Synthesis and Characterization of Lanthanide Amides Bearing Aminophenoxy Ligands and Their Catalytic Activity for the Polymerization of Lactides

A series of neutral lanthanide complexes supported by
aminophenoxy
ligands were synthesized, and their catalytic behavior in the polymerization
of l-lactide and rac-lactide
was explored. The amine elimination reactions of equimolar amounts
of Ln­[N­(TMS)2]3(μ-Cl)­Li­(THF)3 and aminophenol [HONH]1 {[HONH]1 = 2,6-Me2-C6H4NHCH2(3,5-tBu2-C6H2-2-OH)} in toluene gave
the dimeric lanthanide complexes {[ON]1Ln­[ONH]1}2 (Ln = La (1), Nd (2)), whereas
the similar reactions of La­[N­(TMS)2]3(THF)2 or Ln­[N­(TMS)2]3(μ-Cl)­Li­(THF)3 (Ln = Nd, Sm) with the aminophenols [HONH]2 {[HONH]2 = (ο-OCH3-C6H4)­NHCH2(3,5-tBu2-C6H2-2-OH} and [HONH]3 {[HONH]3 = (NC5H4)­NHCH2(3,5-tBu2-C6H2-2-OH)} generated the neutral
aminophenoxy lanthanide amides {[ON]2Ln­[N­(TMS)2]}2 [Ln = La (3), Nd (4), Sm
(5)] and {[ON]3Ln­[N­(TMS)2]­(THF)}2 [Ln = La (6), Nd (7), Sm (8)], respectively, in high isolated yields. These complexes
have been fully characterized. X-ray structural determination revealed
that complexes 1 and 2 have unsolvated centrosymmetric
dimeric structures, in which one hydrogen atom belonging to the amino
group of the ligand is reserved. Complexes 3–5 are isostructural and have an unsolvated dimeric structure.
The coordination geometry around each of the lanthanide metal atoms
can be described as a distorted trigonal bipyramid. Complexes 6 and 7 have a solvated dimeric structure, and
the lanthanide metal centers have distorted capped trigonal-prismatic
geometries. It was found that complexes 3–8 are highly efficient initiators for the ring-opening polymerization
of l-lactide and rac-lactide,
affording polymers with high molecular weights.