Structural Insights into the Nature of Fe 0 and Fe I Low-Valent Species Obtained upon the Reduction of Iron Salts by Aryl Grignard Reagents

Martin Clémancey 1 Thibault Cantat 2 Geneviève Blondin 1 Jean-Marc Latour 1, * Pierre Dorlet 3, 4, * Guillaume Lefèvre 2, *
* Corresponding author
2 LCMCE - Laboratoire de Chimie Moléculaire et de Catalyse pour l'Energie (ex LCCEF)
NIMBE UMR 3685 - Nanosciences et Innovation pour les Matériaux, la Biomédecine et l'Energie (ex SIS2M)
3 LSOD - Stress Oxydants et Détoxication
B3S - Département Biochimie, Biophysique et Biologie Structurale
Abstract : Mechanistic studies of the reduction of Fe III and Fe II salts by aryl Grignard reagents in toluene/tetrahydrofuran mixtures in the absence of a supporting ligand, as well as structural insights regarding the nature of the low-valent iron species obtained at the end of this reduction process, are reported. It is shown that several reduction pathways can be followed, depending on the starting iron precursor. We demonstrate, moreover, that these pathways lead to a mixture of Fe 0 and Fe I complexes regardless of the nature of the precursor. Mö ssbauer and 1 H NMR spectroscopies suggest that diamagnetic 16-electron bisarene complexes such as (η 4-C 6 H 5 Me) 2 Fe 0 can be formed as major species (85% of the overall iron quantity). The formation of a η 6-arene-ligated low-spin Fe I complex as a minor species (accounting for ca. 15% of the overall iron quantity) is attested by Mö ssbauer spectroscopy, as well as by continuous-wave electron paramagnetic resonance (EPR) and pulsed-EPR (HYSCORE) spectroscopies. The nature of the Fe I coordination sphere is discussed by means of isotopic labeling experiments and density functional theory calculations. It is shown that the most likely low-spin Fe I candidate obtained in these systems is a diphenylarene-stabilized species [(η 6-C 6 H 5 Me)Fe I Ph 2 ] − exhibiting an idealized C 2v topology. This enlightens the nature of the lowest valence states accommodated by iron during the reduction of Fe III and Fe II salts by aryl Grignard reagents in the absence of any additional coligand, which so far remained rather unknown. The reactivity of these low-valent Fe I and Fe 0 complexes in aryl−heteroaryl Kumada cross-coupling conditions has also been investigated, and it is shown that the zerovalent Fe 0 species can be used efficiently as a precursor in this reaction, whereas the Fe I oxidation state does not exhibit any reactivity.
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Martin Clémancey, Thibault Cantat, Geneviève Blondin, Jean-Marc Latour, Pierre Dorlet, et al.. Structural Insights into the Nature of Fe 0 and Fe I Low-Valent Species Obtained upon the Reduction of Iron Salts by Aryl Grignard Reagents. Inorganic Chemistry, American Chemical Society, 2017, 56, pp.3834-3848. ⟨10.1021/acs.inorgchem.6b02616⟩. ⟨cea-01492534⟩

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