Our investigations of the interconversion of CO2/H2 and formic acid using Cp*Ir(III) complexes with proton responsive ligands has led to both the reaction rates and catalyst stabilities having been significantly improved. Remarkably, the reversible interconversion has been achieved under mild conditions in environmentally benign water solvent by changing the pH of the aqueous solution. These improved catalysts seem to stem from a bio-inspired design such as the use of proton responsive ligands, ligands with pendent bases or acids for a second-coordination-sphere interaction, electroresponsive ligands, and/or ligands having a hydrogen bonding function with a solvent molecule or an added reagent. By utilizing an acid/base equilibrium for proton removal, the ligand becomes a strong electron donor, resulting in Ir(I) character with a vacant coordination site at the metal center in slightly basic solution. Complemented by DFT calculations, kinetic studies of the rates of formate production using a related family of Ir complexes with and without such functions on the ligand reveal that the rate-determing step for the CO2 hydrogenation is likely to be H2 addition through heterolytic cleavage involving a ‘‘proton relay’’ through the pendent base. The dehydrogenation of formic acid, owing to the proton responsive ligands changing character under acidic pH conditions, is likely to occur by a mechanism with a different rate-determining step. We will present at this symposium more recent progress using complexes with ligands shown in the figure.

Reversible interconversion of CO2/H2 and formic acid using Cp*Ir(III) complexes with proton responsive ligands

253rd National Meeting (2017)

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