Formic acid dehydrogenation with bioinspired iridium complexes: A kinetic isotope effect study and mechanistic insight

Wan Hui Wang, Shaoan Xu, Yuichi Manaka, Yuki Suna, Hide Kambayashi, James T. Muckerman, Etsuko Fujita, Yuichiro Himeda

Research output: Contribution to journalArticle

83 Citations (Scopus)

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. The key controls the mechanism: A deuterium kinetic isotope effect (KIE) study clearly indicates a different mechanism for complexes with OH at different positions of the ligands. The rate-limiting step is β-hydrogen elimination from the iridium-formate intermediate for complexes with OH at ortho positions owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of generation of H2.

Original languageEnglish
Pages (from-to)1976-1983
Number of pages8
JournalChemSusChem
Volume7
Issue number7
DOIs
Publication statusPublished - Jul 2014

Keywords

  • dehydrogenation
  • formic acid
  • hydrogen
  • iridium
  • isotope effects

ASJC Scopus subject areas

  • Environmental Chemistry
  • Chemical Engineering(all)
  • Materials Science(all)
  • Energy(all)

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