How Can We Understand Au₈ Cores and Entangled Ligands of Selenolate- and Thiolate-Protected Gold Nanoclusters Au₂₄(ER)₂₀ and Au₂₀(ER)₁₆ (E = Se, S; R = Ph, Me)? A Theoretical Study

The geometries and electronic structures of selenolate-protected Au nanoclusters, Au₂₄(SeR)₂₀ and Au₂₀(SeR)₁₆, and their thiolate analogues are theoretically investigated with DFT and SCS-MP2 methods, to elucidate the electronic structure of their unusual Au₈ core and the reason why they have the unusual entangled “staple-like” chain ligands. The Au₈ core is understood to be an [Au₄]²⁺ dimer in which the [Au₄]²⁺ species has a tetrahedral geometry with a closed-shell singlet ground state. The SCS-MP2 method successfully reproduced the distance between two [Au₄]²⁺ moieties, but the DFT with various functionals failed it, suggesting that the dispersion interaction is crucial between these two [Au₄]²⁺ moieties. The SCS-MP2-calculated formation energies of these nanocluster compounds indicate that the thiolate staple-like chain ligands are more stable than the selenolate ones, but the Au₈ core more strongly coordinates with the selenolate staple-like chain ligands than with the thiolate ones. Though Au₂₀(SeR)₁₆ has not been reported yet, its formation energy is calculated to be large, suggesting that this compound can be synthesized as a stable species if the concentration of Au­(SeR) is well adjusted. The aurophilic interactions between the staple-like chain ligands and between the Au₈ core and the staple-like chain ligand play an important role for the stability of these compounds. Because of the presence of this autophilic interaction, Au₂₄(SeR)₂₀ is more stable than Au₂₀(SeR)₁₆ and the unusual entangled ligands are involved in these compounds