Boron Oxide B₅O₆ ^- Cluster as a Boronyl-Based Inorganic Analog of Phenolate Anion

Boron oxide clusters have structural richness and exotic chemical bonding. We report a quantum chemical study on the binary B₅O₆ ^- cluster, which is relatively oxygen-rich. A global structural search reveals planar C _2v (¹A₁) geometry as the global minimum structure, featuring a heteroatomic hexagonal B₃O₃ ring as its core. The three unsaturated B sites are terminated by two boronyl (BO) groups and an O^- ligand. The B₅O₆ ^- cluster can be faithfully formulated as B₃O₃(BO)₂O^-. This structure is in stark contrast to that of its predecessors, C _s B₅O₅ ^- and T _d B₅O₄ ^-, both of which have a tetrahedral B center. Thus, there exists a major structural transformation in B₅O _n ^- series upon oxidation, indicating intriguing competition between tetrahedral and heterocyclic structures. The chemical bonding analyses show weak 6π aromaticity in the B₅O₆ ^- cluster, rendering it a boronyl analog of phenolate anion (C₆H₅O^-) or boronyl boroxine. The calculated vertical detachment energy of B₅O₆ ^- cluster is 5.26 eV at PBE0, which greatly surpasses the electron affinities of halogens (Cl: 3.61 eV), suggesting that the cluster belongs to superhalogen anions.