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

Abstract

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 n-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 CrOy 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.