Gold Nanocluster-Catalyzed Semihydrogenation: A Unique Activation Pathway for Terminal Alkynes

Multiple neighboring active sites of an atomically precise copper nanocluster catalyst for efficient bond-forming reactions

Atomically precise copper nanoclusters (NCs) are an emerging class of nanomaterials for catalysis. Their versatile core-shell architecture opens the possibility of tailoring their catalytically active sites. Here, we introduce a core-shell copper nanocluster (CuNC), [Cu29(StBu)13Cl5(PPh3)4H10]tBuSO3 (StBu: tert-butylthiol; PPh3: triphenylphosphine), Cu29NC, with multiple accessible active sites on its shell. We show that this nanocluster is a versatile catalyst for C-heteroatom bond formation (C-O, C-N, and C-S) with several advantages over previous Cu systems. When supported, the cluster can also be reused as a heterogeneous catalyst without losing its efficiency, making it a hybrid homogeneous and heterogeneous catalyst. We elucidated the atomic-level mechanism of the catalysis using density functional theory (DFT) calculations based on the single crystal structure. We found that the cooperative action of multiple neighboring active sites is essential for the catalyst's efficiency. The calculations also revealed that oxidative addition is the rate-limiting step that is facilitated by the neighboring active sites of the Cu29NC, which highlights a unique advantage of nanoclusters over traditional copper catalysts. Our results demonstrate the potential of nanoclusters for enabling the rational atomically precise design and investigation of multi-site catalysts.

Advances for Triangular and Sandwich-Shaped All-Metal Aromatics

Much experimental work has been contributed to all-metal σ, π and δ-aromaticity among transition metals, semimetallics and other metals in the past two decades. Before our focused investigations on the properties of triangular and sandwich-shaped all-metal aromatics, A. I. Boldyrev presented general discussions on the concepts of all-metal σ-aromaticity and σ-antiaromaticity for metallo-clusters. Schleyer illustrated that Nucleus-Independent Chemical Shifts (NICS) were among the most authoritative criteria for aromaticity. Ugalde discussed the earlier developments of all-metal aromatic compounds with all possible shapes. Besides the theoretical predictions, many stable all-metal aromatic trinuclear clusters have been isolated as the metallic analogues of either the σ-aromatic molecule's [H3]+ ion or the π-aromatic molecule's [C3H3]+ ion. Different from Hoffman's opinion on all-metal aromaticity, triangular all-metal aromatics were found to hold great potential in applications in coordination chemistry, catalysis, and material science. Triangular all-metal aromatics, which were theoretically proved to conform to the Hückel (4n + 2) rule and possess the smallest aromatic ring, could also play roles as stable ligands during the formation of all-metal sandwiches. The triangular and sandwich-shaped all-metal aromatics have not yet been specifically summarized despite their diversity of existence, puissant developments and various interesting applications. These findings are different from the public opinion that all-metal aromatics would be limited to further applications due to their overstated difficulties in synthesis and uncertain stabilities. Our review will specifically focus on the summarization of theoretical predictions, feasible syntheses and isolations, and multiple applications of triangular and sandwich shaped all-metal aromatics. The appropriateness and necessities of this review will emphasize and disseminate their importance and applications forcefully and in a timely manner.

Alkyne-Functionalized Platinum Chalcogenide (S, Se) Nanoparticles

Metal chalcogenide nanoparticles play a vital role in a wide range of applications and are typically stabilized by organic derivatives containing thiol, amine, or carboxyl moieties, where the nonconjugated particle-ligand interfaces limit the electronic interactions between the inorganic cores and organic ligands. Herein, a wet-chemistry method is developed for the facile preparation of stable platinum chalcogenide (S, Se) nanoparticles capped with acetylene derivatives (e.g., 4-ethylphenylacetylene, EPA). The formation of Pt-C≡ conjugated bonds at the nanoparticle interfaces, which is confirmed by optical and X-ray spectroscopic measurements, leads to markedly enhanced electronic interactions between the d electrons of the nanoparticle cores and π electrons of the acetylene moiety, in stark contrast to the mercapto-capped counterparts with only nonconjugated Pt-S- interfacial bonds, as manifested in spectroscopic measurements and density functional theory calculations. This study underscores the significance of conjugated anchoring linkages in the stabilization and functionalization of metal chalcogenides, a unique strategy for diverse applications.

Spectroscopic and Chemical Properties of Ionic Liquids: Computational Study

A brief account is given of highlights of our computational efforts - often in collaboration with experimental groups - to understand spectroscopic and chemical properties of ionic liquids (ILs). Molecular dynamics, including their inhomogeneous character, responsible for key spectral features observed in dielectric absorption, infra-red (IR) and fluorescence correlation spectroscopy (FCS) measurements are elucidated. Mechanisms of chemical processes involving imidazolium-based ILs are illustrated for CO2 capture and related reactions, transesterification of cellulose, and Au nanocluster-catalyzed Suzuki cross-coupling reaction with attention paid to differing roles of IL ions. A comparison with experiments is also made.

Surface-exposed silver nanoclusters inside molecular metal oxide cavities

The surfaces of metal nanoclusters, including their interface with metal oxides, exhibit a high reactivity that is attractive for practical purposes. This high reactivity, however, has also hindered the synthesis of structurally well-defined hybrids of metal nanoclusters and metal oxides with exposed surfaces and/or interfaces. Here we report the sequential synthesis of structurally well-defined {Ag₃₀} nanoclusters in the cavity of ring-shaped molecular metal oxides known as polyoxometalates. The {Ag₃₀} nanoclusters possess exposed silver surfaces yet are stabilized both in solution and the solid state by the surrounding ring-shaped polyoxometalate species. The clusters underwent a redox-induced structural transformation without undesirable agglomeration or decomposition. Furthermore, {Ag₃₀} nanoclusters showed high catalytic activity for the selective reduction of several organic functional groups using H₂ under mild reaction conditions. We believe that these findings will serve for the discrete synthesis of surface-exposed metal nanoclusters stabilized by molecular metal oxides, which may in turn find applications in, for example, the fields of catalysis and energy conversion.

Programmable kernel structures of atomically precise metal nanoclusters for tailoring catalytic properties

The unclear structures and polydispersity of metal nanoparticles (NPs) seriously hamper the identification of the active sites and the construction of structure-reactivity relationships. Fortunately, ligand-protected metal nanoclusters (NCs) with atomically precise structures and monodispersity have become an ideal candidate for understanding the well-defined correlations between structure and catalytic property at an atomic level. The programmable kernel structures of atomically precise metal NCs provide a fantastic chance to modulate their size, shape, atomic arrangement, and electron state by the precise modulating of the number, type, and location of metal atoms. Thus, the special focus of this review highlights the most recent process in tailoring the catalytic activity and selectivity over metal NCs by precisely controlling their kernel structures. This review is expected to shed light on the in-depth understanding of metal NCs' kernel structures and reactivity relationships.

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.

Selective Semihydrogenation of Polarized Alkynes by a Gold Hydride Nanocluster

The hydridic hydrogen in nanogold catalysts has long been postulated as an important intermediate in hydrogenation reactions, but it has not been directly observed. Here, we report the synthesis of a new undecagold cluster with a bidentate phosphine ligand. The chelating effects of the bidentate ligand result in a more symmetric Au₁₁ core with two labile Cl^- ligands that can exchange with BH₄^-, leading to a novel undecagold hydride cluster. The new hydride cluster is discovered to readily undergo hydroauration reaction with alkynes containing electron-withdrawing groups, forming key gold-alkenyl semihydrogenation intermediates, which can be efficiently and selectively converted to Z-alkenes under acidic conditions. All key reaction intermediates are isolated and characterized, providing atomic-level insights into the active sites and mechanisms of semihydrogenation reactions catalyzed by gold-based nanomaterials. The hydridic hydrogen in the undecagold cluster is found to be the key to prevent over hydrogenation of alkenes to alkanes. The current study provides fundamental insights into hydrogenation chemistry enabled by gold-based nanomaterials and may lead to the development of efficient catalysts for selective semihydrogenation or functionalization of alkynes.

Identification of the Active Species in Bimetallic Cluster Catalyzed Hydrogenation

Identification of the authentic active species of cluster catalysis is rather challenging, and direct structural evidence is quite valuable and difficult to obtain. Two "isostructural" clusters, Ag₂₅Cu₄Cl₆(dppb)₆(PhC≡C)₁₂(SO₃CF₃)₃ (1) and Ag₂₅Cu₄Cl₆H₈(dppb)₆(PhC≡C)₁₂(SO₃CF₃)₃ (2_H) (dppb is 1,4-bis(diphenylphosphine)butane), have been successfully isolated and structurally characterized. Both these clusters have a centered icosahedron Ag₁₃ core with the same peripheral composition and structure. The only difference is that 2_H has eight hydrides but 1 has none, that is, the kernels are Ag₁₃⁵⁺ and Ag₁₃H₈⁵⁺ in 1 and 2_H, respectively. The catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction is assessed with the two clusters. Cluster 2_H is very active with 100% yield within 2 h, whereas 1 shows a very low conversion (∼8%) under the same conditions. Interestingly, high catalytic activity was observed when 1 was converted to 2_H with the oxidation of H₂O₂ under catalytic conditions. The unprecedented transformation of a reduced nanocluster to an Ag(I)Cu(I) bimetallic cluster compound provides an excellent platform to determine the real active cluster in terms of metal cluster catalysis. The present work presents clear structural evidence that the catalytic performance of metal nanoclusters can be modulated by properly regulating the oxidation state of their constituted metal atoms.

Evidence for triplet-state-dominated luminescence in biicosahedral superatomic molecular Au25 clusters

In photoluminescence (PL) quenching and triplet fusion upconversion experiments with fluorescent organic-molecule quenchers, it was revealed that a rod-shaped, phosphine- and thiolate-protected biicosahedral Au₂₅ cluster (a representative di-superatomic molecule) exhibits only phosphorescence, not fluorescence, at room temperature with an intersystem crossing quantum yield of almost 100%. By virtue of these photophysical properties, this cluster can be used as a triplet sensitizer that undergoes direct singlet-triplet transitions in the near-infrared (NIR) region (730-900 nm), inducing photon upconversion from NIR to visible light.

The Factors Dictating Properties of Atomically Precise Metal Nanocluster Electrocatalysts

Metal nanoparticles occupy an important position in electrocatalysis. Unfortunately, by using conventional synthetic methodology, it is a great challenge to realize the monodisperse composition/structure of metal nanoparticles at the atomic level, and to establish correlations between the catalytic properties and the structure of individual catalyst particles. For the study of well-defined nanocatalysts, great advances have been made for the successful synthesis of nanoparticles with atomic precision, notably ligand-passivated metal nanoclusters. Such well-defined metal nanoclusters have become a type of model catalyst and have shown great potential in catalysis research. In this review, the authors summarize the advances in the utilization of atomically precise metal nanoclusters for electrocatalysis. In particular, the factors (e.g., size, metal doping/alloying, ligand engineering, support materials as well as charge state of clusters) affecting selectivity and activity of catalysts are highlighted. The authors aim to provide insightful guidelines for the rational design of electrocatalysts with high performance and perspectives on potential challenges and opportunities in this emerging field.

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.

Electropolymerization of Metal Clusters Establishing a Versatile Platform for Enhanced Catalysis Performance

Atomically precise metal clusters are attractive as highly efficient catalysts, but suffer from continuous efficiency deactivation in the catalytic process. Here, we report the development of an efficient strategy that enhances catalytic performance by electropolymerization (EP) of metal clusters into hybrid materials. Based on carbazole ligand protection, three polymerized metal-cluster hybrid materials, namely Poly-Cu₁₄ cba, Poly-Cu₆ Au₆ cbz and Poly-Cu₆ Ag₄ cbz, were prepared. Compared with isolated metal clusters, metal clusters immobilizing on a biscarbazole network after EP significantly improved their electron-transfer ability and long-term recyclability, resulting in higher catalytic performance. As a proof-of-concept, Poly-Cu₁₄ cba was evaluated as an electrocatalyst for reducing nitrate (NO₃ ^- ) to ammonia (NH₃ ), which exhibited ≈4-fold NH₃ yield rate and ≈2-fold Faraday efficiency enhancement compared to that of Cu₁₄ cba with good durability. Similarly, Poly-Cu₆ Au₆ cbz showed 10 times higher photocatalytic efficiency towards chemical warfare simulants degradation than the cluster counterpart.

Morphology effect of ceria supports on gold nanocluster catalyzed CO oxidation

The interfacial perimeter is generally viewed as the catalytically active site for a number of chemical reactions over oxide-supported nanogold catalysts. Here, well-defined CeO₂ nanocubes, nanorods and nanopolyhedra are chosen to accommodate atomically precise clusters (e.g. Au₂₅(PET)₁₈) to give different Au cluster-CeO₂ interfaces. TEM images show that Au particles of ∼1.3 nm are uniformly anchored on the ceria surface after annealing in air at 120 °C, which can rule out the size hierarchy of nanogold in CO oxidation studies. The gold nanoclusters are only immobilized on the CeO₂(200) facet in Au₂₅/CeO₂-C, while they are selectively loaded on CeO₂(002) and (111) in the Au₂₅/CeO₂-R and Au₂₅/CeO₂-P catalysts. X-ray photoelectron spectroscopy (XPS) and in situ infrared CO adsorption experiments clearly demonstrate that the gold species in the Au₂₅/CeO₂ samples are similar and partially charged (Au ^δ+, where 0 < δ < 1). It is observed that the catalytic activity decreases in the order of Au/CeO₂-R ≈ Au/CeO₂-P > Au/CeO₄-C in the CO oxidation. And the apparent activation energy over Au₂₅/CeO₂-C (60.5 kJ mol^-1) is calculated to be about two-fold of that over the Au₂₅/CeO₂-R (28.6 kJ mol^-1) and Au₂₅/CeO₂-P (31.3 kJ mol^-1) catalysts. It is mainly tailored by the adsorbed [O] species on the ceria surface, namely, Au₂₅/CeO₂(002) and Au₂₅/CeO₂(111) which were more active than the Au₂₅/CeO₂(200) system in the CO oxidation. These insights at the molecular level may provide guidelines for the design of new oxide-supported nanogold catalysts for aerobic oxidations.

Is Aromaticity a Driving Force in Catalytic Cycles? A Case from the Cycloisomerization of Enynes Catalyzed by All-Metal Aromatic Pd3+ Clusters and Carboxylic Acids

The work details a mechanistic study based on density functional theory modeling on the cycloisomerization of polyunsaturated substrates catalyzed by all-metal aromatic tripalladium complexes and carboxylic acids. These clusters are an emerging class of catalysts for a variety of relevant transformations, including C-C forming processes that occur under mild conditions and display synthetic features complementary to those of established mononuclear complexes. This study is the first computational one devoted to the comprehension of the series of elementary steps involved in a synthetic transformation catalyzed by an all-metal aromatic complex. Present results confirm previous experimental hints on the striking mechanistic differences exerted by these clusters with respect to the usual cyclization pathways of related substrates. Moreover, the catalytic cycle involving present all-metal aromatic clusters closely parallels the mechanism of the aromatic substitution of regular arenes.

The ligand effect of atomically precise gold nanoclusters in tailoring catalytic properties

It is well known that surface ligands are vital layers for ligand-protected Au_n nanoclusters. Improving the knowledge of the relationship between ligands and catalytic properties is a forefront research topic for Au_n nanoclusters. Enormous effort has been devoted to realizing the ligand effect in synthesis, including well-controlled sizes and shapes as well as structural transformation. However, the crucial function of surface ligands has not been addressed yet in catalytic reactions. Here, this review mainly aims to summarize the recent progress concerning the influence of surface ligand layers on catalytic activity and selectivity, based on the various types of ligand protected Au_n nanoclusters. Besides, the potential challenges and opportunities of Au_n nanoclusters are indicated, mainly in terms of surface ligands to guide the improvement of catalytic performances.

Creation of High-Performance Heterogeneous Photocatalysts by Controlling Ligand Desorption and Particle Size of Gold Nanocluster

Recently, the creation of new heterogeneous catalysts using the unique electronic/geometric structures of small metal nanoclusters (NCs) has received considerable attention. However, to achieve this, it is extremely important to establish methods to remove the ligands from ligand-protected metal NCs while preventing the aggregation of metal NCs. In this study, the ligand-desorption process during calcination was followed for metal-oxide-supported 2-phenylethanethiolate-protected gold (Au) 25-atom metal NCs using five experimental techniques. The results clearly demonstrate that the ligand-desorption process consists of ligand dissociation on the surface of the metal NCs, adsorption of the generated compounds on the support and desorption of the compounds from the support, and the temperatures at which these processes occurred were elucidated. Based on the obtained knowledge, we established a method to form a metal-oxide layer on the surface of Au NCs while preventing their aggregation, thereby succeeding in creating a water-splitting photocatalyst with high activity and stability.

Hydride- and halide-substituted Au9(PH3)83+ nanoclusters: similar absorption spectra disguise distinct geometries and electronic structures

Ligands dramatically affect the electronic structure of gold nanoclusters (NCs) and provide a useful handle to tune the properties required for nanomaterials that have high performance for important functions like catalysis. Recently, questions have arisen about the nature of the interactions of hydride and halide ligands with Au NCs: hydride and halide ligands have similar effects on the absorption spectra of Au₉ NCs, which suggested that the interactions of the two classes of ligands with the Au core may be similar. Here, we elucidate the interactions of halide and hydride ligands with phosphine-protected gold clusters via theoretical investigations. The computed absorption spectra using time-dependent density functional theory are in reasonable agreement with the experimental spectra, confirming that the computational methods are capturing the ligand-metal interactions accurately. Despite the similarities in the absorption spectra, the hydride and halide ligands have distinct geometric and electronic effects. The hydride ligand behaves as a metal dopant and contributes its two electrons to the number of superatomic electrons, while the halides act as electron-withdrawing ligands and do not change the number of superatomic electrons. Clarifying the binding modes of these ligands will aid in future efforts to use ligand derivatization as a powerful tool to rationally design Au NCs for use in functional materials.

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.

Cluster From Cluster: A Quantitative Approach to Magic Gold Nanoclusters [Au25 (SR)18 ]. Angew Chem Int Ed Engl 2021;60:14415-14419. [PMID: 33829603 DOI: 10.1002/anie.202103290]

High-yield and large-scale synthesis are highly demanded for the studies of gold nanoclusters. We developed a "cluster from cluster" approach to assemble gold nanoclusters with preformed atomically precise Au₁₃ precursors. This facile approach has proved to be very effective in the synthesis of the well-known magic cluster [Au₂₅ (SR)₁₈ ]^- , which could prepare the target cluster in high yield (overall yield up to ≈100 %) at large scale (gram-scale based on gold). This method can be applied in the synthesis of 10 Au₂₅ clusters with different R groups. This work presents an important approach that may be extended to high-yield and large-scale synthesis of other metal nanoclusters.

Binding of chloroaurate to polytyrosine-PEG micelles leads to an anti-Turkevich pattern of reduction

Here we report formation of gold nanoparticles (GNPs) in micelles of polytyrosine-PEG copolymers that combine the properties of a reducer and a stabilizer. The size and properties of the GNPs were tailored by the excess chloroaurate over the copolymer. The latter quickly formed non-covalent complexes with HAuCl4 and then slowly reduced it to form GNPs. 3 Tyr residues are consumed by reduction of one mole of chloroaurate. The size of the GNPs was controlled by the [Tyr]/[Au(iii)] molar ratio. Small GNPs with D ≅ 8 nm were formed at [Tyr]/[Au(iii)] = 0.5-1.5. 90% of these small GNPs remained bound to the copolymer and could be stored in a lyophilized state. Such polypeptide-gold hybrid materials produced at [Tyr]/[Au(iii)] = 0.5 demonstrated high activity in the catalytic reduction of 4-nitrophenol by sodium borohydride. [Tyr]/[Au(iii)] = 5 led to the formation of large nanoplates (D ≅ 30-60 nm). Thus, in the polymer-based system the GNP size grew in line with the excess of the reducing agent in contrast to Turkevich synthesis of GNPs with citric acid, which also combines the functions of a stabilizer and a reducer. The difference results from the reduction of HAuCl4 in solution according to the Turkevich method and in the micelles of the amphiphilic polymer where the seed growth is limited by the amount of neighboring reducer.

Au101-rGO nanocomposite: immobilization of phosphine-protected gold nanoclusters on reduced graphene oxide without aggregation

Graphene supported transition metal clusters are of great interest for potential applications, such as catalysis, due to their unique properties. In this work, a simple approach to deposit Au₁₀₁(PPh₃)₂₁Cl₅ (Au₁₀₁NC) on reduced graphene oxide (rGO) via an ex situ method is presented. Reduction of graphene oxide at native pH (pH ≈ 2) to rGO was performed under aqueous hydrothermal conditions. Decoration of rGO sheets with controlled content of 5 wt% Au was accomplished using only pre-synthesised Au₁₀₁NC and rGO as precursors and methanol as solvent. High resolution scanning transmission electron microscopy indicated that the cluster size did not change upon deposition with an average diameter of 1.4 ± 0.4 nm. It was determined that the rGO reduction method was crucial to avoid agglomeration, with rGO reduced at pH ≈ 11 resulting in agglomeration. X-ray photoelectron spectroscopy was used to confirm the deposition of Au₁₀₁NCs and show the presence of triphenyl phosphine ligands, which together with attenuated total reflectance Fourier transform infrared spectroscopy, advocates that the deposition of Au₁₀₁NCs onto the surface of rGO was facilitated via non-covalent interactions with the phenyl groups of the ligands. Inductively coupled plasma mass spectrometry and thermogravimetric analysis were used to determine the gold loading and both agree with a gold loading of ca. 4.8-5 wt%. The presented simple and mild strategy demonstrates that good compatibility between size-specific phosphine protected gold clusters and rGO can prevent aggregation of the metal clusters. This work contributes towards producing an agglomeration-free synthesis of size-specific ligated gold clusters on rGO that could have wide range of applications.

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.

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.

ESI-MS Identification of the Cationic Phosphine-Ligated Gold Clusters Au1-22: Insight into the Gold-Ligand Ratio and Abundance of Larger Clusters

Triphenylphosphine (PPh₃)-ligated gold nanoclusters are valuable for a number of potential applications due to their relative ease of synthesis and usefulness in forming advanced cluster architectures. While previous studies have reported cationic PPh₃-ligated gold clusters with core sizes of Au_1-4, Au_6-11, and Au_13-14, there has not been definitive identification by mass spectrometry of many larger clusters in the Au_12-25 range. Herein, we survey a polydisperse solution of cationic PPh₃-ligated gold clusters using high-mass-resolution (M/ΔM = 60,000) electrospray ionization mass spectrometry (ESI-MS). To improve the sensitivity and mass resolution of larger clusters for unambiguous identification, we increased the number of scan averages and reduced the range of mass collection windows to 200 m/z, thereby mitigating potential mass and ion abundance bias resulting from smaller "building block" gold clusters that are present in much higher abundance in solution. In addition to the previously reported clusters, we identify several new species including Au₅(PPh₃)₅⁺, Au₁₂(PPh₃)₉HCl²⁺, Au₁₅(PPh₃)₉Cl²⁺, Au₁₆(PPh₃)₁₀Cl₂²⁺, Au₁₇(PPh₃)₁₁³⁺, Au₁₈(PPh₃)₁₀²⁺, Au₁₉(PPh₃)₁₀Cl²⁺, Au₂₀(PPh₃)₁₂H₃³⁺, Au₂₁(PPh₃)₁₀Cl²⁺, and Au₂₂(PPh₃)₁₀Cl₂²⁺, indicating that a full range of clusters between Au_1-22 may be observed in a single polydisperse solution. Considering all of the clusters observed, our findings provide evidence that the Au_12-14 size range is a critical transition point in cluster nucleation. While smaller clusters exhibit a 1:1 gold-to-ligand ratio, larger clusters (beginning Au_12-14) feature additional gold atoms without an equal number of accompanying ligands. Our results support previous evidence in the literature indicating that the "magic number" icosahedral Au₁₃ geometry is the smallest cluster size where a ligand-less central gold atom is coordinated by a complete shell of 12 surrounding ligated gold atoms, thereby creating a stable "one-shell" cluster. Furthermore, our findings reinforce growing evidence that ligands may be used to actively direct gold cluster size and abundance during synthesis. While for PPh₃-ligated systems the most abundant species are Au_6-9 clusters, we find that for related methyldiphenylphosphine (PPh₂Me) and dimethylphenylphosphine (PPhMe₂)-ligated systems the most abundant cluster sizes are Au_10-11 and Au_12-14, respectively. Together, we demonstrate that reducing the range of m/z collection windows and increasing the number of scan averages dramatically improves instrument sensitivity for cationic gold clusters, enabling thorough characterization of polydisperse solutions that is not possible using conventional techniques.

Clean protocol for deoxygenation of epoxides to alkenes via catalytic hydrogenation using gold

Au NP catalyst combined with triethylphosphite, P(OEt)₃, is remarkably more reactive than solely Au NPs for the selective deoxygenation of epoxides to alkenes.

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.