Synthesis of Alkali Metal Salts of Borylsilyl Anions Utilizing Highly Crowded Silylboranes and Their Properties

The lithium salt of a borylsilyl anion, [Tbt(Mes)SiBpin]–Li+ (11: Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = mesityl, pin = pinacolato), was synthesized by the boron–metal exchange reaction of a diborylsilane, Tbt(Mes)Si(Bpin)2 (8), which was synthesized by the insertion of a highly crowded diarylsilylene, Tbt(Mes)Si: (1), into the B–B bond of a diborane(4) compound, B2pin2. Although a salt of the borylsilyl anion 11 could not be isolated due to its thermal instability, the investigation of the reactivity of the anion revealed that 11 is a possible precursor for Si-functionalized silylboranes as well as Si,Si-difunctionalized silanes. Furthermore, the borylsilyl anion [Tbt(Mes)SiBScat]– (27: Scat = dithiocatecholato), which was synthesized by a procedure similar to that used for 11, showed a reactivity different from that of 11. Theoretical calculations for the borylsilyl anions revealed that the anion 27 has a boratasilene character; namely, the Si–B bond has a double-bond character due to the donation of an electron pair of the negatively charged silicon atom into the vacant p-orbital of the adjacent boron atom.