Efficient Hydrogen Storage and Production Using a Catalyst with an Imidazoline-Based, Proton-Responsive Ligand

A series of new imidazoline-based iridium complexes has been developed for hydrogenation of CO₂ and dehydrogenation of formic acid. One of the proton-responsive complexes bearing two -OH groups at ortho and para positions on a coordinating pyridine ring (3 b) can catalyze efficiently the chemical fixation of CO₂ and release H₂ under mild conditions in aqueous media without using organic additives/solvents. Notably, hydrogenation of CO₂ can be efficiently carried out under CO₂ and H₂ at atmospheric pressure in basic water by 3 b, achieving a turnover frequency of 106 h^-1 and a turnover number of 7280 at 25 °C, which are higher than ever reported. Moreover, highly efficient CO-free hydrogen production from formic acid in aqueous solution employing the same catalyst under mild conditions has been achieved, thus providing a promising potential H₂ -storage system in water.