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  Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin

Tarrago, M., Römelt, C., Nehrkorn, J. P., Schnegg, A., Neese, F., Bill, E., et al. (2021). Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin. Inorganic Chemistry, 60(7), 4966-4985. doi:10.1021/acs.inorgchem.1c00031.

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592_ic1c00031_si_001.pdf (Supplementary material), 2MB
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Tarrago, Maxime1, Author           
Römelt, Christina2, Author           
Nehrkorn, Joscha Paul3, Author           
Schnegg, Alexander3, Author           
Neese, Frank4, Author           
Bill, Eckhard3, Author           
Ye, Shengfa1, 5, Author           
Affiliations:
1Research Group Ye, Max-Planck-Institut für Kohlenforschung, Max Planck Society, ou_2541708              
2Research Department Schlögl, Max Planck Institute for Chemical Energy Conversion, Max Planck Society, ou_3023874              
3Research Department DeBeer, Max Planck Institute for Chemical Energy Conversion, Max Planck Society, ou_3023871              
4Research Department Neese, Max-Planck-Institut für Kohlenforschung, Max Planck Society, ou_2541710              
5State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, ou_persistent22              

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 Abstract: Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO2-to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [FeII(TPP)] complex (1; TPP2– = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin (S = 1) ground state was contradictorily assigned to either a nondegenerate 3A2g state with a (dxy)2(dz2)2(dxz,yz)2 configuration or a degenerate 3Egθ state with a (dxy)2(dxz,yz)3(dz2)1/(dz2)2(dxy)1(dxz,yz)3 configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex 1. Highly correlated wave-function-based computations predicted that 3A2g and 3Egθ are well-isolated from other triplet states by ca. 4000 cm–1, whereas their splitting ΔA–E is on par with the effective spin–orbit coupling (SOC) constant of iron(II) (≈400 cm–1). Therefore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the 3A2g and 3Egθ states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field 57Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, ΔA–E, an angle θ that describes the mixing of (dxy)2(dxz,yz)3(dz2)1 and (dz2)2(dxy)1(dxz,yz)3 configurations, and the ⟨rd–3⟩ expectation value of the iron d orbitals that is necessary to estimate the 57Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of 1 is genuinely multiconfigurational with substantial parentages of both 3A2g (<88%) and 3Eg (>12%), owing to their accidental near-triple degeneracy with ΔA–E = +950 cm–1. As a consequence of this peculiar electronic structure, 1 exhibits a huge effective magnetic moment (4.2 μB at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization (g ≈ 3 and g ≈ 1.7) and a large and positive internal field at the 57Fe nucleus aligned in the xy plane. Further in-depth analyses suggested that g ≫ g is a general spectroscopic signature of near-triple orbital degeneracy with more than half-filled pseudodegenerate orbital sets. Implications of the unusual electronic structure of 1 for CO2 reduction are discussed.

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Language(s): eng - English
 Dates: 2021-01-062021-03-192021-04-05
 Publication Status: Published in print
 Pages: 20
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1021/acs.inorgchem.1c00031
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Title: Inorganic Chemistry
  Abbreviation : Inorg. Chem.
Source Genre: Journal
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Publ. Info: Washington, DC : American Chemical Society
Pages: - Volume / Issue: 60 (7) Sequence Number: - Start / End Page: 4966 - 4985 Identifier: ISSN: 0020-1669
CoNE: https://pure.mpg.de/cone/journals/resource/0020-1669