Journal
CHEMICAL SCIENCE
Volume 4, Issue 3, Pages 1234-1244Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c2sc21923a
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- University of Liverpool
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Formic acid (HCO2H) is an important potential hydrogen storage material, which, in the presence of appropriate catalysts can be selectively dehydrogenated to give H-2 and CO2. In this work, well defined N boolean AND C cyclometallated iridium(III) complexes based on 2-aryl imidazoline ligands are found to be excellent catalysts for the decomposition of HCO2H-NEt3 mixtures to give H-2 and CO2 under mild conditions with high turnover frequencies (up to 147 000 h(-1) at 40 degrees C) and essentially no CO formation. The modular structures of these catalysts have allowed for the construction of structure-activity relationships for the complexes, leading to the rational optimisation of the catalyst structure with respect to both the rate of H-2 production and catalyst lifetime. In particular, the presence of the remote gamma-NH unit in the ligand is shown to be essential for catalytic activity, without which no reaction occurs. Mechanistic studies suggest that the dehydrogenation is rate-limited by the step of hydride protonation, which is made feasible by the gamma-NH unit via an unusual form of long-range metal-ligand bifunctional catalysis involving formic acid-assisted proton hopping.
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