“Template synthesis” of discrete metal clusters with two- or three-dimensional architectures

• Discrete transition-metal clusters can be facilely constructed by “template synthesis” • Well-designed multidentate ligands such as polyoxometalates, multidentate nitrogen- or phosphine-based ligands, or unsaturated hydrocarbons serve as effective templates. • Precise assembly of multiple metal atoms was realized via sequential insertion of metal species into Si–Si bonds of oligosilanes. • Discrete metal clusters exhibited unique reactivity and magnetic property. Discrete transition-metal clusters with metal–metal or metal–oxygen–metal bonds represent a new category of functional materials with numerous unusual properties. For instance, the availability of multiple coordination sites on account of the presence of multiple metal atoms in the metal clusters contributes to cooperative interactions, endowing such clusters with more specific or superior catalytic activity compared to mononuclear complexes or bulk metals. The properties of transition-metal clusters depend on the structural parameters of the clusters, including the number of metal atoms and the overall structural geometry of the cluster. However, conventional synthetic methods for metal clusters, e.g., self-assembly or the reductive assembly of metal species, usually offer only poor control over such structural parameters. In this review, we focus on a recently introduced strategy for the synthesis of metal clusters that can overcome the aforementioned drawbacks, namely, the “template synthesis” of metal clusters, in which well-designed template molecules such as polyoxometalates, multidentate nitrogen- or phosphine-based ligands, or unsaturated hydrocarbons serve as effective multidentate ligands to construct two- or three-dimensional metal clusters in a highly selective manner. In addition, the precise assembly of multiple metal atoms with the aid of oligosilanes through the sequential insertion of metal species into Si–Si bonds will be discussed.