Observation and mechanism of cryo N₂ cleavage by a tantalum cluster

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.