Adsorption of O₂ on Cationic Gold Clusters: Analyses on Bonding Strength and Extent of Activation^†

We explored the adsorption of O₂ on cationic gold clusters by combining cluster reaction experiments and theoretical calculations. Under a mild condition, Au₁₀⁺ can readily adsorb one O₂ molecule forming Au₁₀O₂⁺; Au₂⁺ and Au₄⁺ are low-reactive, while the argon complexes Au₂ArO₂⁺ and Au₄Ar_1,2O₂⁺ can easily form; all other sizes are inactive. Theoretical calculations indicate that the O₂ on the linear or planar structures of Au_n⁺ (n=2–7) and the planar isomers of Au₈⁺ tends to be adsorbed in an end-on manner and non-activated, and so does the O₂ on the large odd sizes of Au_n⁺ with n=9, 11, and 13 determined to be three-dimensional structures. In marked contrast, the O₂ on the three-dimensional isomer of Au₈⁺ and the large even-numbered Au_n⁺ with n=10, 12, and 14 tends to be adsorbed in a side-on manner and apparently activated. Observation of only Au_2,4,10O₂⁺, Au₂ArO₂⁺, and Au₄ArO₂⁺ in the present experiments can be well rationalized using the calculated bonding strengths and the kinetic factors.