Relevance of Single-Transmetalated Resting States in Iron-Mediated Cross-Couplings: Unexpected Role of σ-Donating Additives

Microscopic Reactivity of Phenylferrate Ions toward Organyl Halides

Despite its practical importance, organoiron chemistry remains poorly understood due to its mechanistic complexity. Here, we focus on the oxidative addition of organyl halides to phenylferrate anions in the gas phase. By mass-selecting individual phenylferrate anions, we can determine the effect of the oxidation state, the ligation, and the nuclearity of the iron complex on its reactions with a series of organyl halides RX. We find that Ph₂ Fe(I)^- and other low-valent ferrates are more reactive than Ph₃ Fe(II)^- ; Ph₄ Fe(III)^- is inert. The coordination of a PPh₃ ligand or the presence of a second iron center lower the reactivity. Besides direct cross-coupling reactions resulting in the formation of RPh, we also observe the abstraction of halogen atoms. This reaction channel shows the readiness of organoiron species to undergo radical-type processes. Complementary DFT calculations afford further insight and rationalize the high reactivity of the Ph₂ Fe(I)^- complex by the exothermicity of the oxidative addition and the low barriers associated with this reaction step. At the same time, they point to the importance of changes of the spin state in the reactions of Ph₃ Fe(II)^- .

The crucial and multifaceted roles of main-group cations and their salts in iron-mediated cross-couplings

While a broad variety of iron-catalyzed cross-couplings involve the use of main-group organometallics R-[M] as nucleophiles, the role of the [M]ⁿ⁺ cation in the coupling process is generally disregarded. However, several beneficial effects of [M]ⁿ⁺ cations by themselves or involved in ionic salts used as additives have been observed in such procedures. At the molecular level, interaction of those [M]ⁿ⁺ cations with on-cycle organoiron intermediates can proceed in several ways. Intermolecular interactions can be observed, and also the implication of [M]ⁿ⁺ in the iron's first or second coordination sphere, e.g. by ambiphilic coordination of a [M]-X salt to an R-[Fe] bond. The use of [M]ⁿ⁺ cations in the reaction medium is also a powerful strategy enabling control of the distribution of iron oxidation states within the coupling process.

Iron-catalyzed coupling of three reactants

[Figure: see text].