Nature of Interactions between Boron Clusters: Extended Delocalization and Retention of Aromaticity Post-Oxidation

Polyhedral boron clusters are lauded as 3D aromatics that frequently form interconnected periodic networks resulting in boron-rich borides with metal and non-metals having high thermodynamic stability and hardness. This leads to the question of whether the spherical delocalization of electrons in these clusters is extended across the network as in organic aromatic networks. These borides also frequently show partial oxidation, having fewer electrons than what is mandated by electron counting rules, whose impact on their aromatic stability and geometry remains mysterious. Understanding of the nature of electronic communication between polyhedra in polyhedral borides is largely unknown, though it is crucial for the rational design of advanced materials with desirable mechanical, electronic, and optical properties. Here, we show that electronic delocalization across polyhedral clusters has a significant impact on their structure and stability. Our computational inquiry of closo borane dimers shows substantial variation in conjugation with the ideal electron count. Upon two-electron oxidation, instead of forming exohedral multiple bonding that disrupts the aromaticity, it undergoes subtle geometric transformations that conserve aromaticity. The nature of geometric transformation depends on the highest occupied molecular orbital (HOMO) that is decided locally on the polyhedral degree of the interacting vertices. The prevalence of π-type interactions as the HOMO in tetravalent vertices encourages conjugation across clusters and turns into a macropolyhedral system hosting a rhombic linkage between clusters upon oxidation. In contrast, the σ-type interactions dominate the HOMO of pentavalent vertices that prefer to confine aromaticity within the polyhedra by separating them with localized 3c-2e bonds. Our findings expose the fundamental bonding principles that govern the interaction between boron clusters and will provide the chemical guidance for the design and analysis of polyhedral boride networks with desired properties.