Magnesium complexes supported by salan-like ligands: Synthesis, characterization and their application in the ring-opening polymerization of rac-lactide

Abstract

A series of magnesium silylamido complexes bearing salan-like multidentate aminophenolate ligands, [(L^1–4)MgN(SiMe₃)₂] (1–4) and [(L¹)MgN(SiHMe₂)₂] (1′) (L^1–4 = –OAr¹N(Me)(CH₂)_nN(Me)Ar², n = 2 or 3), have been synthesized via the reaction of Mg[N(SiMe₃)₂]₂ or Mg[N(SiHMe₂)₂] with 1 equiv. of the corresponding aminophenol. The monomeric nature of the magnesium complexes 1, 2, 3 and the main stoichiometric structure of 1′ in the solid state were further confirmed by X-ray diffraction studies, where the four donors of the ligand and the silylamido group coordinated with the magnesium center constructing a distorted trigonal bipyramidal coordination geometry at the metal center. The existence of isomers in complexes 1′ and 4 was verified by the variable temperature ¹H NMR spectroscopy, and the non-coordination of OMe group with the central magnesium in complex 4 was deduced from the comparison of the proton chemical shifts in complex 4 and its proligand. All these complexes efficiently initiated the ring-opening polymerization of rac-lactide with the presence/absence of isopropanol, affording isotactic bias polylactides with relatively broad molecular weight distributions (P_m = 0.59–0.70, M_w/M_n = 1.28–2.21) in toluene at r.t. Declining the steric hindrance of the initiating group from N(SiMe₃)₂ to N(SiHMe₂)₂ increased the catalytic activity considerably. The steric and electronic characteristics of the ancillary ligands showed some influence on the polymerization performance of the corresponding magnesium complexes. The introduction of bulky ortho-substituents on the phenoxy moiety resulted in some changes both in activity and stereoselectivity, and either the elongation of the amine bridge from NCH₂CH₂N to NCH₂CH₂CH₂N or the introduction of chloro-substituent on the phenoxy moiety led to a significant decline of the catalytic activity.