Observation and mechanism of cryo N2 cleavage by a tantalum cluster

Abstract

We explore the cryogenic kinetics of N₂ adsorption to Ta₄⁺ and the infrared signatures of [Ta₄(N₂)_m]⁺ complexes, m = 1–5. This is accomplished by N₂ exposure of isolated ions within a cryogenic ion trap. We find stepwise addition of numerous N₂ molecules to the Ta₄⁺ cluster. Interestingly, the infrared signatures of the [Ta₄(N₂)₁]⁺ and [Ta₄(N₂)₂]⁺ products are special: there are no NN stretching bands. This is consistent with cleavage of the first two adsorbed dinitrogen molecules. DFT calculations reveal intermediates and barriers along reaction paths of N₂ cleavage in support of these experimental findings. We indicate the identified multidimensional path of N₂ cleavage as an across edge-above surface (AEAS) mechanism: initially end-on coordinated N₂ bends towards a neighboring Ta-atom which yields a second intermediate, with a μ₂ bonded N₂ across an edge of the Ta₄⁺ tetrahedron core. Further rearrangement above a Ta–Ta–Ta surface of the Ta₄⁺ tetrahedron results in a μ₃ bonded N₂ ligand. This intermediate relaxes swiftly by ultimate NN cleavage unfolding into the final dinitrido motif. Submerged activation barriers below the entrance channel confirm spontaneous cleavage of the first two dinitrogen molecules (−59 and −33 kJ mol⁻¹, respectively), while cleavage of the third N₂ ligand is kinetically hindered (+55 kJ mol⁻¹). We recognize that substoichiometric N₂ exposure allows for spontaneous activation by Ta₄⁺, while higher N₂ exposure causes self-poisoning.