Gas-phase models for catalysis:: Alkane activation and olefin epoxidation by the triatomic cation Ag2O+

Electrospray ionization of aqueous silver nitrate is used for the preparation of the disilver-oxide cation Ag2O+ in the gas phase. The mass-selected cation is capable of activating C-H bonds of simple alkanes other than methane via H-atom abstraction, i.e., Ag2O+ + R-H -> Ag2OH+ + R center dot (R = C2H5, C3H7, C4H9). Clean O-atom transfer from Ag2O+ is observed with ethene as a neutral reagent, whereas oxygenation and allylic C-H abstraction compete in the case of propene. The gaseous Ag2O+ cation can thus be regarded as a minimalist model for the problems associated with the silver-mediated epoxidation of olefins more complex than ethene itself. The experimental findings are fully supported by the results of quantum chemical studies, thereby providing deep mechanistic insight into the reactions in the idealized gas phase, which also might have implications for further improvements in applied catalysis.