Synthesis and characterization of rare-earth metal complexes supported by a new pentadentate Schiff base and their application in heteroselective polymerization of rac-lactide

Abstract

A series of neutral rare-earth metal aryloxides and amides supported by a new pentadentate (N 2 O 3 ) Salen ligand were synthesized, and their catalytic behaviors for the ring-opening polymerization of rac-lactide (rac-LA) were explored. The protolysis reactions of N,N'-bis(3,5-di-tert-butylsalicylidene)-2,2'diaminodiphenyl ether (LH 2 ) with (ArO) 3 Ln(THF) (ArO = 2,6-Bu t 2 -4-MeC 6 H 2 O) and Ln[N(SiMe 3 ) 2 ] 3 in a 1:1 molar ratio in THF gave the neutral rare-earth metal aryloxides LLn(OAr)(THF) n [n = 0, Ln = Sc 20 (1), Yb (2); n = 1, Ln = Y (3), Sm (4), Nd (5)] and rare-earth metal amides LLnN(SiMe 3 ) 2 [Ln = Yb (6), Y (7)], respectively. X-ray structural determination showed that complexes 1, 2, 6 and 7 have a monomeric structure, in which the coordination geometry around the rare-earth metal atom can be best described as a distorted trigonal prism. Complexes 3 and 5 are a THF-solvated monomer and each of rare-earth metal atoms is seven-coordinated to form a distorted capped trigonal prism. It was found that 25 all of these complexes can efficiently initiate the ring-opening polymerization (ROP) of rac-LA to give heterotactic-rich polylactides (PLAs). The highly heterotactic PLA (P r up to 0.93) was obtained using complex 2 as the initiator at the polymerization temperature of 0 ºC. The observed activity-increasing order of 1 << 2 < 3 < 4 ≈ 5 is in agreement with the order of their ionic radii, whereas the order for stereoselectivity is in the reverse order. The rare-earth metal Salen amides can initiate rac-LA 30 polymerization in a controlled manner, while rare-earth metal Salen aryloxides are less controlled at room temperature. 65 the Salen ligand) complexes are high iso-selective or heteroselective initiators for rac-LA polymerization by changing the

Mononuclear Salen–Sodium Ion Pairs as Catalysts for Isoselective Polymerization of rac-Lactide

“…In the past four years, some sodium/potassium complexes supported with crown ether were explored by us − and Wu and Wang, , which can isoselectively catalyze the ROP of rac -lactide (A and B in Chart ); the isoselectivity even can reach P m = 0.94 at −70 °C . The isoselectivities of sodium/potassium complexes seem to be comparable to some good aluminum, − indium, − lanthanides, ,− zinc, ,− and other complexes ,− which have been extensively investigated. To develop sodium/potassium stereoselective catalysts is valuable for the ROP of rac -lactide because of nontoxicity and low price.…”

Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters

“…Over the past 2 decades, our group have been contributing to the development of the field. Simple arylimino compound 188 ( Figure 34 Rare-earth complexes bearing pentadentate [ONONO]-type salen ligands have also shown notable activity in the ROP of r-LA ( Figure 33) [94]. All catalysts completely converted the monomer within 15-20 min at room temperature with LA:Ln ratios as high as 1000:1, proved more active than previously reported tetradentate Y-salen compounds [95].…”

l- and rac-lactide polymerisation using scandium and aluminium permethylindenyl complexes

“…Me2 SB(Cp,I*)ZrCl(O-2,6-Me-C 6 H 3 ) presented a second-order dependence on L-lactide concentration (k obs = 3.23 M −1 h −1 ) for the polymerisation with reported rac-lactide polymerisation using scandium aryloxide complex supported by a pentadentate (N 2 O 3 ) salen-type ligand in THF at room temperature (71% conversion, 2 h). 25 Scandium alkoxide complexes containing a phosphasalen ligand were found to be inactive for rac-lactide polymerisation attributed to the formation of an unreactive single-lactide insertion product. 26 The effect of temperature on L-lactide polymerisation activity using 4 was studied with polymerisation temperature varied from 60-100°C (Table 3, and Fig.…”