Hydrogen‐Bridged Oligosilanylsilyl Mono‐ and Oligosilanylsilyl Dications

Hydrogen‐bridged oligosilanylsilyl borates 8 [B(C(6)F(5))(4)], 9[B(C(6)F(5))(4)] and diborates 10 [B(C(6)F(5))(4)](2) have been prepared by hydride transfer between α‐ω‐dihydrido‐ (11) and branched tetrahydrido‐oligosilanes (13) and trityl cation. The obtained cyclic intramolecularly stabilized silylium ions 8, 9 and bissilylium ion 10 were characterized by low temperature NMR spectroscopy supported by the results of density functional calculations. The branched Si−H−Si monocation 9 undergoes at low temperatures a fast degenerate rearrangement, which exchanges the Si−H groups with a barrier of 31 kJ mol(−1) via an antarafacial transition state. Reaction of the branched monocation 9 with a second equivalent of trityl cation or of the branched oligosilane 13 with two equivalents of trityl cation, gives at −80 °C the corresponding bissilylium ion 10, an example for a new class of highly reactive poly‐Lewis acids.