Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements

Yoonsu Park, Lei Tian, Sangmin Kim, Tyler P. Pabst, Junho Kim, Gregory D. Scholes, Paul J. Chirik

Research output: Contribution to journalArticlepeer-review


The harvesting of visible light is a powerful strategy for the synthesis of weak chemical bonds involving hydrogen that are below the thermodynamic threshold for spontaneous H2evolution. Piano-stool iridium hydride complexes are effective for the blue-light-driven hydrogenation of organic substrates and contra-thermodynamic dearomative isomerization. In this work, a combination of spectroscopic measurements, isotopic labeling, structure-reactivity relationships, and computational studies has been used to explore the mechanism of these stoichiometric and catalytic reactions. Photophysical measurements on the iridium hydride catalysts demonstrated the generation of long-lived excited states with principally metal-to-ligand charge transfer (MLCT) character. Transient absorption spectroscopic studies with a representative substrate, anthracene revealed a diffusion-controlled dynamic quenching of the MLCT state. The triplet state of anthracene was detected immediately after the quenching events, suggesting that triplet-triplet energy transfer initiated the photocatalytic process. The key role of triplet anthracene on the post-energy transfer step was further demonstrated by employing photocatalytic hydrogenation with a triplet photosensitizer and a HAT agent, hydroquinone. DFT calculations support a concerted hydrogen atom transfer mechanism in lieu of stepwise electron/proton or proton/electron transfer pathways. Kinetic monitoring of the deactivation channel established an inverse kinetic isotope effect, supporting reversible C(sp2)-H reductive coupling followed by rate-limiting ligand dissociation. Mechanistic insights enabled design of a piano-stool iridium hydride catalyst with a rationally modified supporting ligand that exhibited improved photostability under blue light irradiation. The complex also provided improved catalytic performance toward photoinduced hydrogenation with H2and contra-thermodynamic isomerization.

Original languageEnglish
Pages (from-to)407-418
Number of pages12
JournalJACS Au
Issue number2
Publication statusPublished - 2022 Feb 28

Bibliographical note

Funding Information:
This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Catalysis Science program, under Award DE-SC0006498. S.K. acknowledges Samsung Scholarship for partial financial support. G.D.S. is a fellow of the CIFAR BSE Program. L.T. and G. D. S. acknowledge support from BioLEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0019370.

Publisher Copyright:
© 2022 American Chemical Society. All rights reserved.

All Science Journal Classification (ASJC) codes

  • Chemistry (miscellaneous)
  • Analytical Chemistry
  • Organic Chemistry
  • Physical and Theoretical Chemistry


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