Evolution of Ate- organoiron(II) species towards lower oxidation states: role of the steric and electronic factors

Abstract : Ate -iron(II) species such as [Ar 3 Fe II ] ─ (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 0 and Fe I 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 3 Fe II ] ─ by less hindered phenyl ligands was performed, and enlightened the crucial role of both steric and electronic parameters in the formation of the Fe 0 and Fe I oxidation states. The formation of quaternized [Ar 4 Fe II MgBr(THF)] ─ intermediates allows to reduce the bielectronic reductive elimination energy required for the formation of Fe 0 . Similarly, a small steric pressure of the aryl groups in [Ar 3 Fe II ] ─ enables the formation of aryl-bridged [{Fe II (Ar) 2 } 2 (µ-Ar) 2 ] 2─ species, which afford the Fe I oxidation state by bimetallic reductive elimination. These results are supported by 1 H NMR, EPR and 57 Fe-Mössbauer spectroscopies, as well as by DFT calculations.