Reaction of Ta3 - Clusters with Molecular Nitrogen: A Mechanism Investigation

Abstract

Because of the inertness of molecular nitrogen, its practicable activation under mild conditions is a fundamental challenge. Ta₃ ^- is an exceptionally small cluster that reacts with N₂ at room temperature, leading finally to Ta₃N₂ ^-; Ta₃N₂ ^- also could react with N₂ at room temperature, leading finally to Ta₃N₄ ^-, a product of interest in its own right because of its potential as a nitrogen fixation medium. The mechanisms of the Ta₃ ^-- and Ta₃N₂ ^--mediated activation of the N≡N triple bond have been investigated. Our extensive computations elucidate mechanisms for the ready reactions, leading to stepwise cleavage of the N≡N bond. Initial isomeric N₂/Ta₃ ^- complexes, N≡N elongation, undergo a N≡N split over generally low barriers in a highly exothermic process. The nitrogen-atom or molecular exchange reactions found in the Ta₃N₂ ^-/N₂ system may be of paramount importance in both applied and fundamental studies.

Scheme 1. Orbital Interactions between N₂ and Transition Metal (M)

Scheme 2. Possible Modes of N₂ Coordination to Transition Metal

Figure 1

Simplified PES (ΔH_298K in kJ mol^–1) for Ta₃^–/N₂ coupling at the B3LYP/def2-TZVP + ECP level of theory. Key structures including selected geometric parameters are also provided. Bond lengths are given in angstrom and relative energies are given with respect to the entrance asymptote. ΔG (kJ mol^–1) is behind the vertical line.

Figure 2

Simplified PES (ΔH_298K in kJ mol^–1) of Ta₃N₂^–/N₂ coupling at the B3LYP/def2-TZVP + ECP level of theory. Key structures including selected geometric parameters are also provided. Bond lengths are given in angstrom and relative energies are given with respect to the entrance asymptote. ΔG (kJ mol^–1) is behind the vertical line.

Figure 3

Change of relative Gibbs free energy (ΔG, kJ mol^–1) for ³TS1 with temperature (K) at a pressure of 1 atm using B3LYP/def2-TZVP + ECP.