Isolation and Structure of [(Fluorenone ⊙⊖){Na⊕(dme)2}]2—a Touchstone for Radical Contact Ion Pairs?

Single crystals of a radical ion pair salt—the title compound—could be grown after stoichiometric reduction of fluorenone with sodium in aprotic solution under argon at –50°C. The structure determination reveals that a dimeric contact ion pair containing an Na2O2 four‐membered ring is present and shows, for the first time, the bonding between radical anions and countercations: The solvated Na⊕ ion is situated above the plane of the molecule and outside of the πCO plane. This finding further supports the assumption of a π spin delocalization from the radical anion to the metal cation, as derived from the positive ENDOR metal couplings of radical ion pairs in solution.