Highly Robust Hydrogen Generation by Bioinspired Ir Complexes for Dehydrogenation of Formic Acid in Water: Experimental and Theoretical Mechanistic Investigations at Different pH

Hydrogen generation from formic acid (FA), one of the most promising hydrogen storage materials, has attracted much attention due to the demand for the development of renewable energy carriers. Catalytic dehydrogenation of FA in an efficient and green manner remains challenging. Here, we report a series of bioinspired Ir complexes for highly robust and selective hydrogen production from FA in aqueous solutions without organic solvents or additives. One of these complexes bearing an imidazoline moiety (complex 6) achieved a turnover frequency (TOF) of 322 000 h(-1) at 100 degrees C, which is higher than ever reported. The novel catalysts are very stable and applicable in highly concentrated FA. For instance, complex 3 (1 mu mol) affords an unprecedented turnover number (TON) of 2 050 000 at 60 degrees C. Deuterium kinetic isotope effect experiments and density functional theory (DFT) calculations employing a "speciation" approach demonstrated a change in the rate-determining step with increasing solution pH. This study provides not only more insight into the mechanism of dehydrogenation of FA but also offers a new principle for the design of effective homogeneous organometallic catalysts for H-2 generation from FA.