Formation of low-valent Fe$^0$ and Fe$^I$ species in Fe-catalyzed cross-coupling chemistry: key role of ate-Fe$^{II}$ intermediates

Abstract : Ate-iron(lI) species such as [Ar$_3$Fe$^{II}$-(Ar = aryl) are key intermediates in Fe-catalyzed cross-coupling reactions between aryl Grignard reagents (ArMgX) and organic electrophiles.They can be active species in the catalytic cycle, or lead to Feo and Fel oxidation states. These low oxidation states were shown to be catalytically active in some cases, but they mostly lead to unwished organic byproducts. This works relates a study of the evolution of [Ar$_3$Fe$^{II}$]-complexes towards Feo and Fel oxidation states, through $^1$H NMR, EPR and $^{57}$Fe-Môssbauer spectroscopies, as weil as DFT calculations, so as to discuss the role of both steric parameters and spin states in the reduction processes. Such mechanistic insights give a betler understanding of iron-catalyzed C-C bond formation reactions, and can be exploited in the design of new ligands in order to selectively obtain a sole iron oxidation state in a catalytic process.