Mechanistic insights into HCO <inf>2</inf> H dehydrogenation and CO <inf>2</inf> hydrogenation catalyzed by Ir(Cp*) containing tetrahydroxy bipyrimidine ligand: the role of sodium and proton shuttle

Abstract

© The Royal Society of Chemistry. The mechanism of HCO 2 H dehydrogenation catalyzed by [IrCp*(H 2 O)(bpymO 4 H 4 )] 2+ (bpymO 4 H 4 = 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-HCO 2 formation, subsequent hydride transfer, and H 2 formation. HCO 2 H was found to be a more effective proton shuttle than H 2 O for H 2 formation. Under experimental conditions, the highest catalytic reactivity was found at pH 3.5-4.0, where both HCO 2 Na and HCO 2 H were present. At lower pH and low formate concentration, HCO 2 H dehydrogenation tends to proceed via a Na + independent pathway, involving a higher energy barrier. At higher pH, although Na + can mediate hydride transfer and H 2 formation, the low amount of HCO 2 H results in H 2 O as the proton shuttle, which involves a higher energy barrier than that for HCO 2 H proton shuttle. In other words, the catalytic activity of HCO 2 H dehydrogenation by the proton-responsive Ir complexes at different pH values is influenced by the protonation state, involvement of Na + , and the availability of HCO 2 H as a proton shuttle. For the hydrogenation of CO 2 at pH 8.3, the rate determining step is the heterolytic cleavage of H 2 mediated by Na + via a HCO 3− proton shuttle. Our results demonstrate the importance of alkali metal ions in the design of catalysts for efficient, reversible, CO 2 conversion.