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

Polyhedral boron clusters are lauded as 3D-aromatic that frequently form interconnected periodic networks 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 that is mandated by electron counting rules, whose impact on their aromatic stability and geometry remains mysterious. Understanding 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 on 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 HOMO that is decided locally on the polyhedral degree of the interacting vertices. The prevalence of π-type interactions as HOMO in tetravalent vertices encourage conjugation across clusters and turn 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 prefers 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 chemical guidance for the design and analysis of polyhedral boride networks with desired properties.