Electronic shells of a tubular Au26 cluster: a cage–cage superatomic molecule based on spherical aromaticity

Abstract

Gold clusters, which display a variety of unusual geometric structures due to their strong relativistic effects, have attracted much attention. Among them, Au₂₆ has a high-symmetry tubular structure (D_6d) with a large HOMO–LUMO energy gap, but its electronic stability still remains unclear. In this paper, the electronic nature of the Au₂₆ cluster is investigated using the density functional theory method. Depending on the super valence bond model, the tubular Au₂₆ cluster with 26 valence electrons could be viewed as a superatomic molecule composed of two open cages based on spherical aromaticity, and its molecule-like electronic shell closure is achieved via a super triple bond (σ, 2π) between the two cages. Based on this new cage–cage superatomic structural model, a series of similar tubular clusters are predicted from the Au₂₆ skeleton. The two capped Au atoms are replaced by Cu, Ag and In atoms, respectively, to form tubular D_6d Au₂₄Cu₂ and Au₂₄Ag₂ (26e) and Au₂₄In₂ (30e) clusters, where the super triple bonds also exist. Moreover, tubular D_5d Au₂₀In₂ (26e) is obtained by replacing hexatomic Au₆ rings in the bulk of Au₂₄In₂ with pentagonal Au₅ rings. Chemical bonding analysis reveals that there is a super quintuple bond (σ, 2π, 2δ) between two open (Au₁₀In) cages, in accordance with the 26e Li₂₀Mg₃ superatomic molecule composed of two icosahedral superatoms. Our study proposes the new cage–cage structural model of superatomic molecules based on spherical aromaticity, which extends the range of the super valence bonding pattern and gives inferences for further study of superatomic clusters.