Silyl Chalconium Ions: Synthesis, Structure and Application in Hydrodefluorination Reactions

The synthesis of two series of silylated chalconium borates, 9 and 10, which are based on the peri-naphthyl and peri-acenaphthyl framework, is reported (chalcogen (Ch): O, S, Se, Te). NMR investigations of the selenium- and tellurium-containing precursor silanes 3 d–f and 8 d, f revealed a significant through-space J-coupling between the chalcogen nuclei and the Me2SiH group. Experimental and computational results typify the synthesized cations 9 and 10 as chalconium ions. The imposed ring strain weakens the Si−Ch linkage compared to acyclic chalconium ions. This attenuation of the Si−Ch bond strength is more pronounced in the acenaphthene series. Surprisingly, the Si−O bonds in oxonium ions 9 a and 10 a are the weakest Si−Ch linkage in both series. The synthesized silyl chalconium borates are active in hydrodefluorination reactions of alkyl fluorides with silanes. A cooperative activation of the silane by the Lewis acidic (silicon) and by the Lewis basic side (chalcogen) is suggested.