Fixation of CO<sub>2</sub> with Epoxides Catalyzed by Pincer-Type Azo-Aromatic Complexes of Cobalt as Catalysts
Bappaditya Goswami, Manas Khatua, Swati Rani, Robindo Chatterjee, Subhas Samanta
Abstract
Employing a series of azo-aromatic pincer-type cobalt(II) complexes, 1 – 5, and an imine-based cobalt complex, 6, a highly efficient catalytic protocol for the cycloaddition of CO 2 with epoxides at low pressure of CO 2 is reported. The electron-withdrawing group-substituted ligands containing complexes 2 and 4 were most efficient. The catalytic protocol with 2 involved a synergistic participation of an azo-aromatic catalyst (0.1 mol %) and tetra-butyl ammonium iodide (TBAI), a cocatalyst (0.2 mol %) at 90 °C temperature, and 1 bar CO 2 pressure. A very good conversion, high turnover number, and reusability were observed. Complex 4 worked directly in the reaction, and its efficiency was similar to the efficiency of 2 and TBAI. As 2 was synthesized from a cheaper CoCl 2, 2 showed to be more stable than 4; the combination of 2 and TBAI was used for a detailed study. The imine-based complex 6 was less efficient than the corresponding azo-aromatic complex 5 . The catalytic protocol was versatile. It was also very effective for the full conversion of bis-epoxides to bis-carbonates at only 2 bar of CO 2 pressure in 24 h. The reaction mechanism was investigated using various spectroscopic and computational studies.