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

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.