Fixation of CO₂ with Epoxides Catalyzed by Pincer-Type Azo-Aromatic Complexes of Cobalt as Catalysts

Employing a series of azo-aromatic pincer-type cobalt(II) complexes, <b>1</b>-<b>5</b>, and an imine-based cobalt complex, <b>6</b>, a highly efficient catalytic protocol for the cycloaddition of CO₂ with epoxides at low pressure of CO₂ is reported. The electron-withdrawing group-substituted ligands containing complexes <b>2</b> and <b>4</b> were most efficient. The catalytic protocol with <b>2</b> 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 CO₂ pressure. A very good conversion, high turnover number, and reusability were observed. Complex <b>4</b> worked directly in the reaction, and its efficiency was similar to the efficiency of <b>2</b> and TBAI. As <b>2</b> was synthesized from a cheaper CoCl₂, <b>2</b> showed to be more stable than <b>4</b>; the combination of <b>2</b> and TBAI was used for a detailed study. The imine-based complex <b>6</b> was less efficient than the corresponding azo-aromatic complex <b>5</b>. 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 CO₂ pressure in 24 h. The reaction mechanism was investigated using various spectroscopic and computational studies.