Hydrogenation of CO2 to Methanol Catalyzed by Cp*Co Complexes: Mechanistic Insights and Ligand Design

A direct hydride transfer mechanism with three cascade cycles for the conversion of carbon dioxide and dihydrogen to methanol (CO2 + 3H2 → CH3OH + H2O) catalyzed by a half-sandwich cobalt complex [Cp*Co­(bpy-Me)­OH2]2+ (1) is proposed based on density functional theory calculations. The formation of methanediol via hydride transfer from Co to formic acid (4 → TS8,11) is the rate-determining step with a total barrier of 26.0 kcal/mol in free energy. Furthermore, 15 analogues of 1 are constructed by replacing the hydrogen atoms at the two meta and para positions of the bipyridine ligand with different functional groups (1b–1l), the carbon atoms in the bipyridine ligand with nitrogen atoms (1m–1o), and one pyridine ligand with N-heterocyclic carbene (1p). Among all newly proposed complexes, [Cp*Co­(2,2′-bipyrazine)­OH2]2+ (1n) is the most active one with a total barrier of 19.6 kcal/mol in free energy. Such a low barrier indicates 1n is a promising catalyst for efficient conversion of CO2 and H2 to methanol at room temperature.