Ionic cobalt complexes derived from an amine-bis(benzotriazole phenolate) ligand as bifunctional catalysts for copolymerization of epoxides and anhydrides

Abstract

This study reports that a series of structurally well-defined mono- and di-nuclear cobalt complexes containing the amine-bis(benzotriazole phenolate) ligand (amine-BiBTP) have been synthesized and fully characterized. The one-pot reaction of the amine-BiBTP ligand precursor with cobalt nitrate salt in the presence of triethylamine under refluxing methanolic solution generated di-nuclear cobalt(II) complexes [(C1NNBiBTP)2Co2] (1). The mono-Co(II) complex [(C1NNBiBTP)CoR] (R = DMAP (2), tBuPy (3)) could be prepared by 1 with a stoichiometric amount of 4-(dimethylamino)pyridine (DMAP) or 4-tert-butylpyridine (tBuPy) to yield a five-coordinated mononuclear cobalt species. Interestingly, the Co(III) complexes 4–7 resulted from the treatment of 2 or 3 with silver nitrate/trifluoroacetate as the oxidizing agent, and an ionic and mononuclear species formulated as [(C1NNBiBTP)CoR2]X (R = DMAP, X = NO3 (4), R = DMAP, X = CF3COO (5), R = tBuPy, X = NO3 (6), R = tBuPy, X = CF3COO (7)) could be formed. Versatile catalysis towards copolymerization of internal epoxides with anhydride catalyzed by these Co(II)/Co(III) was systematically studied under mild conditions. Experimental results revealed that complex 4 was shown to effectively catalyze copolymerization of cyclohexene oxide (CHO) with phthalic anhydride (PA), resulting in copolymers with a high alternating microstructure possessing ≥ 99% ester-linkage contents.