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Organometallic chemistry: High-valent iron gets homoleptic

Nature Reviews Chemistry volume 1, Article number: 0036 (2017) Cite this article

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The diverse roles that iron has in natural and laboratory environments require it to assume a range of oxidation states — each a guise with a distinct personality. In the +IV state, iron can strip hydrogen from C–H bonds, allowing the installation of a halogen or an oxygenic group. Although such base-metal-catalysed direct functionalizations are attractive and intensely studied, our understanding of Fe(IV) complexes that are often proposed as intermediates has yet to catch up. Homoleptic species are particularly elusive, which increases the significance of the [Fe(alkyl)4] complexes — prepared by disproportionation reactions — reported in Angewandte Chemie International Edition by a team led by Alois Fürstner.

While studying C–C coupling reactions, Fürstner and colleagues considered the reduction of iron catalysts with Grignard reagents. In some cases, Fe(II) or Fe(III) precursors afforded high-valent iron complexes among their products. This is a strange phenomenon: “it was totally unexpected to isolate an oxidized complex from a reaction mixture comprising FeCl2 and cyclohexylmagnesium chloride,” notes Fürstner. The team found that [Fe(cyclohexyl)4] forms alongside Fe metal in a disproportionation reaction that was also used in the synthesis of [Fe(1-norbornyl)4], of which they were unaware at the time. Reflective of the strong donicity of cyclohexyls, the new tetrahedral complex is low-spin (e4t20) because the ligands give rise to a large ligand field splitting (Δt = 40 kcal mol−1). Cyclohexyls are smaller than norbornyls, so the reasonable stability of [Fe(cyclohexyl)4] at −20 °C is interesting given its more exposed Fe(IV) centre and attenuated dispersion interactions. Such interactions appear to tip the Fe(II or III)–Grignard system in favour of disproportionation: [Fe(2-adamantyl)4] could be prepared, but examples with ligands smaller than cyclohexyl could not.

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  • 29 March 2017

    This highlight was originally published with an incorrect date. The error has been fixed in both the HTML and PDF versions.

References

  1. Casitas, A. et al. Two exceptional homoleptic iron(IV) tetraalkyl complexes. Angew. Chem. Int. Ed. http://dx.doi.org/10.1002/anie.201612299 (2017)

  2. Liptrot, D. A. & Power, P. P. London dispersion forces in sterically crowded inorganic and organometallic molecules. Nat. Rev. Chem. 1, 0004 (2017)

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  1. David Schilter
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Schilter, D. Organometallic chemistry: High-valent iron gets homoleptic. Nat Rev Chem 1, 0036 (2017). https://doi.org/10.1038/s41570-017-0036

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