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

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.