[Au<sub>7</sub>]<sup>3+</sup>: A Missing Link in the
Four-Electron Gold Cluster Family
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Abstract
Ligand-stabilized
ultrasmall gold clusters offer a library of diverse
geometrical and electronic structures. Among them, clusters with four
valence electrons form an exceptional but interesting family because
of their unique geometrical structures and optical properties. Here,
we report a novel diphosphine-ligated four-electron Au<sub>7</sub> cluster (<b>2</b>). In good agreement with previous theoretical
predictions, <b>2</b> has a “core+<i>one</i>” structure to exhibit a prolate shape. The absorption spectrum
showed an isolated band, similar to the spectra of Au<sub>6</sub> and
Au<sub>8</sub> clusters with “core+<i>two</i>”
structures. TD-DFT studies demonstrated that the attachment of only
one gold atom to a polyhedral core is sufficient to generate unique
electronic structures and characteristic absorptions. The present
result fills the missing link between Au<sub>6</sub> and Au<sub>8</sub> in the four-electron cluster family, showing that the HOMO–LUMO
gap increases with increasing nuclearity in the case of the tetrahedron-based
“core+<i>exo</i>” clusters