The dodeca-coordinated La©BC molecular wheels: conflicting aromaticity double aromaticity.

The transition-metal centered boron molecular wheels have attracted the attention of chemists. The highest deca-coordination number for central metal atoms was observed in Ta©B and Nb©B molecular wheels. Here, we report a theoretical study of La©BC ( = +1, 0, -1) clusters with the dodeca-coordinated La atom. The La©BC clusters adopt fascinating molecular wheel structures, showing a La atom enclosed by a perfect BC monocyclic ring. The cationic La©BC cluster has a symmetry with the distinctly out-of-plane distortion of the La atom (0.70 Å), which is gradually flattened by the sequential reduction reaction. The distortion of the La atom from the plane in the neutral La©BC cluster decreases to 0.46 Å. The La©BC species turns out to be perfectly planar. Chemical bonding analyses indicate that the neutral La©BC and anionic La©BC possess 10σ and 9π/10π double aromaticity, respectively, obeying the principle of double aromaticity. However, the cationic La©BC has 10σ and 8π conflicting aromaticity, representing a counterexample in planar hyper-coordinated molecular wheels. The dodeca-coordination number in La©BC ( = +1, 0, -1) clusters is unprecedented, which provides a new idea and concept for searching planar hyper-coordinated systems.