Exposing the Delocalized Cu−S π Bonds on the Au 24 Cu 6 (SPh t Bu) 22 Nanocluster and Its Application in Ring‐Opening Reactions

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

Very Low Temperature CO Oxidation over Atomically Precise Au25 Nanoclusters on MnO2

Atomically precise Au25 nanoclusters have garnered significant interest in the field of heterogeneous catalysis due to their remarkable activity and selectivity. However, for the extensively studied reaction of low-temperature CO oxidation, their performance has not been competitive compared to other known gold nanocatalysts. To address this, we deposited Au25(SR)18 (R = CH2CH2Ph) nanoclusters onto a manganese oxide support (Au25/MnO2), resulting in a very stable and highly active catalyst. By optimizing the pretreatment temperature, we were able to significantly enhance the performance of the Au25/MnO2 catalyst, which outperformed most other gold catalysts. Impressively, 100% conversion of CO was achieved at temperatures as low as -50 °C, with 50% conversion being reached below -70 °C. Furthermore, the existence of ligands could also influence the negative apparent activation energy observed at intermediate temperatures. Analysis using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD) techniques indicated that the Au25 nanoclusters remained stable on the catalyst surface even after pretreatment at high temperatures. In-situ modulation excitation spectroscopy (MES) spectra also confirmed that the Au cluster was the active site for CO oxidation, highlighting the potential of atomically precise Au25 nanoclusters as primary active sites at very low temperatures.

Au16 Cd16 (SC6 H11 )20 : A Glance at Structure-Property Relationship

Doping Cd atom(s) into gold clusters is very promising in both theoretical study and practical applications. However, it has long been a challenge to synthesize heavily Cd-doped AuCd bimetallic clusters and thereby reveal their structure-property correlations. Herein a novel AuCd bimetallic cluster: Au16 Cd16 (SC6 H11 )20 (SC6 H11 denotes deprotonated cyclohexanethiol) with a Cd to Au atomic ratio of 1:1 is reported. The precise structure of the cluster determined by single crystal X-ray diffraction demonstrates that it has a unique hexatetrahedron Au14 core and a distinctive shell. Intriguingly, due to the special protecting motifs, the cluster exhibits high stability in various conditions studied, indicating that the geometric structure is crucial in determining the stability of the cluster. Most importantly, the photothermal property of the cluster has been investigated in comparison with those of M13 -kernel (M denotes metal atoms) clusters, and the results imply that the compactness and the Cd atom doping of the core play important roles in dictating the photothermal effect of the cluster. The authors believe that this work will provide some ideas for the rational design of clusters with high stability and excellent photothermal property.

Size Growth of Au4Cu4: From Increased Nucleation to Surface Capping

The size conversion of atomically precise metal nanoclusters is fundamental for elucidating structure-property correlations. In this study, copper salt (CuCl)-induced size growth from [Au₄Cu₄(Dppm)₂(SAdm)₅]⁺ (abbreviated as [Au₄Cu₄S₅]⁺) to [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ (abbreviated as [Au₄Cu₆S₄Cl₃]⁺) (SAdmH = 1-adamantane mercaptan, Dppm = bis-(diphenylphosphino)methane) was investigated via experiments and density functional theory calculations. The [Au₄Cu₄S₅]⁺ adopts a defective pentagonal bipyramid core structure with surface cavities, which could be easily filled with the sterically less hindered CuCl and CuSCy (i.e., core growth) (HSCy = cyclohexanethiol) but not the bulky CuSAdm. As long as the Au₄Cu₅ framework is formed, ligand exchange or size growth occurs easily. However, owing to the compact pentagonal bipyramid core structure, the latter growth mode occurs only for the surface-capped [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ structure (i.e., surface-capped size growth). A preliminary mechanistic study with density functional theory (DFT) calculations indicated that the overall conversion occurred via CuCl addition, core tautomerization, Cl migration, the second [CuCl] addition, and [CuCl]-[CuSR] exchange steps. And the [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ alloy nanocluster exhibits aggregation-induced emission (AIE) with an absolute luminescence quantum yield of 18.01% in the solid state. This work sheds light on the structural transformation of Au-Cu alloy nanoclusters induced by Cu(I) and contributes to the knowledge base of metal-ion-induced size conversion of metal nanoclusters.

Cocrystallization of Two Negatively Charged Dimercaptomaleonitrile-Stabilized Silver Nanoclusters

Studies on the assembly of atomically precise metal nanoclusters (NCs) are of great significance in the nanomaterial field, which has attracted increasing interest in the last few decades. Herein, we report the cocrystallization of two negatively charged atom-precise silver nanoclusters, the octahedral [Ag₆₂(MNT)₂₄(TPP)₆]^8- (Ag₆₂) and the truncated-tetrahedral [Ag₂₂(MNT)₁₂(TPP)₄]^4- (Ag₂₂) in a 1:2 ratio (MNT^2- = dimercaptomaleonitrile, TPP = triphenylphosphine). As far as we know, a cocrystal containing two negatively charged NCs has seldom been reported. Single-crystal structure determinations reveal that the component Ag₂₂ and Ag₆₂ NCs both adopt core-shell structures. In addition, the component NCs were separately obtained by adjusting the synthetic conditions. This work enriches the structural diversity of silver NCs and extends the family of cluster-based cocrystals.

Cd Atom Goes into the Interior of Cluster Induced by Directional Consecutive Assembly of Tetrahedral Units on an Icosahedron Kernel

"Core sliding" in metal nanoclusters drives the reconstruction of external structural units and provides an ideal platform for mapping their precise transformation mechanism and evolution pathway. However, observing the movement behavior of metal atoms in experiments is still challenging because of the uncertain stability of intermediates. In this work, a series of Au-Cd alloy nanoclusters with continuously assembled kernels (one icosahedral building block assembled with 0 to 3 tetrahedral units) were constructed. As the assembly continued, it eventually led to the Cd atom doping into the inner positions of the clusters. Importantly, the Cd doped into the interior of the cluster exhibits a different behavior than the surface or external Cd atoms (dispersion doping vs localized occupy), which provides experimental evidence of the sliding behavior in the nanocluster kernel. Furthermore, density functional theory (DFT) calculations reveal that this sliding behavior in the inner sites of nanoclusters is an energetically favorable process. In addition, these Au-Cd nanoclusters exhibit tunable optical properties with different assembly patterns in their kernels.

Cadmium-Doped and Pincer Ligand-Modified Gold Nanocluster for Catalytic KA2 Reaction

A gold nanocluster Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ (PNP=2,6-bis(diphenylphosphinomethyl)pyridine, SR=4-MeOPhS) consisting of an icosahedral Au₁₃ kernel, two Au₂ CdS₆ staple motifs, and two PNP pincer ligands has been designed, synthesized and well characterized. This cadmium and PNP pincer ligand co-modified gold nanocluster showed high catalytic efficiency in the KA² reaction, featuring high TON, mild reaction conditions, broad substrate scope as well as catalyst recyclability. Comparison of the catalytic performance between Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ and the structurally similar single cadmium (or PNP) modified gold nanoclusters demonstrates that the co-existence of the cadmium and PNP on the surface is crucial for the high catalytic activity of the gold nanocluster. This work would be enlightening for developing efficient catalysts for cascade reactions and discovering the catalytic potential of metal nanoclusters in organic transformations.

A bimetallic Ag15Cu12(S-c-C6H11)18(CH3COO)3 nanocluster featuring an irregular Ag12 kernel

Here, we report the synthesis and atomic structure of a Ag₁₅Cu₁₂(SR)₁₈(CH₃COO)₃·(C₆H₁₄) nanocluster (Ag₁₅Cu₁₂ for short, SR denotes cyclohexanethiol), confirmed by single-crystal X-ray diffraction (SC-XRD), electrospray ionization mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). X-ray crystallographic analysis revealed that Ag₁₅Cu₁₂ consisted of an irregular Ag₁₂ core, stabilized by the Ag₃Cu₁₂(SR)₁₈(CH₃COO)₃ shell. The shell consisted of two nearly planar Cu₃(SR)₆ moieties, three monomeric [-SR-Ag-SR-] units and three Cu₂(CH₃COO) staples. Furthermore, time-dependent density functional theory (TD-DFT) simulation was performed to interpret the optical absorption features of Ag₁₅Cu₁₂. Overall, this work will broaden and deepen the understanding of Ag-Cu alloy nanoclusters.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Switchable catalysis and CO2 sensing by reduction resistant, luminescent copper-thiolate complexes

Metal-thiolate complexes have been the focus of research for several years because of their unique photophysical properties and their use as a precursor for synthesizing various well-defined metal nanoclusters. A rational understanding of their structure-property relationship is necessary to realize their full potential in practical applications. Herein, we demonstrate the synthesis of a unique copper-thiolate complex with reversibly switchable catalytic and photoluminescence (PL) properties. The as-synthesized complex at basic pH (Complex B) showed cyan PL with a strong peak at ∼488 nm (cyan) and a small shoulder peak at ∼528 nm (green). When the pH of the complex was changed to acidic (Complex A), the PL was switched to light green. Such pH-responsive PL properties were demonstrated to be useful for pH and CO₂ sensing. The switchable properties originate from their two distinct structural states at two different pHs. We found that Complex A was resistant to high concentrations of a strong reducing agent, and had an intermediate oxidation state of copper (Cu⁺) with good thermodynamic stability. Furthermore, the switchable catalytic property was investigated with a 4-nitrophenol reduction and 3,3',5,5'-tetramethylbenzidine (TMB) oxidation reaction. The reduction kinetics followed pseudo-first-order, where the catalytic activity was enhanced by more than 10³ times when Complex B was switched to Complex A. A similar trend was also observed for TMB oxidation. Our design strategy demonstrates that redox switchable metal-thiolate complexes could be a powerful candidate for a plethora of applications.

Insights into mechanisms of diphosphine-mediated controlled surface construction on Au nanoclusters

Unraveling the rules governing the size regulation of nanoclusters is of great importance not only in fundamental research, but also in practical applications because of the high structure-property correlation in nanoclusters. Diphosphine-mediated size tailoring is recognized as a powerful method for modulating the size, configuration, and properties of nanoclusters, but the role of diphosphines in these size-controlled processes is still poorly understood due to a lack of systematic studies. Herein, using Au₂₃(SR)₁₆^- as the template for modification, the factors influencing the size-modulation of nanoclusters by diphosphines were systematically investigated. It is revealed that by controlling the length of the diphosphines (from shorter to longer), Au₂₁(SR)₁₂L₂⁺ (L = diphosphine) and Au₂₂(SR)₁₄L can be produced. Moreover, introducing a rigid group into the diphosphines can twist the structural framework or lead to the formation of a new surface motif configuration in the nanoclusters, forming twisted Au₂₂(SR)₁₄L and Au₂₅(SR)₁₆L₂⁺. The size regulation of these nanoclusters enables fine-tuning of the optical properties, including the absorption wavelengths and photoluminescence emission intensity, affording an avenue for precise control of the physicochemical properties of nanoclusters for practical applications.

Insight into the Effects of Chiral Diphosphine Ligands on the Structure and Optical Properties of the Au24Cd2 Nanocluster

Introduction of chiral ligands has been regarded as an effective strategy to obtain nanoclusters with optical purity. However, how the chiral ligands work is still unclear due to the lack of structural comparison between racemic nanoclusters and the corresponding optically active ones. In this work, three structurally related Au₂₄Cd₂ nanoclusters, including one racemic and two homochiral nanoclusters, were synthesized, and their crystal structures were characterized using single-crystal X-ray crystallography (SC-XRD). Based on their crystal structures, the origin of the chirality in Au₂₄Cd₂ was found to be the twist of the kernel and the chiral arrangement of the metal-ligand surface. Au₂₄Cd₂ protected with chiral ligands exhibits a more twisted kernel than the racemic one. Therefore, the chirality of chiral diphosphine was found to transfer from the ligands to the metal-ligand interface and then to the metal core, inducing its distortion to produce enhanced chirality. In addition, the optical properties including optical absorption and circular dichroism of these structurally related Au₂₄Cd₂ nanoclusters were compared.

Regulation of Surface Structure of [Au9Ag12(SAdm)4(Dppm)6Cl6](SbF6)3 Nanocluster via Alloying

Tailoring of specific sites on the nanocluster surface can tailor the properties of nanoclusters at the atomic level, for the in-depth understanding of structure and property relationship. In this work, we explore the regulation of surface structure of [Au₉Ag₁₂(SAdm)₄(Dppm)₆Cl₆](SbF₆)₃ nanocluster via alloying. We successfully obtained the well-determined tri-metal [Au₉Ag₈@Cu₄(SAdm)₄(Dppm)₆Cl₆](SbF₆)₃ by the reaction of [Au₉Ag₁₂(SAdm)₄(Dppm)₆Cl₆](SbF₆)₃ with the Cu^I(SAdm) complex precursor. X-ray crystallography identifies that the Cu dopants prioritily replace the position of the silver capped by Dppm ligand in the motif. The Cu doping has affected the optical properties of Au₉Ag₁₂ alloy nanocluster. DPV spectra, CD spectra and stability tests suggest that the regulation of surface structure via Cu alloying changes the electronic structure, thereby affecting the electrochemical properties, which provides insight into the regulation of surface structure of [Au₉Ag₁₂(SAdm)₄(Dppm)₆Cl₆](SbF₆)₃via alloying.

Cu Doping-Induced Transformation from [Ag62 S12 (SBut )32 ]2+ to [Ag62-x Cux S12 (SBut )32 ]4+ Nanocluster

The change in the valence state of nanocluster can induce remarkable changes in the properties and structure. However, achieving the valence state changes in nanoclusters is still a challenge. In this work, we use Cu²⁺ as dopant to "oxidize" [Ag₆₂ S₁₂ (SBu^t )₃₂ ]²⁺ (4 free electrons) to obtain the new nanocluster: [Ag_62-x Cu_x S₁₂ (SBu^t )₃₂ ]⁴⁺ with 2 free electrons. As revealed by its structure, the [Ag_62-x Cu_x S₁₂ (SBu^t )₃₂ ]⁴⁺ (x=10∼21) has a similar structure to that of [Ag₆₂ S₁₂ (SBu^t )₃₂ ]²⁺ precursor and all the Cu atoms occupy the surface site of nanocluster. It's worth noting that with the Cu atoms doping, the [Ag_62-x Cu_x S₁₂ (SBu^t )₃₂ ]⁴⁺ nanocluster is more stable than [Ag₆₂ S₁₂ (SBu^t )₃₂ ]²⁺ at higher temperature and in electrochemical cycle. This result has laid a foundation for the subsequent application and exploration. Overall, this work reveals crystals structure of a new Ag-Cu nanocluster and offers a new insight into the electron reduction/oxidation of nanocluster.

Multiple Ways Realizing Charge-State Transform in AuCu Bimetallic Nanoclusters with Atomic Precision

Thiolate-protected nanoclusters with different charge states usually show similar structure frameworks but different electronic configurations, which are proved to dramatically affect their properties such as magnetism, photoluminescence, and catalytic activity. Until now, few nanoclusters with alterable charge states have been reported and only some of them are structurally solved, limiting the in-depth studies on their interesting properties. Here, a new AuCu alloy nanocluster [Au₁₈ Cu₃₂ (SPhCl)₃₆ ]^2- (HSPhCl = 4-chlorophenylthiophenol) is synthesized and structurally solved by X-ray crystallography. Interestingly, it is found that this nanocluster can be reduced to another nanocluster with a different charge state, that is, [Au₁₈ Cu₃₂ (SPhCl)₃₆ ]^3- . This change in charge states is clearly proved by X-ray crystallography, electrospray ionization mass spectrometry, thermogravimetric analysis, and electron paramagnetic resonance. Furthermore, several redox methods are carried out to realize the reversible interconversion between these two nanoclusters, including electrochemical redox, introduction of H₂ O₂ /NaBH₄ , and oxidation with silica under air atmosphere. This work offers new insight into the transform progress of charge states with AuCu alloy nanoclusters which contributes to the understanding of the relationship between electronic structure and properties of nanoclusters and further development of AuCu nanoclusters with excellent performance.

Thiolate-Protected Metal Nanoclusters: Recent Development in Synthesis, Understanding of Reaction, and Application in Energy and Environmental Field

Metal nanoclusters (NCs), which are composed of about 250 or fewer metal atoms, possess great potential as novel functional materials. Fundamental research on metal NCs gradually started in the 1960s, and since 2000, thiolate (SR)-protected metal NCs have been the main metal NCs actively studied. The precise and systematic isolation of SR-protected metal NCs has been achieved in 2005. Since then, research on SR-protected metal NCs for both basic science and practical application has rapidly expanded. This review describes this recent progress in the field of SR-protected metal NCs in three areas: synthesis, understanding, and application. Specifically, the recent study of alloy NCs and connected structures composed of NCs is highlighted in the "synthesis" section, recent knowledge on the reactivity of NCs in solution is highlighted in the "understanding" section, and the applications of NCs in the energy and environmental field are highlighted in the "application" section. This review provides insight on the current state of research on SR-protected metal NCs and discusses the challenges to be overcome for further development in this field as well as the possibilities that these materials can contribute to solving the problems facing modern society.

A high-nuclearity CuI/CuII nanocluster catalyst for phenol degradation

Herein, we report a 54-nuclei copper nanocluster, [Cu₅₄S₁₃O₆(^tBuS)₂₀(^tBuSO₃)₁₂] (Cu₅₄), which is the largest atom-precise Cu^I/Cu^II mix-valent cluster reported. The Cu₅₄ nanoclusters supported by TiO₂ exhibit decent photocatalytic activity for phenol degradation under visible light. This work provides a platform to explore the catalytic behaviors of Cu^I/Cu^II nanosystems.

Chiral Inversion and Conservation of Clusters: A Case Study of Racemic Ag32Cu12 Nanocluster

Chiral metal nanoclusters have been widely reported, but their separation and optical stabilization remain challenging. We used a deracemization strategy to accomplish the enantioseparation of a racemic mixture of [Ag₃₂Cu₁₂(CH₃COO)₁₂(SAdm)₁₂(P(CH₃OPh)₃)₄] (M₄₄) in a yield exceeding 50%, forming two optically active [Ag₃₂Cu₁₂(R/S-Cl(CH₃)CHCOO)₁₂(SAdm)₁₂(P(CH₃OPh)₃)₄] (R/S-M₄₄') enantiomers. The optical activity of these products was conserved after exchange of the chiral carboxyl ligands with achiral ligand (Br^-), to give two additional optically active nanoclusters R/S-[Ag₂₈Cu₁₆Br₁₂(SAdm)₁₂(P(CH₃OPh)₃)₄] (R/S(Br)-M₄₄). The crystal structures of the above nanoclusters were determined by single-crystal X-ray crystallography. Based on these structures, the chiral transformation and conservation are mapped out.

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.

Simple and Selective Synthesis of Copper-Containing Metal Nanoclusters Using (PPh3 )2 CuBH4 as Reducing Agent

A simple and selective synthetic protocol, using (PPh₃ )₂ CuBH₄ as reducing agent, for Cu-containing mixed metal nanoclusters (NCs) is reported. Representative NCs include alkynyl-protected [Ag₂₅ Cu₄ (PhCC)₁₂ (PPh₃ )₁₂ Cl₆ H₈ ]³⁺ (1), thiolate-capped [AuCu₁₄ (SR)₁₂ (PPh₃ )₆ ]⁺ (R = 4-flurothiophenol) (2), and phosphine-stabilized [Au₉ Cu₂ (PPh₃ )₈ Cl₂ ]⁺ (3), which are fully characterized by single-crystal X-ray diffraction analysis, electrospray ionization mass, nuclear magnetic resonance (¹ H, ² H, ¹³ C, and ³¹ P NMR), and optical measurements, respectively. This work demonstrates the advantages of using (PPh₃ )₂ CuBH₄ as a reducing agent in the synthesis of Cu-containing heterometallic NCs in terms of versatility as well as high yield and high purity of the products. This work may open the door to utilizing functional metal borohydride, as a new generation of reducing agent for the simple and selective synthesis of metal NCs.

New Routes for Multicomponent Atomically Precise Metal Nanoclusters

Atomically precise metal nanoclusters (NCs), protected by a monolayer of ligands, are regarded as potential building blocks for advanced technologies. They are considered as intermediates between the atomic/molecular regime and the bulk. Incorporation of foreign metals in NCs enhances several of their properties such as catalytic activity, luminescence, and so on; hence, it is of high importance for tuning their properties and broadening the scope of applications. In most of the cases, enhancement in specific properties was observed upon alloying due to the synergistic effect. In the past several years, many alloy clusters have been synthesized, which show a tremendous change in the properties than their monometallic analogs. However, controlling the synthesis and tuning the structures of alloy NCs with atomic precision are major challenges. Various synthetic methodologies have been developed so far for the controlled synthesis of alloy NCs. In this perspective, we have highlighted those diverse synthetic routes to prepare alloys, which include co-reduction, galvanic reduction, antigalvanic reduction, metal deposition, ligand exchange, intercluster reaction, and reaction of NCs with bulk metals. Advancement in synthetic procedures will help in the preparation of alloy NCs with the desired structure and composition. Future perceptions concerning the progress of alloy nanocluster science are also provided.

Construction of a new Au27Cd1(SAdm)14(DPPF)Cl nanocluster by surface engineering and insight into its structure–property correlation

Surface engineering with a functional DPPF ligand and Cd atom is employed on a Au38 nanocluster to obtain a Au–Cd alloy nanocluster, that is, Au27Cd1. The difference in properties between Au38 and Au27Cd1 indicates the importance of the surface structure.

Molecular reactivity of thiolate-protected noble metal nanoclusters: synthesis, self-assembly, and applications

Thiolate-protected noble metal (e.g., Au and Ag) nanoclusters (NCs) are ultra-small particles with a core size of less than 3 nm. Due to the strong quantum confinement effects and diverse atomic packing modes in this ultra-small size regime, noble metal NCs exhibit numerous molecule-like optical, magnetic, and electronic properties, making them an emerging family of "metallic molecules". Based on such molecule-like structures and properties, an individual noble metal NC behaves as a molecular entity in many chemical reactions, and exhibits structurally sensitive molecular reactivity to various ions, molecules, and other metal NCs. Although this molecular reactivity determines the application of NCs in various fields such as sensors, biomedicine, and catalysis, there is still a lack of systematic summary of the molecular interaction/reaction fundamentals of noble metal NCs at the molecular and atomic levels in the current literature. Here, we discuss the latest progress in understanding and exploiting the molecular interactions/reactions of noble metal NCs in their synthesis, self-assembly and application scenarios, based on the typical M(0)@M(i)-SR core-shell structure scheme, where M and SR are the metal atom and thiolate ligand, respectively. In particular, the continuous development of synthesis and characterization techniques has enabled noble metal NCs to be produced with molecular purity and atomically precise structural resolution. Such molecular purity and atomically precise structure, coupled with the great help of theoretical calculations, have revealed the active sites in various structural hierarchies of noble metal NCs (e.g., M(0) core, M-S interface, and SR ligand) for their molecular interactions/reactions. The anatomy of such molecular interactions/reactions of noble metal NCs in synthesis, self-assembly, and applications (e.g., sensors, biomedicine, and catalysis) constitutes another center of our discussion. The basis and practicality of the molecular interactions/reactions of noble metal NCs exemplified in this Review may increase the acceptance of metal NCs in various fields.

Control of single-ligand chemistry on thiolated Au25 nanoclusters

Diverse methods have been developed to tailor the number of metal atoms in metal nanoclusters, but control of surface ligand number at a given cluster size is rare. Here we demonstrate that reversible addition and elimination of a single surface thiolate ligand (-SR) on gold nanoclusters can be realized, opening the door to precision ligand engineering on atomically precise nanoclusters. We find that oxidative etching of [Au₂₅SR₁₈]⁻ nanoclusters adds an excess thiolate ligand and generates a new species, [Au₂₅SR₁₉]⁰. The addition reaction can be reversed by CO reduction of [Au₂₅SR₁₉]⁰, leading back to [Au₂₅SR₁₈]⁻ and eliminating precisely one surface ligand. Intriguingly, we show that the ligand shell of Au₂₅ nanoclusters becomes more fragile and rigid after ligand addition. This reversible addition/elimination reaction of a single surface ligand on gold nanoclusters shows potential to precisely control the number of surface ligands and to explore new ligand space at each nuclearity.

Diverse methods have been developed to tailor the number of metal atoms in metal nanoclusters, but controlling the number of surface ligands is rare. Here, the authors realize reversible addition and elimination of a single thiolate ligand on a gold nanocluster.

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.

Precisely modulating the surface sites on atomically monodispersed gold-based nanoclusters for controlling their catalytic performances

Atomically precise gold nanoclusters protected by ligands are being intensely investigated in current catalysis science, due to the definitive correlation between the catalytic properties and structures at an atomic level. By solving the crystal structures of the nanoclusters, coupled with in situ and ex situ spectroscopy, a very fundamental understanding can be achieved to learn what controls the catalytic activation, active site structure, and catalytic mechanism. Herein, we mainly focus on the recent progress in catalysis controlled by precisely modulating the surface structures of the nanoclusters, including the alteration of the surface motifs, the doping of heterogeneous atoms in the surface of the nanoclusters, and the surface ligand engineering. The article is expected to help not only gain deep insight into the crucial roles of surface motifs of the nanoclusters in regulating the catalytic properties, but also explore the wide catalytic applications of atomically precise nanoclusters by elaborately tailoring the surface of the nanoclusters.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

A novel geometric structure of a nanocluster with an irregular kernel: Ag30Cu14(TPP)4(SR)28

Here, we report a bi-ligand protected bimetallic nanocluster Ag₃₀Cu₁₄(TPP)₄(SR)₂₈. It is composed of an Ag₂₇ kernel and Ag₃Cu₁₄(TPP)₄(SR)₂₈ shell. Typically, the metal atoms are arranged irregularly. Both the core and shell exhibit the characteristics of C₂-symmetry.

The synthesis and structure of the [PdAu13(PPh3)3(SR)7]+ nanocluster

Metal alloy nanoclusters have attracted increasing attention due to the synergistic effect of the foreign atoms. For the first time the synthesis and crystal structure of the [PdAu13(PPh3)3(SR)7]+ nanocluster is reported. The crystal structure of the nanocluster was determined by single crystal X-ray diffraction. The [PdAu13(PPh3)3(SR)7]+ nanocluster has a concave polyhedron Au9Pd kernel, which looks like a girl dancing ballet. The structure shows that [PdAu13(PPh3)3(SR)7]+ has an open shell. Meanwhile, we also carried out ultraviolet-visible (Uv-vis) absorption spectroscopy and fluorescence spectroscopy to study the optical properties of the [PdAu13(PPh3)3(SR)7]+ nanocluster.

Metalation of Catechol-Functionalized Defective Covalent Organic Frameworks for Lewis Acid Catalysis

Covalent organic frameworks (COFs) have emerged as a fascinating crystalline porous material and are widely used in the field of catalysis. However, developing simple approaches to fabricate conjugated COFs with specific functional groups remains a significant challenge. In this study, the construction of defective COF-LZU1 with Lewis acid sites embedded into the frameworks is fulfilled by a facile solvent-assisted ligand exchange method. A monodentate ligand, protocatechualdehyde, is successfully introduced into the skeleton of COF-LZU1, which endows the defects in the structure of COF-LZU1 via replacement of the original coordinated benzene-1,3,5-tricarbaldehyde ligand. As-synthesized defective COF-LZU1 decorated with protocatechualdehyde is rich of free hydroxy groups for chelating with active metal ions. Specifically, after combining with Fe³⁺ , the defective COF-LZU1 shows excellent activity in catalytic alcoholysis of epoxides under mild conditions. The method reported here will open up the opportunity to incorporate different functional groups into COFs and enrich the strategies for creating new types of porous catalysts.