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,H2 ]+ /H2 O, [Ru,O,C]+ /H2 /H2 O, and [Ru,O,H2 ]+ /CH2 O, whereas [RuO3 ]+ exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H2 ). Regarding the reactions with H2 , 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 H2 have been compared with the previously reported results of Group 8 analogues [OsOx ]+ /CH4 /H2 (x=1-3) and the [FeO]+ /H2 system. The electronic origins for their distinctly different reaction behaviors have been addressed.