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

Abstract

The fullerene salt, (TMP⁺)·(C₆₀˙⁻)·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 C₆₀˙⁻ in which each fullerene has three neighbors with interfullerene C⋯C contacts in the 3.13–3.38 Å 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 g₁ = 1.9956 and a linewidth (ΔH) of 2.7 mT and g₂ = 1.9815 and ΔH = 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 × 10⁷ Ω cm, which is probably due to the orientational disorder of the fullerenes and a relatively weak interaction between C₆₀˙⁻ 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⁺)·(C₆₀˙⁻)·TPC.