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 Be2B24+. Global-minimum searches of MB24q and M2B24q (M = alkali/alkaline-earth metals; q = 1+, 0, 1-) reveal that the most stable structure of Be2B24+ is a C2v-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 B24 framework in Be2B24+ can be viewed as consisting of two two-ring B12 tubular structures linked together at one side of the B6 rings along the high-symmetry axis and two offside B6 rings capped by two Be atoms. The Be2-B24 bonding is best described as Be22+ in an excited triplet state, forming two highly polarized covalent bonds with B24- in a quartet spin state. The unique ability of beryllium to make strong covalent and electrostatic interactions makes the Be2B24+ cluster stable in such an unusual geometry.