The dodeca-coordinated La©B8C4+/0/− molecular wheels: conflicting aromaticity versus double aromaticity

Abstract

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©B₁₀⁻ and Nb©B₁₀⁻ molecular wheels. Here, we report a theoretical study of La©B₈C₄^q (q = +1, 0, −1) clusters with the dodeca-coordinated La atom. The La©B₈C₄^q clusters adopt fascinating molecular wheel structures, showing a La atom enclosed by a perfect B₈C₄ monocyclic ring. The cationic La©B₈C₄⁺ 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©B₈C₄ cluster decreases to 0.46 Å. The La©B₈C₄⁻ species turns out to be perfectly planar. Chemical bonding analyses indicate that the neutral La©B₈C₄ and anionic La©B₈C₄⁻ possess 10σ and 9π/10π double aromaticity, respectively, obeying the principle of double aromaticity. However, the cationic La©B₈C₄⁺ has 10σ and 8π conflicting aromaticity, representing a counterexample in planar hyper-coordinated molecular wheels. The dodeca-coordination number in La©B₈C₄^q (q = +1, 0, −1) clusters is unprecedented, which provides a new idea and concept for searching planar hyper-coordinated systems.