Artificial Photosynthesis

Mechanistic insight into CO₂ hydrogenation

Reversible H₂ storage near room temperature and pressure with pH as the ‘switch’ for controlling the direction of the reaction has been demonstrated using a bio-inspired proton-responsive Ir(III) dimer. Several similar mononuclear Ir(III) catalysts for CO₂ hydrogenation were prepared to gain mechanistic insight through investigation of the factors that control the effective generation of formate. These factors include: (1) kinetic isotope effects by water, hydrogen, and bicarbonate; (2) position and number of hydroxyl groups on bpy-type ligands; and (3) mono- vs di-nuclear iridium complexes. We have, for the first time, obtained clear evidence from kinetic isotope effects and computational studies of the involvement of a water molecule in the rate-determining heterolysis of H₂, and accelerated proton transfer by formation of a water bridge in CO₂ hydrogenation catalyzed by bio-inspired complexes bearing a pendent base. A more significant enhancement of the catalytic activity was observed from electron donation by the ligand than on the number of the active metal centers.

ACS Catal. 2013, 3, 856-860, DOI: 10.1021/cs400172j.