Mechanistic insights into HCO2H dehydrogenation and CO2 hydrogenation catalyzed by Ir(Cp*) containing tetrahydroxy bipyrimidine ligand: the role of sodium and proton shuttle

Abstract

The mechanism of HCO₂H dehydrogenation catalyzed by [IrCp*(H₂O)(bpymO₄H₄)]²⁺ (bpymO₄H₄ = 2,2′,6,6′-tetrahydroxy-4,4′-bipyrimidine) was investigated using density functional theory. The relative free energy profiles at various protonation states corrected to pH 3.5 and pH 7.6 suggested that Na⁺ together with the ortho-oxyanion of bipyrimidine facilitates the Ir-HCO₂ formation, subsequent hydride transfer, and H₂ formation. HCO₂H was found to be a more effective proton shuttle than H₂O for H₂ formation. Under experimental conditions, the highest catalytic reactivity was found at pH 3.5–4.0, where both HCO₂Na and HCO₂H were present. At lower pH and low formate concentration, HCO₂H dehydrogenation tends to proceed via a Na⁺ independent pathway, involving a higher energy barrier. At higher pH, although Na⁺ can mediate hydride transfer and H₂ formation, the low amount of HCO₂H results in H₂O as the proton shuttle, which involves a higher energy barrier than that for HCO₂H proton shuttle. In other words, the catalytic activity of HCO₂H dehydrogenation by the proton-responsive Ir complexes at different pH values is influenced by the protonation state, involvement of Na⁺, and the availability of HCO₂H as a proton shuttle. For the hydrogenation of CO₂ at pH 8.3, the rate determining step is the heterolytic cleavage of H₂ mediated by Na⁺via a HCO₃⁻ proton shuttle. Our results demonstrate the importance of alkali metal ions in the design of catalysts for efficient, reversible, CO₂ conversion.