Aromaticity of square planar N42- in the M2N4 (M = Li, Na, K, Rb, or Cs) species

A series of alkali metal-N-4(2-) compounds M2N4 (M = Li, Na, K, Rb, or Cs) have been examined with ab initio and density functional theory (DFT) methods. To our knowledge, these compounds, except for the D-4h bipyramidal Li2N4, are first reported here. The bipyramidal structures with two metal cations above and below the N-4(2-) plane are global minima for all five M2N4 systems. Kinetic analysis shows that the bipyramidal M2N4 species may exist or be characterized due to their significant isomerization or dissociation barriers (39.2-48.6 kcal/mol). Nucleus-independent chemical shifts (NICSs) criteria and the presence of six delocalized pi-electrons (thus following the 4n + 2 rule) confirmed that the square planar N-4(2-) exhibits characteristics of aromaticity for these M2N4 species.