Heterogeneous catalysis involves solid-state catalysts, among which metal nanoparticles occupy an important position. Unfortunately, no two nanoparticles from conventional synthesis are the same at the atomic level, though such regular nanoparticles can be highly uniform at the nanometer level (e.g., size distribution ∼5%). In the long pursuit of well-defined nanocatalysts, a recent success is the synthesis of atomically precise metal nanoclusters protected by ligands in the size range from tens to hundreds of metal atoms (equivalently 1−3 nm in core diameter). More importantly, such nanoclusters have been crystallographically characterized, just like the protein structures in enzyme catalysis. Such atomically precise metal nanoclusters merge the features of well-defined homogeneous catalysts (e.g., ligand-protected metal centers) and enzymes (e.g., protein-encapsulated metal clusters of a few atoms bridged by ligands). The well-defined nanoclusters with their total structures available constitute a new class of model catalysts and hold great promise in fundamental catalysis research, including the atomically precise size dependent activity, control of catalytic selectivity by metal structure and surface ligands, structure−property relationships at the atomic-level, insights into molecular activation and catalytic mechanisms, and the identification of active sites on nanocatalysts. This Review summarizes the progress in the utilization of atomically precise metal nanoclusters for catalysis. These nanocluster-based model catalysts have enabled heterogeneous catalysis research at the single-atom and single-electron levels. Future efforts are expected to achieve more exciting progress in fundamental understanding of the catalytic mechanisms, the tailoring of active sites at the atomic level, and the design of new catalysts with high selectivity and activity under mild conditions.

Recent advances in the synthetic chemistry of atomically precise metal nanoclusters (NCs) have significantly broadened the accessible sizes and structures. Such particles are well defined and have intriguing properties, thus, they are attractive for catalysis. Especially, those NCs with identical size but different core (or surface) structure provide unique opportunities that allow the specific role of the core and the surface to be mapped out without complication by the size effect. Herein, we summarize recent work with isomeric Aun NCs protected by ligands and isostructural NCs but with different surface ligands. The highlighted work includes catalysis by spherical and rod‐shaped Au25 (with different ligands), quasi‐isomeric Au28(SR)20 with different R groups, structural isomers of Au38(SR)24 (with identical R) and Au38S2(SR)20 with body‐centred cubic (bcc) structure, and isostructural [Au38L20(PPh3)4]2+ (different L). These isomeric and/or isostructural NCs have provided valuable insights into the respective roles of the kernel, surface staples, and the type of ligands on catalysis. Future studies will lead to fundamental advances and development of tailor‐made catalysts.

In chemical science, chirality is indeed one of the most extensively studied phenomena. A chiral molecule possesses a pair of enantiomers and sometimes, only one of the enantiomers is effective as a functional unit or drug. [1,2] The mechanism on how the notorious enantiomers of thalidomide lead to teratogenic newborns has been revealed recently. [3] Chirality in coordination complexes has also been generalized in detail. [4] A classic example of molecules with point chirality is that it contains an asymmetric sp 3 carbon atom which is attached with four different atoms or groups. Such a molecule is not superimposable with its mirror image, and for a simple coordination complex, a metal atom-which is at the centerserves as the role of the chiral carbon (i.e., point chirality). As a result, the relative spatial arrangement of the substituents gives either a clockwise or an anticlockwise order, corresponding to the R and S enantiomers. Of note, the R/S notation of chirality has no fixed relation to the d/l notation (for sugars and amino acids). More generally speaking, chiral object is lack of S n symmetry elements; in other words, if a mirror plane (σ) and inversion (i) are both missing in the object, then it becomes chiral. From this definition, irregular objects are all chiral. Objects of C n (with n-fold axis) or D n (with C 2 axis perpendicular to the n-fold axis) symmetry are much more appealing as long-range order is created. Along with organic chiral molecules, [2] chiral complexes, [4] and chiral supramolecular systems with autonomous self-assembly generated by intermolecular noncovalent interactions have been discussed thoroughly in previous reviews. [5-9] Chiral inorganic nanostructures, with intermediate sizes between molecules of Å and objects of micrometers, are of critical significance owing to their tremendous applications in chiral catalysis, chiroptical metamaterials, enantioseparation, biomolecular and enantiomer sensing, as well as medicine. [10-12] For chiral semiconductor nanoparticles (NPs), commonly called quantum dots (QDs), such as cadmium chalcogenides, the chiral surface ligands are attached onto the QDs through covalent bonds, [13-17] and many applications can be designed. [18] The interactions of chiral moieties attached on graphene NPs lead to their helical assembly, enriching the optical and electronic properties of these chiral nanostructures and facilitating their applications. [19-21] The studies on chiral metal NPs greatly overwhelm those on the semiconductor counterparts. Metal NPs are known to have Chirality is ubiquitous in nature and occurs at all length scales. The development of applications for chiral nanostructures is rising rapidly. With the recent achievements of atomically precise nanochemistry, total structures of ligand-protected Au and other metal nanoclusters (NCs) are successfully obtained, and the origins of chirality are discovered to be associated with different parts of the cluster, including the surface ligands (e.g., swirl patterns), the organic-inorganic in...

To realize the molecular design of new functional silver(I) clusters, a new synthetic approach has been proposed, by which the weakly coordinating ligands NO in a Ag thiolate cluster precursor can be substituted by carboxylic ligands while keeping its inner core intact. By rational design, novel atom-precise carboxylic or amino acid protected 20-core Ag(I)-thiolate clusters have been demonstrated for the first time. The fluorescence and electrochemical activity of the postmodified Ag clusters can be modulated by alrestatin or ferrocenecarboxylic acid substitution. More strikingly, when chiral amino acids were used as postmodified ligands, CD-activity was observed for the Ag clusters, unveiling an efficient way to obtain atom-precise chiral silver(I) clusters.

Silver chalcogenolate cluster assembled materials (SCAMs) are a category of promising light-emitting materials the luminescence of which can be modulated by variation of their building blocks (cluster nodes and organic linkers). The transformation of a singly emissive [Ag (SBu ) (CF COO) (bpy) ] (Ag bpy, bpy=4,4'-bipyridine) into a dual-emissive [(Ag (SBu ) (CF COO) (bpy) )] (Ag bpy-2) via cluster-node isomerization, the critical importance of which was highlighted in dictating the photoluminescence properties of SCAMs. Moreover, the newly obtained Ag bpy-2 served to construct visual thermochromic Ag bpy-2/NH by a mixed-linker synthesis, together with dichromatic core-shell Ag bpy-2@Ag bpy-NH -2 via solvent-assisted linker exchange. This work provides insight into the significance of metal arrangement on physical properties of nanoclusters.

Recent efforts in nanoscience to control nanoparticles with atomic precision have met with success in solution-phase chemistry, opening new opportunities. The products, atomically precise nanoclusters (NCs), are not only compositionally well-defined but also structurally precise with unprecedented tailoring over the core and surface for specific functionalities. In this Perspective, we first highlight recent work in metal−hydride NCs for applications in catalytic hydrogenation and then reflect on the catalytic opportunities of atomically precise metal NCs. Metal NCs, as a new class of material, hold great promise for realizing the goals of understanding catalytic mechanisms at the atomic/molecular level (e.g., construction of active sites) and developing rules designing new catalysts with high activity and selectivity for important reactions. Tailoring NC catalysts at the atomic level will bring many exciting opportunities in future catalysis research.

Probing the transition from a metallic state to a molecular state in gold nanoparticles is fundamentally important for understanding the origin of surface plasmon resonance and the nature of the metallic bond. Atomically precise gold nanoclusters are desired for probing such a transition based upon a series of precise sizes with X-ray structures. While the definition of the metallic state in nanoclusters is simple, that is, when the HOMO−LUMO gap (E g ) becomes negligibly small (E g < k B T, where k B is the Boltzmann constant and T the temperature), the experimental determination of ultrasmall E g (e.g., of k B T level) is difficult, and the thermal excitation of valence electrons apparently comes into play in ultrasmall E g nanoclusters. Although a sharp transition from nonmetallic Au 246 (SR) 80 to metallic Au 279 (SR) 84 (SR: thiolate) has been observed, there is still uncertainty about the transition region. Here, we summarize several criteria on determining the metallic state versus the molecular (or nonmetallic) state in gold nanoclusters, including (1) E g determined by optical and electrochemical methods, (2) steady-state absorption spectra, (3) cryogenic optical spectra, (4) transient absorption spectra, (5) excited-state lifetime and power dependence, and (6) coherent oscillations in ultrafast electron dynamics. We emphasize that multiple analyses should be performed and cross-checked in practice because no single criterion is definitive. We also review the photophysics of several gold nanoclusters with nascent surface plasmon resonance. These criteria are expected to deepen the understanding of the metallic to molecular state transition of gold and other metal nanoclusters and also promote the design of functional nanomaterials and their applications.

This work presents the synthesis and intriguing photoluminescence of the Au 42 (PET) 32 (PET = 2-phenylethanethiolate) nanocluster (NC). The Au 42 (PET) 32 NC exhibits dual emission at 875 and 1040 nm, which are revealed to be fluorescence and phosphorescence, respectively. The emission quantum yield (QY) of Au 42 (PET) 32 in dichloromethane is 11.9% at room temperature in air, which is quite rare for thiolate-protected Au NCs. When Au 42 (PET) 32 NCs are embedded in polystyrene films (solid state), the fluorescence was dramatically suppressed while the phosphorescence was significantly enhanced. This divergent behavior is explained by dipolar interaction-induced enhancement of intersystem crossing from singlet to triplet excited state.