Hydrogen-Bridged Oligosilanylsilyl Mono- and Oligosilanylsilyl Dications

Hydrogen-bridged oligosilanylsilyl borates 8 B(C₆F₅)₄], 9 B(C₆F₅)₄] and diborates 10 B(C₆F₅)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.