Anionic Copper Clusters Reacting with NO: An Open-Shell Superatom Cu<sub>18</sub><sup>–</sup>
Baoqi Yin, Qiuying Du, Lijun Geng, Hanyu Zhang, Zhixun Luo, Si Zhou, Jijun Zhao
Abstract
Gas-phase metal clusters have been a subject of research interest for allowing reliable strategies to explore the stability and reactivity of materials at reduced sizes with atomic precision. Here we have prepared well-resolved copper cluster anions Cun– (n = 7–37) and systematically studied their reactivity with O2, NO, and CO. We found remarkable stability of an open-shell cluster Cu18–, which is comparable with the closed-shell clusters Cu17– and Cu19– within the picture of an electronic shell model. Even without having a magic number of valence electrons, intriguingly, the unpaired electron on the singly occupied molecular orbital of Cu18– is mainly contributed by the central copper atom, while the other 18 delocalized valence electrons occupy the lower-energy superatomic orbitals of the cluster. The finding of such an open-shell superatom Cu18–, with an electron configuration of 1S21P61D102S1||1F0, is interesting in the sense that an elementary cluster of coinage metal atoms could still behave as a superatom mimicking coinage metals like silver or gold atoms with an empty f orbital. The superatomic stability of this Cu18– cluster is reinforced by the unique electrostatic interaction between the Cu– core and Cu17 shell, which provides new insights into the chemistry of metal clusters.