Reactivity of Polynuclear Niobium Oxynitride Cluster Anions NbNO (x=0-5) toward N.

The activation of N₂ under mild conditions remains a significant challenge in chemistry. Understanding how the composition of ligands modulates the reactivity of metal centers is pivotal for the rational design of efficient catalysts for nitrogen fixation. Herein, the reactions between polynuclear niobium oxynitride anions NbNO (x=0-5) and N were investigated by employing mass spectrometry, photoelectron imaging spectroscopy, and theoretical calculations. The rate constants of NbNO /N gradually decrease for x=0 to x=4, and then increase again for x=5. The sharp increase of the rate constants of NbO /N corresponds to a decrease in the electron detachment energy of the NbO cluster in the photoelectron spectroscopic experiments. Theoretical calculations suggest that the low-coordinated Nb-Nb sites in NbNO (x=0-5) behaves as the active centers to bind N in the side-on/end-on manner. Mechanistic analysis reveals that reducing the N/O ratio leads to higher electron densities on the active Nb-Nb centers and decreased positive charge on the metal atoms, which hinders the approach of N to the clusters. This finding discloses fundamental insights into the impact of N/O ratio in fine-tuning the reactivity of metal centers toward N adsorption in related catalytic processes.