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Formic Acid Dehydrogenation with Bioinspired Iridium Complexes: A Kinetic Isotope Effect Study and Mechanistic Insight

Authors

  • Dr. Wan-Hui Wang,

    1. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)
    2. Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 (Japan)
    3. Current address: School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221 (PR China)
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  • Dr. Shaoan Xu,

    1. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)
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  • Dr. Yuichi Manaka,

    1. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)
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  • Dr. Yuki Suna,

    1. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)
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  • Dr. Hide Kambayashi,

    1. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)
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  • Dr. James T. Muckerman,

    1. Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000 (USA)
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  • Dr. Etsuko Fujita,

    1. Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000 (USA)
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  • Dr. Yuichiro Himeda

    Corresponding author
    1. National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)
    2. Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 (Japan)
    • National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)===

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Abstract

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 39 500 h−1 at 80 °C in a 1 M 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 β-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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