Probing the viability of oxo-coupling pathways in iridium-catalyzed oxygen evolution

Jonathan Graeupner, Ulrich Hintermair, Daria L. Huang, Julianne M. Thomsen, Mike Takase, Jesus Campos, Sara M. Hashmi, Menachem Elimelech, Gary W. Brudvig, Robert H. Crabtree

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Abstract

A series of Cp*IrIII dimers has been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O2 evolution. The oxidative stability of the precursors towards nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogs. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O2 evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine-pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO4) as the oxidant, the dimers provided significantly lower O2 evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water-oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system.

Original languageEnglish
Pages (from-to)5384–5390
JournalOrganometallics
Volume32
Issue number19
Early online date24 Sep 2013
DOIs
Publication statusPublished - 14 Oct 2013

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Iridium
iridium
viability
Dimers
dimers
Oxygen
oxygen
Ligands
ligands
Bearings (structural)
monomers
Monomers
Nanoparticles
Oxidation
nanoparticles
oxidation
Ruthenium
Water
Chelation
Oxidants

Cite this

Graeupner, J., Hintermair, U., Huang, D. L., Thomsen, J. M., Takase, M., Campos, J., ... Crabtree, R. H. (2013). Probing the viability of oxo-coupling pathways in iridium-catalyzed oxygen evolution. Organometallics, 32(19), 5384–5390. https://doi.org/10.1021/om400658a

Probing the viability of oxo-coupling pathways in iridium-catalyzed oxygen evolution. / Graeupner, Jonathan; Hintermair, Ulrich; Huang, Daria L.; Thomsen, Julianne M.; Takase, Mike; Campos, Jesus; Hashmi, Sara M.; Elimelech, Menachem; Brudvig, Gary W.; Crabtree, Robert H.

In: Organometallics, Vol. 32, No. 19, 14.10.2013, p. 5384–5390.

Research output: Contribution to journalArticle

Graeupner, J, Hintermair, U, Huang, DL, Thomsen, JM, Takase, M, Campos, J, Hashmi, SM, Elimelech, M, Brudvig, GW & Crabtree, RH 2013, 'Probing the viability of oxo-coupling pathways in iridium-catalyzed oxygen evolution', Organometallics, vol. 32, no. 19, pp. 5384–5390. https://doi.org/10.1021/om400658a
Graeupner, Jonathan ; Hintermair, Ulrich ; Huang, Daria L. ; Thomsen, Julianne M. ; Takase, Mike ; Campos, Jesus ; Hashmi, Sara M. ; Elimelech, Menachem ; Brudvig, Gary W. ; Crabtree, Robert H. / Probing the viability of oxo-coupling pathways in iridium-catalyzed oxygen evolution. In: Organometallics. 2013 ; Vol. 32, No. 19. pp. 5384–5390.
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