Towards a selective synthetic route for cobalt amino acid complexes and their application in ring opening polymerization of rac-lactide.

Abstract

Catalysts based on cobalt amino acids and 2,2 bipyridine (bipy) present an attractive and cost-effective alternative as ring opening polymerization catalysts, yet this system remains underexplored despite the advantageous coordination properties of amino acids and bipy as ligands combined with the variety of accessible oxidation states and coordination geometries of cobalt. Here, metal complexes of type [Co(aa)2(bipy)] with amino acids (aa: glycine, leucine and threonine) as ligands are reported. The complexes were characterized spectroscopically (IR, UV-vis and 1H, 13C NMR for diamagnetic species), and by MS spectrometry and elemental analysis. The data reveal that the 2,2 bipyridine acts as a neutral bidentate donor coordinating to the metal ion through two nitrogen atoms and the amino acid acts as a bidentate ligand coordinating through the carboxylate and amino group forming a stable five membered ring and a pseudo-octahedral geometry around the Co center. The activity of the complexes for the ring opening polymerization (ROP) of rac-lactide is presented. The complexes are effective initiators for the ROP of rac-lactide (Kobs = 9.05 × 10−4 s−1) at 100 : 1 [rac-lactide] : [catalyst] 1 M overall concentration of lactide in toluene at 403 K.