Extremes in Oxidizing Power, Acidity, and Basicity

Professor Michl and collaborators prepared several icosahedral monocarba-closo-dodecaborate anions carrying halogen and trifluoromethyl substituents and investigated their oxidation by chemical and electrochemical means and by quantum chemical calculations. These anions are of interest for fuel cell and high-voltage battery applications, because they are unusually weakly nucleophilic and extremely difficult to oxidize, with reversible redox potentials calculated up to 5 V above ferroceneferricenium. In liquid sulfur dioxide, the standard solvent for electrochemical oxidation at highly positive potentials, all oxidations were irreversible. In 1,1,1,3,3,3-hexafluoroisopropan-2-ol, the undecafluorinated anion is oxidized reversibly at 2.43 V above ferroceneferricenium calculated 2.40 V but the radical is too unstable for isolation. Other undecahalogenated anions are oxidized irreversibly. A promising new solvent that dissolves supporting electrolytes and resists oxidation is 1,1,1,3,3-pentafluorobutane. In two cases, the oxidized neutral radicals were sufficiently stable in perfluorohexane solution for spectroscopic characterization, but only up to -40 degrees C. At temperatures closer to ambient, this solvent was oxidized.