Anionic Copper Clusters Reacting with NO: An Open-Shell Superatom Cu18.

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.