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.