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

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 (D6d) 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 D6d Au24Cu2 and Au24Ag2 (26e) and Au24In2 (30e) clusters, where the super triple bonds also exist. Moreover, tubular D5d 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.