Structural and Kinetics Studies of N^O-, O^N^O-, and N^N- Schiff Base Mn Complexes as Highly Active Catalysts in the Ring-Opening Polymerization of rac-Lactides

Abstract

A series of manganese complexes derived from (imino)phenol/pyridine proligands have been synthesized, structurally characterized and used as catalysts in the ring-opening polymerization (ROP) of rac-lactides (rac-LA). Reactions of MnCl₂·4H₂O salt with 2-[((2-hydroxyethyl)imino)methyl]phenol (L1H₂) afford a tetranuclear Mn(III) complex [Mn(L1)Cl]₄ (Mn1). Separately, treatment of MnCl₂·4H₂O with 2-[((2-methoxyethyl)imino)methyl]phenol (L2H) gave the mononuclear Mn(II) complex [Mn(L2)₂(CH₃OH)₂] (Mn2). Further, reactions of (imino)pyridine proligands ((E)-2-((pyridine-2-ylmethylene)amino)ethan-1-ol (L3H) and (E)-N-(2-methoxyethyl)-1-(pyrid-2-yl)methanimine (L4)) with MnCl₂·4H₂O afforded polynuclear Mn(II) complexes [Mn(L3H)Cl₂]₃ (Mn3) and [Mn(L4)Cl₂]₃ (Mn4), respectively. The molecular structure of Mn1 established the tridentate binding mode of the dianionic alkoxy-(imino)phenol proligand ((L1²⁻)) through the phenoxo and pendant-arm alkoxy-oxygen and the imine-nitrogen atoms. In contrast, the molecular structure of Mn2 showed that the ether-(imino)phenol pro-ligand (L2H) is monoanionic (L2⁻) and bidentately coordinated to the Mn(II) metal center through the phenoxo-oxygen and the imine-nitrogen atoms. The dinuclear complex Mn3 contains a neutral N^N bidentately bound proligand (L3H). All the complexes (Mn1–Mn4) formed active catalysts in ROP of rac-LA with the propagation rate constant k_p of up to (4.25 ± 0.15) × 10⁻² L mol⁻¹ s⁻¹. The polymers obtained were atactic biased (P_r = 0.55–0.59), produced with moderate control over average-number molecular weights and were moderately dispersed (Đ up to 1.8) under melt conditions.