How many electrons to stabilize the icosahedral Cu55 core in ligated nanoclusters? The example of [Cu55(NHC)6]

Abstract

While ligand-protected clusters with M55 icosahedral cores are rarely encountered so far, DFT calculations have been performed on a series of bare and ligated Cu55 clusters, including the structurally characterized [Cu55(IDipp)6]. Calculations indicate that the best closed-shell superatomic electron counts for such species are 48, 50, 52 and 56. None of them is a superatom “magic” number. The closest “magic” number, namely 58, which would correspond to full occupation of the 1G level, is highly disfavored. The 48, 50, 52 and 56 counts correspond to 1G partial occupations and can be rationalized from Jahn-Teller distortions away from R3 or Ih symmetry. Similar calculations on related Au55 clusters provided comparable results, with the exception of the 52-electron count, which is not favored as a closed-shell with gold. Neutral ligands such as NHCs, are expected to stabilize efficiently the closed-shell 56-electron count, providing they are able to sterically screen all the “exposed” metal atoms.