Toward Solution Syntheses of the Tetrahedral Au₂₀ Pyramid and Atomically Precise Gold Nanoclusters with Uncoordinated...

A long-standing objective of cluster science is to discover highly stable clusters and to use them as models for catalysts and building blocks for cluster-assembled materials. The discovery of catalytic properties of gold nanoparticles (AuNPs) has stimulated wide interests in gaseous size-selected gold clusters. Ligand-protected AuNPs have also been extensively investigated to probe their size-dependent catalytic and optical properties. However, the need to remove ligands can introduce uncertainties in both the structures and sizes of ligand-protected AuNPs for catalytic applications. Ideal model catalysts should be atomically precise AuNPs with well-defined structures and uncoordinated surface sites as in situ active centers. The tetrahedral ( T_d) Au₂₀ pyramidal cluster, discovered to be highly stable in the gas phase, provided a unique opportunity for such an ideal model system. The T_d-Au₂₀ consists of four Au(111) faces with all its atoms on the surface. Bulk synthesis of T_d-Au₂₀ with appropriate ligands would allow its catalytic and optical properties to be investigated and harnessed. The different types of its surface atoms would allow site-specific chemistry to be exploited. It was hypothesized that if the four corner atoms of T_d-Au₂₀ were coordinated by ligands the cluster would still contain 16 uncoordinated surface sites as potential in situ catalytically active centers. Phosphine ligands were deemed to be suitable for the synthesis of T_d-Au₂₀ to maintain the integrity of its pyramidal structure. Triphenyl-phosphine-protected T_d-Au₂₀ was first observed in solution, and its stability was confirmed both experimentally and theoretically. To enhance the synthetic yield, bidentate diphosphine ligands [(Ph)₂P(CH₂) _nP(Ph)₂ or L ⁿ] with different chain lengths were explored. It was hypothesized that diphosphine ligands with the right chain length might preferentially coordinate to the T_d-Au₂₀. Promising evidence was initially obtained by the formation of the undecagold by the L³ ligand. When the L⁸ diphosphine ligand was used, a remarkable Au₂₂ nanocluster with eight uncoordinated Au sites, Au₂₂(L⁸)₆, was synthesized. With a tetraphosphine-ligand (PP₃), a new Au₂₀ nanocluster, [Au₂₀(PP₃)₄]Cl₄, was isolated with high yield. The crystal structure of the new Au₂₀ core did not reveal the expected pyramid but rather an intrinsically chiral gold core. The surface of the new chiral-Au₂₀ was fully coordinated, and it was found to be highly stable chemically. The Au₂₂(L⁸)₆ nanocluster represents the first and only gold core with uncoordinated gold atoms, providing potentially eight in situ catalytically active sites. The Au₂₂ nanoclusters dispersed on oxide supports were found to catalyze CO oxidation and activate H₂ without ligand removal. With further understanding about the formation mechanisms of gold nanoclusters in solution, it is conceivable that T_d-Au₂₀ can be eventually synthesized, allowing its novel catalytic and optical properties to be explored. More excitingly, it is possible that a whole family of new atomically precise gold nanoclusters can be created with different phosphine ligands.