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

Ate-iron(II) species such as [Ar Fe ] (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 Fe and Fe 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 [Mes Fe ] by less hindered phenyl ligands was performed, and uncovered the crucial role of both steric and electronic parameters in the formation of the Fe and Fe oxidation states. The formation of quaternized [Ar Fe MgBr(THF)] intermediates allows the bielectronic reductive elimination energy required for the formation of Fe to be reduced. Similarly, the small steric pressure of the aryl groups in [Ar Fe ] enables the formation of aryl-bridged [{Fe (Ar) } (μ-Ar) ] species, which afford the Fe oxidation state by bimetallic reductive elimination. These results are supported by H NMR, EPR, and Fe Mössbauer spectroscopies, as well as by DFT calculations.