Synthesis and Characterization of Amine-Bridged Bis(phenolate)lanthanide Alkoxides and Their Application in the Controlled Polymerization of rac-Lactide and rac-β-Butyrolactone

Abstract

A series of neutral lanthanide alkoxides supported by an amine-bridged bis(phenolate) ligand were synthesized, and their catalytic behaviors for the polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of (C(5)H(5))(3)Ln(THF) with amine-bridged bis(phenol) LH(2) [L = Me(2)NCH(2)CH(2)N{CH(2)-(2-OC(6)H(2)Bu(t)(2)-3,5)}(2)] in a 1:1 molar ratio in THF for 1 h and then with 1 equiv each of 2,2,2-trifluoroethanol, benzyl alcohol, and 2-propanol gave the neutral lanthanide alkoxides LLn(OCH(2)CF(3))(THF) [Ln = Y (1), Yb (2), Er (3), Sm (4)], LY(OCH(2)Ph)(THF) (5), and LY(OPr(i))(THF) (6), respectively. These lanthanide alkoxides are sensitive to moisture, and the yttrium complex [(LY)(2)(μ-OPr(i))(μ-OH)] (7) was also isolated as a byproduct during the synthesis of complex 6. Complexes 1-6 were well characterized by elemental analyses and IR and NMR spectroscopy in the cases of complexes 1 and 4-6. The definitive molecular structures of all of these complexes were determined by single-crystal X-ray analysis. It was found that complexes 1-6 can initiate efficiently the ring-opening polymerization of rac-LA and rac-BBL in a controlled manner. For rac-LA, polymerization gave polymers with very narrow molecular weight distributions (PDI ≤ 1.12) and very high heterotacticity (P(r) up to 0.99). The observed activity-increasing order is in agreement with the order of the ionic radii, whereas the order for stereoselectivity is in the reverse order. For rac-BBL polymerization, the resultant polymers have narrow molecular distributions (PDI ≤ 1.26) and high syndiotacticity (P(r) up to 0.83). It is worth noting that the activity-decreasing order Yb > Er > Y >> Sm is observed for rac-BBL polymerization, which is opposite to the order of ionic radii and to the order of activity for rac-LA polymerization. The ionic radii of lanthanide metals have no obvious effect on the stereoselectivity for rac-BBL polymerization, which is quite different from that for rac-LA polymerization. End-group analysis of the oligomer of rac-BBL suggested that elimination side reactions occurred slowly in these systems, which led to chain cleavage and the formation of crotonate (and carboxy) end groups.

Structurally diverse dysprosium and yttrium complexes containing an amine-bis(benzotriazole phenolate) ligand: Synthesis, characterization and catalysis for lactide polymerization

“…Based on the excellent catalytic properties for lactide polymerization catalyzed by wellcharacterized rare-earth metal complexes [12][13][14][15][16], amine-bis(benzotriazole phenolate) dysprosium and yttrium complexes 1-6 are expected to behave as air-stable catalysts for ROP of lactide (LA).…”

“…reported group 3 metal catalysts featuring diamine-bis(phenolate) ligands, and these lanthanide alkoxides (metal = Yb or Er) were found to display not only outstanding performances but also the highest stereoselectivity (P r = 0.99) for heteroselective polymerization of rac-LA [14][15][16]. These group 3/rare-earth catalytic systems with bis(phenolate) ligands gave advantages of good activity, excellent controllability and high heterotactic selectivity of lactide polymerization.…”

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

“…Overall, THF is the optimal solvent, which is consistent with reported results. 24 Almost quantitative yields were obtained with the ratio of monomer to complex 1 increasing from 300 to 1200 (Table 3, entries 4-7). A good yield of 84% was still obtained when the ratio was increased to 1500.…”

Synthesis and characterization of ytterbium guanidinates stabilized by bridged bis(phenolate) ligand and their application for the hydrophosphonylation reaction of aldehydes

“…It was found that the carbon-bridged bis(phenolate) lanthanide alkoxo complexes are efficient initiators for the controlled polymerization of ε-caprolactone [17]. The lanthanide aryloxides and alkoxides stabilized by amine bridged bis(phenolate) ligand are efficient initiators for the stereoselective controlled polymerization of rac-lactide and rac-bbutyrolactone [13,15]. Cationic amidozirconium and titanium complexes bearing an amine-bridged bis(phenolato) ligand are highly efficient catalysts for regioselective intermolecular hydroamination reactions of various terminal alkynes and anilines, with good to excellent yields and 100% Markovnikov selectivity [18].…”