Royal crown-shaped electride Li₃-N₃-Be containing two superatoms: New knowledge on aromaticity

The structure and aromaticity of a royal crown-shaped molecule Li,-N3-Be are studied at the CCSD(T)/ aug-cc-pVDZ level. This molecule is a charge-separated system and can be denoted as Li₃²⁺N ₃^3-Be⁺. It is found that the Li ₃²⁺ ring exhibits aromaticity mainly because the Li ₃²⁺ ring can share the π-electron with the N ₃^-3 ring. The 4n+2 electron counter rule can be satisfied for the Li₃²⁺ subunit if the shared IT valence electron of N₃^3- subunit is also taken into account. This new knowledge on aromaticity of a ring from the interactions between subunits is revealed first time in this paper. Li₃-N₃-Be can be also regarded as a molecule containing two superatoms (Li₃ and N ₃), which may be named as a "superomolecule." Li ₃-N₃-Be is a new metal-nonmetal-metal type sandwich complex. The N₃^3- trianion in the middle repulses the electron clouds of the two metal subunits (mainly to the Li₃ superatom) to generate an excess electron, and thus Li₃-N ₃-Be is also an electride. This phenomenon of the repulsion results in: (a) the HOMO energy level increased, (b) the electron cloud in HOMO distended, (c) the area of the negative NICS value extended, and (d) the VIE value lowered. So the superomolecule Li₃-N₃-Be is not only a new metal-nonmetal-metal type sandwich complex but also a new type electride, which comes from the interaction between the alkali superatom (Li₃) and the nonmetal superatom (N₃).