'Passivated Precursor' Approach to All-Alkynyl-Protected Gold Nanoclusters and Total Structure Determination of Au130

A 'passivated precursor' approach is developed for the efficient synthesis and isolation of all-alkynyl-protected gold nanoclusters. Direct reduction of dpa-passivated precursor Au-dpa (Hdpa=2,2'-dipyridylamine) in one-pot under ambient conditions gives a series of clusters including Au22(C≡CR)18 (R=-C6H4-2-F), Au36(C≡CR)24, Au44(C≡CR)28, Au130(C≡CR)50, and Au144(C≡CR)60. These clusters can be well separated via column chromatography. The overall isolation yield of this series of clusters is 40 % (based on gold), which is much improved in comparison with previous approaches. It is notable that the molecular structure of the giant cluster Au130(C≡CR)50 is revealed, which presents important information for understanding the structure of the mysterious Au130 nanoclusters. Theoretical calculations indicated Au130(C≡CR)50 has a smaller HOMO-LUMO gap than Au130(S-C6H4-4-CH3)50. This facile and reliable synthetic approach will greatly accelerate further studies on all-alkynyl-protected gold nanoclusters.

Pargyline-phosphine copper(I) clusters with tunable emission for light-emitting devices

Three pargyline-phosphine copper(I) clusters, [Cu4(CC-C9H12N)3(PPh3)4](PF6) (1) and [Cu6(CC-C9H12N)4(dppy)4](X)2 (dppy = diphenyl-2-pyridylphosphine; X = PF6 for 2 and X = ClO4 for 3), were synthesized. Their structures were fully characterized using various spectroscopic methods and X-ray crystallography, which showed that the stoichiometry and nature of pargyline and phosphine ligands play an important role in tuning the structure and photophysical features of Cu(I) clusters. Interestingly, clusters 1, 2 and 3 exhibited red, orange and yellow phosphorescence with high quantum yields of 88.5%, 22.0% and 40.2%, respectively, at room temperature. Moreover, clusters 1-3 show distinct temperature-dependent emissions. The excellent luminescence performance of 1 and 3 was designed and employed for the construction of monochrome and white light-emitting devices (LEDs).

Al8 Cluster-Based Metal Halide Frameworks: Balancing Singlet-Triplet Excited States to Achieve White Light and Multicolor Luminescence

Luminescent metal clusters have attracted great interest in current research; however, the design synthesis of Al clusters with color-tunable luminescence remains challenging. Herein, an [Al8 (OH)8 (NA)16 ] (Al8 , HNA = nicotinic acid) molecular cluster with dual luminescence properties of fluorescence and room-temperature phosphorescence (RTP) is synthesized by choosing HNA ligand as phosphor. Its prompt photoluminescence (PL) spectrum exhibits approximately white light emission at room temperature. Considering that halogen atoms can be used to regulate the RTP property by balancing the singlet and triplet excitons, different CdX2 (X- = Cl- , Br- , I- ) are introduced into the reactive system of the Al8 cluster, and three new Al8 cluster-based metal-organic frameworks, {[Al8 Cd3 Cl5 (OH)8 (NA)17 H2 O]·2HNA}n (CdCl2 -Al8 ), {[Al8 Cd4 Br7 (OH)8 (NA)16 CH3 CN]·NA·HNA}n (CdBr2 -Al8 ) and {[Al8 Cd8 I16 (OH)8 (NA)16 ]}n (CdI2 -Al8 ) are successfully obtained. They realize the color tunability from blue to yellow at room temperature. The origination of fluorescence and phosphorescence has also been illustrated by structure-property analysis and theoretical calculation. This work provides new insights into the design of multicolor luminescent metal cluster-based materials and develops advanced photo-functional materials for multicolor display, anti-counterfeiting, and encryption applications.

A concise guide to chemical reactions of atomically precise noble metal nanoclusters

Nanoparticles (NPs) with atomic precision, known as nanoclusters (NCs), are an emerging field in materials science in view of their fascinating structure-property relationships. Ultrasmall noble metal NPs have molecule-like properties that make them fundamentally unique compared with their plasmonic counterparts and bulk materials. In this review, we present a comprehensive account of the chemistry of monolayer-protected atomically precise noble metal nanoclusters with a focus on the chemical reactions, their diversity, associated kinetics, and implications. To begin with, we briefly review the history of the evolution of such precision materials. Then the review explores the diverse chemistry of noble metal nanoclusters, including ligand exchange reactions, ligand-induced structural transformations, and reactions with metal ions, metal thiolates, and halocarbons. Just as molecules do, these precision materials also undergo intercluster reactions in solution. Supramolecular forces between these systems facilitate the creation of well-defined hierarchical assemblies, composites, and hybrid materials. We conclude the review with a future perspective and scope of such chemistry.

Optical, structural, and biological properties of silver nanoclusters formed within the loop of a C-12 hairpin sequence

Silver nanoclusters (AgNCs) are the next-generation nanomaterials representing supra-atomic structures where silver atoms are organized in a particular geometry. DNA can effectively template and stabilize these novel fluorescent AgNCs. Only a few atoms in size - the properties of nanoclusters can be tuned using only single nucleobase replacement of C-rich templating DNA sequences. A high degree of control over the structure of AgNC could greatly contribute to the ability to fine-tune the properties of silver nanoclusters. In this study, we explore the properties of AgNCs formed on a short DNA sequence with a C12 hairpin loop structure (AgNC@hpC12). We identify three types of cytosines based on their involvement in the stabilization of AgNCs. Computational and experimental results suggest an elongated cluster shape with 10 silver atoms. We found that the properties of the AgNCs depend on the overall structure and relative position of the silver atoms. The emission pattern of the AgNCs depends strongly on the charge distribution, while all silver atoms and some DNA bases are involved in optical transitions based on molecular orbital (MO) visualization. We also characterize the antibacterial properties of silver nanoclusters and propose a possible mechanism of action based on the interactions of AgNCs with molecular oxygen.

Revealing the effects of ligands of silver nanoclusters on the interactions between them and ctDNA: Abstraction to visualization

Silver nanoclusters (AgNCs) have been widely applied in the field of biology, drug therapy and cell imaging in the last decade. In order to study the biosafety of AgNCs, GSH-AgNCs and DHLA-AgNCs were synthesized using glutathione (GSH) and dihydrolipoic acid (DHLA) as ligands, and their interactions with calf thymus DNA (ctDNA) from abstraction to visualization were studied. The results of spectroscopy, viscometry and molecular docking demonstrated that GSH-AgNCs mainly bound to ctDNA in a groove mode, while DHLA-AgNCs were both groove and intercalation binding. Fluorescence experiments suggested that the quenching mechanism of both AgNCs to the emission of ctDNA-probe were both in static mode, and thermodynamic parameters demonstrated that the main forces between GSH-AgNCs and ctDNA were hydrogen bonds and van der Waals forces, while hydrogen bonds and hydrophobic forces contributed to the binding of DHLA-AgNCs to ctDNA. The binding strength demonstrated that DHLA-AgNCs bound to ctDNA more strongly than that of GSH-AgNCs. The results of circular dichroism (CD) spectroscopy reflected small effects of both AgNCs on the structure of ctDNA. This study will support the theoretical foundation for the biosafety of AgNCs and have a guiding significance for the preparation and application of AgNCs.

Face-Centered Cubic Silver Nanoclusters Consolidated with Tetradentate Formamidinate Ligands

Growing attention has been paid to nanoclusters with face-centered cubic (fcc) metal kernels, due to its structural similarity to bulk metals. We demonstrate that the use of tetradentate formamidinate ligands facilitate the construction of two fcc silver nanoclusters: [Ag₅₂(5-F-dpf)₁₆Cl₄](SbF₆)₂ (Ag₅₂, 5-F-Hdpf = N,N'-di(5-fluoro-2-pyridinyl)formamidine) and [Ag₅₃(5-Me-dpf)₁₈](NO₃)₅ (Ag₅₃, 5-Me-Hdpf = N,N'-di(5-methyl-2-pyridinyl)formamidine). Single-crystal X-ray structural analysis revealed that the silver atoms in both clusters are in a layer-by-layer arrangement, which can be viewed as a portion of the fcc packing of silver. The nitrogen donors of amidinate ligands selectively passivate the {111} facets. All silver atoms are involved in the fcc packing, that is, no staple motifs are observed due to the linear arrangement of the four N donors of the dpf ligands. The characteristic optical absorption bands of Ag₅₂ and Ag₅₃ have been studied with a time-dependent density functional theory. This work provides a facile access to assembling atomically precise fcc-type nanoclusters and shows the prospect of amidinates as protecting ligands in synthesizing metal nanoclusters.

Alkynyl ligands-induced growth of ultrathin nanowires arrays

Ligands are almost essential in the synthesis of nanostructures. In this work, we introduce the alkynyl ligands into the synthesis of ultrathin gold (Au) nanowires arrays. The strong binding affinity of the alkynyl ligands enables one-dimensional (1D) growth via the active surface growth mechanism. The scope of the ligand generality was systematically investigated, and the alkynyl ligand-induced nanowire growth processes were compared and contrasted with those involving thiolated ligands. While strong ligands are usually difficult to dissociate from the nanostructure surface and therefore problematic for post-synthetic processing, the alkynyl ligands are readily dissociable, making the alkynyl ligand-stabilized Au nanowires potentially more modifiable and applicable. As a demonstration, direct palladium (Pd) deposition on the Au nanowires was successfully carried out without any ligand exchange process.

Alkynyl and halogen co-protected (AuAg)44 nanoclusters: a comparative study on their optical absorbance, structure, and hydrogen evolution performance

We report the synthesis, structure, and electrochemical hydrogen evolution reaction (HER) performance of two alkynyl and halogen coprotected AuAg alloy nanoclusters, namely Au₂₄Ag₂₀(^tBuPh-CC)₂₄Cl₂ (NC 1 for short) and Au₂₂Ag₂₂(^tBuCC)₁₆Br_3.28Cl_2.72 (NC 2 for short). Single crystal X-ray structural analysis revealed that the two nanoclusters possess a rather similar core@shell@shell keplerate metal core configuration to M₁₂@M₂₀@M₁₂ with the main difference in the outermost shell (Au₁₂vs. Au₁₀Ag₂). Interestingly, such a subtle difference in the two-metal-atoms results in different optical absorbance features and drastically different HER performances. Both NCs have excellent long-term stability for the HER, but NC 1 possesses superior activity to NC 2, and density functional theory calculations disclosed that the binding energy of hydrogen to form the key *H intermediate for NC 1 is much lower and hence it adopts a more energetically feasible HER pathway.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Structural transformation and catalytic hydrogenation activity of amidinate-protected copper hydride clusters

Copper hydrides are important hydrogenation catalysts, but their poor stability hinders the practical applications. Ligand engineering is an effective strategy to tackle this issue. An amidinate ligand, N,N'-Di(5-trifluoromethyl-2-pyridyl)formamidinate (Tf-dpf) with four N-donors has been applied as a protecting agent in the synthesis of stable copper hydride clusters: Cu₁₁H₃(Tf-dpf)₆(OAc)₂ (Cu₁₁) with three interfacial μ₅-H and [Cu₁₂H₃(Tf-dpf)₆(OAc)₂]·OAc (Cu₁₂) with three interstitial μ₆-H. A solvent-triggered reversible interconversion between Cu₁₁ and Cu₁₂ has been observed thanks to the flexibility of Tf-dpf. Cu₁₁ shows high activity in the reduction of 4-nitrophenol to 4-aminophenol, while Cu₁₂ displays very low activity. Deuteration experiments prove that the type of hydride is the key in dictating the catalytic activity, for the interfacial μ₅-H species in Cu₁₁ are involved in the catalytic cycle whereas the interstitial μ₆-H species in Cu₁₂ are not. This work highlights the role of hydrides with regard to catalytic hydrogenation activity.

Electrochemical CO2 reduction catalyzed by atomically precise alkynyl-protected Au7Ag8, Ag9Cu6, and Au2Ag8Cu5 nanoclusters: probing the effect of multi-metal core on selectivity

We report the first all-alkynyl-protected Au2Ag8Cu5 cluster, which adopts a M@M8@M6 core configuration similar with Au7Ag8/Ag9Cu6 clusters. The three clusters exhibited strong metal core effect toward CO2RR, which was understood by DFT calculations.

Recent advances in polyoxometalate-based metal-alkynyl clusters

This paper focuses on the recent advances in polyoxometalate-based metal-alkynyl clusters.

A homoleptic alkynyl-protected [Ag9Cu6( t BuC[triple bond, length as m-dash]C)12]+ superatom with free electrons: synthesis, structure analysis, and different properties compared with the Au7Ag8 cluster in the M15 + series

We report the first homoleptic alkynyl-protected AgCu superatomic nanocluster [Ag9Cu6( t BuC[triple bond, length as m-dash]C)12]+ (NC 1, also Ag9Cu6 in short), which has a body-centered-cubic structure with a Ag1@Ag8@Cu6 metal core. Such a configuration is reminiscent of the reported AuAg bimetallic nanocluster [Au1@Ag8@Au6( t BuC[triple bond, length as m-dash]C)12]+ (NC 2, also Au7Ag8 in short), which is also synthesized by an anti-galvanic reaction (AGR) approach with a very high yield for the first time in this study. Despite a similar Ag8 cube for both NCs, structural anatomy reveals that there are some subtle differences between NCs 1 and 2. Such differences, plus the different M1 kernel and M6 octahedron, lead to significantly different optical absorbance features for NCs 1 and 2. Density functional theory calculations revealed the LUMO and HOMO energy levels of NCs 1 and 2, where the characteristic absorbance peaks can be correlated with the discrete molecular orbital transitions. Finally, the stability of NCs 1 and 2 at different temperatures, in the presence of an oxidant or Lewis base, was investigated. This study not only enriches the M15 + series, but also sets an example for correlating the structure-property relationship in alkynyl-protected bimetallic superatomic clusters.

A Photoluminescent Ag10 Cu6 Cluster Stablized by a PNNP Ligand and Phenylacetylides Selectively and Reversibly Senses Ammonia in Air and Water

A photoluminescent bimetallic cluster [Ag₁₀ Cu₆ (bdppthi)₂ (C≡CPh)₁₂ (MeOH)₂ (H₂ O)](ClO₄ )₄ (1, bdppthi=N,N'-bis(diphenylphosphanylmethyl)-tetrahydroimidazole} was synthesized from the PNNP type ligand bdppthi generated in-situ. Upon excitation at 365 nm, 1 exhibited strong phosphorescent emission at 630 nm, which was selectively quenched by NH₃ in air or water. The sensing of NH₃ was rapid and recoverable, with detection limits of 53 ppm (v/v) in N₂ and 21 μmol/L (0.36 ppm, w/w) for NH₃  ⋅ H₂ O in water. Cluster 1 could potentially serve as a bifunctional chemical sensor for the efficient detection of ammonia in waste-gas and waste-water.

Isolation and Structural Elucidation of 15-Nuclear Copper Dihydride Clusters: An Intermediate in the Formation of a Two-Electron Copper Superatom

Highly reactive copper-dihydride clusters, [Cu₁₅ (H)₂ (S₂ CNR₂ )₆ (C₂ Ph)₆ ](PF₆ ) {R = ⁿ Bu (1_H ), ⁿ Pr (2_H ), ⁱ Bu (3_H )}, are isolated during the reaction of [Cu₂₈ H₁₅ {S₂ CNⁿ Bu₂ }₁₂ ](PF₆ ) with ten equivalents of phenylacetylene. They are found to be intermediates in the formation of the earlier reported two-electron superatom [Cu₁₃ (S₂ CNR₂ )₆ (C₂ Ph)₄ ]⁺ . Better yields are obtained by reacting dithiocarbamate sodium salts, [Cu(CH₃ CN)₄ ](PF₆ ), BH₄^- and phenylacetylene. The presence of two hydrides in the isolated clusters is confirmed by the synthesis and characterization of its deuteride analogue [Cu₁₅ (D)₂ (S₂ CNR₂ )₆ (C₂ Ph)₆ ]⁺ , and a single-crystal neutron structure of 2_H . Structural characterization of 1_H reveals a new bicapped icosahedral copper(I) cage encapsulating a linear copper dihydride (CuH₂ )^- unit. Reaction of 3_H with Au(I) salts yields a highly luminescent [AuCu₁₂ (S₂ CNⁱ Bu₂ )₆ (C₂ Ph)₄ ]⁺ cluster.

Inverse heavy-atom effect in near infrared photoluminescent gold nanoclusters

Fluorophores functionalized with heavy elements show enhanced intersystem crossing due to increased spin-orbit coupling, which in turn shortens the fluorescence decay lifetime (τPL). This phenomenon is known as the heavy-atom effect (HAE). Here, we report the observation of increased τPL upon functionalisation of near-infrared photoluminescent gold nanoclusters with iodine. The heavy atom-mediated increase in τPL is in striking contrast with the HAE and referred to as inverse HAE. Femtosecond and nanosecond transient absorption spectroscopy revealed overcompensation of a slight decrease in lifetime of the transition associated with the Au core (ps) by a large increase in the long-lived triplet state lifetime associated with the Au shell, which contributed to the observed inverse HAE. This unique observation of inverse HAE in gold nanoclusters provides the means to enhance the triplet excited state lifetime.

The ligand effect on the interface structures and electrocatalytic applications of atomically precise metal nanoclusters

Metal nanoclusters, also known as ultra-small metal nanoparticles, occupy the gap between discrete atoms and plasmonic nanomaterials, and are an emerging class of atomically precise nanomaterials. Metal nanoclusters protected by different types of ligands, such as thiolates, alkynyls, hydrides, and N-heterocyclic carbenes, have been synthesized in recent years. Moreover, recent experiment and theoretical studies also indicated that the metal nanoclusters show great promise in many electrocatalytic reactions, such as hydrogen evolution, oxygen reduction, and CO₂reduction. The atomically precise nature of their structures enables the elucidation of structure-property relationships and the reaction mechanisms, which is essential if nanoclusters with enhanced performances are to be rationally designed. Particularly, the ligands play an important role in affecting the interface bonding, stability and electrocatalytic activity/selectivity. In this review, we mainly focus on the ligand effect on the interface structure of metal nanoclusters and then discuss the recent advances in electrocatalytic applications. Furthermore, we point out our perspectives on future efforts in this field.

Construction of Silver Clusters Capped by Zwitterionic Ethynide Ligands

A zwitterionic ligand 3-(triethylammonio)propyne (TAP) has been employed to construct nine silver ethynide compounds for the first time. Single-crystal X-ray analyses reveal that compounds 1 and 2 are silver ethynide assemblies based on the Ag₃ subunits and clusters 3-8 are small discrete clusters of Ag₃, Ag₆, Ag₈, and Ag₁₂, respectively, ligated by the bulky TAP ligand with different auxiliary ligands. In addition, upon acquiring the tripod-like ^tBuPO₃^2-, a unprecedented 80 nuclei silver ethynide cluster was isolated and determined to be [(CF₃CO₂)₅@Ag₈₀(TAP)₁₄(^tBuPO₃)₁₆(CF₃CO₂)₂₄]¹⁹⁺ by crystallography and thermogravimetric analysis. The C₁ symmetry of Ag₈₀ was deconstructed to be two [Ag₄₀(TAP)₇(^tBuPO₃)₈(CF₃CO₂)₁₂]¹²⁺ secondary building subunits arranged in a cross way, with five CF₃CO₂^- trapped in the center. These results highlight that the elaborate selection of ethynide ligands is of great importance in the synthesis of novel silver ethynide clusters.

Enriching Structural Diversity of Alkynyl-Protected Gold Nanoclusters with Chlorides

The synthesis and isolation of alkynyl/chloride-protected gold nanoclusters is described. Silica gel column chromatography is effective in isolating gold nanoclusters from the as-synthesized cluster mixture to give the clusters Na[Au₂₅ L₁₈ ] (Au₂₅ ), [HNEt₃ ]₃ [Au₆₇ L₃₂ Cl₄ ] (Au₆₇ ), [HNEt₃ ]₄ [Au₁₀₆ L₄₀ Cl₁₂ ] (Au₁₀₆ ), L=3,5-bis(trifluoromethyl)-phenylacetylide. Au₆₇ and Au₁₀₆ are new clusters; the structures were determined by X-ray single-crystal diffraction. Au₆₇ contains a distorted Au₁₈ Marks decahedron shelled by an irregular Au₃₂ and further protected with two V-shaped Au₂ L₃ , 13 linear AuL₂ staples and 4 chlorides. Au₆₇ is the first structurally determined 34e superatomic gold nanocluster. Au₁₀₆ is composed of 106 Au atoms co-protected by alkynyls and chlorides. It has a Au₇₉ kernel, like in Au₁₀₂ (p-MBA)₄₄ . The surface structure of Au₁₀₆ includes 20 linear Au-alkynyl staples, 5 Cl-Au-Cl and 2 Cl-Au motifs. These three gold nanoclusters show size-dependent electrochemical properties.

First-principles exploration of the versatile configurations at an alkynyl-protected coinage metal(111) interface

Alkynyl groups (R-C[triple bond, length as m-dash]C-) have attracted intense interest recently as alternative ligands to thiolates to protect atomically precise coinage metal nanoclusters, and more than two dozen compositions have been structurally resolved. However, structure determinations indicated that the interface shows strong metal sensitivity, where a staple motif is the common structural feature at the interface of Au-alkynyl nanoclusters, while the bridging motif dominates at the RC[triple bond, length as m-dash]C-/Ag and RC[triple bond, length as m-dash]C-/Cu interface. To understand their interfacial differences, we employed density functional theory (DFT) calculations to examine the versatile interfacial structures between CH3C[triple bond, length as m-dash]C- and the coinage metal surface; both the (111) surface as well as a surface with a metal adatom are investigated. We find that the alkynyl/gold(111) interface does prefer to form the staple motifs, and a linear flat-lying staple motif is preferred. The adatom occupies the bridge position and two CH3C[triple bond, length as m-dash]C- ligands lie diagonally at the fcc hollow sites with the C[triple bond, length as m-dash]C bond interacting with the surface Au by both σ- and π-coordination modes. In contrast, the bridging motif is energetically more favored on Ag(111) and Cu(111). The alkynyl carbons form strong σ, π- or σ-only bonds with the surface Ag/Cu, forming μ3-bridge coordination over the fcc hollow site. The binding strengths have the order of Cu(111) > Ag(111) > Au(111). The difference in structural preference is attributed to the intrinsic metal attributes with the different gaps of energetic penalty for surface energy, adatom creation and vacancy formation. The other two reasons are the differences of alkynyl-metal bond character and vdW interaction. We further show that this structure preference is also the preferred bonding mode of CH3C[triple bond, length as m-dash]C- on M55 clusters and the CH3S-/M(111) interface. Our insights greatly facilitate the structural elucidation and provide useful guidelines for future structure predictions in alkynyl-protected metal nanosystems.

Controllable synthesis and formation mechanism study of homoleptic alkynyl-protected Au nanoclusters: recent advances, grand challenges, and great opportunities

In the past decade, atomically precise coinage metal nanoclusters have been a subject of major interest in nanoscience and nanotechnology because of their determined compositions and well-defined molecular structures, which are beneficial for establishing structure-property relationships. Recently ligand engineering has been extended to alkynyl molecules. Homoleptic alkynyl-protected Au nanoclusters (Au NCs) have emerged as a hotspot of research interest, mainly due to their unique optical properties, molecular configuration, and catalytic functionalities, and more importantly, they are used as a counterpart object for fundamental study to compare with the well-established thiolate Au NCs. In this review, we first summarize the recently reported various controllable synthetic strategies for atomically precise homoleptic-alkynyl-protected Au NCs, with particular emphasis on the ligand exchange method, direct reduction of the precursor, one-pot synthesis, and the synchronous nucleation and passivation strategy. After that, we switch our focus to the formation mechanism and structure evolution process of homoleptic alkynyl-protected Au NCs, where Au144(PA)60 and Au36(PA)24 (PA = phenylacetylide) are given as examples, along with the prediction of the possible formation mechanism of some other cluster molecules. In the end of this review, the outlook and perspective of this rapidly developing field including grand challenges and great opportunities are discussed. This review can stimulate more research efforts towards developing new synthetic strategies to enrich the limited examples and unravel the formation/growth mechanism of homoleptic alkynyl-protected Au NCs.

Chirality and Surface Bonding Correlation in Atomically Precise Metal Nanoclusters

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 interface (e.g., helical stripes), and the kernel. Herein, a unified picture of metal-ligand surface bonding-induced chirality for the nanoclusters is proposed. The different bonding modes of M-X (where M = metal and X = the binding atom of ligand) lead to different surface structures on nanoclusters, which in turn give rise to various characteristic features of chirality. A comparison of Au-thiolate NCs with Au-phosphine ones further reveals the important roles of surface bonding. Compared to the Au-thiolate NCs, the Ag/Cu/Cd-thiolate systems exhibit different coordination modes between the metal and the thiolate. Other than thiolate and phosphine ligands, alkynyls are also briefly discussed. Several methods of obtaining chiroptically active nanoclusters are introduced, such as enantioseparation by high-performance liquid chromatography and enantioselective synthesis. Future perspectives on chiral NCs are also proposed.

New protective ligands for atomically precise silver nanoclusters

Atomically precise silver nanoclusters (NCs) have emerged as a hot topic attracting immense research interest. Protecting ligands are needed for direct capping on cluster surfaces in order to prevent aggregation and to stabilize NCs. It has been demonstrated that protective ligands are critical to determining the sizes, structures and properties of silver NCs. The past decades have witnessed conventionally used organic ligands (thiolates/selenols, phosphines and alkynyls) and inorganic ligands (chalcogens and halogens) being extensively used to passivate NC surfaces. However, only in the most recent years have new-type protecting ligands beyond the conventional ones begun to be introduced in the protecting sphere of new functional silver NCs. The present Frontier article covers the most recent examples of some new protective agents for well-defined silver NCs. We describe four classes of novel silver NCs stabilized by newly-developed surface ligands, namely, nitrogen-donor organic ligands, oxygen-donor inorganic ligands, metalloligands and macrocyclic hosts, paying attention to the synthesis, structures and properties of these silver NCs. This Frontier article will hopefully attract more cluster scientists to explore more freshly ligated atomically precise silver NCs with novel structures and properties in the years ahead. The literature survey in this review is based on publications up to February 2020. Some suggestions for future directions in this field are also given.

Ligand-free gold nanoclusters confined in mesoporous silica nanoparticles for styrene epoxidation

We present a novel approach to produce gold nanoclusters (Au NCs) in the pores of mesoporous silica nanoparticles (MSNs) by sequential and controlled addition of metal ions and reducing agents. This impregnation technique was followed to confine Au NCs inside the pores of MSNs without adding external ligands or stabilizing agents. TEM images show a uniform distribution of monodisperse NCs with an average size of 1.37 ± 0.4 nm. Since the NCs are grown in situ in MSN pores, additional support and high temperature calcination are not required to use them as catalysts. The use of Au NC/MSNs as a catalyst for the epoxidation of styrene in the presence of tert-butyl hydroperoxide (TBHP) as a terminal oxidant resulted in an 88% conversion of styrene in 12 h with a 74% selectivity towards styrene epoxide. Our observations suggest that this remarkable catalytic performance is due to the small size of Au NCs and the strong interaction between gold and the MSNs. This catalytic conversion is environmentally friendly as it is solvent free. We believe our synthetic approach can be extended to other metal NCs offering a wide range of applications.

Investigation of the Effect of a Mixed-Ligand on the Accommodation of a Templating Molecule into the Structure of High-Nucleus Silver Clusters

Advancement of the synthesis and control of the self-assembly process of new high-nucleus silver clusters with desired structures is important for both the material sciences and the many applications. Herein, three new silver clusters, 20-, 22-, and 8-nucleus, based on alkynyl ligands were constructed and their structures were confirmed by single-crystal X-ray diffraction, powder X-ray diffraction, elemental analyses, and Fourier-transform infrared spectroscopy (FT-IR). For the first time, the trivalent tetrahedron anion of AsO₄^3-, as a template, and the surface ligand of Ph₂PO₂H, with new coordination modes, were employed in preparation of the silver clusters. The role of surface ligands and template anions in the size and structure of the clusters was investigated. The presence of the template in the structure of the clusters led to the formation of the high-nucleus clusters. Also, in this report, it was shown that the participation of the template in the assembly of a cluster can be controlled by the surface ligands. UV-vis absorption and luminescent properties of the clusters and the thermal stability of the 8-nucleus cluster were also studied.

Atomic-precision engineering of metal nanoclusters

Ultrasmall metal nanoparticles (below 2.2 nm core diameter) start to show discrete electronic energy levels due to strong quantum confinement effects and thus behave much like molecules. The size and structure dependent quantization induces a plethora of new phenomena, including multi-band optical absorption, enhanced luminescence, single-electron magnetism, and catalytic reactivity. The exploration of such new properties is largely built on the success in unveiling the crystallographic structures of atomically precise nanoclusters (typically protected by ligands, formulated as MnLmq, where M = metal, L = Ligand, and q = charge). Correlation between the atomic structures of nanoclusters and their properties has further enabled atomic-precision engineering toward materials design. In this frontier article, we illustrate several aspects of the precise engineering of gold nanoclusters, such as the single-atom size augmenting, single-atom dislodging and doping, precise surface modification, and single-electron control for magnetism. Such precise engineering involves the nanocluster's geometric structure, surface chemistry, and electronic properties, and future endeavors will lead to new materials design rules for structure-function correlations and largely boost the applications of metal nanoclusters in optics, catalysis, magnetism, and other fields. Following the illustrations of atomic-precision engineering, we have also put forth some perspectives. We hope this frontier article will stimulate research interest in atomic-level engineering of nanoclusters.

Ultrastable hydrido gold nanoclusters with the protection of phosphines

Two phosphine-protected gold nanoclusters with an unprecedented hetero superatomic Au₂₀ core have been isolated. The presence of hydrides is evidenced by mass spectrometry and NMR spectroscopy.

Polynuclear organometallic clusters: synthesis, structure, and reactivity studies

This feature article highlights our recent advances in the controllable synthesis of carbon-centered polynuclear organometallic clusters: from synthesis to transformation, reactivity and mechanism.

The Self-Assembly of [{Ag3(C≡CtBu)2}n]n+ Building Units into a Template-Free Cuboctahedron and Anion-Encapsulating Silver Cages

We describe the controlled synthesis of silver acetylide clusters based on a simple polymeric [Ag₃L₂]⁺ (L = -C≡C^tBu) building block. A linear one-dimensional polymeric structure shows alternating pyramidal motifs and is the basic repeating unit forming discrete molecular cages (pentamers [X⊂Ag₁₅L₁₀]⁴⁺ and hexamers [PF₆⊂Ag₁₈L₁₂]⁵⁺) obtained by incorporating suitable templates. These motifs and a rare template-free cuboctahedral [Ag₁₂L₈]⁴⁺ cluster (tetramer) were crystallographically characterized.

Alkynyl-Protected Au22(C≡CR)18 Clusters Featuring New Interfacial Motifs and R-Dependent Photoluminescence

A series of homoleptic alkynyl-protected gold clusters Au₂₂(C≡CR)₁₈ were newly synthesized using 3-ethynylthiophene (ETP-H), phenylacetylene (PA-H), 3-ethynyltoluene (ET-H), and 3-ethynylanisole (EA-H). Single-crystal X-ray diffraction analysis on Au₂₂(ETP)₁₈ revealed that a bitetrahedral Au₇ core is protected by novel Au₃(ETP)₄ oligomers and a Au₆(ETP)₆ ring, composed of R-C≡C-Au(I)-C≡C-R units bridged by π-Au-π bonds. UV-visible and ¹H NMR spectroscopy revealed that the R groups did not affect the geometric and electronic structures of Au₂₂(C≡CR)₁₈, whereas the photoluminescence quantum yield was dependent on the R group and showed the highest value of 4.6% when R = Ph.

Electrically transmissive alkyne-anchored monolayers on gold

Well-ordered, tightly-packed (surface coverage 0.97 × 10-9 mol cm-2) monolayer films of 1,4-bis((4-ethynylphenyl)ethynyl)benzene (1) on gold are prepared via a simple self-assembly process, taking advantage of the ready formation of alkynyl C-Au σ-bonds. Electrochemical measurements using [Ru(NH3)6]3+, [Fe(CN)6]3-, and ferrocenylmethanol [Fe(η5-C5H4CH2OH)(η5-C5H5)] redox probes indicate that the alkynyl C-Au contacted monolayer of 1 presents a relatively low barrier for electron transfer. This contrasts with monolayer films on gold of other oligo(phenylene ethynylene) derivatives of comparable length and surface coverage, but with different contacting groups. Additionally, a low voltage transition (Vtrans = 0.51 V) from direct tunneling (rectangular barrier) to field emission (triangular barrier) is observed. This low transition voltage points to a low tunneling barrier, which is consistent with the facile electron transport observed through the C-Au contacted self-assembled monolayer of 1.

Surface Chemistry-Mediated Near-Infrared Emission of Small Coinage Metal Nanoparticles

From size-dependent luminescence to localized surface plasmon resonances, the optical properties that emerge from common materials with nanoscale dimensions have been revolutionary. As nanomaterials get smaller, they approach molecular electronic structures, and this transition from bulk to molecular electronic properties is a subject of far-reaching impact. One class of nanomaterials that exhibit particularly interesting optoelectronic features at this size transition are coinage metal (i.e., group 11 elements copper, silver, and gold) nanoparticles with core diameters between approximately 1 to 3 nm (∼25-200 atoms). Coinage metal nanoparticles can exhibit red or near-infrared photoluminescence features that are not seen in either their molecular or larger nanoscale counterparts. This emission has been exploited both as a probe of electronic behavior at the nanoscale as well as in critical applications such as biological imaging and chemical sensing. Interestingly, it has been demonstrated that their photoluminescence figures of merit such as emission quantum yield, energy, and lifetime are largely independent of particle diameter. Instead, emission from particles at this size range depends heavily on the particle surface chemistry, which includes both its metallic composition and the capping ligand architecture. The strong influence of surface chemistry on these emergent optoelectronic phenomena has powerful implications for both the study and use of these particles, primarily due to the theoretically limitless possible surface ligand architectures and metallic compositions. In this Account, we highlight recent work that studies and uses surface chemistry-mediated photoluminescence from coinage metal nanoparticles. Specifically, we emphasize the distinct, as well as synergistic, roles of the nanoparticle capping ligand and the nanoparticle core for controlling and/or enhancing their near-infrared photoluminescence. We then discuss the implications of surface chemistry-mediated photoluminescence as it relates to downstream applications such as energy transfer, sensing, and biological imaging. We conclude by discussing current challenges that remain in the field, including opportunities to develop new particle synthetic routes, analytical tools, and physical frameworks with which to understand small nanoparticle emission. Taken together, we anticipate that these materials will be foundational both in understanding the unique transition from molecular to bulk electronic structures and in the development of nanomaterials that leverage this transition.

The stability enhancement factor beyond eight-electron shell closure in thiacalix[4]arene-protected silver clusters

Destroying coordination open sites may significantly enhance the stability of metal nanoclusters.

We report the synthesis and structures of two 34-atom metal nanoclusters, namely [Ag₃₄(BTCA)₃(C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆ and [AuAg₃₃(BTCA)₃(C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆, where H₄BTCA is p-tert-butylthiacalix[4]arene and tfa is trifluoroacetate. Their compositions and structures have been determined by single-crystal X-ray structural analysis and ESI-MS. The cationic cluster consists of a centered icosahedron M@Ag₁₂ (M = Ag or Au) core that is surrounded by 21 peripheral silver atoms. Surrounding protection is provided by four kinds of ligands, including three BTCA, nine ^tBuC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C, four tfa, and three methanol solvent ligands. It was found that the Ag₅@BTCA μ₅-coordination motif of thiacalixarene is critical for high stability of the title clusters, and extra stability enhancement can be achieved by doping a gold atom at the center of the silver cluster. This work suggests that coordination saturation should be taken into account in addition to electronic and geometric factors for analyzing metal nanocluster stabilities.