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The high abundance, low toxicity and rich redox chemistry of iron has resulted in a surge of iron‐catalyzed organic transformations over the last two decades. Within this area, N‐heterocyclic carbene (NHC) ligands have been widely utilized to achieve high yields across reactions including cross‐coupling and C−H alkylation, amongst others. Central to the development of iron‐NHC catalytic methods is the understanding of iron speciation and the propensity of these species to undergo reduction events, as low‐valent iron species can be advantageous or undesirable from one system to the next. This study highlights the importance of the identity of the NHC on iron speciation upon reaction with EtMgBr, where reactions with SIMes and IMes NHCs were shown to undergo β‐hydride elimination more readily than those with SIPr and IPr NHCs. This insight is vital to developing new iron‐NHC catalyzed transformations as understanding how to control this reduction by simply changing the NHC is central to improving the reactivity in iron‐NHC catalysis.

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.

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.