Magnetic Coupling and Optical Properties of the S6- Dodecakis( trifluoromethyl) fullerene Radical Anions in the Layered Salt ( PPN+)[ C60( CF3)12]

Poly(trifluoromethyl)fullerene S-6-C-60(CF3)(12) was reduced by sodium fluorenone ketyl in the presence of (PPN)Cl (PPN=bis(triphenylphosphine)iminium) to afford the salt (PPN)[C-60(CF3)(12)] (1), which contains C-60(CF3)(12)(.-) radical anions. In the crystal structure of 1, C-60(CF3)(12)(.-) layers alternate with the PPN+ cations. There are short FF contacts between C-60(CF3)(12)(.-) radical anions within the layers but no CC contacts. DFT calculations revealed that the negative charge on C-60(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 S-6-C-60(CF3)(12) due to the formation of radical anions. The solid-state electronic spectrum of 1 exhibits a single broad band at 738nm attributed to C-60(CF3)(12)(.-). Crystals of 1 show a narrow EPR signal with g=2.0025 (H=0.45mT) at 300K. The temperature dependence of the integral intensity follows the Curie-Weiss law with a negative Weiss temperature of -11.8K (30-300K) 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/k(B)=-9.1K. It was assumed that magnetic interaction between the C-60(CF3)(12)(.-) spins in the layers is mediated by short FF contacts.