Magnesium complexes in diverse coordination patterns supported by tetradentate aminophenolate ligands: Synthesis, characterization and application in the stereocontrolled ring-opening polymerization of rac-LA

Abstract

A series of magnesium silylamido complexes supported by tetradentate aminophenolate ligands, LMgN(SiMe3)2 (L={(2-R1)Ar-CH2N[(CH2)2R2]CH2-(4-R4-6-R3)C6H2O−}, R1=NMe2, R2=NiPr2, R3=R4=Cl (1), R3=R4=cumyl (2); R1=NMe2, R2=NEt2, R3=R4=cumyl (3), R3=CPh3, R4=Me (4); R1=NEt2, R2=NEt2, R3=CPh3, R4=Me (5); R1=NMe2, R2=(S)-1-butylpyrrolidine-2-yl, R3=R4=cumyl (6), R3=CPh3, R4=Me (7)), were synthesized and fully characterized, and their catalytic behaviors for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) were explored. The monomeric nature and versatile coordination modes of these complexes in the solid state were further confirmed by X-ray diffraction studies on complexes 1, 3 and 7. In complex 1, the N,N-diisopropylamino group on the alkylamino side arm does not coordinate to the metal centre, whilst the remaining three donors of the aminophenolate ligand and the silylamido group bond with the magnesium centre. By contrast, in complexes 3 and 7, the arylamino moiety does not coordinate to the metal centre, while the alkylamino side arm coordinates. It is worth noting that the introduction of the N,N-dimethyl amino group to the pendant benzyl group realises the diastereoselective synthesis of multichiral aminophenolate magnesium complexes, which has not been achieved by the corresponding tridentate aminophenolate ligand. It was found that most of these magnesium complexes serve as highly active initiators for the ROP of rac-LA. The coordination pattern of the aminophenolate ligand exerted a considerable influence on the activity and stereoselectivity of the corresponding complex toward the polymerization of rac-LA, leading to the production of moderate to high heterotactic biased PLAs. The enantiopure complex 7 showed the highest heteroselectivity among these magnesium complexes (Pr=0.80 in toluene, Pr=0.82 in THF).