Hydrogen-Bridged Oligosilanylsilyl Mono- and Oligosilanylsilyl Dications

Hydrogen-bridged oligosilanylsilyl borates 8 [B(C₆ F₅ )₄ ], 9[B(C₆ F₅ )₄ ] and diborates 10 [B(C₆ F₅ )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.