“Previously, the only known iron compounds had oxidation numbers of +2 and +3,” explains Moritz Malischewski, the study’s first author. These numbers indicate an atom’s ionic charge. Now, for the first time, the researchers from Freie Universität Berlin and Friedrich-Alexander-Universität Erlangen have succeeded in synthesizing, isolating, and studying an iron compound in the rare oxidation state of +4. The result is the molecule [Cp*₂Fe]²⁺. “This is a milestone in chemical synthesis,” says Dr. Karsten Meyer, a professor of inorganic and general chemistry at FAU. “The new molecule is sure to find a place in modern chemistry textbooks. Its synthesis unlocks valuable knowledge and will enable even more interesting teaching,” he continues. The molecule discovered by the researchers is extremely sensitive and reactive.

“Handling the samples is a big challenge, since contact with ambient humidity or conventional organic solvents causes them to break down in just a short time,” explains Malischewski. To synthesize and analyze the molecule, the researchers had to take special measures that are only possible in a few university labs worldwide. For example, the fluorine chemistry team in Berlin had to oxidize the ferrocene with a xenon-fluorine compound in liquid hydrogen fluoride. The researchers were able to prove their success in synthesizing the molecule through structural analyses of a series of different salts.

To analyze the molecule, chemists at FAU crafted special, ultra-durable sample containers that prevent the samples and sensitive analysis apparatus from breaking down. During analysis, the researchers discovered that the new molecule with oxidation number +4 has a special feature. Unlike other ferrocenes, it forms different structures – linear or angled – depending on which anions are used to stabilize the molecule. These structural differences are triggered by supramolecular interactions between the ions. Mario Adelhardt and Jörg Sutter, two chemists from FAU, were also able to explain the unusual electronic structures of this “exotic” compound.