Enhancement of Stannylene Character in Stannole Dianion Equivalents Evidenced by NMR and Mössbauer Spectroscopy and Theoretical Studies of Newly Synthesized Silyl-Substituted Dilithiostannoles

Dilithiostannoles, which are aromatic
tin-containing ring compounds,
were proposed to have stannylene character, as judged from their NMR
analysis. We herein report on the synthesis of silyl-substituted dilithiostannoles,
which were characterized by NMR spectroscopy and X-ray diffraction
analysis. The silyl-substituted derivatives also exhibit features
characteristic of aromatic dilithiostannoles such as 7Li
NMR signals at high-field area and no C–C bond alternation
in the stannole rings. Theoretical calculations and the 119Sn NMR chemical shifts revealed that the stannylene character in
the silyl-substituted dilithiostannoles is enhanced due to greater
interaction between 5p (Sn) and LUMO (butadiene) in comparison to
those in alkyl and aryl derivatives. The 119Sn Mössbauer
spectra of dilithiostannoles were measured for the first time, indicating
that each of the tin atoms in dilithiostannoles can be characterized
as having Sn­(II) character.