Stabilization of fullerene-like boron cages by transition metal encapsulation

The stabilization of fullerene-like boron (B) cages in the free-standing form has been long sought after and a challenging problem. Studies that have been carried out for more than a decade have confirmed that the planar or quasi-planar polymorphs are energetically favored ground states over a wide range of small and medium-sized B clusters. Recently, the breakthroughs represented by Nat. Chem., 2014, 6, 727 established that the transition from planar/quasi-planar to cage-like B-n clusters occurs around n = similar to 38-40, paving the way for understanding the intriguing chemistry of B-fullerene. We herein demonstrate that the transition demarcation, n, can be significantly reduced with the help of transition metal encapsulation. We explore via extensive first-principles swarm-intelligence based structure searches the free energy landscapes of B-24 clusters doped by a series of transition metals and find that the low-lying energy regime is generally dominated by cage-like isomers. This is in sharp contrast to that of bare B-24 clusters, where the quasi-planar and rather irregular polyhedrons are prevalent. Most strikingly, a highly symmetric B cage with D-3h symmetry is discovered in the case of Mo or W encapsulation. The endohedral D-3h cages exhibit robust thermodynamic, dynamic and chemical stabilities, which can be rationalized in terms of their unique electronic structure of an 18-electron closed-shell configuration. Our results indicate that transition metal encapsulation is a feasible route for stabilizing medium-sized B cages, offering a useful roadmap for the discovery of more B fullerene analogues as building blocks of nanomaterials.