CaB11−: A unique four-ring conjugated dual aromatic system

Abstract

At the B3LYP/6–311G(d) level, we obtained the ground state structures of CaB_n^0/− (n = 2–12). Observing the evolution characteristics of these structures, we found that the transition from two-dimensional to three-dimensional structures occurs when the number of boron atoms equals 6 (for the neutral system) and 7 (for the anionic system). For the three-dimensional structure, the Ca atom is always positioned above the plane or quasi-plane formed by the boron atoms (at the vertex position), similar to an umbrella-like structure. In most cases, the neutral and anionic structures with the same number of boron atoms are consistent. The only exceptions are CaB₆⁻ and CaB₁₁⁻, where the neutral structure is three-dimensional, but upon adding an electron, they transform into a two-dimensional planar structure. This peculiarity persists in the stability analysis, and according to the results, the CaB₁₁⁻ structure with C_2V symmetry is the most stable. A detailed analysis of the bonding situation in CaB₁₁⁻ reveals that it is a planar structure with four conjugated rings, where adjacent atoms in each ring are connected by covalent bonds, B-B bonds are in the form of covalent single bonds, and Ca-B bonds are in the form of covalent half-bonds. The charge on the Ca atom unexpectedly transfers to the neighboring boron atoms; however, the valence electrons on Ca do not completely transfer, resulting in a + 2 charge, but rather retain some valence electrons, which interact with the valence electrons of the adjacent boron atoms to form half-bonds. The main interaction between Ca and B is still ionic bonding. All four rings are aromatic, with the central ring having the strongest aromaticity. The entire CaB₁₁⁻ exhibits dual aromaticity of σ/π types.