Synthesis, Properties, and Reactions of a Series of Stable Dialkyl-Substituted Silicon−Chalcogen Doubly Bonded Compounds

The first dialkyl-substituted silicon−chalcogen doubly bonded compounds [R₂SiX; R₂ = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl, X = S (4), Se (5), and Te (6) ] were synthesized by the reactions of an isolable dialkylsilylene R₂Si:  (3) with phosphine sulfide, elemental selenium, and elemental tellurium, respectively. Systematic changes of characteristics of silicon−chalcogen double bonds are elucidated by X-ray analysis, UV−vis spectroscopy, and DFT calculations. In the solid state, the unsaturated silicon atom in 4−6 adopts planar geometry and the extent of the shortening of SiX double bonds from the corresponding Si−X single bonds decreases in the order 4 > 5 > 6. In the absorption spectra of 4−6, π → π* transition bands are observed distinctly in addition to n → π* transition bands. Both the n → π* and π → π* transitions are red-shifted in the order 4 < 5 < 6, and the difference between the energies of the two transitions is kept almost constant among 4−6. The tendency is explained using the qualitative perturbation theory and is reproduced by the DFT calculations for model silanechalcogenones. Addition reactions of water, methanol, and isoprene to 4−6 are reported.