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A number of doped boron clusters with metals have been achieved with intriguing electronic and structural properties. In this work, we found the inverse sandwich complexes of B7M2−, B8M2, and B9M2+ (M = Zr, Hf) to be a global minimum, where the M2 motif perpendicularly embeds in the B7, B8, and twist B9 monocyclic boron rings. The interactions between M2 and boron rings, including the dp–π interactions and the electrostatic interactions originating from the charge transfer from the M to the boron rings, provide driving forces that stabilize such title complexes. The effective charge transfer gives rise to perfectly delocalized (3π + 3σ) orbitals, leading to double aromaticity of the whole species, especially when the three delocalized σ orbitals of the boron rings are perpendicularly mixed with one negligible σ and two π bonds of the M2 motifs, giving rise to less pronounced and nonclassical bonding interactions between two short-contact M atoms. [ABSTRACT FROM AUTHOR]

Electron deficiency of boron atom has led to the abundant chemical properties of boron clusters, such as intriguing structures, unique multi-center bonding and electronic properties, as well as the structural evolution from planar to three-dimensional forms. Tubular boron clusters or all-boron fullerenes serve as an important indicator for the structural evolution of boron clusters. Moreover, tubular boron clusters can be considered as precursors for boron nanotubes. However, the anionic tubular boron clusters have been theoretically identified to be less stable in comparison to the planar or quasi-planar forms, which is clearly disadvantageous for the gas-phase experimental characterization. In this study, we follow the strategy that the stability of the tubular boron clusters can be significantly enhanced by doping alkali metals, suggesting MB22− (M = Na, K) as the largest anionic boron nanotube predicted until now. The title tubular boron clusters were more stable than the quasi-planar forms because of the double aromaticity with the delocalized π + σ electrons in boron motifs as well as the optimal electrostatic interactions between the cationic M+ and anionic tubular boron motifs, originating from the strong charge transfer from the alkali metals to the boron motifs. [ABSTRACT FROM AUTHOR]