Potassium complexes supported by monoanionic tetradentate amino-phenolate ligands: synthesis, structure and catalysis in the ring-opening polymerization of rac-lactide

Abstract

A series of potassium complexes bearing monoanionic tetradentate amino-phenolate ligands, [LK] (L = {(2-R)CHCHN[(CH)R]CH(4-R-6-R)CHO-}, R = NMe, R = NEt, R = CPh, R = Me (1); R = R = NEt, R = CPh, R = Me (2); R = NMe, R = NEt, R = R = cumyl (4); R = R = OMe, R = Bu, R = Me (6); L = (2-NMe)CHCHN[[CH-(S)-1-butylpyrrolidinyl]CH(4-Me-6-CPh)CHO-] (3)), have been synthesized via reactions of KN(SiMe) and 1 equiv. of the corresponding aminophenols. The solid-state structures of typical complexes 4 and 6 are determined via X-ray diffraction studies, which reveal the dinuclear nature of these complexes. By contrast, DOSY measurements of 1, 4 and 6 suggest that these complexes are monomeric in solution. It is noteworthy that the coordination chemistry of these potassium complexes is versatile, which is closely related to the nature of the ortho-substituent of the phenolate ring, as indicated by the results of the corresponding spectroscopic studies. In the presence of PrOH, 1-4 and 6 could initiate the polymerization of 500 equiv. of rac-lactide to achieve high monomer conversions within several minutes but afford atactic PLAs with slightly isotactic-enriched microstructures (P = 0.58-0.60). Experimental results also demonstrated that a bulky trityl substituent at the ortho-position of the phenolate ring of the ligand framework is beneficial for the enhancement of the activities of these potassium complexes.

Metal Complexes as Catalysts/Moderators for Polymerization Reactions

“…Numerous alkali metal aryloxide species (ArOM) were shown to efficiently polymerize lactide under mild conditions. [257][258][259][260] The steric and electronic properties of the supported aryloxide ligand play a crucial role in ROP activity and stereoselectivity, with the preferred use of multidentate aryloxide ligands, such as 4.1-4.3 (Figure 4.2), [261][262][263][264][265] to control and limit catalyst aggregation. Remarkably, salts 4.3, in which the Na + or K + metal ion is supported by an ancillary crown ether and a sterically demanding phenoxide, are highly active catalysts for the iso-selective ROP of rac-lactide, producing chain-length-controlled isotactic PLA (complete conversion of 500 equiv LA, 10 min, RT, 10 equiv BnOH, Pm = 0.82).…”

Ring-Opening Polymerization of L-Lactide Catalyzed by Potassium-Based Complexes: Mechanistic Studies

“…In most reported cases of potassium complexes as ROP catalysts, the ligands present are aryloxide [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ] and less frequently O- and/or N-donors such as amino-phenolates [ 21 , 22 ] amidinates [ 23 ] and quinolinolates [ 24 ]. Other neutral donor ligand such as sulfonamides [ 25 ] or calixarenes [ 26 ] have also been explored.…”

Gallium (III) Complexes Based on Aminobisphenolate Ligands: Extremely High Active ROP-Initiators from Well-Known and Easily Accessible Compounds

“…The latter is very important for catalysis applications [ 13 ]. Aminobisphenolate complexes of main group elements (for example, Al [ 14 , 15 , 16 , 17 , 18 , 19 ], alkali metals [ 20 , 21 , 22 ], Ca [ 21 ], Mg [ 23 ]), transition metals (for example, Ti [ 19 ]) and lanthanides [ 24 , 25 ] have been reported to catalyze ROP reactions, but by now there is absolutely no information not only about catalysis but also about the synthesis of such gallium complexes in general. At the same time, gallium is a non-toxic metal if one may consider the concentrations of metal compounds that remain in the polymer during its synthesis [ 26 ], in addition, there are examples of the use of some gallium complexes in medicine [ 27 ], and gallium complexes of various structures (for example, diamido-ether dianionic ligands, (aminomethyl)phenolate monoanionic ligands, 8-quinolinolato monoanionic ligands, salan dianionic ligands, salen dianionic ligands, bis(imino)phenoxide monoanionic ligands) are active in ROP as initiators, [ 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ] thus aminobisphenolate gallium derivatives should be considered as potentially promising ROP initiators and interesting targets for their synthesis and investigation of their behavior in ROP.…”