Evolution of Ate‐Organoiron(II) Species towards Lower Oxidation States: Role of the Steric and Electronic Factors

Ate‐iron(II) species such as [Ar3FeII]− (Ar=aryl) are key intermediates in Fe‐catalyzed couplings between aryl nucleophiles and organic electrophiles. They can be active species in the catalytic cycle, or lead to Fe0 and FeI oxidation states, which can themselves be catalytically active or lead to unwished organic byproducts. Analysis of the reactivity of the intermediates obtained by step‐by‐step displacement of the mesityl groups in high‐spin [Mes3FeII]− by less hindered phenyl ligands was performed, and uncovered the crucial role of both steric and electronic parameters in the formation of the Fe0 and FeI oxidation states. The formation of quaternized [Ar4FeIIMgBr(THF)]− intermediates allows the bielectronic reductive elimination energy required for the formation of Fe0 to be reduced. Similarly, the small steric pressure of the aryl groups in [Ar3FeII]− enables the formation of aryl‐bridged [{FeII(Ar)2}2(μ‐Ar)2]2− species, which afford the FeI oxidation state by bimetallic reductive elimination. These results are supported by 1H NMR, EPR, and 57Fe Mössbauer spectroscopies, as well as by DFT calculations. 0, 1, or 2‐electron chemistry: The different elementary pathways connecting iron(II) ate hydrocarbyls to the lower iron(I/0) oxidation states have been investigated. A critical discussion of the steric and electronic effects governing these transformations is proposed, based on EPR, 1H NMR, and Mössbauer spectroscopies, as well as on DFT calculations (see scheme).