Structure and magnetic properties of the ionic fullerene salt (TMP+)•(C60•-)•C6H5CN containing layers of monomeric C60•- radical anions

The fullerene salt, (TMP+)center dot(C-60(center dot-))center dot C6H5CN (1), with a layered structure was obtained as single crystals (TMP+ is the N,N,N'-trimethylpiperazinium cation). The fullerene layers alternating with the TMP+-C6H5CN layers along the c-axis have a honeycomb arrangement of C-60(center dot-) in which each fullerene has three neighbors with interfullerene C center dot center dot center dot C contacts in the 3.13-3.38 angstrom range. Despite the dense packing, the fullerene anions showed no intermolecular bond formation, even when down to helium temperatures. The structure of 1 was solved at 120 and 90 K. The fullerene molecules are disordered between three orientations at 120 K. The ordering transition observed in the 100-90 K range is accompanied by the trebling of the unit cell to provide three crystallographically independent fullerenes. The salt shows an asymmetric EPR signal down to 4 K which can be fitted by two Lorentzian lines with g(1) = 1.9956 and a linewidth (Delta H) of 2.7 mT and g(2) = 1.9815 and Delta H = 5.1 mT at room temperature. The ordering transition results in the abrupt narrowing of both lines in the 100-90 K range. The temperature dependence of the magnetic susceptibility of 1 follows the Curie-Weiss law with a negative Weiss temperature of -11 K, indicating an antiferromagnetic interaction of spins in the fullerene layers. Salt 1 is non-conducting with a room temperature resistivity of 7 x 10(7) Omega cm, which is probably due to the orientational disorder of the fullerenes and a relatively weak interaction between C-60(center dot-) in the layers. Indeed, the AM1 molecular orbital calculations show essentially smaller and more anisotropic overlap integrals between fullerenes than those in the layered metal (MDABCO(+))center dot(C-60(center dot-))center dot TPC.