Electronic Structure and Optical Properties of the Intrinsically Chiral 16-Electron Superatom Complex [Au₂₀(PP₃)₄]⁴⁺

Abstract

The recently solved crystal structure of the [Au₂₀(PP₃)₄]­Cl₄ cluster (PP₃: tris­(2-(diphenylphophino)­ethyl)­phosphine) is examined using density functional theory (DFT). The Au₂₀ core of the cluster is intrinsically chiral by the arrangement of the Au atoms. This is in contrast to the chirality of thiolate-protected gold clusters, in which the protecting Au-thiolate units are arranged in chiral patterns on achiral cores. We interpret the electronic structure of the [Au₂₀(PP₃)₄]­Cl₄ cluster in terms of the superatom complex model. The 16-electron cluster cannot be interpreted as a dimer of 8-electron clusters (which are magic). Instead, a superatomic electron configuration of 1S² 1P⁶ 1D⁶ 2S² is found. The 2S band is strongly stabilized, and the 1D states are nondegenerate with a large gap. Ligand protection of the (Au₂₀)⁴⁺ core leads to a significant increase of the HL-gap and thus stabilization. We also tested a charge of +II, which would give rise to an 18-electron superatom complex. Our results indicate that the 16-electron cluster is indeed more stable. We also investigate the optical properties of the cluster. The experimental absorption spectrum is well-reproduced by time-dependent DFT. Prominent transitions are analyzed by time-dependent density-functional perturbation theory. The intrinsic chirality of the cluster is compared to that of Au₃₈(SR)₂₄. We observe that the chiral arrangement of the protecting Au-SR units in Au₃₈(SR)₂₄ has very strong influence on the strength of the CD spectra, whereas phosphine protection in the title compound does not