When Bigger is Better: Lanthanum Complexes of Bis(phenoxy-amidine) FAlen Ligands as Unique Catalysts for the...

Discrete yttrium and lanthanum complexes supported by new bis(phenoxy-amidine) ligands have been prepared in high yields. Some of these so-called FAlen ligands, when coordinated onto the large La³⁺ in contrast to the small Y³⁺, can adopt an η³ coordination mode of both amidine moieties. These La-FAlen complexes, either previously isolated or conveniently prepared in situ, act as highly efficient ROP catalysts of simple and functional β-propiolactones rac-BL^R (R = Me, CH₂OiPr, CH₂OSitBuMe₂) under mild conditions, returning the corresponding linear polymers PBL^R with high control over the molecular weights and narrow dispersities. More uniquely, when substituents with appropriate bulkiness are installed on the phenoxy and amidine moieties, the polymers all feature high isotacticity (P_m up to 0.93). High activity and isoselectivity are maintained, no matter the nature and polarity of the solvent used.