Rare earth metal complexes supported by 1,ω-dithiaalkanediyl-bridged bis(phenolato) ligands: synthesis, characterization and ring-opening polymerization catalysis of l-lactide

Abstract

Monomeric rare earth metal bis(phenolato) complexes [(L^s,s)Ln{N(SiHMe₂)₂}(THF)] (1a–4c) were isolated from the reaction of silylamido complexes [Ln{N(SiHMe₂)₂}₃(THF)_x] (Ln = Sc, x = 1; Ln = Y, Lu, x = 2) and one equivalent of tetradentate 1,ω-dithiaalkanediyl-bridged bis(phenol)s etbmpH₂, ptbmpH₂, edtbpH₂ and pdtbpH₂ in moderate to high yields. In contrast to the unsymmetrical scandium complexes 1a and 3a, the scandium complex 2a, the yttrium complexes 1b and 4b as well as the lutetium complexes 1c–4c show C_s or C₂ symmetry due to the relatively fast dissociation of THF on the NMR time scale at room temperature. The monomeric structures of the complexes 2a and 4b were confirmed by X-ray diffraction studies. The six-coordinate central metal with the tetradentate ligand, the silylamido group, and one THF, adopts a C₁ symmetrical configuration with trans(O,O) or cis(O,O) orientation of the two oxygen donors of the ligand. Distorted octahedral and trigonal prismatic coordination geometries are found for 2a and 4b. Substitution reaction with 2,2,6,6-tetramethyl-3,5-heptanedione afforded the corresponding complexes 6b, 6c, 7c and 8b with dimeric structures and with trityl alcohol the alkoxide complex [(etbmp)Y(OCPh₃)(THF)] (9). All new complexes efficiently initiated the ring-opening polymerization of L-lactide in THF. High molecular weight poly(L-lactide)s with narrow molecular weight distributions (M_w/M_n 1.15–1.41) were obtained using complexes 1a–4c. Dimeric β-diketonato complexes were only active in the presence of THF or excess isopropanol.