A homoleptic alkynyl-protected [Ag9Cu6(tBuCC)12]+ superatom with free electrons: synthesis, structure analysis, and different properties compared with the Au7Ag8 cluster in the M15+ series

Abstract

We report the first homoleptic alkynyl-protected AgCu superatomic nanocluster [Ag₉Cu₆(^tBuCC)₁₂]⁺ (NC 1, also Ag₉Cu₆ in short), which has a body-centered-cubic structure with a Ag₁@Ag₈@Cu₆ metal core. Such a configuration is reminiscent of the reported AuAg bimetallic nanocluster [Au₁@Ag₈@Au₆(^tBuCC)₁₂]⁺ (NC 2, also Au₇Ag₈ in short), which is also synthesized by an anti-galvanic reaction (AGR) approach with a very high yield for the first time in this study. Despite a similar Ag₈ cube for both NCs, structural anatomy reveals that there are some subtle differences between NCs 1 and 2. Such differences, plus the different M₁ kernel and M₆ octahedron, lead to significantly different optical absorbance features for NCs 1 and 2. Density functional theory calculations revealed the LUMO and HOMO energy levels of NCs 1 and 2, where the characteristic absorbance peaks can be correlated with the discrete molecular orbital transitions. Finally, the stability of NCs 1 and 2 at different temperatures, in the presence of an oxidant or Lewis base, was investigated. This study not only enriches the M₁₅⁺ series, but also sets an example for correlating the structure–property relationship in alkynyl-protected bimetallic superatomic clusters.