Mechanistic Insights into Iridium Catalyzed Disproportionation of Formic Acid to CO2 and Methanol: A DFT Study

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The disproportionation of formic acid to methanol catalyzed by a half-sandwich iridium complex, [Cp*Ir(bpy-Me)OH2]2+, was computationally investigated by using density functional theory. A newly proposed mechanism features three interrelated catalytic cycles, the dehydrogenation of formic acid to CO2 and H2, the hydrogenation of formic acid to formaldehyde with the formation of water, and the hydrogenation of formaldehyde to methanol. Methanol assisted proton transfer and direct C–O bond cleavage after hydroxyl deprotonation in two competitive pathways for the formation of formaldehyde are the rate-determining steps in the whole catalytic reaction. Calculation results indicate that the formation of formaldehyde from methanediol through direct cleavage of a C–O bond after hydroxyl deprotonation has a free energy barrier of 25.9 kcal/mol, which is 1.9 kcal/mol more favorable than methanol assisted proton transfer.