Synthesis and characterization of Co(iii) amidoamine complexes: Influence of substituents of the ligand on catalytic cyclic carbonate synthesis from epoxide and carbon dioxide


A series of amidoamine ligands (1) and their cobalt(iii) complexes (2) were synthesized and characterized by various spectroscopic techniques including 1H-NMR and X-ray crystallographic techniques. X-ray crystallography shows that one of the complexes, 2a, forms a chiral coordination polymer due to bridge formation with Li+ associated with the complex, although the ligand is achiral. Complex 2 was employed for catalytic synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2) in a solvent free condition. A strong influence of the substituents on the ligand 1 was revealed by the varied activity of complex 2. The presence of electron withdrawing groups such as chloro (2b) and nitro (2c) increases the Lewis acidity of the catalyst, which, in turn, enhances the catalytic activity of 2. An electron withdrawing group containing complexes (2b and 2c) showed exceptionally high catalytic activity with a turnover frequency (TOF) of 662 and 602 h-1 respectively at 130°C and 300 psig CO2 pressure. On the other hand, our studies indicate that a catalyst with an electron releasing group (2d) showed relatively lower activity with a TOF of 488 h-1 under similar reaction conditions. Our results show that cobalt(iii) complexes follow the reactivity order of 2d < 2a < 2c < 2b.



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Dalton Transactions