Structural diversity of cobalt(II) and copper(II) complexes with aminomethylpyridine/quinoline derivatives: application in rac-lactide polymerization

A series of Co(II) and Cu(II) complexes supported by N,N′-bidentate ligands derived from aminomethylpyridine and aminomethylquinoline were synthesized and characterized. The ligands 2-(piperidin-1-ylmethyl)pyridine (L_A), 4-(pyridin-2-yl)methyl)morpholine (L_B), 2-(piperidin-1-ylmethyl)quinoline (L_C), and 4-(quinolin-2-yl)methyl)morpholine (L_D) reacted with [CoCl₂·6H₂O] or [CuCl₂·2H₂O] in a 1:1 M ratio to yield the corresponding dichloro complexes, formulated as [L_nMX₂] (where L_n = L_A–L_D; M = Co, Cu). X-ray diffraction analysis revealed that all [L_nCoCl₂] complexes (L_n = L_A–L_C) adopt distorted tetrahedral geometries. In contrast, the copper complexes displayed ligand-dependent geometries: [L_CCuCl₂] is distorted square-planar, while L_B afforded dimeric [{CuCl(L_B)}₂(μ-Cl)₂] species with distorted square-pyramidal copper centers. The ring-opening polymerization of rac-lactide was efficiently catalyzed by the in-situ-generated [L_nM(OⁱPr)₂] species (L_n = L_A–L_D; M = Co, Cu) upon treatment with LiOⁱPr. Catalytic activity was observed at both 0 °C and 25 °C, producing poly(lactide) (PLA) with pronounced heterotacticity. High monomer conversions were achieved; however, the resulting polymers exhibited moderately broad dispersities (PDIs = 1.22–1.38) and relatively low molecular weights, indicative of a moderate degree of polymerization control.