A Tin Analogue of the Cycloheptatrienyl Anion: Synthesis, Structure, and Further Reduction to Form a Dianionic Species

The synthesis and electronic structure of a tin analogue of the cycloheptatrienyl anion are described. The reaction of 5,5-diphenyldibenzo­[b,f]­stannepin with lithium at low temperatures reductively cleaved a Sn–Ph bond to give a lithium salt of the stannepinyl anion, the structure of which was established by X-ray diffraction analysis. Elongating the reaction time resulted in further reduction of the other Sn–Ph bond to generate a dilithium species, the formation of which was confirmed by a trapping reaction using MeI. Unlike general cycloheptatrienyl anions, the monoanionic species was proved to be nonaromatic on the basis of the NMR and X-ray diffraction studies. When the reduction was carried out at room temperature, decomposition of the seven-membered ring followed by Z/E isomerization promoted by lithium proceeded to generate (E)-2,2′-dilithiostilbene, which forms a dimer in the crystalline state.