Structural studies of N, O donor (imino)/(amino)pyridine mononuclear zinc(II) complexes as catalysts in the ring-opening polymerization reactions of rac-lactide

• N^N^O donor ligands were used to synthesize mononuclear zinc(II) complexes. • The complexes form active catalysts in the ring-opening polymerization of rac -lactide. • Moderate to low molecular weight polymers with narrow distributions were produced. • Kinetics studies showed pseudo-first-order kinetics with respect to the monomer. • Activated-monomer mechanistic pathway was established. Four zinc(II) complexes based on N, O donor (imino)/(amino)pyridine ligands were synthesized and used as initiators in the ring opening polymerization (ROP) of racemic -lactide ( rac -LA). Treatments of (imino)pyridine ligands (( E )-2-((pyridine-2-ylmethylene)amino)ethan-1-ol ( L1H ) and ( E )-N-(2-methoxyethyl)-1-(pyrid-2-yl)methanimine ( L2 )) with ZnCl 2 in a 1:1 ratio afforded the respective complexes [Zn( L1H )Cl 2 ] ( Zn1 ) and [Zn( L2 )Cl 2 ] ( Zn2 ) in moderate yields. Similarly, reactions of (amino)pyridine ligands (2-((pyridine-2-ylmethyl) amino) ethan-1-ol ( L3H 2 ) and 2-methoxy- N -(pyridine-2-ylmethyl)ethan-1-amine ( L4H )) with ZnCl 2 in equimolar amounts afforded the corresponding mononuclear complexes [Zn( L3H 2 )Cl 2 ] ( Zn3 ) and [Zn( L4H )Cl 2 ] ( Zn4 ). The molecular structures revealed that in complexes Zn1, Zn2 and Zn4 , the ligands L1H, L2 , and L4H adopt tridentate coordination modes to give five-coordinate mononuclear zinc(II) structures. All the complexes formed active catalysts in the ROP of rac -LA under solvent-free industrial conditions with apparent rate constants k app of up to 4.64×10 -4 s. -1 The catalytic efficiency of the complexes was largely influenced by electronic property of the coordinated ligands. Kinetics studies using complexes Zn1 and Zn3 revealed pseudo -first dependency on the rac -LA monomer. The complexes produced atactic-biased (poly)lactides (PLAs) with narrow distributions ( Đ = 1.60-1.62) and average molecular weights M n of up to 3530 g mol. -1 Mechanistic studies using NMR spectroscopy showed that the ROP reactions followed coordination-insertion mechanism and activated-monomer mechanistic pathways in absence and in the presence of benzyl alcohol as a co-initiator respectively.