chengliang2009

Gas-phase clusters are deemed to be σ-aromatic when they satisfy the 4n+2 rule of aromaticity for delocalized σ electrons and fulfill other requirements known for aromatic systems. While the range of n values was shown to be quite broad when applied to short-lived clusters found in molecular-beam experiments, stability of all-metal cluster-like fragments isolated in condensed phase was previously shown to be mainly ascribed to two electrons (n=0). In this work, the applicability of this concept is extended towards solid-state compounds by demonstrating a unique example of a storable compound, which was isolated as a stable [K([2.2.2]crypt)]⁺ salt, featuring a [Au₂Sb₁₆]⁴⁻ cluster core possessing two all-metal aromatic AuSb₄ fragments with six delocalized σ electrons each (n=1). This discovery pushes the boundaries of the original idea of Kekulé and firmly establishes the usefulness of the σ-aromaticity concept as a general idea for both small clusters and solid-state compounds.

A very unique [Au₂Sb₁₆]⁴⁻ complex was isolated as a stable salt and characterized by single-crystal X-ray diffraction. Its rod-like Sb₁₆ framework is the largest unfragmented Sb_n polyanion isolated from solution, which was possible due to the σ-aromatic stabilization through quenching with two Au atoms. The [Au₂Sb₁₆]⁴⁻ anion is the first solid-state all-metal σ-aromatic cluster with two 6-σ-electron aromatic fragments.