Fullerenium Salts: New C60 Cation Based Materials

Introduction The large variety of compounds referred to as fullerides are intensively investigated due to their exciting electronic and transport properties, such as high-Tc superconductivity, molecular magnetism, and ionic conductivity[1-4]. These C60 charge-transfer salts invariably contain C60 in its anionic form. In fact, the observation of six reversible reduction states[5] confirms that C60 is an exceptionally good electron acceptor, but, consequently, also a bad electron donor. However, the interest in producing a fullerenium salt, namely a solid in which the fullerenes are in the oxidized state, is kept rather high, prompted by the noticeable electronic properties expected in a “hole-doped” C60 compound[6, 7]. Although the oxidation of C60 has already been reported in the gas phase[8] and in solution[9], the preparation of an ionic solid containing C60 as a cation has proved to be quite a challenging task, due to the extreme reactivity of the C60 species. Here, we report the synthesis and the extensive characterization of the compound C60(AsF6)2, where fullerenes are in the 2+ oxidized state. We show that in this system the extremely reactive C60 cations, among the most reactive molecular ions in nature, organize themselves to form a novel 1D polymer architecture. Either Nuclear Magnetic Resonance (NMR), SQUID magnetometry and DC conductivity show that this polymer is a diamagnetic semiconductor. These results establish a new class of C60 based materials, with still unexplored physical and chemical properties.