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Issue 32, 2020
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Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

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Abstract

Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure–reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(II) hydride with an IrII–H bond dissociation free energy around 44 kcal mol−1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(III) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer.

Graphical abstract: Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

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Supplementary files

Article information


Submitted
30 Mar 2020
Accepted
27 Jul 2020
First published
06 Aug 2020

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2020,11, 8582-8594
Article type
Edge Article

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

M. R. Schreier, B. Pfund, X. Guo and O. S. Wenger, Chem. Sci., 2020, 11, 8582
DOI: 10.1039/D0SC01820A

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