Theoretical Prediction of the Smallest Sized Tricyclic-Boron Oxide B₆O₆²⁺

The discovery of cyclic boron oxide clusters has prompted investigations into their distinctive structures and bonding characteristics. Notably, the majority of reported cyclic boron oxide structures consist predominantly of four to six-membered rings. In this study, we employ theoretical methods to predict the global-minimum (GM) structure of B₆O₆²⁺. Our analyses, including global-minimum searches and calculations using B3LYP, PBE, PBE0, and single-point CCSD(T), reveal that the D_2h B₆O₆²⁺ (¹A_g) configuration represents a planar and tricyclic structure, resulting from the fusion of B₃O₂/B₄O₂/B₃O₂ units. Remarkably, this structure establishes the B₆O₆²⁺ cluster as the smallest boron oxide cluster with a planar tricyclic motif. Further bonding analysis indicates that B₆O₆²⁺ is a weakly antiaromatic system with 12π delocalized electrons. The reported B₆O₆ has a planar structure with a 6-membered B₃O₃ ring and 6 π electrons distributed over the ring. Because of the absence of two electrons from the highest occupied molecular orbital (HOMO) of neutral B₆O₆, the structure of B₆O₆²⁺ is distinctly different from that of B₆O₆.