Ligand engineering for active and durable, atomically-precise coinage-metal nanocluster catalysts

Metal nanoclusters (NCs) composed of fewer than 200 atoms have emerged as novel catalysts due to their specific physicochemical properties and structural fluxionalities, which differ significantly from those of the corresponding bulk or nanoparticles. Among the various types of NCs, those composed of coinage metals (Au, Ag, Cu) have garnered particular interest due to their relatively simple electronic structures and the discovery of size-specific catalysis by Haruta and Hutchings. Given the substantial alterations in the fundamental properties of coinage-metal NCs that result from the addition or removal of even a single atom, it is imperative to synthesize them with atomic precision to establish the structure-catalysis relationship and to maximize their potential in catalysis. The protection by organic ligands, such as thiolates, phosphines, alkynyls, and N-heterocyclic carbenes, plays a pivotal role in the precise control of NC size, the enhancement of NC stability against aggregation, and the determination of their geometric structures through single-crystal X-ray diffractometry. However, the ligand-protected metal NCs typically exhibit poor catalysis, primarily due to the absence of active sites for chemical transformation. A growing body of research has demonstrated that the employment of designed ligands facilitates the circumvention of the trade-off between activity and durability in the catalytic application of ligand-protected metal NCs. The application of ligand engineering facilitates the modulation of the geometry, density, and electronic structure of active sites. Furthermore, it enables the characterization of key intermediates through a range of spectroscopic methodologies. This review article provides a synopsis of recent examples of ligand engineering, with the objective of gaining deeper insight into catalytic mechanisms on coinage-metal NCs. This article also discusses prospects and challenges for the development of superior metal NC catalysts.