Synthesis and characterization of bimetallic lanthanide–alkali metal complexes stabilized by aminophenoxy ligands and their catalytic activity for the polymerization of 2,2-dimethyltrimethylene carbonate

Abstract

Electronic properties of the aminophenolate groups have obvious effect on the synthesis of aminophenolate lanthanide–lithium complexes. Amine elimination reactions of Ln[N(SiMe₃)₂]₃(μ-Cl)Li(THF)₃ with lithium aminophenolates [ArNHCH₂(3,5-^tBu₂C₆H₂-2-O)Li(THF)]₂ (Ar = p-ClC₆H₄, [ONH]^Cl-p; p-BrC₆H₄, [ONH]^Br-p) in tetrahydrofuran (THF) in a 1 : 2 molar ratio gave the bimetallic lanthanide–lithium amido complexes [NO]^Cl-p₂Ln[N(SiMe₃)₂][Li(THF)]₂ (Ln = Y (1), Yb (2)), and [NO]^Br-p₂Ln[N(SiMe₃)₂][Li(THF)]₂ (Ln = Y (3), Yb (4)). When the Ar groups are p-MeOC₆H₄, ([ONH]^MeO-p) and o-MeOC₆H₄ ([ONH]^MeO-o), similar reactions generated the homoleptic lanthanide–lithium complexes [NO]^MeO-p₃Ln[Li(THF)]₃ (Ln = Y (5), Yb (6)) and [NO]^MeO-o₂Ln[Li(THF)] (Ln = Y (7), Yb (8)) in high isolated yields, respectively. Whereas the bimetallic lanthanide–lithium amido complexes [NO]^Cl-o₂Ln[N(SiMe₃)₂][Li(THF)]₂ (Ln = Y (9), Yb (10)) can be obtained in good yields, when the Ar group is o-ClC₆H₄ ([ONH]^Cl-o). All of these complexes were well characterized. X-ray structure determination revealed that these complexes have solvated monomeric structures. In complexes 1–4, 9, and 10, the lanthanide atom is five-coordinated by two oxygen atoms and two nitrogen atoms from two aminophenoxy ligands and one nitrogen atom from N(SiMe₃)₂ group to form a distorted trigonal bipyramidal geometry, whereas in complexes 5–8, the central lanthanide atom is six-coordinated by oxygen atoms, and nitrogen atoms from the aminophenoxy ligands to form a distorted octahedron. It was found that complexes 1–10 are highly efficient initiators for the ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC), affording the polymers with high molecular weights, and the homoleptic heterobimetallic lanthanide complexes showed apparently high activity.