Dinitrogen Fixation and Reduction by TaNH 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 N binding and reduction at the well-defined TaNH and TaN 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 N can be adsorbed and completely activated by TaNH and TaN clusters, yielding to the products TaNH and TaN, but the reactivity of TaNH is five times higher than that of the dehydrogenated TaN clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N activation reactions mediated by TaNH and TaN; two and three Ta atoms are active sites and also electron donors for the N reduction, respectively. Although the hydrogen atom in TaNH is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of TaNH, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N 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 N reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N fixation and activation or NH synthesis.