The dodeca-coordinated La (c) B8C4+/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 (c) B-10(-) and Nb (c) B-10(-) molecular wheels. Here, we report a theoretical study of La (c) B8C4q (q = +1, 0, -1) clusters with the dodeca-coordinated La atom. The La (c) B8C4q clusters adopt fascinating molecular wheel structures, showing a La atom enclosed by a perfect B8C4 monocyclic ring. The cationic La (c) B8C4+ cluster has a C-4v symmetry with the distinctly out-of-plane distortion of the La atom (0.70 angstrom), which is gradually flattened by the sequential reduction reaction. The distortion of the La atom from the plane in the neutral La (c) B8C4 cluster decreases to 0.46 angstrom. The La (c) B8C4- species turns out to be perfectly planar. Chemical bonding analyses indicate that the neutral La (c) B8C4 and anionic La (c) B8C4- possess 10 sigma and 9 pi/10 pi double aromaticity, respectively, obeying the principle of double aromaticity. However, the cationic La (c) B8C4+ has 10 sigma and 8 pi conflicting aromaticity, representing a counterexample in planar hyper-coordinated molecular wheels. The dodeca-coordination number in La (c) B8C4q (q = +1, 0, -1) clusters is unprecedented, which provides a new idea and concept for searching planar hyper-coordinated systems.