Size-Dependent Reactivity of Chromium Oxide Cluster Anions (CrO₃)₁₋₄O⁻ towards Alkanes

The reactivity of chromium oxide cluster anions (CrO₃)₁₋₄O⁻ towards low carbon (C₁−C₄) alkanes has been experimentally investigated at 298 K by employing a homemade ship-lock-type reactor coupled with a time-of-flight mass spectrometer. The results demonstrate that while CrO₄⁻ and Cr₂O₇⁻ clusters could abstract a hydrogen atom from C₂H₆ and CH₄, respectively, Cr₃O₁₀⁻ and Cr₄O₁₃⁻ clusters were found to be inert towards <i>n</i>-C₄H₁₀ within the detection limit. Theoretical results reveal that CrO₄⁻ and Cr₂O₇⁻ clusters possess delocalized chromium-bonded oxygen radicals (Cr−O^−•), which rationalizes the hydrogen atom abstraction reactions between (CrO₃)_1,2O⁻ clusters and alkanes. However, the active sites of (CrO₃)_3,4O⁻ clusters evolve to peroxide species (O₂²⁻), which exhibit inferior activity compared to O^−• radicals. The increase of Cr-3d orbital energy driven by the more negative charge around CrO<i>_y</i> unit formed via downsizing the cluster size has been proposed to account for favorable reduction of O₂²⁻ and selective generation of reactive O^−• radicals in small-sized (CrO₃)_1,2O⁻ clusters. This study not only enriches the chemistry of metal-bonded O^−• radicals toward alkane activation under mild conditions, but also provides a new insight into the conversion between O₂²⁻ and O^−• radicals over metal oxides.