Synthesis and characterization of rare-earth metal guanidinates stabilized by amine-bridged bis(phenolate) ligands and their application in the controlled polymerization of rac-lactide and rac-β-butyrolactone

Abstract

Eight rare-earth metal guanidinates supported by a versatile family of chelating amine-bridged bis(phenolate) ligands were synthesized. Metathesis reactions of rare-earth metal chlorides [LnClL¹(THF)] stabilized by amine-bridged bis(phenolate) ligand L¹ with in situ generated lithium guanidinates in a 1 : 1 molar ratio in THF afforded ytterbium guanidinates YbL¹ [R₂NC(NR¹)₂] [R¹ = –ⁱPr, R₂N = –NⁱPr₂ (1), –N(CH₂)₅ (2)]. Insertion reactions of the yttrium amides bearing bridged bis(phenolate) ligands with 1 equiv of N,N′-diisopropylcarbodiimide (DIC) yielded six yttrium guanidinates YL¹ [(SiHMe₂)₂NC(NⁱPr)₂] (3), YL²[(SiHMe₂)₂NC(NⁱPr)₂](THF) (4), YL³[(SiHMe₂)₂NC(NⁱPr)₂] (5), YL⁴[(SiHMe₂)₂NC(NⁱPr)₂] (6), YL⁵ [(SiHMe₂)₂NC(NⁱPr)₂] (7), YL⁶[(SiHMe₂)₂NC(NⁱPr)₂] (8), respectively. The behaviors of complexes 1–8 in the polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were also explored. It was found that complexes 1–8 efficiently initiated the ring-opening polymerization (ROP) of rac-LA and rac-BBL in a controlled manner, providing highly heterotactic polylactide (P_r up to 0.99) and highly syndiotactic poly(3-hydroxybutyrate) (P_r up to 0.82). The framework of the bridge played a significant role in governing the stereoselectivity, while guanidinate groups work as initiating groups.