Observation of Boron Carbonyl Complexes B11(CO)n+ (n = 1-6) and B15(CO)n+ (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+ (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+ (n = 1-6) and B15(CO)n+ (n = 1-5) with π and σ conflicting aromaticity analogous to benzene C6H6 and cyclooctatetraene C8H8 in π-bonding, respectively, with B15(CO)n+ 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)n+ (6π + 8σ) exhibit global 6π aromaticity and 8σ antiaromaticity, while B15(CO)n+ (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)n+ and B15(CO)n+ with conflicting aromaticity are found to possess obviously larger average coordination energies per CO than the previously observed BCAs B13(CO)n+.