On the oxidation state of iron in iron-mediated C–C couplings

Multinuclear iron–phenyl species in reactions of simple iron salts with PhMgBr: identification of Fe4(μ-Ph)6(THF)4 as a key reactive species for cross-coupling catalysis

“…Lastly, it is interesting to consider the origin of the reactivity differences observed for [FePh 2 (μ-Ph)] 2 2– and Fe 4 (μ-Ph) 6 (THF) 4 . Since previous studies have proposed an Fe( i ) active species for cross-coupling with PhMgBr and simple ferric salts (such as [PhFe I (acac)(THF)] – ), 18 – 25 the presence of two formally iron( i ) sites in the mixed valent tetranuclear iron complex 2a might suggest iron reduced below iron( ii ) is important for reactivity in the isolated multimetallic complexes. Specifically, the THF ligation differences between complexes 2b and 2c demonstrate the ability of the tetranuclear complexes to lose a THF ligand to generate an open coordination position for reaction with electrophile.…”

“…Motivated by the critical need to define the iron–phenyl species involved in catalysis, several recent studies have employed NMR and electron paramagnetic resonance (EPR) spectroscopies, as well as density functional theory (DFT) calculations to investigate iron speciation in reactions of simple ferric salts and PhMgBr. 18 – 25 Such studies have hypothesized the formation of mononuclear Fe I and Fe 0 species in situ . 18 – 25 Alternatively, extended X-ray absorption fine structure (EXAFS) studies have led to the proposed formation of Fe II dimers in situ in these reactions, whereas mass spectrometry studies have suggested that iron–phenyl-ate species of even higher nuclearity can form.…”

“… 18 – 25 Such studies have hypothesized the formation of mononuclear Fe I and Fe 0 species in situ . 18 – 25 Alternatively, extended X-ray absorption fine structure (EXAFS) studies have led to the proposed formation of Fe II dimers in situ in these reactions, whereas mass spectrometry studies have suggested that iron–phenyl-ate species of even higher nuclearity can form. 26 , 27 While these studies represent important contributions towards the identification of some of the in situ formed iron–phenyl species, the resulting lack of consensus on the nature of these species ( e.g.…”

Iron(II) Active Species in Iron–Bisphosphine Catalyzed Kumada and Suzuki–Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides

“…Such artificial contributions to biphenyl generation independent of the presence of reduced iron species in solution are important to be aware of since biphenyl counting methods to estimate the average oxidation state of iron in solution have been previously employed in the literature. 36 , 57 …”

X‐ray Spectroscopic Verification of the Active Species in Iron‐Catalyzed Cross‐Coupling Reactions

“…The oxidation state obtained from XANES spectra and GC analysis is Fe I , which is in agreement with the recent study of Norrby et al who also found that the Grignard reagent alone can only reduce the iron pre-catalyst to Fe I . [36] The XAS results presented allow the assessment of the non-spectroscopic mechanistic studies of iron cross-coupling reactions reported to date. Based on the oxidation state of Fe I for the active species determined by XANES and GC analysis, it is possible to preclude hypotheses 2, [10] 3 [11] and 4, [12] because these species have oxidation states of ÀII and + II.…”