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

摘要

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 LnN(TMS)_2 _3(μ-Cl)Li(THF)_3 with PARA-H_2, PARA-H _2 = 22,6-~iPr_2C_6H _3NHC(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-{LnN(SiMe_3)_2_2}_2 Ln = Nd(1), Sm(2), Y(3) in high isolated yields. Similar reaction of NdN(TMS) _2_3(μ-Cl)Li(THF)_3 with META-H2, META-H_2 = 22,6-~iPr_2C_6H _3NHC(Me)C(H)C(Me)N-(meta-phenylene) at 90°C in toluene for about 48 h gave META-{NdN(SiMe_3)_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 (Me _3Si)_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.