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 [LnClL 1 (THF)] stabilized by amine-bridged bis(phenolate) ligand L 1 with in situ generated lithium guanidinates in a 1 : 1 2 ) 5 (2)]. Insertion reactions of the yttrium amides bearing bridged bis(phenolate) ligands with 1 equiv of N,N 0 -diisopropylcarbodiimide (DIC) yielded six yttrium guanidinates 2 ] (8), respectively. The behaviors of complexes 1-8 in the polymerization of rac-lactide (LA) and rac-b-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.

Low-Symmetry β-Diketimine Aryloxide Rare-Earth Complexes: Flexible, Reactive, and Selective

“…Poly(3-hydroxybutyrate) (P3HB) is the most common member of the large class of naturally occurring and biodegradable poly(hydroxyalkanoates), 34 and its stereospecific synthesis from the racemic four-membered lactone, rac- b,36 Given the exceptional reactivity and selectivity exhibited by RE III catalysts supported by aminophenolate ligand fragments 30,33e,37 and the well-defined protonolysis and insertion reactivity displayed by 1-RE, we explored the reactivity of 1-RE, 2-RE, and 3-La in the stereospecific ROP of rac-BBL (Table 2). Despite examples of active RE III amide 33e,37e,38 and guanidinate 39 initiators, 1-RE and 3-La displayed low efficiency at RT over 22 h (entries 1, 2, and 8). Metal alkoxides often display higher efficiencies in the ROP of cyclic esters because of lower steric profiles and structural similarity to the propagating chain.…”

“…Despite examples of active RE III amide ,, and guanidinate initiators, 1-RE and 3-La displayed low efficiency at RT over 22 h (entries 1, 2, and 8). Metal alkoxides often display higher efficiencies in the ROP of cyclic esters because of lower steric profiles and structural similarity to the propagating chain. ,,, Unfortunately, 2-RE displayed marginal improvements in reactivity and limited stereocontrol (entries 3 and 5).…”

Neutral and Cationic Zirconium Complexes Bearing Multidentate Aminophenolato Ligands for Hydrophosphination Reactions of Alkenes and Heterocumulenes

“…Amine-bridged bis­(phenolato) ligands are a promising class of ancillary ligands that well stabilize metal centers . Moreover, ligand modifications can be easily achieved through changes in substituents on the nitrogen atom and/or aromatic rings, which influences complex structures and catalytic activities.…”

Precise Microstructure Control in Poly(hydroxybutyrate-co-lactic Acid) Copolymers Prepared by an Yttrium Amine Bis(phenolate) Complex

“…Since this monomer contains a stereocentre it again offers the opportunity for stereocontrol in the polymerization, to produce either isotactically or syndiotactically enriched polymer as opposed to atactic material. 11 Yttrium and lanthanide amine bis(phenolate) complexes have been extensively explored in the homopolymerizations of rac-LA, [17][18][19][20][21][22][23][24][25][26][27][28][29][30] and rac-β-BL [30][31][32] and related 4-membered lactones. 19,33,34 This family of complexes, and, in particular, the yttrium analogues, have been shown to exert exceptional stereocontrol in polymerization reactions, generally producing highly heterotactic PLA and syndiotactic P3HB.…”

“…Yttrium and lanthanide amine bis­(phenolate) complexes have been extensively explored in the homopolymerizations of rac -LA, − and rac -β-BL − , and related 4-membered lactones. ,, This family of complexes, and, in particular, the yttrium analogues, have been shown to exert exceptional stereocontrol in polymerization reactions, generally producing highly heterotactic PLA and syndiotactic P3HB . In addition, they are able to mediate reactions in a controlled manner, producing polymers with predictable molecular weights, based on the monomer to initiator ratio, and very narrow dispersities.…”