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

The consecutive discoveries of double-ring (DR) tubular D10d B20 and D2d 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 D2d 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 D2d 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 C2v B13+ (B3@B10+) with typical π-aromaticity analogous to benzene's but about ten times less reactive than both quasi-planar Cs B11+ (B2@B9+) and C2v 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.