Methyl viologen (MV2+)(Cl−)2 is reduced by cesium anthracene to form neutral MV0 whose interaction with C60 produces (MV˙+)2(C60−)2·2.5C6H4Cl2·0.5C6H6 (1). X-ray diffraction analysis shows that C60˙− radical anions form singly-bonded (C60−)2 dimers at 100 K and are arranged in zig-zag layers. According to EPR, the dimers dissociate in the 235–375 K range. Salt 1 contains two paramagnetic MV˙+ and C60˙− species in the monomeric phase with strong exchange interaction. This interaction is switched off at the dimerization of C60˙− and magnetically isolated MV˙+ pairs are formed. Spins of MV˙+ are strongly antiferromagnetically coupled in the pairs with the Weiss temperature of −32 K and J/kB = −20 K. MV˙+ shows narrow EPR signal with g = 2.0039 and the linewidth of about 0.2 mT (T <100 K). A plot of natural logarithm of the dimerization constant vs. reverse temperature (at 235–375 K) allows the estimation of the energy of single C–C bonds in the (C60−)2 dimers as 38.2 ± 0.8 kJ mol−1.
