Calculated hydride and fluoride affinities of a series of carbenium and silylium cations in the gas phase and in C6H5Cl solution.

We report the hydride and fluoride affinities for a group of silylium and carbenium cations. With comparable substituents on the central atom, the silylium cations have the higher fluoride affinity, whereas the carbenium ions have the higher hydride affinity. In the first approximation, the hydride and the fluoride affinities vary in parallel with changes in substitution, but the deviations from linear correspondence of hydride and fluoride affinities are more pronounced for carbenium ions. The hydride and fluoride affinities of silylium cations are very similar, whereas for carbenium ions, the hydride affinities are 35-60 kcal  mol(-1) higher than fluoride affinities. These results are placed in the context of studies of hydrodefluorination of aliphatic C-F bonds enabled by silylium carborane catalysts [C. Douvris, O. V. Ozerov, Science 2008, 321, 1188]. The abstraction of fluoride from perfluoroalkanes by a trialkylsilylium cation is neither thermodynamically favorable nor kinetically accessible and, if at all possible, will require a much more fluorophilic silylium cation.