Phenoxyamidine Zn and Al Complexes: Synthesis, Characterization, and Use in the Ring-Opening Polymerization of Lactide

Herein, we report the synthesis of new ditopic ligands, which consist of a phenoxy group and N,N,N'-trisubstituted amidines linked by a methylene spacer (L1-L4). Their coordination chemistry has been investigated with Zn(II) and Al(III). Alkane elimination route between the phenol-amidine proligands (L1H-L4H) and Et2Zn led to dinuclear complexes [(L1-L4)ZnEt](2) (1a-4a) in which the Zn centers are chelated by phenoxyamidine ligands and bridged through the oxygen atom of the phenoxy groups. Salt metathesis reaction between two equivalents of the sodium amidine phenate L1Na and ZnCl2 led to a bis-chelate chiral spiro-complex (L1(2)Zn) 1a'. Analogous alkane elimination route between AlMe3 and the phenol-amidine proligands L1H-L4H allowed the preparation of the mononuclear complexes [(L1-L4)AlMe2] (1b-4b). The phenoxyamidine-Al and -Zn complexes have been characterized by NMR spectroscopy, elemental analysis, and/or high-resolution ESI-MS. The solid state structures of the proligands [L1H(2)] [Br] and L2H as well as of six complexes have been established by single crystal X-ray diffraction analysis. Fluxional properties of the proligands L1H-L2H and of the complexes la and 2b have been investigated by VT NMR experiments. In the presence of an alcohol source, complexes 1a-4a and 1b-4b were used as initiators for the controlled ring-opening polymerization (ROP) of rac-lactide to afford atactic polylactic acid (PLA).