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

Abstract

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<inf>A</inf>), 4-(pyridin-2-yl)methyl)morpholine (L<inf>B</inf>), 2-(piperidin-1-ylmethyl)quinoline (L<inf>C</inf>), and 4-(quinolin-2-yl)methyl)morpholine (L<inf>D</inf>) reacted with [CoCl<inf>2</inf>·6H<inf>2</inf>O] or [CuCl<inf>2</inf>·2H<inf>2</inf>O] in a 1:1 M ratio to yield the corresponding dichloro complexes, formulated as [L<inf>n</inf>MX<inf>2</inf>] (where L<inf>n</inf> = L<inf>A</inf>–L<inf>D</inf>; M = Co, Cu). X-ray diffraction analysis revealed that all [L<inf>n</inf>CoCl<inf>2</inf>] complexes (L<inf>n</inf> = L<inf>A</inf>–L<inf>C</inf>) adopt distorted tetrahedral geometries. In contrast, the copper complexes displayed ligand-dependent geometries: [L<inf>C</inf>CuCl<inf>2</inf>] is distorted square-planar, while L<inf>B</inf> afforded dimeric [{CuCl(L<inf>B</inf>)}<inf>2</inf>(μ-Cl)<inf>2</inf>] species with distorted square-pyramidal copper centers. The ring-opening polymerization of rac-lactide was efficiently catalyzed by the in-situ-generated [L<inf>n</inf>M(OiPr)<inf>2</inf>] species (L<inf>n</inf> = L<inf>A</inf>–L<inf>D</inf>; 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. © 2026 Elsevier Ltd