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Ferrous and Ferric N-Heterocyclic Carbene Complexes: Characterization and Applications in Photoredox Catalysis
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.ORCID iD: 0000-0001-9975-6577
2025 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
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 [en]
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: urn:nbn:se:uu:diva-544629ISBN: 978-91-513-2331-2 (print)OAI: oai:DiVA.org:uu-544629DiVA, id: diva2:1919185
Public defence
2025-02-06, Polhemsalen, Room 10134, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2025-01-15 Created: 2024-12-07 Last updated: 2025-01-15
List of papers
1. High turnover photocatalytic hydrogen formation with an Fe(iii) N-heterocyclic carbene photosensitiser
Open this publication in new window or tab >>High turnover photocatalytic hydrogen formation with an Fe(iii) N-heterocyclic carbene photosensitiser
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2022 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 58, no 35, p. 5351-5354Article in journal (Refereed) Published
Abstract [en]

Herein we report the first high turnover photocatalytic hydrogen formation reaction based on an earth-abundant Fe-III-NHC photosensitiser. The reaction occurs via reductive quenching of the (LMCT)-L-2 excited state that can be directly excited with green light and employs either Pt-colloids or [Co(dmgH)(2)pyCl] as proton reduction catalysts and [HNEt3][BF4] and triethanolamine/triethylamine as proton and electron donors. The outstanding photostability of the Fe-III-NHC complex enables turnover numbers >1000 without degradation.

Place, publisher, year, edition, pages
Royal Society of ChemistryRoyal Society of Chemistry (RSC), 2022
National Category
Physical Chemistry Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-483680 (URN)10.1039/d2cc01016j (DOI)000777682500001 ()35373799 (PubMedID)
Funder
Swedish Foundation for Strategic ResearchKnut and Alice Wallenberg FoundationSwedish Research CouncilSwedish Energy Agency
Available from: 2022-09-05 Created: 2022-09-05 Last updated: 2024-12-07Bibliographically approved
2. Base-promoted homolytic aromatic substitution (BHAS) reactions and hydrodehalogenations driven by green light and an iron(III)-NHC photoredox catalyst
Open this publication in new window or tab >>Base-promoted homolytic aromatic substitution (BHAS) reactions and hydrodehalogenations driven by green light and an iron(III)-NHC photoredox catalyst
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(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-544433 (URN)
Available from: 2024-12-04 Created: 2024-12-04 Last updated: 2024-12-07
3. Aminomethylations of electron-deficient compounds: bringing iron photoredox catalysis into play
Open this publication in new window or tab >>Aminomethylations of electron-deficient compounds: bringing iron photoredox catalysis into play
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2024 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 15, no 30, p. 12077-12085Article in journal (Refereed) Published
Abstract [en]

The α-functionalisation of N-containing compounds is an area of broad interest in synthetic chemistry due to their presence in biologically active substances among others. Visible light-induced generation of nucleophilic α-aminoalkyl radicals as reactive intermediates that can be trapped by electron-deficient alkenes presents an attractive and mild approach to achieve said functionalisation. In this work, [Fe(III)(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate), an N-heterocyclic carbene (NHC) complex based on Earth-abundant iron, was used as photoredox catalyst to efficiently drive the formation of α-aminoalkyl radicals from a range of different α-trimethylsilylamines and their subsequent addition to a number of electron-deficient alkenes under green light irradiation. Mechanistic investigations elucidated the different reaction steps of the complete photocatalytic cycle. In terms of yields and substrate scope, we show that [Fe(III)(phtmeimb)2]PF6 can compete with noble metal photoredox catalysts, for instance outcompeting archetypal [Ru(bpy)3]Cl2 under comparable reaction conditions, illustrating that iron photocatalysts can efficiently facilitate photoredox reactions of synthetic value.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2024
National Category
Organic Chemistry Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-540961 (URN)10.1039/d4sc02612h (DOI)001261895400001 ()39092117 (PubMedID)
Funder
Swedish Foundation for Strategic Research, EM16-0067Knut and Alice Wallenberg Foundation, 2018.0074Swedish Research Council, 2020-05058Swedish Research Council, 2020-03207Swedish Energy Agency, P48747-1LMK StiftelsenSten K Johnson Foundation
Available from: 2024-10-25 Created: 2024-10-25 Last updated: 2024-12-07Bibliographically approved
4. Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)–N-heterocyclic carbene complex
Open this publication in new window or tab >>Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)–N-heterocyclic carbene complex
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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
National Category
Theoretical Chemistry Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-483786 (URN)10.1039/d2sc02122f (DOI)000834974500001 ()
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
5. Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited
Open this publication in new window or tab >>Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited
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2023 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 14, no 37, p. 10129-10139Article in journal (Refereed) Published
Abstract [en]

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [Fe-II(ImP)(2)] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its (MLCT)-M-3 (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(II) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(II) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher ((MLCT)-M-2) state of its Fe(III) analogue [Fe-III(ImP)(2)](+), which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe-III(ImP)(2)](+), we confirmed however that higher ((MLCT)-M-2) states of [Fe-III(ImP)(2)](+) are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [Fe-III(ImP)(2)](+) prepared by us only shows red emission from its lower (LMCT)-L-2 state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe-III(ImP)(2)](+) is instead attributed to an impurity, most probably a photoproduct of the Fe(II) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [Fe-III(ImP)(2)](+) itself.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2023
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-514757 (URN)10.1039/d3sc02806b (DOI)001062236300001 ()37772113 (PubMedID)
Funder
Swedish Foundation for Strategic Research, EM16- 0067Knut and Alice Wallenberg Foundation, KAW, 2018.0074Swedish Research Council, VR, 2020-03207Swedish Energy Agency, P48747-1Sten K Johnson FoundationRoyal Physiographic Society in LundSwedish Research Council, VR, 2020-05058
Available from: 2023-10-24 Created: 2023-10-24 Last updated: 2024-12-07Bibliographically approved
6. Homo- and Heteroleptic Iron N-Heterocyclic Carbene Complexes with Cyclometalating Ligands
Open this publication in new window or tab >>Homo- and Heteroleptic Iron N-Heterocyclic Carbene Complexes with Cyclometalating Ligands
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(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-544628 (URN)
Available from: 2024-12-06 Created: 2024-12-06 Last updated: 2024-12-16
7. Electron withdrawing groups with opposing influence on excited state properties of iron cyclometalated carbene complexes
Open this publication in new window or tab >>Electron withdrawing groups with opposing influence on excited state properties of iron cyclometalated carbene complexes
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(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-544534 (URN)
Available from: 2024-12-05 Created: 2024-12-05 Last updated: 2024-12-07
8. Heteroleptic Ferrous and Ferric Tetra-N-Heterocyclic Carbene Complexes - A Comparative Study
Open this publication in new window or tab >>Heteroleptic Ferrous and Ferric Tetra-N-Heterocyclic Carbene Complexes - A Comparative Study
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(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-544533 (URN)
Available from: 2024-12-05 Created: 2024-12-05 Last updated: 2024-12-07
9. Shining light on the ferrous analogue: Excited state dynamics of an Fe(II) hexa-carbene scorpionate complex
Open this publication in new window or tab >>Shining light on the ferrous analogue: Excited state dynamics of an Fe(II) hexa-carbene scorpionate complex
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-544429 (URN)
Available from: 2024-12-04 Created: 2024-12-04 Last updated: 2024-12-07

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