No abstract

A new class of beryllium-boron clusters, beryllo-borospherene, is described herein theoretically. When beryllium is gradually added to the B 12 motif, it undergoes drastic structural modifications. The global minimum of the Be 4 B 12 + cluster is an Archimedean beryllo-borospherene in a 2 A 1 electronic ground state, composed of four boron triangles linked at each corner, resulting in a truncated tetrahedron with four B 6 rings capped with four beryllium atoms. Beryllium forms strong bonding with the boron clusters through strong electrostatic and covalent interactions. For instance, the bonding between a beryllium atom and Be 3 B 12 unit is best described as a Be + fragment in a 2 P excited state forming a strong and polarized electron-sharing bond with Be 3 B 12 , followed by several dative interactions by employing its vacant s, p, and very high-lying d orbitals.Counterintuitively, for an s-block element, the p orbitals of beryllium are the most crucial atomic orbitals for bonding rather than s orbitals.

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 higherring analogues are very scarce. We report herein the first example of a four-ring tubular boron motif in the cagelike global minimum of Be 2 B 24 + . Globalminimum searches of MB 24 q and M 2 B 24 q (M = alkali/alkaline-earth metals; q = 1+, 0, 1−) reveal that the most stable structure of Be 2 B 24 + is a C 2v -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 24 framework in Be 2 B 24 + can be viewed as consisting of two two-ring B 12 tubular structures linked together at one side of the B 6 rings along the highsymmetry axis and two offside B 6 rings capped by two Be atoms. The Be 2 −B 24 bonding is best described as Be 2 2+ in an excited triplet state, forming two highly polarized covalent bonds with B 24 − in a quartet spin state. The unique ability of beryllium to make strong covalent and electrostatic interactions makes the Be 2 B 24 + cluster stable in such an unusual geometry.

Planar hexacoordination (ph) is only rarely reported in the literature. So far, only a few neutral and cationic molecules possessing phE (E = C, Si, B, Al, Ga) in the most stable isomer are predicted theoretically. Present electronic structure calculations report hitherto unknown anionic planar hexcoordinate beryllium and magnesium, phBe/Mg, as the most stable isomer. Global minimum searches show that the lowest energy structure of BeC6M3− (M = Al, Ga) and MgC6M3− (M = Ga, In, Tl) is the D3h symmetric phBe/Mg clusters, where beryllium/magnesium is covalently bonded with six carbon centers and M is located in a bridging position between two carbon centers. These global minimum phBe/Mg clusters are highly kinetically stable against isomerization, facilitating the experimental confirmation by photoelectron spectroscopy. Noteworthy is the fact that the phBe/Mg center is linked with carbon centers through three 7c–2e delocalized σ bonds and three 7c–2e π bonds, making the cluster double aromatic (σ + π) in nature. The bonding between the Be/Mg and outer ring moiety can be best expressed as an electron-sharing σ-bond between the s orbital of Be+/Mg+ and C6M32− followed by three dative interactions involving empty pπ and two in-plane p orbitals of Be/Mg. Furthermore, Lewis basic M centers of the title clusters can be passivated through the complexation with bulky Lewis acid, 9-boratriptycene, lowering the overall reactivity of the cluster, which can eventually open up the possibility of their large-scale syntheses.

Planar boron clusters have often been regarded as "π-analogous" to aromatic arenes because of their similar delocalized π-bonding. However, unlike arenes such as C 5 H 5 À and C 6 H 6 , boron clusters have not previously shown the ability to form sandwich complexes. In this study, we present the first sandwich complex involving beryllium and boron, B 7 Be 6 B 7 . The global minimum of this combination adopts a unique architecture having a D 6h geometry, featuring an unprecedented monocyclic Be 6 ring sandwiched between two quasi-planar B 7 motifs. The thermochemical and kinetic stability of B 7 Be 6 B 7 can be attributed to strong electrostatic and covalent interactions between the fragments. Chemical bonding analysis shows that B 7 Be 6 B 7 can be considered as a [B 7 ] 3À [Be 6 ] 6 + [B 7 ] 3À complex. Moreover, there is a significant electron delocalization within this cluster, supported by the local diatropic contributions of the B 7 and Be 6 fragments.