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 RuO_x]⁺ (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]⁺/H₂O, Ru,O,C]⁺/H₂/H₂O, and Ru,O,H₂]⁺/CH₂O, 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 RuO_x]⁺ (x=1–3) with CH₄ and H₂ have been compared with the previously reported results of Group 8 analogues OsO_x]⁺/CH₄/H₂ (x=1–3) and the FeO]⁺/H₂ system. The electronic origins for their distinctly different reaction behaviors have been addressed.