Ta₂⁺-mediated ammonia synthesis from N₂ and H₂ at ambient temperature

In a full catalytic cycle, bare Ta₂⁺ in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber-Bosch-like process starting from N₂ and H₂ at ambient temperature. This finding is the result of extensive quantum chemical calculations supported by experiments using Fourier transform ion cyclotron resonance MS. The planar Ta₂N₂⁺, consisting of a four-membered ring of alternating Ta and N atoms, proved to be a key intermediate. It is formed in a highly exothermic process either by the reaction of Ta₂⁺ with N₂ from the educt side or with two molecules of NH₃ from the product side. In the thermal reaction of Ta₂⁺ with N₂, the N≡N triple bond of dinitrogen is entirely broken. A detailed analysis of the frontier orbitals involved in the rate-determining step shows that this unexpected reaction is accomplished by the interplay of vacant and doubly occupied d-orbitals, which serve as both electron acceptors and electron donors during the cleavage of the triple bond of N≡N by the ditantalum center. The ability of Ta₂⁺ to serve as a multipurpose tool is further shown by splitting the single bond of H₂ in a less exothermic reaction as well. The insight into the microscopic mechanisms obtained may provide guidance for the rational design of polymetallic catalysts to bring about ammonia formation by the activation of molecular nitrogen and hydrogen at ambient conditions.