| 1. |
- Garg, Nitish Kumar, et al.
(author)
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A mononuclear iron(iii) complex with unusual changes of color and magneto-structural properties with temperature : Synthesis, structure, magnetization, multi-frequency ESR and DFT study
- 2022
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In: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 51:6, s. 2338-2345
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Journal article (peer-reviewed)abstract
- From the reaction of 2-hydroxy-6-methylpyridine (L) with iron(ii) tetrafluoroborate, a new mononuclear iron(iii) octahedral complex [FeL6](BF4)3 has been isolated. The color of the complex reversibly changed from red at room temperature to yellow-orange at the liquid nitrogen temperature. Magnetization measurements indicate that iron(iii) in [FeL6](BF4)3 is in a high-spin state S = 5/2, from room temperature to 1.8 K. The high-spin ground state of iron(iii) is also confirmed by DFT calculations. Although the spin-crossover of the complex is not observed, X-band and multifrequency high-field/high-frequency electron spin resonance (ESR) spectroscopy shows rather uncommon iron(iii) spectra at room temperature and an unusual change with cooling. Spectral simulations reveal that the S = 5/2 ground state multiplet of the complex can be characterized by the temperature independent axial zero-field splitting parameter of |D| = +2 GHz (0.067 cm-1) while the value of the rhombic parameter E of the order of some tenths MHz increases on lowering the temperature. Single crystal X-ray diffraction (SCXRD) shows that the iron(iii) coordination geometry does not change with temperature while supramolecular interactions are temperature dependent, influencing the iron(iii) rhombicity. Additionally, the DFT calculations show temperature variation of the HOMO-LUMO gap, in agreement with the changes of color and ESR-spectra of the iron(iii) complex with temperature.
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| 2. |
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| 3. |
- Lindh, Linnea, et al.
(author)
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Multifaceted Deactivation Dynamics of Fe(II) N-Heterocyclic Carbene Photosensitizers
- 2023
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In: Journal of Physical Chemistry A. - 1089-5639. ; 127:48, s. 10210-10222
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Journal article (peer-reviewed)abstract
- Excited state dynamics of three iron(II) carbene complexes that serve as prototype Earth-abundant photosensitizers were investigated by ultrafast optical spectroscopy. Significant differences in the dynamics between the investigated complexes down to femtosecond time scales are used to characterize fundamental differences in the depopulation of triplet metal-to-ligand charge-transfer (3MLCT) excited states in the presence of energetically accessible triplet metal-centered (3MC) states. Novel insights into the full deactivation cascades of the investigated complexes include evidence of the need to revise the deactivation model for a prominent iron carbene prototype complex, a refined understanding of complex 3MC dynamics, and a quantitative discrimination between activated and barrierless deactivation steps along the 3MLCT → 3MC → 1GS path. Overall, the study provides an improved understanding of photophysical limitations and opportunities for the use of iron(II)-based photosensitizers in photochemical applications.
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| 4. |
- Lindh, Linnea, et al.
(author)
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Side-group switching between metal-to-ligand charge-transfer and metal-centered excited state properties in iron(II) N-heterocyclic carbene complexes
- 2024
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In: Coordination Chemistry Reviews. - 0010-8545. ; 506
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Research review (peer-reviewed)abstract
- Fe(II) N-heterocyclic carbene (NHC) complexes have emerged over the last decade as a promising class of light-harvesting complexes for a variety of photochemical applications relying on the presence of high-energy excited states of mainly charge-transfer character with excited state lifetimes of tens of picoseconds or longer. Recent spectroscopic investigations have significantly refined the understanding of some of the key prototype complexes of this kind and highlighted the subtle balance between population of triplet metal-to-ligand charge-transfer (3MLCT) and triplet metal-centered (3MC) states as a key issue to better understand and ultimately control the excited state dynamics in these complexes. To present a broader perspective on this issue, we here re-examine and discuss the excited state properties of a series of complexes with different side-groups on a common Fe NHC scaffold. Both the steady-state absorption spectrum and excited state dynamics are influenced by the side-group substitution, and the changes are rationalized based on shifting of the lowest metal-to-ligand charge-transfer (MLCT) state in energy based on the electron-withdrawing or electron-donating properties of the side-groups. Only electron-withdrawing substituents such as carboxylic acid groups ensured that the majority excited population stays in the 3MLCT state for ∼20 ps rather than rapidly converting into metal-centered (MC) states. In other complexes, the 3MLCT state survived <300 fs after which the 3MC state was populated for ∼10 ps. The transient absorption results also show that the dynamics can be switched in a simple manner by deprotonating the carboxylic acid group, which renders some of the complexes pH-sensitive. For the here discussed complexes, the results from transient absorption measurements indicate that the 3MLCT and 3MC states were close enough in energy to enable the side-group to determine the photophysics. The emerging understanding of the 3MLCT-3MC balance, as well as the nature and properties of the 3MC state in these complexes with intermediate ligand field strength is used to provide a broader fundamental perspective required to improve the ligand-design of Fe carbene complexes for issues such as to ensure a long-lived 3MLCT state.
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| 5. |
- Manojveer, Seetharaman, et al.
(author)
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Ligand-Promoted [Pd]-Catalyzed α-Alkylation of Ketones through a Borrowing-Hydrogen Approach
- 2023
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In: ChemistryOpen. - : Wiley. - 2191-1363. ; 12:1
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Journal article (peer-reviewed)abstract
- A new class of palladium complexes bearing bidentate 2-hydroxypyridine based ligands have been prepared and fully characterized. The applications of these new complexes towards ketone alkylation reactions with alcohols through a metal-ligand cooperative borrowing-hydrogen (BH) process were demonstrated.
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