Three salts containing the fullerene tetra-anion C ₆₀ ^4- - Synthesis, X-ray single-crystal structure determination and EPR investigation

A synthesis for four-fold negatively charged fullerides in solution is presented. Three salts containing discrete C ₆₀ ^4- anions were synthesized by the reduction of C ₆₀ in solution using rubidium-mercury amalgams and rubidium suboxide both in the presence of elemental mercury. The three new salts, [Rb ₆DMF ₁₄(C ₆H ₁₃N ₂O ₂) ₂]·C ₆₀ (1), [Rb(diaza-18-crown-6)] ₄·C ₆₀·(en) _4.1 (2), and [Rb(benzo-18-crown-6)] ₄·C ₆₀ (3), were characterized by single-crystal X-ray diffraction. The results clearly indicate a charge of 4- for the fulleride anions. In 1 the fulleride units are ordered, and their distortion from I _h symmetry shows similarities to binary alkali metal fullerides that contain C ₆₀ ^4- anions. In the crystal structures of 2 and 3 the C ₆₀ ^4- anions show a rotational disorder. In all structures the 6:6 bond lengths within the fulleride are strongly enlarged compared to the ones in neutralC ₆₀. EPR measurements reveal a singlet state for the C ₆₀ ^4- anion.