NO+ dissolved in various fluorobenzenes. The more hydrogen atoms are replaced by fluoride atoms, the lower the interaction of the positive ions with the electrons and the paler the colour of the solution. Photo: Dr. Malte Sellin
A team of scientists headed by Professor Ingo Krossing, Professor of Molecular and Coordination Chemistry at the University of Freiburg's Institute of Inorganic and Analytical Chemistry, has succeeded in significantly increasing the potential for oxidation of and positive ions. While the potential of these positive ions in conventional solvents and anions is up to +0.65 / +1.0 V vs. Fc+/0, the scientists demonstrated potentials of up to +1.50 / +1.52 V vs. Fc+/0. This was achieved with the use of particularly weakly interacting solvents and anions, with the work group focusing on strategic and polarity-maximising substituted fluorinated benzene derivatives. This new approach will in future enable redox reactions even with hard-to-oxidise systems or entirely new applications in the field of electrocatalysis or redox shuttles/mediators. The team's results have been published in Nature Communications.
The weaker the interaction with the positive ion, the stronger the potential for oxidation
Ag+ and NO+ positive ions are oxidising agents widely used in chemistry and materials research. With the right conditions they can selectively remove electrons from substrates. Since these positive ions are very small and have a high charge density, they interact strongly with their environment. And it is this strong interaction with the environment, for example the anion or the solvent, that leads to the potential for oxidation of these positive ions being massively reduced. In order to maximise the oxidation power of the dissolved positive ions, the scientists used particularly weakly coordinating anions (WCA) and solvents.
As solvents, the work group resorted to fluorinated benzene derivatives. To understand the properties of this molecule class, Dr. Johannes Hunger from the Max Planck Institute for Polymer Research assisted the research and determined the extremely important values of the dielectric constants as a solvent property. This revealed that in particular the twofold to fourfold fluorinated aromatic compounds in this aspect displayed higher values than conventional solvents such as dichloromethane or acetone.
While benzene itself or single fluorinated benzene still interacts strongly with the Ag+ and NO+ positive ions, the interaction with all other fluorine atoms reduces. "Besides the electrochemical measurements we have determined the solid-state structures of compounds of the solvents and the positive ions using single-crystal X-ray diffraction and were able to show the interaction reduces the greater the degree of fluorination," explains co-author Dr. Malte Sellin.
"These almost undisturbed particles and their high potential for oxidation allow previously unachievable reactions," says Krossing. "This enables a large number of new elementary chemical studies and potentially also entirely new applications. In future we'll understand even better how molecules behave in an oxidised state - simply because we can now also produce and study them."
- Original publication: Armbruster, Sellin, Seiler, Würz, Oesten, Schmucker, Sterbak, Fischer, Radtke, Hunger, Krossing (2024): Pushing the redox potentials of deelectronators to highly positive values using solvent effects and weakly coordinating anions. Nature Communications. https://doi.org/10.1038/s41467-024-50669-3
- Professor Ingo Krossing holds the Chair of Molecular and Coordination Chemistry at the University of Freiburg's Institute of Inorganic and Analytical Chemistry and is a member of the Living, Adaptive and Energy-autonomous materials Systems (livMatS) Cluster of Excellence.
- Dr. Malte Sellin was a doctoral student under Ingo Krossing and is now a postdoc with the Chair of Molecular and Coordination Chemistry.
- The research has been funded by the German Research Association (DFG) (project numbers 431116391, 281091989, 350173756) and by the European Research Council (ERC) (grant agreement ID 101052935).
