Journal
CHEMSUSCHEM
Volume 7, Issue 7, Pages 1976-1983Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/cssc.201301414
Keywords
dehydrogenation; formic acid; hydrogen; iridium; isotope effects
Funding
- Japan Science and Technology Agency (JST), ACT-C
- U.S. Department of Energy [DE-AC02-98CH10886]
- Division of Chemical Sciences, Geosciences & Biosciences, Office of Basic Energy Sciences
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Highly efficient hydrogen generation from dehydrogenation of formic acid is achieved by using bioinspired iridium complexes that have hydroxyl groups at the ortho positions of the bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)(H2O)] SO4 (TH4BPM: 2,2', 6,6'-tetrahydroxyl-4,4'-bipyrimidine; Cp*: pentamethylcyclopentadienyl) has a high turnover frequency of 39500 h(-1) at 80 degrees C in a 1M aqueous solution of HCO2H/HCO2Na and produces hydrogen and carbon dioxide without carbon monoxide contamination. The deuterium kinetic isotope effect study clearly indicates a different rate-determining step for complexes with hydroxyl groups at different positions of the ligands. The rate-limiting step is beta-hydrogen elimination from the iridium-formate intermediate for complexes with hydroxyl groups at ortho positions, owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of hydrogen generation. In contrast, the reaction of iridium hydride with a proton to liberate hydrogen is demonstrated to be the rate-determining step for complexes that do not have hydroxyl groups at the ortho positions.
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