Structural diversity in B₇O₇^+/0/− clusters: emergence of a seven-membered ring and an inverted heart-like ring stabilized by 3c–2e localized bonds

Based upon global-minimum searches and first-principles electronic structure calculations, we present the global minimum (GM) structures of B₇O₇^+/0/- systems. The cationic cluster innovatively showcases an inverted heart-like ring structure. For the neutral cluster, the GM structure introduces a seven-membered ring for the first time, forming a planar double-ring framework by fusing a B₃O₂ five-membered ring with a B₄O₃ seven-membered ring. The anionic cluster's structure is similar to that of B₆O₆^-, but the inclusion of an additional BO unit leads to a three-dimensional structure. Further chemical bonding analysis indicates that the B₇O₇⁺ structure exhibits dual aromaticity, and the 3c-2e σ aromaticity of the B3 unit contributes to the enhanced stability of the overall structure. Additionally, B₇O₇ has a competing co-existing isomer. Its GM structure is predominantly characterized by 10 π aromaticity, contrasting with the isomer exhibiting 11 π/2 σ aromaticity. Similarly, the GM structure of B₇O₇^- displays a slightly lower energy due to its 6 π aromatic nature compared to the 4 π antiaromatic isomer.