Dinitrogen Fixation and Reduction by Ta₃N₃H_0,1^- Cluster Anions at Room Temperature: Hydrogen-Assisted Enhancement of Reactivity

Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N₂ binding and reduction at the well-defined Ta₃N₃H^- and Ta₃N₃^- gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N₂ can be adsorbed and completely activated by Ta₃N₃H^- and Ta₃N₃^- clusters, yielding to the products Ta₃N₅H^- and Ta₃N₅^-, but the reactivity of Ta₃N₃H^- is five times higher than that of the dehydrogenated Ta₃N₃^- clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N₂ activation reactions mediated by Ta₃N₃H^- and Ta₃N₃^- two and three Ta atoms are active sites and also electron donors for the N₂ reduction, respectively. Although the hydrogen atom in Ta₃N₃H^- is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of Ta₃N₃H^-, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N₂ activation and the important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N₂ reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N₂ fixation and activation or NH₃ synthesis.