The dodeca-coordinated La©B 8 C 4 +/0/- molecular wheels: conflicting aromaticity versus 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 D 10h Ta©B 10 - and Nb©B 10 - molecular wheels. Here, we report a theoretical study of La©B 8 C 4 q ( q = +1, 0, -1) clusters with the dodeca-coordinated La atom. The La©B 8 C 4 q clusters adopt fascinating molecular wheel structures, showing a La atom enclosed by a perfect B 8 C 4 monocyclic ring. The cationic La©B 8 C 4 + cluster has a C 4v 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©B 8 C 4 cluster decreases to 0.46 Å. The La©B 8 C 4 - species turns out to be perfectly planar. Chemical bonding analyses indicate that the neutral La©B 8 C 4 and anionic La©B 8 C 4 - possess 10σ and 9π/10π double aromaticity, respectively, obeying the principle of double aromaticity. However, the cationic La©B 8 C 4 + has 10σ and 8π conflicting aromaticity, representing a counterexample in planar hyper-coordinated molecular wheels. The dodeca-coordination number in La©B 8 C 4 q ( q = +1, 0, -1) clusters is unprecedented, which provides a new idea and concept for searching planar hyper-coordinated systems.