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

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 Cu ( = 7-37) and systematically studied their reactivity with O, NO, and CO. We found remarkable stability of an open-shell cluster Cu, which is comparable with the closed-shell clusters Cu and Cu 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 Cu 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 Cu, with an electron configuration of 1S1P1D2S||1F, 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 Cu cluster is reinforced by the unique electrostatic interaction between the Cu core and Cu shell, which provides new insights into the chemistry of metal clusters.