Structural and spectroscopic characterization of an Fe(VI) bis(imido) complex

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Science  16 Oct 2020:
Vol. 370, Issue 6514, pp. 356-359
DOI: 10.1126/science.abd3054

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Nitrogen lifts iron to hexavalence

The myriad ways that iron can interact with oxygen have been amply studied in biochemical and geochemical contexts. More recently, chemists have explored the extent to which nitrogen can likewise stabilize iron in high oxidation states. Martinez et al. now report that an iron center coordinated by carbene ligands can react with an organic azide to form a pentavalent bis(imido) complex with two Fe=N bonds. One-electron oxidation then accessed the Fe(VI) oxidation state. Both compounds were sufficiently stable for crystallographic characterization.

Science, this issue p. 356


High-valent iron species are key intermediates in oxidative biological processes, but hexavalent complexes apart from the ferrate ion are exceedingly rare. Here, we report the synthesis and structural and spectroscopic characterization of a stable Fe(VI) complex (3) prepared by facile one-electron oxidation of an Fe(V) bis(imido) (2). Single-crystal x-ray diffraction of 2 and 3 revealed four-coordinate Fe centers with an unusual “seesaw” geometry. 57Fe Mössbauer, x-ray photoelectron, x-ray absorption, and electron-nuclear double resonance (ENDOR) spectroscopies, supported by electronic structure calculations, support a low-spin (S = 1/2) d3 Fe(V) configuration in 2 and a diamagnetic (S = 0) d2 Fe(VI) configuration in 3. Their shared seesaw geometry is electronically dictated by a balance of Fe-imido σ- and π-bonding interactions.

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