Quadruple C-H Bond Activations of Methane by Dinuclear Rhodium Carbide Cation [Rh2C3](+)

The structure of the [Rh2C3](+) ion and its reaction with CH4 in the gas phase have been studied by infrared photodissociation spectroscopy and mass spectrometry in conjunction with quantum chemical calculations. The [Rh2C3](+) ion is characterized to have an unsymmetrical linear [Rh-C-C-CRh](+) structure existing in two nearly isoenergetic spin states. The [Rh2C3](+) ion reacts with CH4 at room temperature to form [Rh2C]+ + C3H4 and [Rh2C2H2](+) + C2H2 as the major products. In addition to the [Rh2C](+) ion, the [(Rh2C)-C-13](+) ion is formed at about one-half of the [Rh2C](+) intensity when the isotopic-labeled (CH4)-C-13 sample is used. The production of [(Rh2C)-C-13](+) indicates that the linear C-3 moiety of [Rh2C3](+) can be replaced by the bare carbon atom of methane with all four C-H bonds being activated. The calculations suggest that the overall reactions are thermodynamically exothermic, and that the two Rh centers are the reactive sites for C-H bond activation and hydrogen atom transfer reactions.