Size-Dependent Reactivity of Chromium Oxide Cluster Anions (CrO _\textbf3 ) _\textbf1-\textbf4 O ^\mathit\boldsymbol- towards Alkanes

The reactivity of chromium oxide cluster anions (CrO _3 ) _1-4 O ^- towards low carbon (C _1 - C _4 ) 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 CrO _4 ^- and Cr _2 O _7 ^- clusters could abstract a hydrogen atom from C _2 H _6 and CH _4 , respectively, Cr _3 O _10 ^- and Cr _4 O _13 ^- clusters were found to be inert towards n -C _4 H _10 within the detection limit. Theoretical results reveal that CrO _4 ^- and Cr _2 O _7 ^- clusters possess delocalized chromium-bonded oxygen radicals (Cr - O ^-\cdot ), which rationalizes the hydrogen atom abstraction reactions between (CrO _3 ) _1, 2 O ^- clusters and alkanes. However, the active sites of (CrO _3 ) _3, 4 O ^- clusters evolve to peroxide species (O _2 ^2- ), which exhibit inferior activity compared to O ^-\cdot radicals. The increase of Cr-3d orbital energy driven by the more negative charge around CrO _y unit formed via downsizing the cluster size has been proposed to account for favorable reduction of O _2 ^2- and selective generation of reactive O ^-\cdot radicals in small-sized (CrO _3 ) _1, 2 O ^- clusters. This study not only enriches the chemistry of metal-bonded O ^-\cdot radicals toward alkane activation under mild conditions, but also provides a new insight into the conversion between O _2 ^2- and O ^-\cdot radicals over metal oxides.