Magnetic coupling and optical properties of the S6-dodecakis(trifluoromethyl)fullerene radical anions in the layered salt (PPN(+))[C60(CF3)12(·-)].

Poly(trifluoromethyl)fullerene S6-C60(CF3)12 was reduced by sodium fluorenone ketyl in the presence of (PPN)Cl (PPN = bis(triphenylphosphine)iminium) to afford the salt (PPN)[C60(CF3)12] (1), which contains C60(CF3)12(·-) radical anions. In the crystal structure of 1, C60(CF3)12(·-) layers alternate with the PPN(+) cations. There are short F⋅⋅⋅F contacts between C60(CF3)12(·-) radical anions within the layers but no C⋅⋅⋅C contacts. DFT calculations revealed that the negative charge on C60(CF3)12(·-) is distributed mainly between sp(2) carbon and fluorine atoms, whereas spin density is localized mainly on the fullerene-cage sp(2) carbon atoms. IR and UV/Vis/NIR spectra in the solid state and solution showed characteristic changes relative to those of neutral S6-C60(CF3)12 due to the formation of radical anions. The solid-state electronic spectrum of 1 exhibits a single broad band at 738 nm attributed to C60(CF3)12(·-). Crystals of 1 show a narrow EPR signal with g = 2.0025 (ΔH = 0.45 mT) at 300 K. The temperature dependence of the integral intensity follows the Curie-Weiss law with a negative Weiss temperature of -11.8 K (30-300 K) indicating antiferromagnetic interaction of spins. This dependence was approximated by the Heisenberg model for one-dimensional chains of antiferromagnetically interacting spins with exchange interaction J/kB = -9.1 K. It was assumed that magnetic interaction between the C60(CF3)12(·-) spins in the layers is mediated by short F⋅⋅⋅F contacts.