Mechanistic Insight through Factors Controlling Effective Hydrogenation of CO2 Catalyzed by Bioinspired Proton-Responsive Iridium(III) Complexes

Reversible H-2 storage near room temperature and pressure with pH as the switch for controlling the direction of the reaction has been demonstrated (Nat. Chem., 2012, 4, 383-388). Several bioinspired proton-responsive 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 dinuclear 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 H-2 and accelerated proton transfer by formation of a water bridge in CO2 hydrogenation catalyzed by bioinspired complexes bearing a pendent base. Furthermore, contrary to expectations, 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.