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 language | English |
|---|---|
| Pages (from-to) | 1976-1983 |
| Number of pages | 8 |
| Journal | ChemSusChem |
| Volume | 7 |
| Issue number | 7 |
| DOIs | |
| Publication status | Published - 2014 |
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Keywords
- dehydrogenation
- formic acid
- hydrogen
- iridium
- isotope effects
ASJC Scopus subject areas
- Energy(all)
- Environmental Chemistry
- Materials Science(all)
- Chemical Engineering(all)
- Medicine(all)
Cite this
Formic acid dehydrogenation with bioinspired iridium complexes : A kinetic isotope effect study and mechanistic insight. / Wang, Wan Hui; Xu, Shaoan; Manaka, Yuichi; Suna, Yuki; Kambayashi, Hide; Muckerman, James; Fujita, Etsuko; Himeda, Yuichiro.
In: ChemSusChem, Vol. 7, No. 7, 2014, p. 1976-1983.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Formic acid dehydrogenation with bioinspired iridium complexes
T2 - A kinetic isotope effect study and mechanistic insight
AU - Wang, Wan Hui
AU - Xu, Shaoan
AU - Manaka, Yuichi
AU - Suna, Yuki
AU - Kambayashi, Hide
AU - Muckerman, James
AU - Fujita, Etsuko
AU - Himeda, Yuichiro
PY - 2014
Y1 - 2014
N2 - 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.
AB - 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.
KW - dehydrogenation
KW - formic acid
KW - hydrogen
KW - iridium
KW - isotope effects
UR - http://www.scopus.com/inward/record.url?scp=84905241149&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84905241149&partnerID=8YFLogxK
U2 - 10.1002/cssc.201301414
DO - 10.1002/cssc.201301414
M3 - Article
C2 - 24840600
AN - SCOPUS:84905241149
VL - 7
SP - 1976
EP - 1983
JO - ChemSusChem
JF - ChemSusChem
SN - 1864-5631
IS - 7
ER -