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

Abstract

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 → TS_8,11) 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.