Negative Electron Binding Energy of the Au₁₃ Core in Au₂₅(SC₆H₄CO₂H)₁₈ Achieved by Ligand Deprotonations

We elucidated how the electronic structure of [Au₂₅(<i>p</i>MBA)₁₈-<i>n</i>H⁺]^{(<i>n</i>+1)-} (<i>p</i>MBA = 4-SC₆H₄CO₂H and <i>n</i> = 0-4), with a superatomic Au₁₃(8e) core, changes with the number of deprotonated <i>p</i>MBA ligands (<i>n</i>) using anion photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The photoelectron (PE) spectrum of non-deprotonated [Au₂₅(<i>p</i>MBA)₁₈]^- exhibited a band with the lowest electron binding energy (EBE) of 4.32 eV. This band was assigned to the electron detachment from 1P orbitals delocalized over the Au₁₃ core. For <i>n</i> = 1, a weak PE band originating from molecular orbitals localized in the CO₂^- moiety of a <i>p</i>MBA ligand appeared in the EBE region lower than the band originating from the 1P orbital. The EBE of the 1P orbitals of the Au₁₃ core decreased almost linearly with <i>n</i>, from 4.32 eV for <i>n</i> = 0 to -0.84 eV for <i>n</i> = 4. This remarkable reduction in the EBE by more than 5 eV was attributed to electrostatic repulsion from the localized negative charge at the CO₂^- group(s).