Proton Delivery and Removal in [Ni(P R 2 N R' 2 ) 2 ] 2+ Hydrogen Production and Oxidation Catalysts
To examine the role of proton delivery and removal in the electrocatalytic oxidation and production of hydrogen by [Ni(PR2NR´)2]2+ (where PR2NR´2 is 1,5-R´-3,7-R-1,5-diaza-3,7-diphosphacyclooctane), we report experimental and theoretical studies of the intermolecular proton exchange reactions underlying the isomerization of [Ni(PCy2NBn2H)2]2+ (Cy = cyclohexyl, Bn = benzyl) species formed during the stochiometric oxidation of H2 by [NiII(PCy2NBn2)2]2+ or the protonation of [Ni0(PCy2NBn2)2]. The three isomers formed differ by the position of the N-H bond with respect to the nickel (endo-endo, endo-exo, or exo-exo) and only the endo-endo isomer is catalytically active. We have found that the rate of isomerization is limited by proton removal from and delivery to the complex. In particular, steric hindrance disfavors the catalytically active protonation site (endo to the metal) in favor of inactive protonation (exo to the metal). The ramifications to catalysis of poor accessibility of the endo site and protonation at the exo site are discussed. In hydrogen oxidation, deprotonation of the sterically hindered endo position by an external base may lead to slow catalytic turnover. As for hydrogen production, the limited accessibility of the endo position can result in the formation of exo protonated species, which must undergo one or more isomerization steps to generate the catalytically active endo protonated species. These studies highlight the importance of precise proton delivery, and the mechanistic details described herein will guide future catalyst design. This research was carried out in the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. WJS was funded by the DOE Office of Science Early Career Research Program through the Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle. Computational resources were provided at W. R. Wiley Environmental Molecular Science Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory; the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory; and the Jaguar supercomputer at Oak Ridge National Laboratory (INCITE 2008-2011 award supported by the Office of Science of the U.S. DOE under Contract No. DE-AC0500OR22725).
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Oak Ridge Leadership Computing Facility (OLCF); Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1057353
- Report Number(s):
- PNNL-SA-88737; 39981; 44681; KC0307010
- Journal Information:
- Journal of the American Chemical Society, Vol. 134, Issue 47; ISSN 0002-7863
- Publisher:
- American Chemical Society (ACS)
- Country of Publication:
- United States
- Language:
- English
Similar Records
Water-Assisted Proton Delivery and Removal in bio-inspired Hydrogen Production Catalysts
Electrocatalytic Hydrogen Production by [Ni(7PPh2NH)2]2+: Removing the Distinction Between Endo- and Exo- Protonation Sites