Unveiling the Structural Evolution of Europium-Doped Boron Clusters: From Half Sandwich to a Novel 22-Coordinate Molecular Drum

In recent years, lanthanide-doped boron clusters have attracted considerable attention. However, the structural evolution and stability of medium-sized boron clusters doped with Eu atoms remain largely unknown. In this study, a combination of particle swarm optimization and density functional theory is employed to systematically investigate the structural evolution and stability of Eu-doped boron clusters, EuB_n (n = 1–24). The results show that the most stable configurations of the EuB_n clusters transitioned from planar geometries to half-sandwich molecular structures as the cluster size increased. Furthermore, for larger cluster sizes, such as n = 18, 20, and 22, the lowest-energy configurations transitioned to tubular and drum-type molecules featuring double B rings. Notably, the EuB₂₂ cluster has been identified as a twenty-two-coordinate complex, which is the highest coordination number achieved in lanthanide-doped boron-cluster chemistry. The research not only uncovers the structural evolution of Eu-doped boron clusters but also delves into their electronic characteristics, offering valuable insights into the chemistry of rare-earth-element-doped boron clusters.