Observation of double-ring tubular B₂₀(CO)_<i>n</i>⁺ (<i>n</i> = 1–8): emergence of 2D-to-3D transition in boron carbonyl complexes

The consecutive discoveries of double-ring (DR) tubular <i>D</i>_10d B₂₀ and <i>D</i>_2d B₂₀⁺ as embryos of single-walled boron nanotubes have attracted considerable attention in the past two decades. Joint chemisorption experiments and first-principles theory investigations performed herein indicate that, as the only isomer of the monocation observed in gas-phase experiments, DR tubular <i>D</i>_2d B₂₀⁺ can react with CO successively under ambient conditions to form a series of DR tubular boron carbonyl monocations B₂₀(CO)_<i>n</i>⁺ up to <i>n</i> = 8, presenting the largest boron carbonyl complexes observed to date, which mark the 2D-to-3D transition in boron carbonyl complexes. DR tubular <i>D</i>_2d B₂₀⁺ with twenty peripheral boron atoms is found to be about ten times more reactive to chemisorb the first CO than the experimentally known quasi-planar <i>C</i>_2v B₁₃⁺ (B₃@B₁₀⁺) with typical π-aromaticity analogous to benzene's but about ten times less reactive than both quasi-planar <i>C</i>_s B₁₁⁺ (B₂@B₉⁺) and <i>C</i>_2v B₁₅⁺ (B₄@B₁₁⁺) with <i>σ</i> and π conflicting aromaticity. Extensive theoretical calculations and analyses unveil the chemisorption pathways, potential energy profiles, and chemical bonding patterns of DR tubular B₂₀(CO)_<i>n</i>⁺ and its neutral counterpart B₂₀(CO)_<i>n</i>, both of which appear to be tubularly aromatic in nature.