X-ray Crystal Structure of Au38-xAgx(SCH2CH2Ph)24 Alloy Nanomolecules

Controlled aggregation of Pt/PtH/Rh/RhH doped silver superatomic nanoclusters into 16-electron supermolecules

The assembly of discrete superatomic nanoclusters into larger constructs is a significant stride towards developing a new set of artificial/pseudo-elements. Herein, we describe a novel series of 16-electron supermolecules derived from the combination of discrete 8-electron superatomic synthons containing interstitial hydrides as vertex-sharing building blocks. The symmetric (RhH)2Ag33[S2P(OPr)2]17 (1) and asymmetric PtHPtAg32[S2P(OPr)2]17 (2) are characterized by ESI-MS, SCXRD, NMR, UV-vis absorption spectra, electrochemical and computational methods. Cluster 1 represents the first group 9-doped 16-electron supermolecule, composed of two icosahedral (RhH)@Ag12 8-electron superatoms sharing a silver vertex. Cluster 2 results from the assembly of two distinct icosahedral units, Pt@Ag12, and (PtH)@Ag12. In both cases, the presence of the interstitial hydrides is unprecedented. The stability of the supermolecules is investigated, and 2 spontaneously transforms into Pt2Ag33[S2P(OPr)2]17 (3) with thermal treatment. The lability of the hydride within the icosahedral framework in solution at low-temperature was confirmed by the VT-NMR.

Enzyme-mimic catalytic activities and biomedical applications of noble metal nanoclusters

Noble metal (e.g., Au and Ag) nanoclusters (NCs), which exhibit structural complexity and hierarchy comparable to those of natural proteins, have been increasingly pursued in artificial enzyme research. The protein-like structure of metal NCs not only ensures enzyme-mimic catalytic activity, including peroxidase-, catalase-, and superoxide dismutase-mimic activities, but also affords an unprecedented opportunity to correlate the catalytic performance with the cluster structure at the molecular or atomic levels. In this review, we aim to summarize the recent progress in programming and demystify the enzyme-mimic catalytic activity of metal NCs, presenting the state-of-the-art understandings of the structure-property relationship of metal NC-based artificial enzymes. By leveraging on a concise anatomy of the hierarchical structure of noble metal NCs, we manage to unravel the structural origin of the catalytic performance of metal NCs. Noteworthily, it has been proven that the surface ligands and metal-ligand interface of metal NCs are instrumental in influencing enzyme-mimic catalytic activities. In addition to the structure-property correlation, we also discuss the synthetic methodologies feasible to tailoring the cluster structure at the atomic level. Prior to the closure of this review with our perspectives in noble metal NC-based artificial enzymes, we also exemplify the biomedical applications based on the enzyme-mimic catalysis of metal NCs with the theranostics of kidney injury, brain inflammation, and tumors. The fundamental and methodological advancements delineated in this review would be conducive to further development of metal NCs as an alternative family of artificial enzymes.

Ion mobility-tandem mass spectrometry of bulky tert-butyl thiol ligated gold nanoparticles

Gold nanoparticles (AuNPs) synthesized in the 1-3 nm range have a specific number of gold core atoms and outer protecting ligands. They have become one of the "hot topics" in recent decades because of their interesting physical and chemical properties. The characterization of their structures is usually achieved by crystal X-ray diffraction although the structures of some AuNPs remain unknown because they have not been successfully crystallized. An alternative method for studying the structure of AuNPs is electrospray ionization-ion mobility-tandem mass spectrometry (ESI-IM-MSMS). This research evaluated how effectively ESI-IM-MSMS using the commercially available Waters Synapt XS instrument yielded useful structural information from two AuNPs; Au23 (S-tBu)16 and Au30 (S-tBu)18 . The study used the maximum range of available collision energies along with ion mobility separation to measure the energy-dependence of the product ions and their drift times which is a measure of their spatial size. For Au23 (S-tBu)16 , the dissociation gave the masses of the outer protecting monomeric [RS-Au-SR] and trimeric [SR-Au-SR-Au-SR-Au-SR] staples where R = tBu, and complete dissociation of the outer layer Au and tBu groups to reveal the Au15 S8 core. For Au30 (S-tBu)18 , the dissociation products was primarily through the loss of the partial ligands S-tBu and tBu from the outer protecting layer and the loss of single Au4 (S-tBu)4 unit. These results showed the that ESI-IM-MSMS analysis of the smaller Au23 (S-tBu)16 gave information on all it major structural components whereas for Au30 (S-tBu)18 , the overall structural information was limited to the ligands of the outer layer.

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.

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

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

Master key to coinage metal nanoclusters treasure chest: 38-metal clusters

Atomically precise metal nanoclusters with specific chemical compositions have become a popular research topic due to their precise structures, attractive properties, and wide range of applications in various fields. Currently, among more than 100 reported metal nanoclusters with precise formulas, 38-atom coinage metal nanoclusters stand out due to their unique structural diversities, such as face-centered cubic (FCC) and body-centered cubic (BCC) arrangements. Among them, the formation of the metal cores includes vertex-sharing, face-fusion, and FCC cubes fusion. Due to their geometrical features, 38-atom coinage metal nanoclusters exhibit attractive properties, making them an ideal model for exploring structure-property relationships. Therefore, 38-atom coinage metal nanoclusters are a universal key to the treasure trove of nanoclusters, which can open almost all fields and are of great research significance. This paper focuses on the structure of 38-atom coinage metal nanoclusters and reviews the preparation and crystallization methods, excellent properties, and practical applications. Finally, future research prospects and development opportunities are provided.

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.

Structure Determination of the Cl-Enriched [Ag52(SAdm)31Cl13]2+ Nanocluster

Cl atoms can serve as the innermost core, the peripheral ligand, or the counterions of metal nanoclusters. Herein, we report the structural determination a Cl-enriched [Ag₅₂(SAdm)₃₁Cl₁₃]²⁺. The ratio of Cl to AdmSH is quite high compared to those of other nanoclusters. Structurally, nine Cl atoms, existing at the interlayer of the inner kernel and the surface motif, serve as the bridging ligands to sustain the robustness of the whole structure. Interestingly, four Cl atoms on the motif structure can be substituted by Br. This work allows us to clear the regulation of Cl ligands in the structural construction of metal nanoclusters.

Raman Spectroscopic Fingerprints of Atomically Precise Ligand Protected Noble Metal Clusters: Au38 (PET)24 and Au38-x Agx (PET)24

Distinct Raman spectroscopic signatures of the metal core of atomically precise, ligand-protected noble metal nanoclusters are reported using Au₃₈ (PET)₂₄ and Au_38-x Ag_x (PET)₂₄ (PET = 2-phenylethanethiolate, -SC₂ H₄ C₆ H₅ ) as model systems. The fingerprint Raman features (occurring <200 cm^-1 ) of these clusters arise due to the vibrations involving metal atoms of their Au₂₃ or Au_23-x Ag_x cores. A distinct core breathing vibrational mode of the Au₂₃ core has been observed at 90 cm^-1 . Whereas the breathing mode shifts to higher frequencies with increasing Ag content of the cluster, the vibrational signatures due to the outer metal-ligand staple motifs (between 200 and 500 cm^-1 ) do not shift significantly. DFT calculations furthermore reveal weak Raman bands at higher frequencies compared to the breathing mode, which are associated mostly with the rattling of two central gold atoms of the bi-icosahedral Au₂₃ core. These vibrations are also observed in the experimental spectrum. The study indicates that low-frequency Raman spectra are a characteristic fingerprint of atomically precise clusters, just as electronic absorption spectroscopy, in contrast to the spectrum associated with the ligand shell, which is observed at higher frequencies.

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.

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.

Design of a fluorescent and clickable Ag38(SRN3)24 nanocluster platform: synthesis, modeling and self-assembling

Fluorescent atomically precise Ag₃₈(11-azido-2-ol-undecane-thiolate)₂₄ nanoclusters are easily prepared using sodium ascorbate as a "green" reducer and are extensively characterized by way of elemental analyses, ATR-FTIR, XRD, SAXS, UV-vis, fluorescence spectroscopies, and theoretical modeling. The fluorescence and the atomically determined stoichiometry and structure, the facile and environmentally green synthesis, together with the novel presence of terminal azido groups in the ligands which opens the way to "click"-binding a wide set of molecular species, make Ag₃₈(11-azido-2-ol-undecane-thiolate)₂₄ nanoclusters uniquely appealing systems for biosensing, recognition and functionalization in biomedicine applications and in catalysis.

Experimental Confirmation of a Topological Isomer of the Ubiquitous Au25(SR)18 Cluster in the Gas Phase

High-resolution electrospray ionization ion mobility mass spectrometry has revealed a gas-phase isomer of the ubiquitous, extremely well-studied Au₂₅(SR)₁₈ cluster both in anionic and cationic form. The relative abundance of the isomeric structures can be controlled by in-source activation. The measured collision cross section of the new isomer agrees extremely well with a recent theoretical prediction (MatusM. F.; et al. Chem. Commun.2020, 56, 8087) corresponding to a Au₂₅(SR)₁₈^– isomer that is energetically close and topologically connected to the known ground-state structure via a simple rotation of the gold core without breaking any Au–S bonds. The results imply that the structural dynamics leading to isomerization of thiolate-protected gold clusters may play an important role in their gas-phase reactions and that isomerization could be controlled by external stimuli.

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.

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.

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.

Crystal Structure of Au30-xAgx(S-tBu)18 and Effect of the Ligand on Ag Alloying in Gold Nanomolecules

We report the X-ray crystal structure of the Au_30-xAg_x(S-tBu)₁₈ alloy and the effect of the ligand on alloying site preferences. Gold-silver nanoalloys prepared by co-reduction of metal salts are known to have only partial Ag occupancies. Interestingly, Au_30-xAg_x(S-tBu)₁₈ has 100% Ag occupancy at two sites on the core surface as well as partial Ag occupancies on the surface, capping, and staples sites. The Au_30-xAg_x(S-tBu)₁₈ (x = 1-5) composition was confirmed by X-ray diffraction and electrospray ionization mass spectrometry studies. Thiolate ligands can be categorized into three classes on the basis of the groups at the α-position as aliphatic, aromatic, and bulky thiols. The effect of the ligand on Ag doping can be clearly seen in the crystal structures of Au_36-xAg_x(SPh-tBu)₂₄ and Au_38-xAg_x(SCH₂CH₂Ph)₂₄ when compared with that of Au_30-xAg_x(S-tBu)₁₈. Ag is preferentially doped onto the core surface when the ligand is aliphatic, and Ag is doped in both core surface and staple metal sites when the ligand is aromatic or bulky.

Solvent-mediated assembly of atom-precise gold-silver nanoclusters to semiconducting one-dimensional materials

Bottom-up design of functional device components based on nanometer-sized building blocks relies on accurate control of their self-assembly behavior. Atom-precise metal nanoclusters are well-characterizable building blocks for designing tunable nanomaterials, but it has been challenging to achieve directed assembly to macroscopic functional cluster-based materials with highly anisotropic properties. Here, we discover a solvent-mediated assembly of 34-atom intermetallic gold-silver clusters protected by 20 1-ethynyladamantanes into 1D polymers with Ag-Au-Ag bonds between neighboring clusters as shown directly by the atomic structure from single-crystal X-ray diffraction analysis. Density functional theory calculations predict that the single crystals of cluster polymers have a band gap of about 1.3 eV. Field-effect transistors fabricated with single crystals of cluster polymers feature highly anisotropic p-type semiconductor properties with ≈1800-fold conductivity in the direction of the polymer as compared to cross directions, hole mobility of ≈0.02 cm² V^-1 s^-1, and an ON/OFF ratio up to ≈4000. This performance holds promise for further design of functional cluster-based materials with highly anisotropic semiconducting properties.

Doping Copper Atoms into the Nanocluster Kernel: Total Structure Determination of [Cu30Ag61(SAdm)38S3](BPh4)

Doping active metal (i.e., Cu) into the kernel of noble metal nanoclusters (i.e., Au/Ag nanocluster) remains challenging in the synthesis of alloy nanoclusters. Herein, we report the synthesis and the total structure determination of a bimetallic [Ag₆₁Cu₃₀(SAdm)₃₈S₃]BPh₄ (Ag₆₁Cu₃₀) nanocluster. The Ag₆₁Cu₃₀ nanocluster is composed of an Ag₁₃@Cu₃₀ kernel which is further capped by a peripheral Ag₄₈(SAdm)₃₈S₃ shell. The icosidodecahedron Cu₃₀ middle layer connects the innermost icosahedral Ag₁₃ core and Ag atoms at the outermost Ag₄₈(SR)₃₈S₃ shell, demonstrating that the Cu atoms in the Cu₃₀ layer are in a metallic state.

Chiral Inversion of Thiolate-Protected Gold Nanoclusters via Core Reconstruction without Breaking a Au-S Bond

On the basis of density functional theory computations of the well-known chiral Au₃₈(SR)₂₄ nanocluster and its Pd- and Ag-doped derivatives, we propose here a mechanism for chiral inversion that does not require the breaking of a metal–sulfur bond at the metal–ligand interface but features a collective rotation of the gold core. The calculated energy barriers for this mechanism for Au₃₈ and Pd-doped Au₃₈ are in the range of 1–1.5 eV, significantly lower than barriers involving the breakage of Au–S bonds (2.5 eV). For Ag-doped Au₃₈, barriers for both mechanisms are similar (1.3–1.5 eV). Inversion barriers for a larger chiral Au₁₄₄(SR)₆₀ are much higher (2.5−2.8 eV). Our computed barriers are in good agreement with racemization barriers estimated from existing experiments for bare and doped Au₃₈. These results highlight the sensitivity of chiral inversion to the size, structure, and metal composition of the metal core and sensitivity to the detailed structure of the metal–thiolate interface. Our work also predicts that enantiopure Au₁₄₄(SR)₆₀ clusters would be promising materials for applications requiring high resistance to chiral inversion.

Atomic-Level Doping of Metal Clusters

Atomically precise noble metal (mainly silver and gold) nanoclusters are an emerging category of promising functional materials for future applications in energy, sensing, catalysis, and nanoelectronics. These nanoclusters are protected by ligands such as thiols, phosphines, and hydride and have sizes between those of atoms and plasmonic nanoparticles. In metallurgy, the properties of a pure metal are modified by the addition of other metals, which often offers augmented characteristics, making them more utilizable for real-life applications. In this Account, we discuss how the incorporation of various metal atoms into existing protected nanoclusters tunes their structure and properties. The process of incorporating metals into an existing cluster is known as doping; the product is known as a doped cluster, and the incorporated metal atom is called a dopant/foreign atom. We first present a brief historical overview of protected clusters and the need for doping and explain (with examples) the difference between an "alloy" and a "doped" cluster, which are two frequently confused terms. We then discuss several commonly observed challenges in the synthesis of doped clusters: (i) doping produces a mixture of compositions that prevents the growth of single crystals; (ii) doping with foreign atoms sometimes changes the overall composition and structure of the parent cluster; and (iii) doping beyond a certain number of foreign atoms decomposes the doped cluster. After delineating the challenges, we review a few potential synthetic methods for doped clusters: (i) the co-reduction method, (ii) the galvanic exchange method, (iii) ligand-induced conversion of bimetallic clusters to doped clusters, and (iv) intercluster reactions. As a foreign atom is able to occupy different positions within the structure of the parent cluster, we examine the structural relationship between the parent clusters and their different foreign-atom-doped clusters. We then show how doping enhances the stability, luminescence, and catalytic properties of clusters. The enhancement factor highly depends on the number and nature of the foreign atoms, which can also alter the charge state of the parent cluster. Atomic-level doping of foreign atoms in the parent cluster is confirmed by high-resolution electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry techniques and single-crystal X-ray diffraction methods. The photophysical properties of the doped clusters are investigated using both time-dependent and steady-state luminescence and optical absorption spectroscopies. After presenting an overview of atomic-level doping in metal clusters and demonstrating its importance for enriching the chemistry and photophysics of clusters and extending their applications, we conclude this Account with a brief perspective on the field's future.

Rethinking Heterometal Doping in Ligand-Protected Metal Nanoclusters

Heterometal doping is a promising avenue toward tailoring properties of ligand-protected metal nanoclusters for specific applications. Though successful doping has been demonstrated in several structures, the underlying reasons for the dopant preference on occupying specific locations on the nanocluster with different concentrations remain unclear. In this study we apply our thermodynamic stability model, originally developed for ligand-protected monometallic nanoclusters, to rationalize the synthetic accessibility, dopant location, and concentrations of various heterometals on ligand-protected Au nanoclusters. Importantly, we demonstrate that the thermodynamic stability theory is a significant step forward in accurately describing doping effects on nanoclusters using first-principles calculations. With our computational predictions being in excellent agreement with a series of experiments, we introduce the thermodynamic stability theory as a new method for bimetallic nanocluster prediction.

Customizing the Structure, Composition, and Properties of Alloy Nanoclusters by Metal Exchange

The properties of metal materials can be greatly enriched by including various elements to generate alloys. The galvanic replacement represents a classical method for the preparation of both bulk- and nanoalloy materials. The difference of the electrochemical potential between the two metals acts as the driving force for the galvanic replacement reaction. However, this classical rule partially fails at the ultrasmall size scale, for that novel chemistry emerges by the decrease of the size of materials down to less than 3 nm due to the strong quantum effect. In this Account, we discuss an emerging topic of nanochemistry, the metal exchange in atomically precise ultrasmall (<3 nm) metal nanoparticles (or nanoclusters). The metal exchange method uses different types of metal sources (e.g., AuBrPPh₃ or AgSR complexes) to react with templating metal nanoclusters (e.g., Au₂₅(SR)₁₈^-), and finally alloy nanoclusters are produced. We demonstrate that the metal exchange reaction between metal nanoclusters and metal complexes does not follow the classical metal activity sequence (i.e., Fe > Cd > Co > Ni > Pb > Cu > Hg > Ag > Pd > Pt > Au) and such metal exchange reactions in the nanocluster range is, to a large extent, related with the electron shell closing and the structural stability of nanoclusters. In the subsequent sections, we present effective control over the number, position, and distribution of the dopants. The shape and structure of the final alloy products can be tailored by recently developed metal exchange methods. More importantly, modulation and enhancement of the properties of NCs through metal exchange are realized. For example, the largely increased quantum yield and the significantly improved catalytic activity. In addition, we shall also discuss the real-time characterization of the metal exchange reaction by the combination of UV-vis absorption spectroscopy, high resolution electrospray ionization mass spectrometry (ESI-MS), matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and single crystal X-ray diffraction (SC-XRD). By controlling the charge of the templating metal nanoclusters and the different types of metal complexes, the driving force of metal exchange has been studied, which is considered to be the thermodynamics rather than the electrochemical potential. In summary, the metal exchange reactions in the ultrasmall nanocluster range are totally different compared with the case of larger-sized metal nanoparticles. Depending on this novel method, atomically precise alloy nanoclusters can be prepared by reacting the nanocluster composed of inert metal (such as Au) with complexes of high-activity metals (e.g., Cd/Hg/Cu/Ag). We anticipate that future research on the metal exchange will contribute to the fundamental understanding of reaction behavior of metal atoms in ultrasmall nanoclusters and to the design of alloy nanoclusters with enhanced properties.

Atomic and Isomeric Separation of Thiolate-Protected Alloy Clusters

Techniques to control the chemical compositions and geometric structures of alloy clusters are indispensable to understand the correlation between the structures and physical/chemical properties of alloy clusters. In this study, we established a method to separate thiolate-protected 25-atom gold-silver alloy clusters (Au_{25- x}Ag _x(SR)₁₈) according to their chemical composition and structural isomer. Furthermore, using this method, we revealed that an isomeric distribution of the products exists in Au_{25- x}Ag _x(SR)₁₈ ( x ≥ 2) and that the distribution of these isomers depends on the synthesis method and standing time in solution. In this study, it was also demonstrated that the continuous discretization of the electronic structure is induced by the Ag substitution. This method can also be used to separate mixtures of [Au₂₄M(SR)₁₈]⁰ (M = Au, Pt, or Pd) and other Au-Ag alloy clusters ([Au_{36- x}Ag _x(SR)₂₄]⁰ and [Au_{38- x}Ag _x(SR)₂₄]⁰). This method is expected to be used to obtain comprehensive knowledge of the structural-property correlation of alloy clusters.

Synthesis, structural characterization and transformation of an eight-electron superatomic alloy, [Au@Ag19{S2P(OPr)2}12]

Controlling the metal nanoclusters with atomic precision is highly difficult and further studies on their transformation reactions are even more challenging. Herein we report the controlled formation of a silver alloy nanocluster [AuAg19{S2P(OnPr)2}12] (1) from an Ag20 template via a galvanic exchange route. X-ray structural analysis reveals that the alloy structure comprises of a gold-centered Ag12 icosahedron, Au@Ag12, capped by seven silver atoms. Interestingly upon reacting with one equiv. of silver(i) salt, (1) can transform into a higher nuclearity nanocluster, [Au@Ag20{S2P(OnPr)2}12]+ (2). The conversion process is studied via ESI mass spectrometry and 31P NMR spectroscopy. This kind of size-structural transformation at the single atom level is quite remarkable. Furthermore, the compositions of all the doped nanoclusters (1, 2) were fully characterized with ESI-MS and EDS. The blue shift depicted in the UV-visible and emission spectra of the doped nanoclusters (1, 2) compared with the precursor, Ag20, demonstrates that the doping atoms have significant effects on the electronic structures.

Dopant-Dependent Electronic Structures Observed for M2Au36(SC6H13)24 Clusters (M = Pt, Pd)

Heteroatom doping is a powerful means to tune the optical and electronic properties of gold clusters at the atomic level. We herein report that doping a Au₃₈ cluster with Pt and Pd atoms leads to core-doped [Pt₂Au₃₆(SC₆H₁₃)₂₄]^2- and [Pd₂Au₃₆(SC₆H₁₃)₂₄]⁰, respectively. Voltammetric investigations show that these clusters exhibit drastically different electronic structures; whereas the HOMO-LUMO gap of [Pt₂Au₃₆(SC₆H₁₃)₂₄]^2- is found to be 0.95 V, that of [Pd₂Au₃₆(SC₆H₁₃)₂₄]⁰ is drastically decreased to 0.26 V, suggesting Jahn-Teller distortion of the 12-electron cluster. Density functional investigations confirm that the HOMO-LUMO gap of the Pd-doped cluster is indeed reduced. Analysis of the optimized geometry for the 12-electron [Pd₂Au₃₆(SC₆H₁₃)₂₄]⁰ reveals that the rod-like M₂Au₂₁ core becomes more flattened upon Pd-doping. Reversible geometrical interconversion between [Pt₂Au₃₆(SC₆H₁₃)₂₄]⁰ and [Pt₂Au₃₆(SC₆H₁₃)₂₄]^2- is clearly demonstrated by manipulating the oxidation state of the cluster.

On the mechanism of rapid metal exchange between thiolate-protected gold and gold/silver clusters: a time-resolved in situ XAFS study

The fast metal exchange reaction between Au₃₈ and Ag_xAu_38-x nanoclusters in solution at -20 °C has been studied by in situ X-ray absorption spectroscopy (time resolved quick XAFS) in transmission mode. A cell was designed for this purpose consisting of a cooling system, remote injection and mixing devices. The capability of the set-up is demonstrated for second and minute time scale measurements of the metal exchange reaction upon mixing Au₃₈/toluene and Ag_xAu_38-x/toluene solutions at both Ag K-edge and Au L₃-edge. It has been proposed that the exchange of gold and silver atoms between the clusters occurs via the SR(-M-SR)_n (n = 1, 2; M = Au, Ag) staple units in the surface of the reacting clusters during their collision. However, at no point during the reaction (before, during, after) evidence is found for cationic silver atoms within the staples. This means that either the exchange occurs directly between the cores of the involved clusters or the residence time of the silver atoms in the staples is very short in a mechanism involving the metal exchange within the staples.

High-performance liquid chromatography mass spectrometry of gold and alloy clusters protected by hydrophilic thiolates

In this work, we found two hydrophilic interaction liquid chromatography (HILIC) columns for high-performance liquid chromatography (HPLC) suitable for the high-resolution separation of hydrophilic metal clusters. The mass distributions of the product mixtures of hydrophilic metal clusters were evaluated via HPLC mass spectrometry (LC/MS) using these HILIC columns. Consequently, we observed multiple clusters that had not been previously reported for glutathionate (SG)-protected gold clusters (Au_n(SG)_m). Additionally, we demonstrated that Au_n-xM_x(SG)_m alloy clusters (M = Ag, Cu, or Pd) in which part of the Au in the Au_n(SG)_m cluster is replaced by a heteroelement can be synthesized, similar to the case of hydrophobic alloy clusters. It is easy to evaluate the mass distributions of hydrophilic metal clusters using this method. Thus, remarkable progress in the synthesis techniques of hydrophilic metal clusters through the use of this method is anticipated, as is the situation for hydrophobic metal clusters.

Synthesis of Au38(SCH2CH2Ph)24, Au36(SPh-tBu)24, and Au30(S-tBu)18 Nanomolecules from a Common Precursor Mixture

Phenylethanethiol protected nanomolecules such as Au₂₅, Au₃₈, and Au₁₄₄ are widely studied by a broad range of scientists in the community, owing primarily to the availability of simple synthetic protocols. However, synthetic methods are not available for other ligands, such as aromatic thiol and bulky ligands, impeding progress. Here we report the facile synthesis of three distinct nanomolecules, Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-tBu)₂₄, and Au₃₀(S-tBu)₁₈, exclusively, starting from a common Au_n(glutathione)_m (where n and m are number of gold atoms and glutathiolate ligands) starting material upon reaction with HSCH₂CH₂Ph, HSPh-tBu, and HStBu, respectively. The systematic synthetic approach involves two steps: (i) synthesis of kinetically controlled Au_n(glutathione)_m crude nanocluster mixture with 1:4 gold to thiol molar ratio and (ii) thermochemical treatment of the purified nanocluster mixture with excess thiols to obtain thermodynamically stable nanomolecules. Thermochemical reactions with physicochemically different ligands formed highly monodispersed, exclusively three different core-size nanomolecules, suggesting a ligand induced core-size conversion and structural transformation. The purpose of this work is to make available a facile and simple synthetic method for the preparation of Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-tBu)₂₄, and Au₃₀(S-tBu)₁₈, to nonspecialists and the broader scientific community. The central idea of simple synthetic method was demonstrated with other ligand systems such as cyclopentanethiol (HSC₅H₉), cyclohexanethiol(HSC₆H₁₁), para-methylbenzenethiol(pMBT), 1-pentanethiol(HSC₅H₁₁), 1-hexanethiol(HSC₆H₁₃), where Au₃₆(SC₅H₉)₂₄, Au₃₆(SC₆H₁₁)₂₄, Au₃₆(pMBT)₂₄, Au₃₈(SC₅H₁₁)₂₄, and Au₃₈(SC₆H₁₃)₂₄ were obtained, respectively.

The tetrahedral structure and luminescence properties of Bi-metallic Pt1Ag28(SR)18(PPh3)4 nanocluster

The atomic-structure characterization of alloy nanoclusters (NCs) remains challenging but is crucial in order to understand the synergism and develop new applications based upon the distinct properties of alloy NCs. Herein, we report the synthesis and X-ray crystal structure of the Pt1Ag28(S-Adm)18(PPh3)4 nanocluster with a tetrahedral shape. Pt1Ag28 was synthesized by reacting Pt1Ag24(SPhMe2)18 simultaneously with Adm-SH (1-adamantanethiol) and PPh3 ligands. A tetrahedral structure is found in the metal framework of Pt1Ag28 NC and an overall surface shell (Ag16S18P4), as well as discrete Ag4S6P1 motifs. The Pt1Ag12 kernel adopts a face-centered cubic (FCC) arrangement, which is observed for the first time in alloy nanoclusters in contrast to the commonly observed icosahedral structure of homogold and homosilver NCs. The Pt1Ag28 nanocluster exhibits largely enhanced photoluminescence (quantum yield QY = 4.9%, emission centered at ∼672 nm), whereas the starting material (Pt1Ag24 NC) is only weakly luminescent (QY = 0.1%). Insights into the nearly 50-fold enhancement of luminescence were obtained via the analysis of electronic dynamics. This study demonstrates the atomic-level tailoring of the alloy nanocluster properties by controlling the structure.

Gold-doped silver nanocluster [Au3Ag38(SCH2Ph)24X5]2- (X = Cl or Br)

Here we report the single-crystal structure, experimental and theoretical characterization of a 41-metal atom Au-Ag alloy nanocluster [Au₃Ag₃₈(SCH₂Ph)₂₄X₅]^2- (1, X = Cl or Br). The nanocluster 1 is co-protected by thiolate and halogen atoms and features an all-metallic face-fused biicosahedral Au₂@AuAg₂₀ rod-like kernel enwrapped by the outermost Ag₁₈(SCH₂Ph)₂₄X₃ shell. Two sites on the surface of the biicosahedral kernel are partially occupied by Au and Ag atoms. The outer Ag₁₈(SCH₂Ph)₂₄X₃ shell is composed of two Ag₆S₆ cycles at the two poles and one Ag₆S₂X₃ arc at the equator with both 2- and 3-coordinated Ag atoms, which has not been observed in gold or silver nanoclusters ever before. Theoretical calculations elucidate its electronic structure as well as optical properties, thus producing informative correlations between its structure and properties. This nanocluster exhibits near-infrared (NIR) emission around 825 nm. This work (i) snapshots a rare crystal structure of an Au-doped silver alloyed nanocluster; (ii) gives a deep insight to understand how the capping ligand or anions affect the structure of the alloy nanocluster; and (iii) provides precise information about gold atom doping site that is very significant in the recognition of potential active catalytic sites of the alloy nanoparticles.

The solely motif-doped Au36-xAgx(SPh-tBu)24 (x = 1-8) nanoclusters: X-ray crystal structure and optical properties

We report the observation of new doping behavior in Au36-xAgx(SR)24 nanoclusters (NCs) with x = 1 to 8. The atomic arrangements of Au and Ag atoms are determined by X-ray crystallography. The new gold-silver bimetallic NCs share the same framework as that of the homogold counterpart, i.e. possessing an fcc-type Au28 kernel, four dimeric AuAg(SR)3 staple motifs and twelve simple bridging SR ligands. Interestingly, all the Ag dopants in the Au36-xAgx(SR)24 NCs are selectively incorporated into the surface motifs, which is in contrast to the previously reported Au-Ag alloy structures with the Ag dopants preferentially displacing the core gold atoms. This distinct doping behavior implies that the previous assignments of an fcc Au28 core with four dimers and 12 bridging thiolates for Au36(SR)24 are more justified than other assignments of core vs. surface motifs. The UV-Vis adsorption spectrum of Au36-xAgx(SR)24 is almost the same as that of Au36(SR)24, indicating that the Ag dopants in the motifs do not change the optical properties. The similar UV-Vis spectra are further confirmed by TD-DFT calculations. DFT also reveals that the energies of the HOMO and LUMO of the motif-doped AuAg alloy NC are comparable to those of the homogold Au36 NC, indicating that the electronic structure is not disturbed by the motif Ag dopants. Overall, this study reveals a new silver-doping mode in alloy NCs.

Understanding Au∼98Ag∼46(SR)60 nanoclusters through investigation of their electronic and local structure by X-ray absorption fine structure

Synchrotron radiation-based X-ray absorption fine structure (XAFS) of thiol-stabilized Au_∼98Ag_∼46(SR)₆₀ nanoclusters suggests that Au atoms preferred to occupy the metal core sites while the Ag atoms were mainly on the surface.

Thiolated Au18 cluster: preferred Ag sites for doping, structures, and optical and chiroptical properties

Silver doping of thiolated Au₁₈ cluster occurs in the inner core.

Structures and magnetism of mono-palladium and mono-platinum doped Au25(PET)18nanoclusters

Herein we report the crystal structures of [Au₂₄Pt(PET)₁₈]⁰and [Au₂₄Pd(PET)₁₈]⁰as well as the main source of magnetism in [Au₂₅(PET)₁₈]⁰.

Silver migration between Au38(SC2H4Ph)24 and doped AgxAu38−x(SC2H4Ph)24 nanoclusters

A fast redistribution of metal atoms occurs upon collision between Ag_xAu_38−x and Au₃₈ nanoclusters in solution.