Zn(II)- and Mg(II)-Complexes of a Tridentate {ONN} Ligand: Application to Poly(lactic acid) Production and Chemical Upcycling of Polyesters

The synthesis and characterization of two homoleptic Zn­(II)- and Mg­(II)-complexes based on a simple tridentate {NNO} ligand are reported. The production of biocompatible atactic poly­(lactic acid) (PLA) under industrially relevant melt conditions is demonstrated, noting high activity for Zn­(1)2 at room temperature in CH2Cl2 (TOF = 184 h–1). Mg­(1)2 and Zn­(1)2 were shown to facilitate rapid PLA methanolysis into methyl lactate (Me-LA) under mild conditions, achieving up to 85% Me-LA yield within 30 min at 50 °C in THF. Further kinetic analysis found Mg­(1)2 and Zn­(1)2 to exhibit k app values of 0.23 ± 0.0076 and 0.15 ± 0.0029 min–1, respectively {8 wt % cat. loading}, among the highest reported thus far. Zn­(1)2 retained excellent activity for both poly­(ethylene terephthalate) (PET) and poly­(ε-caprolactone) (PCL) degradations, demonstrating catalyst versatility. Various upcycling strategies (e.g., methanolysis, glycolysis, and aminolysis) were employed to achieve a broad substrate scope, which included bis­(2-hydroxyethyl) terephthalate (BHET), high value terephthalamides, and methyl 6-hydroxyhexanoate. Optimal glycolysis conditions using Zn­(1)2 enabled 64% BHET yield within 1 h at 180 °C, a rare example of PET glycolysis mediated by a discrete homogeneous metal-based catalyst. The application of such catalysts for PET aminolysis and PCL methanolysis has been reported for the first time.