On the Oxidation of the Three-Dimensional Aromatics [B12X12]2- (X=F, Cl, Br, I)

The perhalogenated closo-dodecaborate dianions [B12X12](2-) (X=H, F, Cl, Br, I) are three-dimensional counterparts to the two-dimensional aromatics C6X6 (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B12H12](2-) and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo-dodecaborate dianions [B12X12](2-) with the strong oxidizer AsF5 in liquid sulfur dioxide (lSO(2)) yielded the corresponding radical anions [B12X12](-) (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso-boranes B12X12 (X=Cl, Br). These compounds were characterized by single-crystal X-ray diffraction of dark blue B12Cl12 and [Na(SO2)(6)][B12Br12]B12Br12. Sublimation of the crude reaction products that contained B12X12 (X=Cl, Br) resulted in pure dark blue B12Cl12 or decomposition to red B9Br9, respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2-TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B12X12](2-) dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B12X12](2-) dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo-dodecaborate dianions [B12X12](2-) (X=F, Cl, Br, I) by cyclic and Osteryoung square-wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B12X12](2-) (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi-reversible (with oxidation potentials in the range between +1.68 and +2.29V (lSO(2), versus ferrocene/ferrocenium (Fc(0/+)))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71V (lSO(2), versus Fc(0/+))). [B12I12](2-) showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.