Large B7 Triangles in Hollow Spherical Trihedral Metallo-borospherenes and Their Endohedral Complexes of B20TMn (TM = Sc, Y; n = 3, 4): a Theoretical Characterization

The spherical trihedral metallo-borospherene D_3h B₁₈Ln₃^- (Ln = La, Tb), in which both Ln and B atoms are integral parts of the cage surface, represents a brand new geometric structure. On the basis of particle swarm optimization searches and density functional theory calculations, a large spherical trihedral metallo-borospherene D_3h B₂₀TM₃ (TM = Sc, Y) is predicted herein, of which the B₂₀ framework can be viewed as two equivalent B₇ triangles, rather than two routine equivalent B₆ triangles in B₁₈Ln₃^-, connected by three B₂ units. More interestingly, an additional TM can be embedded into the center of surface cage, first designing the co-existence of empty and endohedral spherical trihedral metallo-borospherene, of which the stability is further enhanced and the energy gaps of 1.25 and 1.20 eV are enlarged to 2.86 and 2.61 eV at the PBE0 level for Sc and Y as dopants, respectively. Detailed orbital and bonding analyses show that B₂₀TM₄ resembles B₁₈Ln₃^-, except for the six missing 2c-2e B-B σ bonds, the 10 more 3c-2e B-B σ bonds, and the lack of one delocalized π bonds for the B₂₀ skeleton, where the absence of two totally delocalized Sc-B bonds lead to the relative instability of B₂₀Sc₃. The novel hollow geometric structure can be expanded to their charged states with one or three negative charges and to the systems doped by three Ti, Zr, or Hf atoms. Their infrared and Raman spectra are theoretically simulated to facilitate the experimental characterizations in future. Our findings enrich the family of trihedral metallo-borospherene and provide a B₇ triangular motif for the B-framework.