Hydrogen Production and Storage on a Formic Acid/Bicarbonate Platform using Water-Soluble N-Heterocyclic Carbene Complexes of Late Transition Metals

The synthesis and characterization of two water-soluble bis-N-heterocyclic carbene (NHC) complexes of rhodium and iridium is presented. Both compounds are active in H-2 generation from formic acid and in hydrogenation of bicarbonate to formate. The rhodium derivative is most active in both reactions, reaching a TOF of 39 000 h(-1) and a TON of 449 000 for H-2 production. The catalytic hydrogenation reactions were carried out in an autoclave system and analyzed using the integrated peak areas in the H-1 NMR spectra. Decomposition of formic acid was investigated using a Fisher-Porter bottle equipped with a pressure transducer. Long-term stability for hydrogen evolution was tested by surveillance of the gas flow rate. The procedure does not require any additives like amines or inert gas conditions. Density functional theory calculations in agreement with experimental results suggest a bicarbonate reduction mechanism involving a second catalyst molecule, which provides an external hydride acting as reducing agent.