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Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)–N-heterocyclic carbene complex
Lund Univ, Ctr Anal & Synth CAS, Dept Chem, SE-22100 Lund, Sweden..ORCID iD: 0000-0001-5773-1742
Lund Univ, Ctr Anal & Synth CAS, Dept Chem, SE-22100 Lund, Sweden..ORCID iD: 0000-0002-6031-3311
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.ORCID iD: 0000-0001-9975-6577
Lund Univ, Ctr Anal & Synth CAS, Dept Chem, SE-22100 Lund, Sweden..ORCID iD: 0000-0001-6148-2259
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2022 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 13, no 32, p. 9165-9175Article in journal (Refereed) Published
Abstract [en]

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) (LMCT)-L-2 and Fe(ii) (MLCT)-M-3 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) (LMCT)-L-2 state by a sacrificial electron donor and subsequent excitation of the Fe(ii) product to its highly reducing (MLCT)-M-3 state. This route is shown to be more efficient than the alternative, where oxidative quenching of the less reducing Fe(iii) (LMCT)-L-2 state by the alkyl halide drives the reaction, in the absence of a sacrificial electron donor.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2022. Vol. 13, no 32, p. 9165-9175
National Category
Theoretical Chemistry Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-483786DOI: 10.1039/d2sc02122fISI: 000834974500001OAI: oai:DiVA.org:uu-483786DiVA, id: diva2:1693309
Funder
Swedish Foundation for Strategic Research, EM16-0067Knut and Alice Wallenberg Foundation, 2018.0074Swedish Research Council, 2020-03207Swedish Research Council, 2020-05058Swedish Energy AgencyWenner-Gren Foundations
Note

Title in Web in Science: Photoredox catalysis via consecutive (LMCT)-L-2- and (MLCT)-M-3-excitation of an Fe(iii/ii)-N-heterocyclic carbene complex

Available from: 2022-09-06 Created: 2022-09-06 Last updated: 2024-12-07Bibliographically approved
In thesis
1. Ferrous and Ferric N-Heterocyclic Carbene Complexes: Characterization and Applications in Photoredox Catalysis
Open this publication in new window or tab >>Ferrous and Ferric N-Heterocyclic Carbene Complexes: Characterization and Applications in Photoredox Catalysis
2025 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Iron complexes are emerging as favourable substitutes to noble metal complexes as photocatalysts due to iron being earth-abundant and inexpensive. Much of the recent progress has been enabled by the strong electron-donating character of N-heterocyclic carbene (NHC) ligands that strongly destabilizes metal-centred (MC) states of FeNHC complexes and thereby greatly extends the lifetimes of their charge transfer (CT) states that are otherwise rapidly deactivated via low-lying MC states.

The first part of this thesis successfully employed FeNHC complexes in different photoredox catalysis (PRC) reactions and an example of high-turnover catalytic hydrogen production. The latter was accomplished with the benchmark ferric bis-tridentate scorpionate complex [FeIII(phtmeimb)2]+ (phtmeimb = phenyl(tris(3-methylimidazol-1-ylidene))borate) which has a 2LMCT state lifetime of two nanoseconds and excellent photostability. It was further employed in two PRC reactions that yielded synthetically-useful organic compounds, where fast and efficient reductive quenching of the 2LMCT state by various amine donors with cage escape yields between 2 and 22 % were observed. A tris-bidentate complex with favourable excited-state (ES) redox properties and lifetimes in both oxidation states, [FeII,III(btz)3]2+,3+ (btz = 3,3’-dimethyl-1,1’-bis(p-tolyl)-4,4’-bis(1,2,3-triazol-5-ylidene)), was employed in a two-photon PRC reaction utilizing both oxidative and reductive quenching steps, making the PRC reaction overall more efficient. 

The second part of this thesis describes the electrochemical and photophysical characterization of novel FeNHC complexes with three different motifs in view of their potential suitability as photocatalysts. (i) For a series of ferric bis-tridentate complexes with cyclometalating ligands, not only were their emissive 2LMCT states with lifetimes of hundreds of picoseconds approaching values previously obtained with the [FeIII(phtmeimb)2]+ motif, their electrochemical and ES properties were more tunable by substituent effects. (ii) For the (NHC)4(bpy)2 bis-tridentate complexes [Fe(btz)2bpy]2+,3+ (bpy = 2,2'-bipyridyl) and [Fe(btz)2mbpy]2+,3+ (mbpy = 4,4'-dimethyl-2,2'-bipyridyl), both ferrous and ferric analogues offered insufficient ES lifetimes on the order of ten ps. The ferrous mbpy variant featured however a more long-lived, presumably MC state that deserves further characterization, also in regard to its potential reactivity. (iii) For the ferrous analogue of [FeIII(phtmeimb)2]+, the strikingly-short picosecond 3MLCT state lifetime concludes that even the phtmeimb- ligand with superior σ-donating ability cannot sufficiently prevent the relatively high-energy 3MLCT state from rapid deactivation. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2025. p. 156
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2482
Keywords
Iron, Earth-Abundant, N-Heterocyclic Carbene, Photoredox Catalysis, Excited State Electron Transfer, Characterization, Photophysics
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-544629 (URN)978-91-513-2331-2 (ISBN)
Public defence
2025-02-06, Polhemsalen, Room 10134, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
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Available from: 2025-01-15 Created: 2024-12-07 Last updated: 2025-01-15

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