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 roc-lactide was explored. The amine elimination reactions of equimolar amounts of Ln[N(TMS)(2)](3)(mu-Cl)Li(THF)(3) and aminophenol [HONH](1) {[HONH](1) = 2,6-Me-2-C6H4NHCH2(3,5-Bn-t(2)-C6H2-2-OH)} in toluene gave the dimeric lanthanide complexes {[ON](1)Ln[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)(mu-Cl)Li(THF)(3) (Ln = Nd, Sm) with the aminophenols [HONH](2) {[HONH](2) = (o-OCH3-C6H4)NHCH2(3,5-Bu-t(2)-C6H2-2-OH} and [HONH](3) ([HONH](3) = (NC5H4)NHCH2(3,5-Bu-t(2)-C6H2-2-OH)} generated the neutral aminophenoxy lanthanide amides {[ON](2)Ln[N(TMS)(2)]}(2) [Ln = La (3), Nd (4), Sm (5)) and {[ON](3)Ln[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.