Synthesis of mono(guanidinate) rare earth metal bis(amide) complexes and their performance in the ring-opening polymerization of l-lactide and rac-lactide

Abstract

One-pot salt metathesis reaction of anhydrous LnCl3 with one equivalent of guanidinate lithium [(SiMe3)2NC(NCy)2]Li (Cy = cyclohexyl), following the introduction of two equivalents of NaN(SiMe3)2 or LiN(SiHMe2)2 in THF at room temperature, afforded a family of mono(guanidinate) rare earth metal bis(amide) complexes [(SiMe3)2NC(NCy)2]Ln[N(SiRMe2)2]2(THF)n (R = Me, n = 0, Ln = Nd (1), Lu (2), Y (3); R = H, n = 1, Ln = Y (4)) in 62–79% isolated yields. These complexes were characterized by elemental analysis, NMR spectroscopy (except for 1 for its strong paramagnetic property of Nd3+), FT-IR spectroscopy, and X-ray single crystal diffraction. Single crystal structural determination revealed that the central metals in 1–3 are four-coordinated with a distorted tetrahedral geometry, while that in 4 is five-coordinated with a distorted trigonal bipyramidal geometry. These complexes could serve as highly active initiators for L-lactide polymerization in toluene at 50 °C. In addition, they also showed high activity towards rac-lactide polymerization in THF at room temperature, affording a moderate level of heterotactic polymers (Pr > 0.70). Employing 4 as an initiator, a controllable polymerization fashion was observed both in L-lactide polymerization and rac-lactide polymerization under certain conditions. The polymerization proceeded via a coordination–insertion mechanism verified experimentally by end group analysis of the oligomer.