Designing a Four-Ring Tubular Boron Motif through Metal Doping.

Tubular boron clusters represent a class of extremely unusual geometries that can be regarded as a key indicator for the 2D-to-3D boron structural evolution as well as the embryos for boron nanotubes. While a good number of pure boron or metal-doped boron tubular clusters have been reported so far, most of them are two-ring tubular structures, and their higher-ring analogues are very scarce. We report herein the first example of a four-ring tubular boron motif in the cagelike global minimum of BeB. Global-minimum searches of MB and MB (M = alkali/alkaline-earth metals; = 1+, 0, 1-) reveal that the most stable structure of BeB is a -symmetric cage having a four-ring tubular boron moiety, whereas it is a high-lying isomer for those having a two/three-ring tubular boron motif for all other systems. The B framework in BeB can be viewed as consisting of two two-ring B tubular structures linked together at one side of the B rings along the high-symmetry axis and two offside B rings capped by two Be atoms. The Be-B bonding is best described as Be in an excited triplet state, forming two highly polarized covalent bonds with B in a quartet spin state. The unique ability of beryllium to make strong covalent and electrostatic interactions makes the BeB cluster stable in such an unusual geometry.