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-beta-butyrolactone

摘要

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 [LnClL(1)(THF)] stabilized by amine-bridged bis(phenolate) ligand L-1 with in situ generated lithium guanidinates in a 1 : 1 molar ratio in THF afforded ytterbium guanidinates YbL1 [R2NC(NR1)(2)] [R-1 = -Pr-i, R2N = -(NPr2)-Pr-i (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)(2)NC((NPr)-Pr-i)(2)] (3), YL2[(SiHMe2)(2)NC((NPr)-Pr-i)(2)](THF) (4), YL3[(SiHMe2)(2)NC((NPr)-Pr-i)(2)] (5), YL4[(SiHMe2)(2)NC((NPr)-Pr-i)(2)] (6), YL5 [(SiHMe2)(2)NC((NPr)-Pr-i)(2)] (7), YL6[(SiHMe2)(2)NC((NPr)-Pr-i)(2)] (8), respectively. The behaviors of complexes 1-8 in the polymerization of rac-lactide (LA) and rac-beta-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.