Quadruple C–H Bond Activations of Methane by Dinuclear Rhodium Carbide Cation [Rh(2)C(3)](+)

[Image: see text] The structure of the [Rh(2)C(3)](+) ion and its reaction with CH(4) in the gas phase have been studied by infrared photodissociation spectroscopy and mass spectrometry in conjunction with quantum chemical calculations. The [Rh(2)C(3)](+) ion is characterized to have an unsymmetrical linear [Rh–C–C–C–Rh](+) structure existing in two nearly isoenergetic spin states. The [Rh(2)C(3)](+) ion reacts with CH(4) at room temperature to form [Rh(2)C](+) + C(3)H(4) and [Rh(2)C(2)H(2)](+) + C(2)H(2) as the major products. In addition to the [Rh(2)C](+) ion, the [Rh(2)(13)C](+) ion is formed at about one-half of the [Rh(2)C](+) intensity when the isotopic-labeled (13)CH(4) sample is used. The production of [Rh(2)(13)C](+) indicates that the linear C(3) moiety of [Rh(2)C(3)](+) 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.