Modeling dinitrogen activation by lithium: a mechanistic investigation of the cleavage of N2 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(2) at room temperature, leading finally to (NLi(3))(n), 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(2) with various model clusters, Li(2), Li(4), Li(6), and Li(8), leading to stepwise cleavage of the NN bond during dinitrogen reduction, N(2)(0) to 2 N(3-). Initial isomeric N(2)-Li(n) 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(2) 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.