Fixation of\nCO<sub>2</sub> with Epoxides Catalyzed\nby Pincer-Type Azo-Aromatic Complexes of Cobalt as Catalysts

Employing a series of azo-aromatic pincer-type cobalt(II)\ncomplexes, <b>1</b>–<b>5</b>, and an imine-based\ncobalt complex, <b>6</b>, a highly efficient catalytic protocol\nfor the cycloaddition\nof CO<sub>2</sub> with epoxides at low pressure of CO<sub>2</sub> is\nreported. The electron-withdrawing group-substituted ligands containing\ncomplexes <b>2</b> and <b>4</b> were most efficient. The\ncatalytic protocol with <b>2</b> involved a synergistic participation\nof an azo-aromatic catalyst (0.1 mol %) and tetra-butyl ammonium iodide\n(TBAI), a cocatalyst (0.2 mol %) at 90 °C temperature, and 1\nbar CO<sub>2</sub> pressure. A very good conversion, high turnover\nnumber, and reusability were observed. Complex <b>4</b> worked\ndirectly in the reaction, and its efficiency was similar to the efficiency\nof <b>2</b> and TBAI. As <b>2</b> was synthesized from\na cheaper CoCl<sub>2</sub>, <b>2</b> showed to be more stable\nthan <b>4</b>; the combination of <b>2</b> and TBAI was\nused for a detailed study. The imine-based complex <b>6</b> was\nless efficient than the corresponding azo-aromatic complex <b>5</b>. The catalytic protocol was versatile. It was also very effective\nfor the full conversion of bis-epoxides to bis-carbonates at only\n2 bar of CO<sub>2</sub> pressure in 24 h. The reaction mechanism was\ninvestigated using various spectroscopic and computational studies.