Diphosphine-protected ultrasmall gold nanoclusters: opened icosahedral Au₁₃ and heart-shaped Au₈ clusters

Due to distinctive quantum confinement effects, ultrasmall gold nanoparticles usually exhibit interesting electronic structure and molecular-like properties. However, the lack of atomically-precise structural information makes the understanding of them almost impossible, such as understanding the relationships between their compositions and unique properties. Herein, by reducing a diphosphine Au^I precursor (Au₂(dppm)₂Cl₂; dppm = Ph₂PCH₂PPh₂) with or without a S^2- releasing reagent, we enriched our knowledge of the members in the families of Au₁₃ and Au₈ by the structural determinations of two new dppm-protected gold nanoclusters, [Au₁₃(dppm)₆]⁵⁺ (SD/Au1) and [Au₈(dppm)₄S₂]²⁺ (SD/Au2), respectively. Within SD/Au1, the Au₁₃ kernel significantly deviates from the ideal I_h icosahedron by the elongation of three surface Au-Au bonds to ∼3.5 Å, giving it C₃ symmetry, whereas SD/Au2 has a novel heart-shaped C₂ symmetric Au₈S₂ core (central Au₄ tetrahedron + two Au₂S units) protected by four μ₂-dppm ligands in the outer shell. Of note, SD/Au1 represents a rare Au₁₃ nanocluster with an opened icosahedral geometry, and SD/Au2 shows a new edge-shared "core + 4exo" structure type that has never been observed before. The electronic structures and optical absorption spectra of these systems are correlated with time-dependent density functional theory (TDDFT) calculations. Based on the spherical jellium model, the stability of the Au₁₃ and Au₈ nanoclusters can be ascribed to 8- and 2-electron superatoms with 1S²1P⁶ and 1S² configurations, respectively. Interestingly, the cluster SD/Au2 exhibits bright yellow luminescence with an emission maximum at 591 nm that slightly hypsochromically shifts to 581 nm upon cooling to 93 K. Our findings not only enrich the family of diphosphine-protected ultrasmall gold nanoclusters, but also demonstrate the rich variations of gold kernels during the transformation from a simple Au^I precursor to Au nanoclusters.