Observation and mechanism of cryo N₂ cleavage by a tantalum cluster

We explore the cryogenic kinetics of N-2 adsorption to Ta-4(+) and the infrared signatures of [Ta-4(N-2)(m)](+) complexes, m = 1-5. This is accomplished by N-2 exposure of isolated ions within a cryogenic ion trap. We find stepwise addition of numerous N-2 molecules to the Ta-4(+) cluster. Interestingly, the infrared signatures of the [Ta-4(N-2)(1)](+) and [Ta-4(N-2)(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 N-2 cleavage in support of these experimental findings. We indicate the identified multidimensional path of N-2 cleavage as an across edge-above surface (AEAS) mechanism: initially end-on coordinated N-2 bends towards a neighboring Ta-atom which yields a second intermediate, with a mu(2) bonded N-2 across an edge of the Ta-4(+) tetrahedron core. Further rearrangement above a Ta-Ta-Ta surface of the Ta-4(+) tetrahedron results in a mu(3) bonded N-2 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 N-2 ligand is kinetically hindered (+55 kJ mol(-1)). We recognize that substoichiometric N-2 exposure allows for spontaneous activation by Ta-4(+), while higher N-2 exposure causes self-poisoning.