Phosphasalen Yttrium Complexes: Highly Active and Stereoselective Initiators for Lactide Polymerization

Preparation and characterization of three yttrium alkoxide
complexes
with new phosphasalen ligands are reported. The phosphasalens are
analogues of the well-known salen ligands but with iminophosphorane
donors replacing the imine functionality. The three yttrium alkoxide
complexes show mono- and dinuclear structures in the solid state,
depending on the substituents on the ligand. The new ligands and complexes
are characterized using multinuclear NMR spectroscopy, mass spectrometry,
elemental analysis, and single-crystal X-ray diffraction experiments.
The complexes are all rapid initiators for lactide polymerization;
they show excellent polymerization control on addition of exogeneous
alcohol. The mononuclear complex shows extremely rapid rates and a
high degree of stereocontrol in rac-lactide polymerization,
yielding heterotactic PLA (Ps of 0.9).
The phosphasalens are, therefore, excellent ligands for lactide ring-opening
polymerization catalysis showing superior rates and stereocontrol
versus salen ligands, which may be related to their excellent donating
ability and the high degrees of steric protection they can confer.