Study makes iron(VI) structure crystal clear

High oxidation states of iron are kind of like giant squid: they’re thought to exist, but we have only snapshots of evidence. The iron(VI) state occurs in metalloenzyme intermediates and in a scant handful of compounds, such as the ferrate ion (Fe^VIO₄)²⁻, used as ecofriendly water treatments. Scientists have been able to characterize these oddities only through spectroscopy methods, which don’t give definitive data on structure. Now Jeremy Smith and coworkers at Indiana University Bloomington have made the answer crystal clear. The team synthesized and structurally characterized an Fe(VI) bis(imido) compound with a four-coordinate seesaw shape (Science 2020, DOI: 10.1126/science.abd3054). Smith says the compound is not very reactive, perhaps because of the bulky bidentate bis(carbene)borate and imido ligands. However, these strong electron-donating groups help stabilize the electron-poor Fe center. The researchers first synthesized an Fe(V) complex from an Fe(I) compound and an aryl organoazide. Then the team isolated this compound and oxidized it further to get the Fe(VI) bis(imido) in 81% yield. This work “shows that you can get to this kind of oxidation state with the right design,” Smith says. The finding might lead the way to more environmentally friendly oxidants and new catalytic species.