definitive structures were confirmed by an X-ray crystal structure analysis. The coordination environment and coordination geometry around the metal atoms are similar in these complexes. Each of the metal atoms is four-coordinated with two nitrogen atoms from the N,N-chelating β-diketiminate unit, and two nitrogen atoms from two (Me₃Si)₂N– groups to form a distorted tetrahedron. These complexes can serve as highly active initiators for L-lactide polymerization in toluene. In addition, they also showed high activity towards rac-lactide polymerization in THF at room temperature, giving heterotactic-enriched polymers (P_r ≈ 0.70), and complex 4 displays obviously higher activity in comparison with complex 1.