Interconversion Between Superatomic 6-Electron and 8-Electron Configurations of M@Au24(SR)18 Clusters (M = Pd, Pt)

Kyuju Kwak, Qing Tang, Minseok Kim, De-en Jiang*, and Dongil Lee*
J. Am. Chem. Soc., 2015, 137 (33), pp 10833–10840

Abstract

The exceptional stability of thiolate-protected Au25 clusters, [Au25(SR)18], arises from the closure of superatomic electron shells, leading to a noble-gas-like 8-electron configuration (1S21P6). Here we present that replacing the core Au atom with Pd or Pt results in stable [MAu24(SR)18]0 clusters (M = Pd, Pt) having a superatomic 6-electron configuration (1S21P4). Voltammetric studies of [PdAu24(SR)18]0 and [PtAu24(SR)18]0 reveal that the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps of these clusters are 0.32 and 0.29 eV, respectively, indicating their electronic structures are drastically altered upon doping of the foreign metal. Density functional investigations confirm that the HOMO–LUMO gaps of these clusters are indeed smaller, respectively 0.33 and 0.32 eV, than that of [Au25(SR)18] (1.35 eV). Analysis of the optimized geometries for the 6-electron [MAu24(SR)18]0 clusters shows that the MAu12 core is slightly flattened to yield an oblate ellipsoid. The drastically decreased HOMO–LUMO gaps observed are therefore the result of Jahn–Teller-like distortion of the 6-electron [MAu24(SR)18]0 clusters, accompanying splitting of the 1P orbitals. These clusters become 8-electron [MAu24(SR)18]2– clusters upon electronic charging, demonstrating reversible interconversion between the 6-electron and 8-electron configurations of MAu24(SR)18.