Silaphenolates and silaphenylthiolates: two unexplored unsaturated silicon compound classes influenced by aromaticity.

Monosilicon analogs of phenolates and phenylthiolates are studied by quantum chemical calculations. Three different silaphenolates and three different silaphenylthiolates are possible; the ortho-, meta-, and para-isomers. For the silaphenolates, the meta-isomer is the thermodynamically most stable, regardless if the substituent R at Si is H, t-Bu or SiMe(3). However, with R = H and SiMe(3) the energy differences between the three isomers are small, whereas with R = t-Bu the meta-isomer is ~5 kcal/mol more stable than the ortho-isomer. For the silaphenylthiolates the ortho-isomer is of lowest energy, although with R = H the ortho- and meta-isomers are isoenergetic. The calculated nucleus independent chemical shifts (NICS) indicate that the silaphenolates and silaphenylthiolates are influenced by aromaticity, but they are less aromatic than the parent silabenzene. The geometries and charge distributions suggest that all silaphenolates and silaphenylthiolates to substantial degrees are described by resonance structures with an exocyclic C=O double bond and a silapentadienyl anionic segment. Indeed, they resemble the all-carbon phenolate and phenylthiolate. Silaphenylthiolates are less bond alternate and have slightly more negative NICS values than analogous silaphenolates, suggesting that this compound class is a bit more aromatic. Dimerization of the silaphenolates and silaphenylthiolates is hampered due to intramolecular Coulomb repulsion in the dimers, and silaphenolates with a moderately bulky SiMe(3) group as substituent at Si should prefer the monomeric form.