Issue 32, 2022

Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)–N-heterocyclic carbene complex

Abstract

Fe–N-heterocyclic carbene (NHC) complexes attract increasing attention as photosensitisers and photoredox catalysts. Such applications generally rely on sufficiently long excited state lifetimes and efficient bimolecular quenching, which leads to there being few examples of successful usage of Fe–NHC complexes to date. Here, we have employed [Fe(III)(btz)3]3+ (btz = (3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene))) in the addition of alkyl halides to alkenes and alkynes via visible light-mediated atom transfer radical addition (ATRA). Unlike other Fe–NHC complexes, [Fe(III/II)(btz)3]3+/2+ benefits from sizable charge transfer excited state lifetimes ≥0.1 ns in both oxidation states, and the Fe(III) 2LMCT and Fe(II) 3MLCT states are strong oxidants and reductants, respectively. The combined reactivity of both excited states enables efficient one-electron reduction of the alkyl halide substrate under green light irradiation. The two-photon mechanism proceeds via reductive quenching of the Fe(III) 2LMCT state by a sacrificial electron donor and subsequent excitation of the Fe(II) product to its highly reducing 3MLCT state. This route is shown to be more efficient than the alternative, where oxidative quenching of the less reducing Fe(III) 2LMCT state by the alkyl halide drives the reaction, in the absence of a sacrificial electron donor.

Graphical abstract: Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)–N-heterocyclic carbene complex

Supplementary files

Article information

Article type
Edge Article
Submitted
13 apr 2022
Accepted
09 jul 2022
First published
20 jul 2022
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2022,13, 9165-9175

Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(III/II)–N-heterocyclic carbene complex

A. Ilic, J. Schwarz, C. Johnson, L. H. M. de Groot, S. Kaufhold, R. Lomoth and K. Wärnmark, Chem. Sci., 2022, 13, 9165 DOI: 10.1039/D2SC02122F

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