Modeling Dinitrogen Activation by Lithium: A Mechanistic Investigation of the Cleavage of N<sub>2</sub> by Stepwise Insertion into Small Lithium Clusters

Because of the inertness of molecular nitrogen, its practicable activation under mild conditions is a fundamental challenge. Nature can do it easily; chemists should be able to achieve comparable success. Lithium is exceptional among the main group elements in that it slowly reacts with N<sub>2</sub> at room temperature, leading finally to (NLi<sub>3</sub>)<sub><i>n</i></sub>, lithium nitride, a product of interest in its own right, because of its potential as a hydrogen storage medium. We explored this remarkably facile dinitrogen activation reaction by using model lithium clusters. Our extensive computations elucidate mechanisms for the ready reactions of N<sub>2</sub> with various model clusters, Li<sub>2</sub>, Li<sub>4</sub>, Li<sub>6</sub>, and Li<sub>8</sub>, leading to stepwise cleavage of the NN bond during dinitrogen reduction, N<sub>2</sub><sup>0</sup> to 2 N<sup>3−</sup>. Initial isomeric N<sub>2</sub>−Li<sub><i>n</i></sub> complexes, retaining NN triple bonds, undergo cluster insertion/reduction processes over generally low barriers. A minimum of eight lithium atoms are needed to cleave the triple bonded nitrogen completely in a highly exothermic process. Moreover, we provide an explanation for the exceptional reactivity of N<sub>2</sub> with Li, compared to the other alkali metals, e.g., Na and K. Li is a very strong reducing agent as its nitrides have the highest atomization energy, the shortest M−N bond distance, and the largest M−N charge separation as well as interaction energy. Our study delineates the general manner in which molecular nitrogen can be activated sequentially by electron transfer and bond elongation, to give a series of increasingly reduced complexes. We conclude that lithium incorporation into complexes might facilitate the development of nitrogen fixation catalysts.