Toward Solution Syntheses of the Tetrahedral Au20 Pyramid and Atomically Precise Gold Nanoclusters with Uncoordinated Sites

Metal-ligand bond in group-11 complexes and nanoclusters

Density functional theory is used to study geometric, energetic, and electronic properties of metal-ligand bonds in a series of group-11 metal complexes and ligand-protected metal clusters. We study complexes as the forms of M-L (L = SCH3, SC8H9, PPh3, NHCMe, NHCEt, NHCiPr, NHCBn, CCMe, CCPh) and L1-M-L2 (L1 = NHCBn, PPh3, and L2 = CCPh). Furthermore, we study clusters denoted as [M13L6Br6]- (L = PPh3, NHCMe, NHCEt, NHCiPr, NHCBn). The systems were studied at the standard GGA level using the PBE functional and including vdW corrections via BEEF-vdW. Generally, Au has the highest binding energies, followed by Cu and Ag. PBE and BEEF-vdW functionals show the order Ag-L > Au-L > Cu-L for bond lengths in both M-L complexes and metal clusters. In clusters, the smallest side group (CH3) in NHCs leads to the largest binding energy whereas no significant variations are seen concerning different side groups of NHC in M-L complexes. By analyzing the projected density of states and molecular orbitals in complexes and clusters, the M-thiolate bonds were shown to have σ and π bond characteristics whereas phosphines and carbenes were creating σ bonds to the transition metals. Interestingly, this analysis revealed divergent behavior for M-alkynyl complexes: while the CCMe group displayed both σ and π bonding features, the CCPh ligand was found to possess only σ bond properties in direct head-to-head binding configuration. Moreover, synergetic effects increase the average binding strength to the metal atom significantly in complexes of two different ligands and underline the potential of adding Cu to synthesize structurally richer cluster systems. This study helps in understanding the effects of different ligands on the stability of M-L complexes and clusters and suggests that PPh3 and NHCs-protected Cu clusters are most stable after Au clusters.

On the halide aggregation into the [Au4(PPh3)4]4+ cluster core. Insights from structural, optical and interaction energy analysis in [(Ph3PAu)4X2]2+ and [(Ph3PAu)4X]3+ species (X = Cl-, Br-, I-)

The aggregation of halide atoms into gold clusters offers an interesting scenario for the development of novel metal-based cavities for anion recognition and sensing applications. Thus, further understanding of the different contributing terms leading to efficient cluster-halide aggregation is relevant to guide their synthetic design. In this report, we evaluate the formation of [(Ph3PAu)4X2]2+ and [(Ph3PAu)4X]3+ species (X = Cl-, Br-, I-) in terms of different energy contributions underlying the stabilization of the cluster-halide interaction, and the expected UV-vis absorption profiles as a result of the variation in frontier orbital arrangements. Our results denote that a non-planar Au4 core shape enables enhanced halide aggregation, which is similar for Cl-, Br-, and I-, in comparison to the hypothetical planar Au4 counterparts. The electrostatic nature of the interaction involves a decreasing ion-dipole term along with the series, and for iodine species, higher-order electrostatic contributions become more relevant. Hence, the obtained results help in gaining further understanding of the different stabilizing and destabilizing contributions to suitable cluster-based cavities for the incorporation of different monoatomic anions.

Site-Recognition-Induced Structural and Photoluminescent Evolution of the Gold-Pincer Nanocluster

The induced structural transformation provides an efficient way to precisely modulate the fine structures and the corresponding performance of gold nanoclusters, thus constituting one of the important research topics in cluster chemistry. However, the driving forces and mechanisms of these processes are still ambiguous in many cases, limiting further applications. In this work, based on the unique coordination mode of the pincer ligand-stabilized gold nanocluster Au8(PNP)4, we revealed the site-recognition mechanism for induced transformations of gold nanoclusters. The "open nitrogen sites" on the surface of the nanocluster interact with different inducers including organic compounds and metals and trigger the conversion of Au8(PNP)4 to Au13 and Au9Ag4 nanoclusters, respectively. Control experiments verified the site-recognition mechanism, and the femtosecond and nanosecond transient absorption spectroscopy revealed the electronic and photoluminescent evolution accompanied by the structural transformation.

Understanding the decomposition process of the Pt1Ag24(SPhCl2)18 nanocluster at the atomic level

We report the decomposition of the Pt1Ag24(SPhCl2)18 nanocluster into a crown-like Pt1Ag4(SR)8 (SR = 2,4-SPhCl2 and 4-SPhBr) complex. UV-vis spectra and single crystal X-ray diffraction were used to track the structure-conversion process. Based on the total structure, the differences in ligand exchange rates at different sites and the effects on the stability were mapped out. This work can not only help us understand the ligand exchange behavior of the clusters, but also provide experimental support for the design of stable metal clusters.

Tiara-like Hexanuclear Nickel-Platinum Alloy Nanocluster

Tiara-like metal nanoclusters (TNCs) have attracted a great deal of attention because of their high stability and easy synthesis under atmospheric conditions as well as their high activity in various catalytic reactions. Alloying is one of the methods that can be used to control the physicochemical properties of nanoclusters, but few studies have reported on alloy TNCs. In this study, we synthesized alloy TNCs [NixPt6-x(PET)12, where x = 1-5 and PET = 2-phenylethanethiolate] consisting of thiolate, nickel (Ni), and platinum (Pt). We further evaluated the stability, geometric structure, and electronic structure by high-performance liquid chromatography and density functional theory calculations. The results revealed that NixPt6-x(PET)12 has a distorted structure and is therefore less stable than single-metal TNCs.

Reduction-Oxidation Cascade Strategy for Reforming a Au13-Kerneled Gold Thiolate Nanocluster

Gold nanoclusters protected by thiolate ligands are ideal models for investigating the structure-property correlation of nanomaterals. Introducing relatively weak coordinating ligands into gold thiolate nanoclusters and thus reforming their structures is beneficial for further releasing their activities. However, controlling the selectivity of the process is a challenging task. In this work, we report a cascade strategy for deeply and purposefully reforming the structures of gold thiolate nanoclusters, exemplified by a Au13-kerneled Au23 nanocluster. Specifically, weakly coordinated triphenylphosphine was utilized to reduce (activate) the surface of Au23, enabling its further structural reformation by the following oxidation step. A structurally distinctive Au20 nanocluster was obtained based on this reduction-oxidation cascade strategy. Mechanism studies reveal that both the reduction and oxidation steps and their working sequence are critical for the transformation. Theoretical and experimental results all indicate that the deep structural reformation results in the evolution of the electronic and photoluminescent properties of the gold thiolate nanocluster.

Nanocluster Surface Microenvironment Modulates Electrocatalytic CO2 Reduction

The catalytic activity and product selectivity of the electrochemical CO2 reduction reaction (eCO2 RR) depend strongly on the local microenvironment of mass diffusion at the nanostructured catalyst and electrolyte interface. Achieving a molecular-level understanding of the electrocatalytic reaction requires the development of tunable metal-ligand interfacial structures with atomic precision, which is highly challenging. Here, the synthesis and molecular structure of a 25-atom silver nanocluster interfaced with an organic shell comprising 18 thiolate ligands are presented. The locally induced hydrophobicity by bulky alkyl functionality near the surface of the Ag25 cluster dramatically enhances the eCO2 RR activity (CO Faradaic efficiency, FECO : 90.3%) with higher CO partial current density (jCO ) in an H-cell compared to Ag25 cluster (FECO : 66.6%) with confined hydrophilicity, which modulates surface interactions with water and CO2 . Remarkably, the hydrophobic Ag25 cluster exhibits jCO as high as -240 mA cm-2 with FECO >90% at -3.4 V cell potential in a gas-fed membrane electrode assembly device. Furthermore, this cluster demonstrates stable eCO2 RR over 120 h. Operando surface-enhanced infrared absorption spectroscopy and theoretical simulations reveal how the ligands alter the neighboring water structure and *CO intermediates, impacting the intrinsic eCO2 RR activity, which provides atomistic mechanistic insights into the crucial role of confined hydrophobicity.

Photoluminescence control by atomically precise surface metallization of C-centered hexagold(i) clusters using N-heterocyclic carbenes

The properties of metal clusters are highly dependent on their molecular surface structure. The aim of this study is to precisely metallize and rationally control the photoluminescence properties of a carbon(C)-centered hexagold(i) cluster (CAu^I₆) using N-heterocyclic carbene (NHC) ligands with one pyridyl, or one or two picolyl pendants and a specific number of silver(i) ions at the cluster surface. The results suggest that the photoluminescence of the clusters depends highly on both the rigidity and coverage of the surface structure. In other words, the loss of structural rigidity significantly reduces the quantum yield (QY). The QY in CH₂Cl₂ is 0.04 for [(C)(Au^I-BIPc)₆Ag^I₃(CH₃CN)₃](BF₄)₅ (BIPc = N-isopropyl-N'-2-picolylbenzimidazolylidene), a significant decrease from 0.86 for [(C)(Au^I-BIPy)₆Ag^I₂](BF₄)₄ (BIPy = N-isopropyl-N'-2-pyridylbenzimidazolylidene). This is due to the lower structural rigidity of the ligand BIPc because it contains a methylene linker. Increasing the number of capping Ag^I ions, i.e., the coverage of the surface structure, increases the phosphorescence efficiency. The QY for [(C)(Au^I-BIPc²)₆Ag^I₄(CH₃CN)₂](BF₄)₆ (BIPc² = N,N'-di(2-pyridyl)benzimidazolylidene) recovers to 0.40, 10-times that of the cluster with BIPc. Further theoretical calculations confirm the roles of Ag^I and NHC in the electronic structures. This study reveals the atomic-level surface structure-property relationships of heterometallic clusters.

Structural water molecules dominated p band intermediate states as a unified model for the origin on the photoluminescence emission of noble metal nanoclusters: from monolayer protected clusters to cage confined nanoclusters

In the past several decades, noble metal nanoclusters (NMNCs) have been developed as an emerging class of luminescent materials due to their superior photo-stability and biocompatibility, but their luminous quantum yield is relatively low and the physical origin of the bright photoluminescence (PL) of NMNCs remain elusive, which limited their practical application. As the well-defined structure and composition of NMNCs have been determined, in this mini-review, the effect of each component (metal core, ligand shell and interfacial water) on their PL properties and corresponded working mechanism were comprehensively introduced, and a model that structural water molecules dominated p band intermediate state was proposed to give a unified understanding on the PL mechanism of NMNCs and a further perspective to the future developments of NMNCs by revisiting the development of our studies on the PL mechanism of NMNCs in the past decade.

Intermediate Silver Doping of Au25(SR)18: Variation of Electronic, Optical, and Chiroptical Properties along Au25-xAgx(SH)18- (x = 0-12) Stoichiometry from DFT Calculations

The silver analogue of the prominent Au₂₅(SR)₁₈ nanocluster reveals the possibility of finding "gold"-like behavior despite their different nature, in addition to the common features among molecular AgNP. Herein, we explore the effect of successive additions of silver atoms reaching an intermediate Ag/Au doping ratio where the parent gold cluster exhibits properties from both elements. Our results show a more favorable situation as the Ag/Au ratio increases along the Au_25-xAg_x(SH)₁₈^- (x = 0-12) clusters, with structural distortions mainly centered at the ligand-protected shell. The calculated optical spectrum shows that from the Au₁₉Ag₆ species, a plasmon-like peak appears along species with a doping ratio above 25%, where all the silver atoms are located within the M₁₂ icosahedron. In addition, the chiral properties were explored, showing mild optical activity from the calculated circular dichroism spectra due to the distorted ligand-shell avoiding a centrosymmetric structure. Thus, an intermediate doping ratio ascribed to a specific structural layer can recover inherent properties to both elements in the binary Au_25-xAg_x(SH)₁₈^- series, suggesting the possibility of having clusters with dual properties at a certain degree of element exchange. This can be useful for further exploration theoretically and synthetically toward different and larger-nuclearity clusters.

A first glance into mixed phosphine-stibine moieties as protecting ligands for gold clusters

Atomically precise gold clusters have attracted considerable research interest as their tunable structure-property relationships have resulted in widespread applications, from sensing and biomedicine to energetic materials and catalysis. In this article, the synthesis and optical properties of a novel [Au₆(SbP₃)₂][PF₆]₂ cluster are reported. Despite the lack of spherical symmetry in the core, the cluster shows exceptional thermal and chemical stability. Detailed structural attributes and optical properties are evaluated experimentally and theoretically. This, to the best of our knowledge, is the first report of a gold cluster protected via synergistic multidentate coordination of stibine (Sb) and phosphine moieties (P). To further show that the latter moieties give a set of unique properties that differs from monodentate phosphine-protected [Au₆(PPh₃)₆]²⁺, geometric structure, electronic structure, and optical properties are analyzed theoretically. In addition, this report also demonstrates the critical role of overall-ligand architecture in stabilizing mixed ligand-protected gold clusters.

Key factors for connecting silver-based icosahedral superatoms by vertex sharing

Metal nanoclusters composed of noble elements such as gold (Au) or silver (Ag) are regarded as superatoms. In recent years, the understanding of the materials composed of superatoms, which are often called superatomic molecules, has gradually progressed for Au-based materials. However, there is still little information on Ag-based superatomic molecules. In the present study, we synthesise two di-superatomic molecules with Ag as the main constituent element and reveal the three essential conditions for the formation and isolation of a superatomic molecule comprising two Ag_13-xM_x structures (M = Ag or other metal; x = number of M) connected by vertex sharing. The effects of the central atom and the type of bridging halogen on the electronic structure of the resulting superatomic molecule are also clarified in detail. These findings are expected to provide clear design guidelines for the creation of superatomic molecules with various properties and functions.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Structure and assembly of a hexanuclear AuNi bimetallic nanocluster

An Au₄Ni₂ nanocluster containing a square-planar [-PPh₂-Au-S-Au-]₂ ring and two nickel-pincer arms is reported here. Abundant intra- and inter-cluster noncovalent interactions promote the assembly of the nanocluster into a porous framework material. The assembly-dependent unique solubility and photoluminescence were also investigated.

Au 20 ( t Bu 3 P) 8 : A Highly Symmetric Metalloid Gold Cluster in Oxidation State 0

Metalloid gold clusters have unique properties with respect to size and structure and are key intermediates in studying transitions between molecular compounds and the bulk phase of the respective metal. In the following, the synthesis of the all‐phosphine protected metalloid cluster Au₂₀(^tBu₃P)₈, solely built from gold atoms in the oxidation state of 0 is reported. Single‐crystal X‐ray analysis revealed a highly symmetric hollow cube‐octahedral arrangement of the gold atoms, resembling gold bulk structure. Quantum‐chemical calculations illustrated the cluster can be described as a 20‐electron superatom. Optical properties of the compound have shown molecular‐like behavior.

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.

Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

Direct conversion of methane (CH₄) to C_1-2 liquid oxygenates is a captivating approach to lock carbons in transportable value-added chemicals, while reducing global warming. Existing approaches utilizing the transformation of CH₄ to liquid fuel via tandemized steam methane reforming and the Fischer-Tropsch synthesis are energy and capital intensive. Chemocatalytic partial oxidation of methane remains challenging due to the negligible electron affinity, poor C-H bond polarizability, and high activation energy barrier. Transition-metal and stoichiometric catalysts utilizing harsh oxidants and reaction conditions perform poorly with randomized product distribution. Paradoxically, the catalysts which are active enough to break C-H also promote overoxidation, resulting in CO₂ generation and reduced carbon balance. Developing catalysts which can break C-H bonds of methane to selectively make useful chemicals at mild conditions is vital to commercialization. Single atom catalysts (SACs) with specifically coordinated metal centers on active support have displayed intrigued reactivity and selectivity for methane oxidation. SACs can significantly reduce the activation energy due to induced electrostatic polarization of the C-H bond to facilitate the accelerated reaction rate at the low reaction temperature. The distinct metal-support interaction can stabilize the intermediate and prevent the overoxidation of the reaction products. The present review accounts for recent progress in the field of SACs for the selective oxidation of CH₄ to C_1-2 oxygenates. The chemical nature of catalytic sites, effects of metal-support interaction, and stabilization of intermediate species on catalysts to minimize overoxidation are thoroughly discussed with a forward-looking perspective to improve the catalytic performance.

A Heteroleptic Gold Hydride Nanocluster for Efficient and Selective Electrocatalytic Reduction of CO2 to CO

It has been a long-standing challenge to create and identify the active sites of heterogeneous catalysts, because it is difficult to precisely control the interfacial chemistry at the molecular level. Here we report the synthesis and catalysis of a heteroleptic gold trihydride nanocluster, [Au₂₂H₃(dppe)₃(PPh₃)₈]³⁺ [dppe = 1,2-bis(diphenylphosphino)ethane, PPh₃ = triphenylphosphine]. The Au₂₂H₃ core consists of two Au₁₁ units bonded via six uncoordinated Au sites. The three H atoms bridge the six uncoordinated Au atoms and are found to play a key role in catalyzing electrochemical reduction of CO₂ to CO with a 92.7% Faradaic efficiency (FE) at -0.6 V (vs RHE) and high reaction activity (134 A/g_Au mass activity). The CO current density and FE_CO remained nearly constant for the CO₂ reduction reaction for more than 10 h, indicating remarkable stability of the Au₂₂H₃ catalyst. The Au₂₂H₃ catalytic performance is among the best Au-based catalysts reported thus far for electrochemical reduction of CO₂. Density functional theory (DFT) calculations suggest that the hydride coordinated Au sites are the active centers, which facilitate the formation of the key *COOH intermediate.

Isomer dynamics of the [Au6(NHC-S)4]2+ nanocluster

The use of metal nanoclusters is strongly reliant on their size and configuration; hence, studying the potential isomers of a cluster is extremely beneficial in understanding their performance. In general, the prediction and identification of isomer structures and their properties can be challenging and computationally expensive. Our work describes an investigation to find local isomers for the previously experimentally characterized small gold cluster [Au₆(NHC-S)₄]²⁺ protected by bidentate mixed carbene-thiolate ligands. We employ the molecular dynamics simulation method where the interatomic forces are calculated from density functional theory. We find several isomers that are more stable than the isomer corresponding to the experimental crystal structure, as well as a significant impact of the finite-temperature atom dynamics on the electronic structure and optical properties. Our work highlights the growing need to investigate ligand-stabilized metal clusters to uncover isomerism and temperature effects on their properties.

Exploring the materials space in the smallest particle size range: From heterogeneous catalysis to electrocatalysis and photocatalysis

Ultrasmall clusters of subnanometer size can possess unique and even unexpected physical and chemical propensities which make them interesting in various fields of basic science and for potential applications, such...

Atomically dispersed metal catalysts on nanodiamond and its derivatives: synthesis and catalytic application

Atomically dispersed metal catalysts (ADMCs) have attracted increasing interest in the field of heterogeneous catalysis. As sub-nanometric catalysts, ADMCs have exhibited remarkable catalytic performance in many reactions. ADMCs are classified into two categories: single atom catalysts (SACs) and atomically dispersed clusters with a few atoms. To stabilize the highly active ADMCs, nanodiamond (ND) and its derivatives (NDDs) are promising supports. In this Feature Article, we have introduced the advantages of NDDs with a highly curved surface and tunable surface properties. The controllable defective sites and oxygen functional groups are known as the anchoring sites for ADMCs. Tunable surface acid-base properties enable ADMCs supported on NDDs to exhibit unique selectivity towards target products and an extended lifetime in many reactions. In addition, we have firstly overviewed the recent advances in the synthesis strategies for effectively fabricating ADMCs on NDDs, and further discussed how to achieve the atomic dispersion of metal precursors and stabilize the as-formed metal atoms against migration and agglomeration based on NDDs. And then, we have also systematically summarized the advantages of ADMCs supported on NDDs in reactions, including hydrogenation, dehydrogenation, aerobic oxidation and electrochemical reaction. These reactions can also effectively guide the design of ADMCs. The recent progress in understanding the effect of structure of active centers and metal-support interactions (MSIs) on the catalytic performance of ADMCs is particularly highlighted. At last, the possible research directions in ADMCs are forecasted.

A 20-core copper(I) nanocluster as electron-hole recombination inhibitor on TiO2 nanosheets for enhancing photocatalytic H2 evolution

For the design of atom-precise copper nanoclusters, besides the exploration of their aesthetic cage-like architectures, their structural modulation and potential applications are being extensively explored. Herein, an atom-precise 20-core copper(I)-alkynyl nanocluster (UJN-Cu20) protected by ethinyloestradiol ligands issynthesized. By virtue of outer-shell hydroxyl groups, UJN-Cu20 could be uniformly modified on the surface of TiO₂ nanosheets via hydrogen bonding interactions, thus forming an efficient nanocomposite photocatalyst for hydrogen evolution. By constructing a Z-scheme heterojunction, the photocatalytic hydrogen evolution activity of the nanocomposite (13 mmol g^-1 h^-1) significantly improved as compared to that of TiO₂ nanosheets (0.4 mmol g^-1 h^-1). As a narrow bandgap cocatalyst, UJN-Cu20 is confirmed to effectively inhibit the electron-hole recombination on the surface of the TiO₂ nanosheet, which provides a new concept for the design of copper cluster-assisted effective photocatalysts.

Effects of ligands on (de-)enhancement of plasmonic excitations of silver, gold and bimetallic nanoclusters: TD-DFT+TB calculations

Metal nanoclusters can be synthesized in various sizes and shapes and are typically protected with ligands to stabilize them. These ligands can also be used to tune the plasmonic properties of the clusters as the absorption spectrum of a protected cluster can be significantly altered compared to the bare cluster. In this paper, we computationally investigate the influence of thiolate ligands on the plasmonic intensity for silver, gold and alloy clusters. Using time-dependent density functional theory with tight-binding approximations, TD-DFT+TB, we show that this level of theory can reproduce the broad experimental spectra of Au₁₄₄(SR)₆₀ and Ag₅₃Au₉₁(SR)₆₀ (R = CH₃) compounds with satisfactory agreement. As TD-DFT+TB does not depend on atom-type parameters we were able to apply this approach on large ligand-protected clusters with various compositions. With these calculations we predict that the effect of ligands on the absorption can be a quenching as well as an enhancement. We furthermore show that it is possible to unambiguously identify the plasmonic peaks by the scaled Coulomb kernel technique and explain the influence of ligands on the intensity (de-)enhancement by analyzing the plasmonic excitations in terms of the dominant orbital contributions.

The synthesis and characterization of a new diphosphine-protected gold hydride nanocluster

Gold is the most inert metal and does not form a bulk hydride. However, gold becomes chemically active in the nanometer scale and gold nanoparticles have been found to exhibit important catalytic properties. Here, we report the synthesis and characterization of a highly stable ligand-protected gold hydride nanocluster, [Au₂₂H₃(dppee)₇]³⁺ [dppee = bis(2-diphenylphosphino) ethyl ether]. A synthetic method is developed to obtain high purity samples of the gold trihydride nanocluster with good yields. The properties of the new hydride cluster are characterized with different experimental techniques, as well as theoretical calculations. Solid samples of [Au₂₂H₃(dppee)₇]³⁺ are found to be stable under ambient conditions. Both experimental evidence and theoretical evidence suggest that the Au₂₂H₃ core of the [Au₂₂H₃(dppee)₇]³⁺ hydride nanocluster consists of two Au₁₁ units bonded via two triangular faces, creating six uncoordinated Au sites at the interface. The three H atoms bridge the six uncoordinated Au atoms at the interface. The Au₁₁ unit behaves as an eight-electron trivalent superatom, forming a superatom triple bond (Au₁₁ ≡ Au₁₁) in the [Au₂₂H₃(dppee)₇]³⁺ trihydride nanocluster assisted by the three bridging H atoms.

[Ag23Pd2(PPh3)10Cl7]0: A new family of synthesizable bi-icosahedral superatomic molecules

Icosahedral noble-metal 13-atom nanoclusters (NCs) can form connected structures, which can be regarded as superatomic molecules, by vertex sharing. However, there have been very few reports on the superatomic molecules formed using silver (Ag) as the base element. In this study, we synthesized [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ (Pd = palladium, PPh₃ = triphenylphosphine, Cl = chloride), in which two icosahedral 13-atom NCs are connected, and elucidated its geometric and electronic structures to clarify what type of superatomic molecules can be synthesized. The results revealed that [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ is a synthesizable superatomic molecule. Single crystal x-ray diffraction analysis showed that the metal-metal distances in and between the icosahedral structures of [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ are slightly shorter than those of previously reported [Ag₂₃Pt₂(PPh₃)₁₀Cl₇]⁰, whereas the metal-PPh₃ distances are slightly longer. On the basis of several experiments and density functional theory calculations, we concluded that [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ and previously reported [Ag₂₃Pt₂(PPh₃)₁₀Cl₇]⁰ are more stable than [Ag₂₅(PPh₃)₁₀Cl₇]²⁺ because of their stronger superatomic frameworks (metal cores). These findings are expected to lead to clear design guidelines for creation of new superatomic molecules.

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.

Modulating catalytic activity of human topoisomerase II α enzyme by fluorescent gold nanoclusters

Precise monitoring of the enzyme activity by a suitable modulator is one of the very fundamental aspects of drug designing that provides the opportunity to overcome the challenges of several diseases. Herein, inhibition of human Topoisomerase IIα enzyme which serves as a potential target site for several anti-cancer drugs is demonstrated by using ultra-small size gold nanoclusters (Au NCs) with the dimension comparable with size of the active site of the enzyme. Molecular dynamics simulation results demonstrate that the Au NCs strongly interact with the human Topo IIα enzyme at its active site or allosteric site depending on forms of enzyme. Additionally, binding energy and interaction profile provides the molecular basis of understanding of interactions of ultra-small size Au NCs and human Topo IIα enzyme. Enthalpy change (ΔH) and binding constant (K) are measured based on a sequential binding model of the Au NCs with the enzyme as demonstrated by the ITC study. Moreover, the in-vitro inhibition study of the catalytic activity of the enzyme and gel electrophoresis indicates that the ultra-small size Au NCs may be used as a potent inhibitor of human Topo IIα enzyme.

Toward the Formation of N-Heterocyclic-Carbene-Protected Gold Clusters of Various Nuclearities. A Comparison with Their Phosphine-Protected Analogues from Density Functional Theory Calculations

The structure and bonding of a series of selected phosphine-protected gold clusters (Au_n-P) of nuclearity varying from n = 6 to 13 were investigated by density functional theory (DFT) calculations and compared to those of the hypothetical homologues in which phosphines were replaced by N-heterocyclic carbene (NHC) analogues (Au_n-C). Both the Au_n-P and Au_n-C series exhibit similar stabilities and structural features, except for n = 6, where some differences are noted. The NHC ligands are found to be even slightly more strongly bonded to the gold core (by a few kilocalories per mole per ligand) than phosphines. Investigation of the optical properties of both series using time-dependent DFT calculations indicates similarities in the nature and energies of the UV-vis optical transitions and, consequently, relatively similar shapes of the simulated spectra, with a general blue-shift tendency when going from Au_n-P to Au_n-C. The fluorescence behavior observed experimentally for some of the Au_n-P species is expected to occur also for their Au_n-C analogues, which can be extended to other carbene-ligand-protected nanoclusters. Our results show that it should be possible to stabilize gold clusters with NHC ligands, in relation to the seminal Au₁₃-ligand-protected core, offering novel building blocks for the design of nanostructured materials with various properties.

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.

The precise editing of surface sites on a molecular-like gold catalyst for modulating regioselectivity

It is extremely difficult to precisely edit a surface site on a typical nanoparticle catalyst without changing other parts of the catalyst. This precludes a full understanding of which site primarily determines the catalytic properties. Here, we couple experimental data collection with theoretical analysis to correlate rich structural information relating to atomically precise gold clusters with the catalytic performance for the click reaction of phenylacetylene and benzyl azide. We also identify a specific surface site that is capable of achieving high regioselectivity. We further conduct site-specific editing on a thiolate-protected gold cluster by peeling off two monomeric RS–Au–SR motifs and replacing them with two Ph₂P–CH₂–PPh₂ staples. We demonstrate that the surface Au–Ph₂P–CH₂–PPh₂–Au motifs enable extraordinary regioselectivity for the click reaction of alkyne and azide. The editing strategy for the surface motifs allows us to exploit previously inaccessible individual active sites and elucidate which site can explicitly govern the reaction outcome.

Editing surface motifs on gold cluster catalysts achieves high regioselectivity for the click reactions of azides and alkynes.

γ-Alumina-supported Pt17 cluster: controlled loading, geometrical structure, and size-specific catalytic activity for carbon monoxide and propylene oxidation

Although Pt is extensively used as a catalyst to purify automotive exhaust gas, it is desirable to reduce Pt consumption through size reduction because Pt is a rare element and an expensive noble metal. In this study, we successfully loaded a Pt₁₇ cluster on γ-alumina (γ-Al₂O₃) (Pt₁₇/γ-Al₂O₃) using [Pt₁₇(CO)₁₂(PPh₃)₈]Cl _n (n = 1, 2) as a precursor. In addition, we demonstrated that Pt is not present in the form of an oxide in Pt₁₇/γ-Al₂O₃ but instead has a framework structure as a metal cluster. Moreover, we revealed that Pt₁₇/γ-Al₂O₃ exhibits higher catalytic activity for carbon monoxide and propylene oxidation than γ-Al₂O₃-supported larger Pt nanoparticles (Pt_NP/γ-Al₂O₃) prepared using the conventional impregnation method. Recently, our group discovered a simple method for synthesizing the precursor [Pt₁₇(CO)₁₂(PPh₃)₈]Cl _n . Furthermore, Pt₁₇ is a Pt cluster within the size range associated with high catalytic activity. By combining our established synthesis and loading methods, other groups can conduct further research on Pt₁₇/γ-Al₂O₃ to explore its catalytic activities in greater depth.

All-thiolate-stabilized Ag42 nanocluster with a tetrahedral kernel and its transformation to an Ag61 nanocluster with a bi-tetrahedral kernel

Two new silver nanoclusters, formulated as Ag42(SBu^t)₂₄ and Ag61(SC₆H₁₁)₄₀Cl were prepared by a NaSbF₆-mediated two-phase ligand exchange method. A size conversion from Ag42 to Ag61 were achieved via cyclohexanethiol etching.

Heterogeneous metal alloy engineering: embryonic growth of M13icosahedra in Ag-based alloy superatomic nanoclusters

Alloying is an effective tool to comprehend the packing mechanism and adjust the properties of nanomaterials.

Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au20 clusters

The free-standing Au₂₀ cluster has a unique tetrahedral shape and a large HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of around 1.8 electron volts. The "magic" Au₂₀ has been intensively used as a model system for understanding the catalytic and optical properties of gold nanoclusters. However, direct real-space ground-state characterization at the atomic scale is still lacking, and obtaining fundamental information about the corresponding structural, electronic, and dynamical properties, is challenging. Here, using cluster-beam deposition and low-temperature scanning tunneling microscopy, atom-resolved topographic images and electronic spectra of supported Au₂₀ clusters are obtained. We demonstrate that individual size-selected Au₂₀ on ultrathin NaCl films maintains its pyramidal structure and large HOMO-LUMO gap. At higher cluster coverages, we find sintering of the clusters via Smoluchowski ripening to Au_20n agglomerates. The evolution of the electron density of states deduced from the spectra reveals gap reduction with increasing agglomerate size.

Elucidating ligand effects in thiolate-protected metal clusters using Au24Pt(TBBT)18 as a model cluster

2-Phenylethanethiolate (PET) and 4-tert-butylbenzenethiolate (TBBT) are the most frequently used ligands in the study of thiolate (SR)-protected metal clusters. However, the effect of difference in the functional group between these ligands on the fundamental properties of the clusters has not been clarified. We synthesized [Au24Pt(TBBT)18]0, which has the same number of metal atoms, number of ligands, and framework structure as [Au24Pt(PET)18]0, by replacing ligands of [Au24Pt(PET)18]0 with TBBT. It was found that this ligand exchange is reversible unlike the case of other metal-core clusters. A comparison of the geometrical/electronic structure and stability of the clusters between [Au24Pt(PET)18]0 and [Au24Pt(TBBT)18]0 revealed three things with regard to the effect of ligand change from PET to TBBT on [Au24Pt(SR)18]0: (1) the induction of metal-core contraction and Au-S bond elongation, (2) no substantial effect on the HOMO-LUMO gap but a clear difference in optical absorption in the visible region, and (3) the decrease of stabilities against degradation in solution and under laser irradiation. By using these two clusters as model clusters, it is expected that the effects of the structural difference of ligand functional-groups on the physical properties and functions of clusters, such as catalytic ability and photoluminescence, would be clarified.

Metal Nanoclusters Stabilized by Selenol Ligands

The past decades have witnessed great advances in controllable synthesis, structure determination, and property investigation of metal nanoclusters. Selenolated nanoclusters, a special branch in the nanocluster family, have attracted great interest in these years. The electronegativity and atomic radius of selenium is different from sulfur, and thus the selenolated nanoclusters are anticipated to display different electronic/geometric structures and distinct chemical/physical properties relative to their thiolated analogues. This review covers the syntheses, structures, and properties of selenolated nanoclusters (including Au, Ag, Cu, and alloy nanoclusters). Ligand effects (between SeR and SR) on nanocluster properties, including optical absorption, stability, and electrochemical properties, are disclosed as well. At the end of the review, a scope for improvements and future perspectives of selenolated nanoclusters is highlighted. The review hopefully opens up new horizons for cluster scientists to synthesize more selenolated nanoclusters with novel structures and properties. This review is based on publications available up to May 2019.

An Au22(L8)6 nanocluster with in situ uncoordinated Au as a highly active catalyst for O2 activation and CO oxidation

Ligand-capped gold nanoclusters with atomic precision have attracted great interest as a new type of nanocatalyst to elucidate mechanisms and establish structure-reactivity correlations. In most cases, however, the catalytic activity of fully protected gold nanoclusters is severely hindered due to the blocking effects of the ligands. Alternatively, partially protected gold nanoclusters with the creation of in situ coordination unsaturated (cus) Au are highly promising for nanocatalysis. In this study, via density functional theory (DFT) calculations, we investigated the reactivity of a diphosphine-protected Au22(L8)6 nanocluster using oxygen activation and CO oxidation as the testing probes. The results showed that the cus Au at the interface shows strong adsorption and activation of oxygen, promoting the dissociation of two O2 molecules into fully oxidized Au22(L8)6O4 nanoclusters with a moderate activation barrier (0.6-0.7 eV). The adsorption of additional O2 prefers molecular adsorption and locates at the terminal Au11 unit around the Au-P framework. Furthermore, our results indicate that the Au22(L8)6 cluster shows very high activity and a low energy barrier (0.51 eV) for CO oxidation. The facile O2 activation and CO oxidation over the Au22(L8)6 nanocatalyst will aid the rational design of partially protected metal nanoclusters with coordination unsaturated metal centers in oxidation or other catalytic reactions for important practical applications.