Size-dependent reactivity of chromium oxide cluster anions (CrO3)1-4O- towards alkanes

The reactivity of chromium oxide cluster anions (CrO3)(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 CrO4- and Cr2O7- clusters could abstract a hydrogen atom from C2H6 and CH4, respectively, Cr3O10- and Cr4O13- clusters were found to be inert towards n-C-4 H-10 within the detection limit. Theoretical results reveal that CrO4- and Cr2O7- clusters possess delocalized chromium-bonded oxygen radicals (Cr-O-center dot), which rationalizes the hydrogen atom abstraction reactions between (CrO3)(1;2) O- clusters and alkanes. However, the active sites of (CrO3)(3;4)O- clusters evolve to peroxide species (O-2(2)), which exhibit inferior activity compared to O-center dot radicals. The increase of Cr-3d orbital energy driven by the more negative charge around CrOy 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-center dot radicals in small-sized (CrO3)(1;2) O- clusters. This study not only enriches the chemistry of metal-bonded O-center dot radicals toward alkane activation under mild conditions, but also provides a new insight into the conversion between O-2(2-) and O-center dot radicals over metal oxides.