Spherical trihedral metallo-borospherenes

Abstract

The discovery of borospherenes unveiled the capacity of boron to form fullerene-like cage structures. While fullerenes are known to entrap metal atoms to form endohedral metallofullerenes, few metal atoms have been observed to be part of the fullerene cages. Here we report the observation of a class of remarkable metallo-borospherenes, where metal atoms are integral parts of the cage surface. We have produced La₃B₁₈^- and Tb₃B₁₈^- and probed their structures and bonding using photoelectron spectroscopy and theoretical calculations. Global minimum searches revealed that the most stable structures of Ln₃B₁₈^- are hollow cages with D_3h symmetry. The B₁₈-framework in the Ln₃B₁₈^- cages can be viewed as consisting of two triangular B₆ motifs connected by three B₂ units, forming three shared B₁₀ rings which are coordinated to the three Ln atoms on the cage surface. These metallo-borospherenes represent a new class of unusual geometry that has not been observed in chemistry heretofore.

Fig. 1. Photoelectron spectrum of La₃B₁₈^–.

a At 193 nm. b The simulated spectrum.

Fig. 2. Photoelectron spectrum of Tb₃B₁₈^–.

a At 193 nm. b The simulated spectrum.

Fig. 3. The global minimum structure of La₃B₁₈^– (D_3h, ¹A₁) at the PBE0/TZP level.

a The C₃ axis is along the page vertically. b The C₃ axis is perpendicular to the page.

Fig. 4. The Kahn–Sham molecular orbital correlation diagram for La₃B₁₈^– (D_3h, ¹A₁).

It shows the interactions between the 5d orbitals of the three La atoms and the group orbitals of the B₁₈ moiety.

Fig. 5. Schematic pathways for the formation of the D_3h Ln₃B₁₈^– metallo-borospherenes.

Two pathways for the construction of the B₁₈ framework and its bonding with the three Ln atoms are depicted. a The fused B₁₀ ring pathway. b The B₂-linked triangular B₆ pathway.

Fig. 6. Chemical bonding analyses of La₃B₁₈^– (D_3h¹A₁).

The analyses were done using the AdNDP method. ON stands for occupation number.