A homoleptic alkynyl-protected [Ag(9)Cu(6)((t)BuC[triple bond, length as m-dash]C)(12)](+) superatom with free electrons: synthesis, structure analysis, and different properties compared with the Au(7)Ag(8) cluster in the M(15)(+) series

We report the first homoleptic alkynyl-protected AgCu superatomic nanocluster [Ag(9)Cu(6)((t)BuC[triple bond, length as m-dash]C)(12)](+) (NC 1, also Ag(9)Cu(6) in short), which has a body-centered-cubic structure with a Ag(1)@Ag(8)@Cu(6) metal core. Such a configuration is reminiscent of the reported AuAg bimetallic nanocluster [Au(1)@Ag(8)@Au(6)((t)BuC[triple bond, length as m-dash]C)(12)](+) (NC 2, also Au(7)Ag(8) 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 Ag(8) cube for both NCs, structural anatomy reveals that there are some subtle differences between NCs 1 and 2. Such differences, plus the different M(1) kernel and M(6) 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 M(15)(+) series, but also sets an example for correlating the structure–property relationship in alkynyl-protected bimetallic superatomic clusters.