Thermal Activation of CH4 and H2 as Mediated by the Ruthenium Oxide Cluster Ions [RuOx]+ (x=1-3): On the Influence of Oxidation States

Thermal gas-phase reactions of the ruthenium-oxide clusters [RuOx](+) (x=1-3) with methane and dihydrogen have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. For methane activation, as compared to the previously studied [RuO](+)/CH4 couple, the higher oxidized Ru systems give rise to completely different product distributions. [RuO2](+) brings about the generations of [Ru,O,C,H-2](+)/H2O, [Ru,O,C](+)/H-2/H2O, and [Ru,O,H-2](+)/CH2O, whereas [RuO3](+) exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H-2). Regarding the reactions with H-2, as compared to CH4, both [RuO](+) and [RuO2](+) react similarly inefficiently with oxygen-atom transfer being the main reaction channel; in contrast, [RuO3](+) is inert toward dihydrogen. Theoretical analysis reveals that the reduction of the metal center drives the overall oxidation of methane, whereas the back-bonding orbital interactions between the cluster ions and dihydrogen control the H-H bond activation. Furthermore, the reactivity patterns of [RuOx](+) (x=1-3) with CH4 and H-2 have been compared with the previously reported results of Group 8 analogues [OsOx](+)/CH4/H-2 (x=1-3) and the [FeO](+)/H-2 system. The electronic origins for their distinctly different reaction behaviors have been addressed.