BxGe120/+ Clusters with x=1-4: Germanium Tubes Stabilized by Three and Four Boron Dopants

Some boron-doped germanium clusters BxGe12q with x = 1, 2, 3, and 4 and q = 0, 1 were designed as stabilized double ring tubes. While the B2Ge12 constitutes the smallest deltahedral germanium cluster, both B3Ge12+ and B4Ge12 clusters present us, for the first time, with an endohedral tubular motif in which either the B-3 or the B-4 cycle is encapsulated inside a Ge-12 hexagonal prism tube. Both B-3 and B-4 units thus satisfy a geometry requirement to create an endohedral structure within the Ge-12 double ring. Keeping their high symmetry, both B-3 and B-4 units generate delocalized bonds upon interaction with the Ge-12 tubular framework and thereby induce an aromatic character for the resulting B3Ge12+ and B4Ge12, respectively. Their aromaticity was probed by the magnetic responses of electron densities. Such a tubular aromaticity appears to greatly contribute to the high thermodynamic stability of the binary hexagonal germanium tubes.