Halogenated benzene cation radicals

The halogenated benzenes C(6)HF(5), 2,4,6-C(6)H(3)F(3), 2,3,5,6-C(6)H(2)F(4), C(6)F(6), C(6)Cl(6), C(6)Br(6), and C(6)I(6) were converted into their corresponding cation radicals by using various strong oxidants. The cation-radical salts were isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy and by single-crystal X-ray diffraction. The thermal stability of the cation radicals increased with decreasing hydrogen content. As expected, the cation radicals C(6)HF(5)](+) and 2,3,5,6-C(6)H(2)F(4)](+) had structures with the same geometry as C(6)HF(5) and 2,3,5,6-C(6)H(2)F(4)]. In contrast, the cation radicals C(6)F(6)](+), C(6)Cl(6)](+), and possibly also C(6)Br(6)](+) exhibited Jahn-Teller-distorted geometries in the crystalline state. In the case of C(6)F(6)(+)Sb(2)F(11)(-), two low-symmetry geometries were observed in the same crystal. Interestingly, the structures of the cation radicals 2,4,6-C(6)H(3)F(3)](+) and C(6)I(6)(+) did not exhibit Jahn-Teller distortions. DFT calculations showed that the explanation for the lack of distortion of these cations from the D(3h) or D(6h) symmetry of the neutral benzene precursor was different for 2,4,6-C(6)H(3)F(3)](+) than for C(6)I(6)](+).