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Abstract
High-valent iron compounds in oxidation states of +4 and +5 have been proposed as key
intermediates in numerous biological processes catalyzed by metalloenzymes.
Bioinorganic chemists have successfully synthesized a wide range of high-valent iron
complexes stabilized by strong electron-donating ancillary ligands in combination with
strong π-donor ligands such as oxido (O2-), imido (RN2-) or nitride (N3-). These
complexes are typically synthesized using traditional group-transfer two-electron
oxidants, such as iodosylbenzene (PhIO) and tosylimidoiodanes (i.e., PhINTS).
However, studies exploring the reactivity of iron complexes with other iodine(III)
compounds, such as azide, cyanide, trifluoromethyl or fluoro λ3-iodanes, remain scarce.
The activation of iodine(III) compounds with iron complexes offers an opportunity to
develop environmentally friendly bond-forming catalytic reactions, owing to the high
abundance, low cost, and low toxicity of iron. Despite their potential, the broader
application of iron-catalyzed group transfer reactions in sustainable synthesis is limited
by the lack of mechanistic understanding.
The synthesis of Fe(IV) complexes remains challenging due to their inherent thermal
instability. In particular, limited examples of organometallic Fe(IV) complexes containing
metal-carbon -bonds have been isolated and thoroughly characterized by
spectroscopic methods. Organometallic iron(IV) complexes have been proposed as
catalytically active species in only few reactions, such as in Gif processes or in certain
C–C cross-coupling reactions. Aiming at designing organometallic iron(IV) compounds
that are relevant to catalytic C–C and C–X bond forming reactions, the carbon atom of
the Fe–C bond cannot belong to the ancillary ligand but rather should be part of a labile
exogenous substrate to enable intermolecular reactions.
This work focuses on the synthesis of organoiron(IV) complexes featuring exogenous
and labile Fe–C bonds to perform mechanistic investigations on C–C bond forming
reactions. In addition, gaining knowledge in the synthesis and reactivity of organoiron(IV)
complexes could pave the way for designing catalytic cycles proceeding via
organoiron(IV) intermediates. Furthermore, this thesis explores the use of iodine(III)
reagents to synthesize high-valent iron complexes through oxidative group-transfer
reactions. The use of the electron-donating nitrogen-based ligand tris(N-tert
butyldimethylsilyl-2-amidoethyl)amine (N3N′3-) facilitates the synthesis of iron complexes
in high oxidation state. Thus, the investigation of the reactivity of iron(II) and iron(III)
precursors bearing the N3N’3- ligand offers an opportunity to synthesize iron(IV)
complexes using iodine(III) reagents by oxidative group-transfer reactions.
In Chapter III, the investigation of the reactivity of iron(II) and iron(III) complexes bearing
the N3N'3- ligand with cyanobenziodoxolone (CBX) and cyano-3,3-dimethyl-1,2
benziodoxole (CDBX) is presented. This work demonstrates that CBX and CDBX react
with these iron complexes as cyano-transfer agents and one-electron oxidants. Whereas
the reaction of CDBX with iron(II) affords the synthesis of iron(III) cyanide complexes,
the oxidation of iron(II) with the most oxidizing CBX forms a highly electrophilic and
thermally unstable Fe(IV) cyanide complex. Single-crystal X-ray diffraction confirms the
solid-state structure of the Fe(III) and Fe(IV) cyanide complexes, while their electronic
structures are elucidated through 57Fe Mössbauer spectroscopy and electron
paramagnetic resonance (EPR) spectroscopy, and computational analyses.
The synthesis of Fe(IV) alkynylide complexes through the reaction of iron(II) with
ethynylbenziodoxo(on)es (EBX) was hampered by the complex spectroscopic analysis
of the crude reactions. Therefore, in Chapter IV, the Fe(IV) alkynylide complexes are
synthesized through a more conventional approach, which is based on the
transmetalation of an iron(III) complex bearing the N3N′3- ligand with lithium alkynylides,
followed by one-electron oxidation. The alkynylferrates(III) and Fe(IV) alkynylides
prepared in this thesis are rare examples of organoiron(IV) complexes where the C
based ligand is not part of the chelating ligand. Their electronic structure has been
thoroughly characterized by nuclear magnetic resonance (NMR), electronic
paramagnetic resonance (EPR), 57Fe Mössbauer spectroscopy, X-ray emission (XES)
and absorption (XAS) spectroscopies, as well as computational studies. While Fe(III)
alkynylferrates show very limited reactivity towards C–C bond formation, Fe(IV)
alkynylides decompose to give 1,3-diynes at room temperature. Based on mechanistic
investigations, it is proposed that the Fe(IV) alkynylides undergo bimolecular C–C
reductive elimination to form the 1,3-diynes. These findings contribute to the
fundamental understanding of high-valent organoiron complexes and their role in C–C
bond-forming processes, offering new pathways for their application in catalysis and
synthetic methodologies.
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Dates
Created: 2025Issued: 2025-11-06Updated: 2025-11-06
Faculty
Fachbereich Chemie
Language
eng
Data types
DoctoralThesis
Keywords
Organische ChemieOrganic ChemistryOrganometallic Chemistry
DFG-subjects
Metallorganische Chemie
DDC-Numbers
540
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Souilah El Hadri, Charafa (000-0003-4066-6168): Synthesis and Characterization of High-Valent Organoiron complexes. : Philipps-Universität Marburg 2025-11-06. DOI: https://doi.org/10.17192/z2025.0658.
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