Observation of boron carbonyl complexes B₁₁(CO)⁺ (n = 1-6) and B₁₅(CO)⁺ (n = 1-5) with conflicting aromaticity

Boron carbonyl complexes have received considerable attention in recent years due to their unique structures and bonding. With inspiration from the newly observed first boron carbonyl aromatics (BCAs) B₁₃(CO)⁺ (n = 1-7) and based on joint chemisorption experiments and first-principles theory investigations, we report herein observation of the first boron carbonyl complexes B₁₁(CO)⁺ (n = 1-6) and B₁₅(CO)⁺ (n = 1-5) with π and σ conflicting aromaticity analogous to benzene C₆H₆ and cyclooctatetraene C₈H₈ in π-bonding, respectively, with B₁₅(CO)⁺ being the largest boron carbonyl complexes observed to date, enriching the structures and bonding of boron carbonyls effectively. B₁₁⁺ and B₁₅⁺ which can chemisorb up to six and five CO molecules under ambient conditions, respectively, are found to be much more reactive towards the first CO than the magic-number aromatic B₁₃⁺. Extensive theoretical analyses unveil both the chemisorption pathways and potential energy profiles of these interesting species. Detailed bonding pattern analyses show that quasi-planar B₁₁(CO)⁺ (6π + 8σ) exhibit global 6π aromaticity and 8σ antiaromaticity, while B₁₅(CO)⁺ (8π + 10σ + 4σ) possess global 8π antiaromaticity and 10σ aromaticity, as well as local 4σ antiaromaticity for the B₄ core, inheriting the delocalized bonding patterns of their parent B₁₁⁺ (B₂@B₉⁺) and B₁₅⁺ (B₄@B₁₁⁺) monocations, respectively. Both naked B₁₁⁺ and B₁₅⁺ appear to have much higher chemisorption reaction rates toward CO than B₁₃⁺, while their carbonyl complexes B₁₁(CO)⁺ and B₁₅(CO)⁺ with conflicting aromaticity are found to possess obviously larger average coordination energies per CO than the previously observed BCAs B₁₃(CO)⁺.