Silicon in a Negatively Charged Shell: Anions of Spirosilabifluorene

Mono- and dianions of a polycyclic compound with a central sp3-hybridized silicon atom, spirosilabifluorene (C24H16Si, 1), were prepared by reduction with alkali metals. The salts containing 1•– and 12– anions were isolated and studied by single-crystal X-ray diffraction. The lithium salt of the C24H16Si•– radical monoanion ([Li­(THF)4+]­[1•–], 2) exists as a solvent-separated ion pair in the solid state. Substantially different geometrical parameters were found for each of the fluorene groups within the C24H16Si•– anion of 2 due to asymmetric charge distribution. The C24H16Si2– dianion was isolated in the form of its sodium ([{Na­(THF)3+}­{Na­(THF)+(12–)], 3) or potassium ([{K­(THF)+}2(12–)], 4) salt. The environment at the central silicon atom in the dianion is flattened in comparison to the monoanion and neutral compound, with the angle between the two fluorene planes measured at 55° in 12– vs 89° in 1•– and 83° in 10. The aggregation of dianions and alkali-metal counterions leads to the formation of dimeric units and 1D polymeric chains in the solid sodium and potassium salts, respectively. The structure of the cesium salt 5, containing both mono- and dianions in the crystal lattice, was also studied by X-ray diffraction. Complexes 2–5 were investigated by ESR and variable-temperature multinuclear NMR spectroscopy. Theoretical investigations at the PBE0, MP2, and multireference NEVPT2 levels of theory for the C24H16Sin– (n = 0–2) species revealed the conjugation of two fluorene units over the central silicon atom and a singlet ground state for the dianion.