Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra and X-ray diffraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the heteroselective ring-opening polymerization (ROP) of rac-lactide (P_r up to 0.77) with high catalytic activities and a controlled polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also efficiently catalyze the ROP of rac-β-butyrolactone to produce syndiotactic polymers (P_r up to 0.73) while their salalen analogues were inert, revealing the special effects of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo-hydroxo-bridged complexes maintained their binuclear structures in the polymerization.