Reversible dissociation of singly-bonded (C60−)2 dimers in (MV˙+)2(C60−)2·solvent salt containing paramagnetic methyl viologen MV˙+ radical cations

Abstract

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/k_B = −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.