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

Abstract

The consecutive discoveries of double-ring (DR) tubular D_10d B₂₀ and D_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 D_2d B₂₀⁺ can react with CO successively under ambient conditions to form a series of DR tubular boron carbonyl monocations B₂₀(CO)_n⁺ up to n = 8, presenting the largest boron carbonyl complexes observed to date, which mark the 2D-to-3D transition in boron carbonyl complexes. DR tubular D_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 C_2v B₁₃⁺ (B₃@B₁₀⁺) with typical π-aromaticity analogous to benzene's but about ten times less reactive than both quasi-planar C_s B₁₁⁺ (B₂@B₉⁺) and C_2v B₁₅⁺ (B₄@B₁₁⁺) with σ 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)_n⁺ and its neutral counterpart B₂₀(CO)_n, both of which appear to be tubularly aromatic in nature.