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.