Observation of competing nitrogen activation in metal tricarbon anions MC₃ ^- (M = Os, Ir, Pt)

Metal carbides hold significant potential as catalytic and functional materials. However, the absence of explicit directives hinders investigations of the reaction mode of metal carbide clusters. In this study, we employ size-specific photoelectron velocity-map imaging spectroscopy to explore the reactivity of metal tricarbon clusters MC₃ ^- (M = Os, Ir, Pt) in nitrogen activation. The experimental results reveal two competing modes of nitrogen activation: cleavage of the N[triple bond, length as m-dash]N bond with formation of a stable C-N bond, and chemisorption. IrC₃ ^- exhibits coexistence of dual nitrogen activation mechanisms, while OsC₃ ^- achieves nitrogen activation through cleavage of the N[triple bond, length as m-dash]N bond and PtC₃ ^- employs chemisorption-mediated activation of dinitrogen. Further theoretical analysis suggests that the activation of N₂ by MC₃ ^- (M = Os, Ir, Pt) decreases as the 5d orbital energy of the metal atoms decreases. Additionally, the chemisorption mode becomes more dominant, consistent with the experimental results. These findings are promising for advancing nitrogen activation and have important implications for the development of related single-atom catalysts with isolated metal atoms dispersed on supports.