Formic acid dehydrogenation with bioinspired iridium complexes: a kinetic isotope effect study and mechanistic insight.
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)(H2 O)]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 HCO2 H/HCO2 Na 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.
National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan); Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 (Japan); Current address: School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221 (PR China).
Wan-Hui
WH
Wang
Shaoan
S
Xu
Yuichi
Y
Manaka
Yuki
Y
Suna
Hide
H
Kambayashi
James T
JT
Muckerman
Etsuko
E
Fujita
Yuichiro
Y
Himeda
D040761:Biomimetic Materials / Q000737:chemistry*; D003903:Deuterium / Q000737:chemistry*; D005561:Formates / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007495:Iridium / Q000737:chemistry*; D007700:Kinetics; D009942:Organometallic Compounds / Q000737:chemistry*
D005561:Formates; D009942:Organometallic Compounds; C030544:formic acid; D007495:Iridium; D006859:Hydrogen; D003903:Deuterium
dehydrogenation; formic acid; hydrogen; iridium; isotope effects