Ligand-protected gold clusters: the structure, synthesis and applications

Modern concepts of the structure and properties of atomic gold clusters protected by thiolate, selenolate, phosphine and phenylacetylene ligands are analyzed. Within the framework of the superatom theory, the 'divide and protect' approach and the structure rule, the stability and composition of a cluster are determined by the structure of the cluster core, the type of ligands and the total number of valence electrons. Methods of selective synthesis of gold clusters in solution and on the surface of inorganic composites based, in particular, on the reaction of $\mathrm{Au}_n$ with $\mathrm{RS}$, $\mathrm{RSe}$, $\mathrm{PhC\equiv C}$, $\mathrm{Hal}$ ligands or functional groups of proteins, on stabilization of clusters in cavities of the $\alpha$-, $\beta$ and $\gamma$-cyclodextrin molecules ($\mathrm{Au}_{15}$ and $\mathrm{Au}_{25}$) and on anchorage to a support surface ($\mathrm{Au_{25}/SiO}_2$, $\mathrm{Au_{20}/C}$, $\mathrm{Au_{10}/FeO}_x$) are reviewed. Problems in this field are also discussed. Among the methods for cluster structure prediction, particular attention is given to the theoretical approaches based on the density functional theory (DFT). The structures of a number of synthesized clusters are described using the results obtained by X-ray diffraction analysis and DFT calculations. A possible mechanism of formation of the $\mathrm{SR(AuSR})_n$ 'staple' units in the cluster shell is proposed. The structure and properties of bimetallic clusters $\mathrm M_x\mathrm{Au}_n\mathrm L_m$ ($\mathrm{M=Pd}$, $\mathrm{Pt}$, $\mathrm{Ag}$, $\mathrm{Cu}$) are discussed. The $\mathrm{Pd}$ or $\mathrm{Pt}$ atom is located at the centre of the cluster, whereas $\mathrm{Ag}$ and $\mathrm{Cu}$ atoms form bimetallic compounds in which the heteroatom is located on the surface of the cluster core or in the 'staple' units. The optical properties, fluorescence and luminescence of ligand-protected gold clusters originate from the quantum effects of the $\mathrm{Au}$ atoms in the cluster core and in the oligomeric $\mathrm{SR(AuSR})_x$ units in the cluster shell. Homogeneous and heterogeneous reactions catalyzed by atomic gold clusters are discussed in the context of the reaction mechanism and the nature of the active sites.
The bibliography includes 345 references.