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.