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Synthetic methods that utilise iron to facilitate C–H bond activation to yield new C–C and C–heteroatom bonds continue to attract significant interest. However, the development of these systems is still hampered by a limited molecular-level understanding of the key iron intermediates and reaction pathways that enable selective product formation. While recent studies have established the mechanism for iron-catalysed C–H arylation from aryl-nucleophiles, the underlying mechanistic pathway of iron-catalysed C–H activation/functionalisation systems which utilise electrophiles to establish C–C and C–heteroatom bonds has not been determined. The present study focuses on an iron-catalysed C–H allylation system, which utilises allyl chlorides as electrophiles to establish a C–allyl bond. Freeze-trapped inorganic spectroscopic methods ( 57 Fe Mössbauer, EPR, and MCD) are combined with correlated reaction studies and kinetic analyses to reveal a unique and rapid reaction pathway by which the allyl electrophile reacts with a C–H activated iron intermediate. Supporting computational analysis defines this novel reaction coordinate as an inner-sphere radical process which features a partial iron–bisphosphine dissociation. Highlighting the role of the bisphosphine in this reaction pathway, a complementary study performed on the reaction of allyl electrophile with an analogous C–H activated intermediate bearing a more rigid bisphosphine ligand exhibits stifled yield and selectivity towards allylated product. An additional spectroscopic analysis of an iron-catalysed C–H amination system, which incorporates N -chloromorpholine as the C–N bond-forming electrophile, reveals a rapid reaction of electrophile with an analogous C–H activated iron intermediate consistent with the inner-sphere radical process defined for the C–H allylation system, demonstrating the prevalence of this novel reaction coordinate in this sub-class of iron-catalysed C–H functionalisation systems. Overall, these results provide a critical mechanistic foundation for the rational design and development of improved systems that are efficient, selective, and useful across a broad range of C–H functionalisations. 

Structural characterization of the ionic title complex, [MgBr(THF) 5 ][Co(dpbz) 2 ]·2THF [THF is tetrahydrofuran, C 4 H 8 O; dpbz is 1,2-bis(diphenylphosphanyl)benzene, C 30 H 24 P 2 ], revealed a well-separated cation and anion co-crystallized with two THF solvent molecules that interact with the cation via weak C—H...O contacts. The geometry about the cobalt center is pseudotetrahedral, as is expected for a d 10 metal center, only deviating from an ideal tetrahedral geometry because of the restrictive bite angles of the bidentate phosphane ligands. Three THF ligands of the cation and one co-crystallized THF solvent molecule are each disordered over two orientations. In the extended structure, the cations and THF solvent molecules are arranged in (100) sheets that alternate with layers of anions, the latter of which show various π-interactions, which may explain the particular packing arrangement. 

Structural characterization of the ionic complexes [FeCl 2 (C 26 H 22 P 2 ) 2 ][FeCl 4 ]·0.59CH 2 Cl 2 or [(dppen) 2 FeCl 2 ][FeCl 4 ]·0.59CH 2 Cl 2 (dppen = cis -1,2-bis(diphenylphosphane)ethylene, P 2 C 26 H 22 ) and [FeCl 2 (C 30 H 24 P 2 ) 2 ][FeCl 4 ]·CH 2 Cl 2 or [(dpbz) 2 FeCl 2 ][FeCl 4 ]·CH 2 Cl 2 (dpbz = 1,2-bis(diphenylphosphane)benzene, P 2 C 30 H 24 ) demonstrates trans coordination of two bidentate phosphane ligands (bisphosphanes) to a single iron(III) center, resulting in six-coordinate cationic complexes that are balanced in charge by tetrachloridoferrate(III) monoanions. The trans bisphosphane coordination is consistent will all previously reported molecular structures of six coordinate iron(III) complex cations with a (PP) 2 X 2 ( X = halido) donor set. The complex with dppen crystallizes in the centrosymmetric space group C 2/ c as a partial-occupancy [0.592 (4)] dichloromethane solvate, while the dpbz-ligated complex crystallizes in the triclinic space group P 1 as a full dichloromethane monosolvate. Furthermore, the crystal studied of [(dpbz) 2 FeCl 2 ][FeCl 4 ]·CH 2 Cl 2 was an inversion twin, whose component mass ratio refined to 0.76 (3):0.24 (3). Beyond a few very weak C—H...Cl and C—H...π interactions, there are no significant supramolecular features in either structure. 

The first direct syntheses, structural characterizations, and reactivity studies of multinuclear iron–phenyl species formed upon reaction of Fe(acac)₃and PhMgBr in THF are described.

 

The effects of β‐hydrogen‐containing alkyl Grignard reagents in simple ferric salt cross‐couplings have been elucidated. The reaction of FeCl₃with EtMgBr in THF leads to the formation of the cluster species [Fe₈Et₁₂]²⁻, a rare example of a structurally characterized metal complex with bridging ethyl ligands. Analogous reactions in the presence of NMP, a key additive for effective cross‐coupling with simple ferric salts and β‐hydrogen‐containing alkyl nucleophiles, result in the formation of [FeEt₃]⁻. Reactivity studies demonstrate the effectiveness of [FeEt₃]⁻in rapidly and selectively forming the cross‐coupled product upon reaction with electrophiles. The identification of iron‐ate species with EtMgBr analogous to those previously observed with MeMgBr is a critical insight, indicating that analogous iron species can be operative in catalysis for these two classes of alkyl nucleophiles.

 

The effects of β‐hydrogen‐containing alkyl Grignard reagents in simple ferric salt cross‐couplings have been elucidated. The reaction of FeCl₃with EtMgBr in THF leads to the formation of the cluster species [Fe₈Et₁₂]²⁻, a rare example of a structurally characterized metal complex with bridging ethyl ligands. Analogous reactions in the presence of NMP, a key additive for effective cross‐coupling with simple ferric salts and β‐hydrogen‐containing alkyl nucleophiles, result in the formation of [FeEt₃]⁻. Reactivity studies demonstrate the effectiveness of [FeEt₃]⁻in rapidly and selectively forming the cross‐coupled product upon reaction with electrophiles. The identification of iron‐ate species with EtMgBr analogous to those previously observed with MeMgBr is a critical insight, indicating that analogous iron species can be operative in catalysis for these two classes of alkyl nucleophiles.

 

The use ofN‐methylpyrrolidone (NMP) as a co‐solvent in ferric salt catalyzed cross‐coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross‐coupled product in reactions utilizing alkyl Grignard reagents. Despite the critical importance of NMP, the molecular level effect of NMP on in situ formed and reactive iron species that enables effective catalysis remains undefined. Herein, we report the isolation and characterization of a novel trimethyliron(II) ferrate species, [Mg(NMP)₆][FeMe₃]₂(1), which forms as the major iron species in situ in reactions of Fe(acac)₃and MeMgBr under catalytically relevant conditions where NMP is employed as a co‐solvent. Importantly, combined GC analysis and⁵⁷Fe Mössbauer spectroscopic studies identified1as a highly reactive iron species for the selective formation generating cross‐coupled product. These studies demonstrate that NMP does not directly interact with iron as a ligand in catalysis but, alternatively, interacts with the magnesium cations to preferentially stabilize the formation of1over [Fe₈Me₁₂]⁻cluster generation, which occurs in the absence of NMP.