Proton delivery and removal in [Ni(PR 2NR 2 2)2]2+ hydrogen production and oxidation catalysts

Molly Ohagan, Ming Hsun Ho, Jenny Y. Yang, Aaron Appel, M. Rakowski Dubois, Simone Raugei, Wendy J. Shaw, Daniel L DuBois, R Morris Bullock

Research output: Contribution to journalArticle

92 Citations (Scopus)

Abstract

To examine the role of proton delivery and removal in the electrocatalytic oxidation and production of hydrogen by [Ni(PR 2NR 2 2)2]2+ (where PR 2NR2 2 is 1,5-R2-3,7-R-1,5-diaza-3,7- diphosphacyclooctane), we report experimental and theoretical studies of the intermolecular proton exchange reactions underlying the isomerization of [Ni(PCy 2NBn 2H)2] 2+ (Cy = cyclohexyl, Bn = benzyl) species formed during the oxidation of H2 by [NiII(PCy 2N Bn 2)2]2+ or the protonation of [Ni0(PCy 2NBn 2) 2]. Three protonated isomers are formed (endo/endo, endo/exo, or exo/exo), which differ in the position of the N-H bond's with respect to nickel. The endo/endo isomer is the most productive isomer due to the two protons being sufficiently close to the nickel to proceed readily to the transition state to form/cleave H2. Therefore, the rate of isomerization of the endo/exo or exo/exo isomers to generate the endo/endo isomer can have an important impact on catalytic rates. We have found that the rate of isomerization is limited by proton removal from, or delivery to, the complex. In particular, the endo position is more sterically hindered than the exo position; therefore, protonation exo to the metal is kinetically favored over endo protonation, which leads to less catalytically productive pathways. In hydrogen oxidation, deprotonation of the sterically hindered endo position by an external base may lead to slow catalytic turnover. For hydrogen production catalysts, the limited accessibility of the endo position can result in the preferential formation of the exo protonated isomers, which must undergo one or more isomerization steps to generate the catalytically productive endo protonated isomer. The results of these studies highlight the importance of precise proton delivery, and the mechanistic details described herein will be used to guide future catalyst design.

Original languageEnglish
Pages (from-to)19409-19424
Number of pages16
JournalJournal of the American Chemical Society
Volume134
Issue number47
DOIs
Publication statusPublished - Nov 28 2012

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Hydrogen production
Isomers
Protons
Hydrogen
Oxidation
Catalysts
Isomerization
Protonation
Nickel
Deprotonation
Theoretical Models
Metals
Ion exchange

ASJC Scopus subject areas

  • Chemistry(all)
  • Catalysis
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Proton delivery and removal in [Ni(PR 2NR 2 2)2]2+ hydrogen production and oxidation catalysts. / Ohagan, Molly; Ho, Ming Hsun; Yang, Jenny Y.; Appel, Aaron; Dubois, M. Rakowski; Raugei, Simone; Shaw, Wendy J.; DuBois, Daniel L; Bullock, R Morris.

In: Journal of the American Chemical Society, Vol. 134, No. 47, 28.11.2012, p. 19409-19424.

Research output: Contribution to journalArticle

Ohagan, Molly ; Ho, Ming Hsun ; Yang, Jenny Y. ; Appel, Aaron ; Dubois, M. Rakowski ; Raugei, Simone ; Shaw, Wendy J. ; DuBois, Daniel L ; Bullock, R Morris. / Proton delivery and removal in [Ni(PR 2NR 2 2)2]2+ hydrogen production and oxidation catalysts. In: Journal of the American Chemical Society. 2012 ; Vol. 134, No. 47. pp. 19409-19424.
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AU - Appel, Aaron

AU - Dubois, M. Rakowski

AU - Raugei, Simone

AU - Shaw, Wendy J.

AU - DuBois, Daniel L

AU - Bullock, R Morris

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