Cage π-bond: A New Non-Covalent Interaction between Boron Clusters and Aromatic Rings and Its Applications

Abstract

Exploration and comprehension of new chemical bonding is one of the central tasks in chemistry. To date, non-covalent interac-tions based on sigma- and π-bonding molecules such as hydrogen bonds and π···π interactions have been extensively investigated. However, the research on chemical bonding involving multicenter multielectron skeletons like boron clusters is much less report-ed. Here, a new type of non-covalent interaction, nido-cage···π bond, is discovered based on the boron cluster C2B9H12- and an aromatic π system. The X-ray diffraction studies indicate that the nido-cage···π bonding presents the same parallel-displaced or T-shaped geometries as a π···π interaction does. The contacting distance between the cage and the pi ring varies with the type and the substituent of the aromatic ring. The quantum chemical calculations reveal that this nido-cage···π non-covalent interaction shares a similar nature to the conventional anion···π or π···π bond found in classical aromatic ring systems. Besides, effective electronic communication between the boron cluster and the π unit is observed through the nido-cage···π interaction, which leads to unique photophysical properties such as both aggregation-induced emission in the amorphous state and aggregation-caused quenching in the crystalline state in one molecule. This demonstrates distinct emission behaviors from the conventional organic luminogens. The present work not only offers an overall understanding on this new non-covalent interaction, but also opens a door to investigate its properties and further applications