Zinc complexes supported by claw-type aminophenolate ligands: synthesis, characterization and catalysis in the ring-opening polymerization of rac-lactide

Abstract

A series of zinc silylamido complexes bearing claw-type multidentate aminophenolate ligands, [LZnN(SiMe₃)₂] (L = –OAr¹-CH₂N[(CH₂)_nNR₂]CH₂Ar², n = 2 or 3; R = Me or Et (1a–3a, 5a, 7a and 8a); L = –OC₆H₂-4,6-^tBu₂-2-CH₂N[(CH₂)₂OMe]₂ (9a)), have been synthesized via the reaction of Zn[N(SiMe₃)₂]₂ and 1 equiv. of corresponding aminophenol. The reaction of Zn[N(SiMe₃)₂]₂ with the proligand L⁶H (2-{N-(2-methoxybenzyl)-N-[3-(N′,N′-dimethylamino)propyl]aminomethyl}-4-methyl-6-tritylphenol) resulted in the formation of bisphenolate zinc complex 6 regardless of the stoichiometric ratio of the two starting materials. Complex 4b with an initiating group of 3-tert-butyl-2-methoxy-5- methylbenzyloxy was obtained and further studied via the X-ray diffraction method to be monomeric. Zinc ethyl complex 2c was also prepared from the reaction of ZnEt₂ and 1 equiv. of proligand L²H as the representative complex with an alkyl initiating group. All zinc silylamido complexes efficiently initiated the ring-opening polymerization of rac-lactide in the presence or absence of isopropanol at ambient temperature. The steric and electronic characteristics of the ancillary ligands significantly influenced the polymerization performance of the corresponding zinc complexes. The introduction of bulky ortho- substituents on the phenoxy moiety resulted in an apparent decrease of catalytic activity while a slightly enhanced isotactic selectivity. Meanwhile, the elongation of the pendant amine arm to three-carbon-atom linkage led to significant decline of the catalytic activity in the absence of isopropanol. The zinc ethyl complex 2c was not such an efficient initiator as the silylamido ones, but the alkoxy complex 4b gave an obviously faster and better controlled polymerization when compared to the zinc silylamido complexes.