Supramolecular Intercluster Compounds Consisting of Gold Clusters and Keggin Anions

Counteranion-induced structural isomerization of phosphine-protected PdAu8 and PtAu8 clusters

Controlling the geometric structures of metal clusters through structural isomerization allows for tuning of their electronic state. In this study, we successfully synthesized butterfly-motif [PdAu₈(PPh₃)₈]²⁺ (PdAu8-B, B means butterfly-motif) and [PtAu₈(PPh₃)₈]²⁺ (PtAu8-B) by the structural isomerization from crown-motif [PdAu₈(PPh₃)₈]²⁺ (PdAu8-C, C means crown-motif) and [PtAu₈(PPh₃)₈]²⁺ (PtAu8-C), induced by association with anionic polyoxometalate, [Mo₆O₁₉]^2- (Mo6) respectively, whereas their structural isomerization was suppressed by the use of [NO₃]^- and [PMo₁₂O₄₀]^3- as counter anions. DR-UV-vis-NIR and XAFS analyses and density functional theory calculations revealed that the synthesized [PdAu₈(PPh₃)₈][Mo₆O₁₉] (PdAu8-Mo6) and [PtAu₈(PPh₃)₈][Mo₆O₁₉] (PtAu8-Mo6) had PdAu8-B and PtAu8-B respectively because PdAu8-Mo6 and PtAu8-Mo6 had bands in optical absorption at the longer wavelength region and different structural parameters characteristic of the butterfly-motif structure obtained by XAFS analysis. Single-crystal and powder X-ray diffraction analyses revealed that PdAu8-B and PtAu8-B were surrounded by six Mo6 with rock salt-type packing, which stabilizes the semi-stable butterfly-motif structure to overcome high activation energy for structural isomerization.

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

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

Synthesis and solution isomerization of water-soluble Au9 nanoclusters prepared by nuclearity conversion of [Au11(PPh3)8Cl2]Cl

Water-soluble gold nanoclusters (AuNCs) are popular in biomedical applications such as bioimaging, labelling, drug delivery, and biosensing. Despite their widespread applications, the synthesis of water-soluble phosphine-capped AuNCs is not as straightforward as their organic-soluble equivalents. Organic soluble phosphine-passivated [Au₉(L)₈]³⁺ are 6-electron closed-shell AuNCs that are generally prepared via the reduction of a phosphine-Au(I) complex by NaBH₄. A similar approach attempted for the water-soluble ligand triphenylphosphine monosulfonate (TPPMS) using [AuTPPMS]Cl resulted in a mixture of cluster sizes that required gel electrophoresis or fractional precipitation to isolate the Au₉ product. In this work, we report the synthesis of water-soluble [Au₉(L)₈]³⁺ nanoclusters in high yield through the biphasic ligand exchange of [Au₁₁(PPh₃)₈Cl₂]Cl with water-soluble phosphines such as TPPMS and 4-(diphenylphosphino)benzoic acid (DPPBA). The small molecule byproducts can be completely removed by size-based separation methods, like size exclusion chromatography or dialysis, as confirmed by ³¹P and ¹H nuclear magnetic resonance (NMR) as well as diffusion ordered spectroscopy (DOSY). Furthermore, [Au₉(DPPBA)₈]Cl₃ underwent a visible pH- and temperature-induced isomerization in ethanol between the 'crown' and 'butterfly' isomers of [Au₉(L)₈]³⁺ which has not been previously reported. Cytotoxicity evaluation of these water-soluble nanoclusters gave CC₅₀ values of 36 μg mL^-1 and 70 μg mL^-1 against A549 human alveolar epithelial cells, and 30 μg mL^-1 and 40 μg mL^-1 against NIH/3T3 mouse fibroblast cells for [Au₉(TPPMS)₈]Cl₃ and [Au₉(DPPBA)₈]Cl₃, respectively. For comparison, auranofin, an FDA-approved gold drug, is more than an order of magnitude more toxic with a CC₅₀ value of 7.7 μg mL^-1 against A549 cells.

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

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

Thermal stability of crown-motif [Au9(PPh3)8]3+ and [MAu8(PPh3)8]2+ (M = Pd, Pt) clusters: Effects of gas composition, single-atom doping, and counter anions

The thermal behaviors of ligand-protected metal clusters, [Au₉(PPh₃)₈]³⁺ and [MAu₈(PPh₃)₈]²⁺ (M = Pd, Pt) with a crown-motif structure, were investigated to determine the effects of the gas composition, single-atom doping, and counter anions on the thermal stability of these clusters. We successfully synthesized crown-motif [PdAu₈(PPh₃)₈][HPMo₁₂O₄₀] (PdAu8-PMo12) and [PtAu₈(PPh₃)₈][HPMo₁₂O₄₀] (PtAu8-PMo12) salts with a cesium-chloride-type structure, which is the same as the [Au₉(PPh₃)₈][PMo₁₂O₄₀] (Au9-PMo12) structure. Thermogravimetry-differential thermal analysis/mass spectrometry analysis revealed that the crown-motif structure of Au9-PMo12 was decomposed at ∼475 K without weight loss to form Au nanoparticles. After structural decomposition, the ligands were desorbed from the sample. The ligand desorption temperature of Au9-PMo12 increased under 20% O₂ conditions because of the formation of Au nanoparticles and stronger interaction of the formed O=PPh₃ than PPh₃. The Pd and Pt single-atom doping improved the thermal stability of the clusters. This improvement was due to the formation of a large bonding index of M-Au and a change in Au-PPh₃ bonding energy by heteroatom doping. Moreover, we found that the ligand desorption temperatures were also affected by the type of counter anions, whose charge and size influence the localized Coulomb interaction and cluster packing between the cationic ligand-protected metal clusters and counter anions.

Toward quantitative electronic structure in small gold nanoclusters

Ligand-protected gold nanoclusters (AuNCs) feature a dense but finite electronic structure that can be rationalized using qualitative descriptions such as the well-known superatomic model and predicted using quantum chemical calculations. However, the lack of well-resolved experimental probes of a AuNC electronic structure has made the task of evaluating the accuracy of electronic structure descriptions challenging. We compare electronic absorption spectra computed using time-dependent density functional theory to recently collected high resolution experimental spectra of Au₉(PPh₃)₈ ³⁺ and Au₈(PPh₃)₇ ²⁺ AuNCs with strikingly similar features. After applying a simple scaling correction, the computed spectrum of Au₈(PPh₃)₇ ²⁺ yields a suitable match, allowing us to assign low-energy metal-metal transitions in the experimental spectrum. No similar match is obtained after following the same procedure for two previously reported isomers for Au₉(PPh₃)₈ ³⁺, suggesting either a deficiency in the calculations or the presence of an additional isomer. Instead, we propose assignments for Au₉(PPh₃)₈ ³⁺ based off of similarities Au₈(PPh₃)₇ ²⁺. We further model these clusters using a simple particle-in-a-box analysis for an asymmetrical ellipsoidal superatomic core, which allows us to reproduce the same transitions and extract an effective core size and shape that agrees well with that expected from crystal structures. This suggests that the superatomic model, which is typically employed to explain the qualitative features of nanocluster electronic structures, remains valid even for small AuNCs with highly aspherical cores.

Cocrystals of Atomically Precise Noble Metal Nanoclusters

Cocrystallization is a phenomenon involving the assembly of two or more different chemical entities in a lattice, occurring typically through supramolecular interactions. In this concept, recent advancements in the cocrystallization of atomically precise noble metal clusters and their potential future directions are presented. Different strategies to create coassemblies of thiolate-protected noble metal nanoclusters are presented first. An approach is the simultaneous synthesis, and cocrystallization of two clusters having similar structures. A unique pair of clusters found recently, namely Ag₄₀ and Ag₄₆ with same core but different shell are taken to illustrate this. In another category, the case of the same core is presented, namely Ag₁₁₆ with different shells, as in a mixture of Ag₂₁₀ and Ag₂₁₁ . Next, an intercluster reaction is presented to create cocrystals through selective crystallization of the reaction products. The coexistence of competing effects, magic sizes, and magic electron shells in a coassembly of alloy nanoclusters is discussed next. Finally, an assembly strategy for nanoclusters using electrostatic interactions is described. This concept is concluded with a future perspective on the emerging possibilities of such solids. Advancements in this field will certainly help the development of novel materials with exciting properties.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver

Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)_x L_y ]^z , have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)_x L_y ]^z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)_x L_y ]^z . This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)_x L_y ]^z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single-crystal X-ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand-protected Au/Ag clusters as new artificial elements on a nanometer scale.

Controlling the Crystallographic Packing Modes of Pt1Ag28 Nanoclusters: Effects on the Optical Properties and Nitrogen Adsorption-Desorption Performances

We herein report the manipulation of the crystallographic packing modes of Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ nanoclusters by altering counterions as different polyoxometalates (POMs). Specifically, the Cl^- anion of the presynthesized Pt₁Ag₂₈ nanocluster was substituted by POM anions including [Mo₆O₁₉]^2-, [W₆O₁₉]^2-, or [PW₁₂O₄₀]^3-. The crystal lattices of these Pt₁Ag₂₈ nanoclusters with diverse anions showed distinct packing modes and thus manifested remarkably distinguishable crystalline-state optical properties and nitrogen adsorption-desorption performances. Overall, the combination of intercluster control in this work and intracluster control reported previously (the control over metal-ligand within the nanocluster framework) accomplished a more comprehensive manipulation over the M₂₉(SR)₁₈(PR'₃)₄ nanocluster system, which enables us to further grasp the structure-property correlations at the atomic level.

N4Mg6M (M = Li, Na, K) superalkalis for CO2 activation

Superatoms have exciting properties, including diverse functionalization, redox activity, and magnetic ordering, so the resulting cluster-assembled solids hold the promise of high tunability, atomic precision, and robust architectures. By utilizing adamantane-like clusters as building blocks, a new class of superatoms N₄Mg₆M (M = Li, Na, K) is proposed here. The studied superalkalis feature low adiabatic ionization energies, an antibonding character in the interactions between magnesium and nitrogen atoms, and highly delocalized highest occupied molecular orbital (HOMO). Consequently, the N₄Mg₆M superalkalis might easily lose their HOMO electrons when interacting with superhalogen electrophiles to form stable superatom [superalkali]⁺[superhalogen]^- compounds. Moreover, the studied superalkalis interact strongly with carbon dioxide, and the resulting N₄Mg₆M/CO₂ systems represent two strongly interacting ionic fragments (i.e., N₄Mg₆M⁺ and CO₂ ^-). In turn, the electron affinity of the N₂ molecule (of -1.8 eV) is substantially lower than that observed for carbon dioxide (EA = -0.6 eV) and consequently, the N₂ was found to form the weakly bound [N₄Mg₆M][N₂] complex rather than the desired ionic [N₄Mg₆M]⁺[N₂]^- product. Thus, the N₄Mg₆M superalkalis have high selectivity over N₂ when it comes to CO₂ reduction and also are themselves stable. We believe that the results described within this paper will be useful for understanding CO₂ activation, which is the first step for producing fuels from CO₂. Moreover, we demonstrate that designing novel superatomic systems and exploring their physicochemical features might be used to create desirable functional materials.

Cyclodextrin-Assisted Hierarchical Aggregation of Dawson-type Polyoxometalate in the Presence of {Re6Se8} Based Clusters

The association of metallic clusters (CLUS) and polyoxometalates (POM) into hierarchical architectures is achieved using γ-cyclodextrin (γ-CD) as a supramolecular connector. The new self-assembled systems, so-called CLUSPOM, are formed from Dawson-type polyoxometalate [P₂W₁₈O₆₂]^6- and electron-rich rhenium clusters. It is worth noting that a cluster-based cation [{Re₆Se₈}(H₂O)₆]²⁺ on one hand and a cluster-based anion on the other hand [{Re₆Se₈}(CN)₆]^4- can be associated with the anionic POM. In the absence of the supramolecular connector, a "CLUSPOM salt" was obtained from aqueous solution of the cationic cluster and the polyoxometalate. In this solid, the arrangement between the polymetallic building blocks is mainly governed by long-range Coulombic interactions. In the presence of γ-CD, the Dawson anion and the cationic cluster are assembled differently, forming a hierarchical supramolecular solid, K₂[{Re₆Se₈}(H₂O)₆]₂{[P₂W₁₈O₆₂]@2γ-CD}·42H₂O, where the organic macrocycle acts as a ditopic linker between the inorganic building blocks. In such an edifice, the short-range molecular recognition dominates the long-range Coulombic interactions leading to a specific three-dimensional organization. Interestingly, the assembling of anionic POM [P₂W₁₈O₆₂]^6- with the anionic rhenium cluster [{Re₆Se₈}(CN)₆]^4- is also achieved with γ-CD despite the repulsive forces between the nanosized anions. The resulting solid, K₁₀{[{Re₆Se₈}(CN)₆]@2γ-CD}[P₂W₁₈O₆₂]·33H₂O, is built from 1:2 inclusion complexes {[{Re₆Se₈}(CN)₆]@2γ-CD}^4- linked by a POM unit interacting with the exterior wall of the organic macrocycle. Multinuclear NMR and small-angle X-ray scattering investigations support supramolecular preorganization in aqueous solution prior to crystallization.

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.

Superatomic Ligand-Field Splitting in Ligated Gold Nanoclusters

Gold nanoclusters are attractive because of their electronic and optical properties. Many theoretical models have been proposed to explain their electronic structures through an electron-counting approach. However, subtle features may not be well explained by electron-counting rules. In this work, we have discovered a unique example of ligand-controlled skeletal bonding in two recently reported gold nanoclusters with very similar compositions and geometries. We have shown that the superatomic orbitals of the common kernel of the two clusters undergo different ligand-field splitting because of the different ligand-field strengths in the two clusters. Such a difference is clearly revealed by constructing the Jellium orbitals via an orbital alignment process, and a subsequent localization of the Jellium orbitals allows us to obtain localized bonding models. Finally, on the basis of localized bonding models, we predict the existence of a ligated gold cluster with a [Au₃₂]⁴⁺ kernel.

High Precision Electronic Spectroscopy of Ligand-Protected Gold Nanoclusters: Effects of Composition, Environment, and Ligand Chemistry

Atomically precise gold nanoclusters (AuNCs) are a class of nanomaterials valued for their electronic properties and diverse structural features. While the advent of X-ray crystallography of AuNCs has revealed their geometric structures with high precision, detailed electronic structure analysis is challenged by environmental, compositional, and thermal averaging effects present in electronic spectra of typical samples. To circumvent these challenges, we have adapted mass spectrometer-based electronic absorption spectroscopy techniques to acquire high-resolution electronic spectra of atomically precisely defined nanoclusters separated from a synthetic mixture. Here we discuss recent results using this approach to link the surface chemistry of triphenylphosphine-protected AuNCs to their electronic structure and expand on key elements of the experiment and the link between these gas-phase measurements and solution-phase behavior of AuNCs. Chemically derivatized Au₈(P(p-X-Ph)₃)₇²⁺ and Au₉(P(p-X-Ph)₃)₈³⁺ clusters, where X = -H, -CH₃, or -OCH₃, are used to derive systematic trends in the response of the electronic spectrum to the electron-donating character of the ligand shell. We find a linear relationship between the substituent Hammett parameter σ_p and the transition energy between both sets of clusters' highest occupied and lowest unoccupied molecular orbitals, a transition that is localized in the metal core within the limits of the superatomic model. The similarity of the mass-selective and solution-phase UV/vis spectra of Au₉(PPh₃)₈³⁺ indicates that the interpretation of these experiments is transferable to the condensed phase. He and N₂ environments are introduced to a series of isovalent clusters as a subtle probe of discrete environmental effects over electronic structure. Strikingly, select bands in the UV/vis spectrum respond strongly to the identity of the environment, which we interpret as a state-selective indicator of interfacially relevant electronic transitions. Physically predictable trends such as these will aid in building molecular design principles necessary for the development of novel materials based on nanoclusters.

Making the unconventional μ2-P bridging binding mode more conventional in phosphinine complexes

As compared to the normal η¹-P σ-complexes or η⁶-phosphinine π-complexes, the rare μ²-P bridging binding mode of phosphinines can be tuned by employing electron donating substitute.

Phosphinines, as aromatic heterocycles, usually engage in coordination as η¹-P σ-complexes or η⁶-phosphinine π-complexes. The μ²-P bridging coordination mode is rarely observed. With the aim to study the effect of different electronic configurations of phosphinines on the coordination modes, a series of anionic phosphinin-2-olates and neutral phosphinin-2-ols were prepared with moderate to high yield. Then the coordination chemistry of these two series was studied in detail towards coinage metals (Au(i) and Cu(i)). It is observed that the anionic phosphinin-2-olates possess a higher tendency to take a bridging position between two metal centers compared to the neutral phosphinin-2-ols. Based on these experimental findings bolstered by DFT calculations, some insight is gained on how the unconventional μ²-P phosphinine bridging coordination mode can be made more conventional and used for the synthesis of polynuclear complexes.

Characterization of chemically modified gold and silver clusters in gas phase

Atomically precise Au and Ag clusters protected by organic ligands can be viewed as chemically modified Au/Ag superatoms and have attracted interest as promising building units of functional materials and ideal platforms for studying the size-dependent evolution of structures and properties. Their structures, stability, and physicochemical properties have been characterized in solution and solid (or crystalline) phases by various methods conventionally used in materials science. However, novel and complementary information on their intrinsic stability and structures can be obtained by applying a variety of gas-phase methods, including mass spectrometry, ion mobility mass spectrometry, collision- or surface-induced dissociation mass spectrometry, photoelectron spectroscopy, and photodissociation mass spectrometry, to the chemically modified Au/Ag superatoms isolated in the gas phase. This perspective describes our recent efforts in the gas-phase studies on chemically synthesized Au/Ag superatoms.

Hydride Doping of Chemically Modified Gold-Based Superatoms

Atomically size-selected gold (Au) clusters protected by organic ligands or stabilized by polymers provide an ideal platform to test fundamental concepts and size-specific phenomena, such as the superatomic concept and metal-to-nonmetal transition. Recent studies revealed that these stabilized Au clusters take atomlike quantized electronic structures and can be viewed as chemically modified Au superatoms. An analogy between Au and hydrogen (H) atoms is an interesting proposal made for bare Au clusters: a Au atom at a low-coordination site of a Au cluster can be replaced with a H atom while retaining the structural motif and electronic structure. However, this proposal has not been experimentally proved in chemically modified Au superatoms while a recent theoretical study predicted the formation of [HAu₂₅(SR)₁₈]⁰ (RS = thiolate). This Account summarizes our recent studies on the interaction of hydride(s) with two types of chemically modified Au-based superatoms: (1) the Au cores of [Au₉(PPh₃)₈]³⁺ and [PdAu₈(PPh₃)₈]²⁺ formally described as (Au₉)³⁺ and (PdAu₈)²⁺, respectively, and (2) Au₃₄ cluster stabilized by poly( N-vinyl-2-pyrrolidone) (PVP). The (Au₉)³⁺ and (PdAu₈)²⁺ cores correspond to oblate-shaped superatoms with six electrons and a coordinatively unsaturated site at the center, whereas the Au₃₄ cluster in PVP is viewed as a nearly spherical superatom having a closed electronic structure with 34 electrons and multiple uncoordinated sites on the surface. Through this study, we aimed to deepen our understanding on the role of a hydride in the formation processes of Au superatoms, the effect of adsorbed hydride(s) on the electronic structure of Au superatoms, and the activity of adsorbed hydrogen species for hydrogenation catalysis. Mass spectrometry and nuclear magnetic resonance spectroscopy demonstrated that a single hydride (H^-) was selectively doped to (Au₉)³⁺ and (PdAu₈)²⁺ upon reactions with BH₄^- to form (HAu₉)²⁺ and (HPdAu₈)⁺, respectively. Density functional theory (DFT) calculations showed that (HAu₉)²⁺ and (HPdAu₈)⁺ were more spherical than the original superatoms and had a closed electronic structure with eight electrons. The hydride-doped (HAu₉)²⁺ was selectively converted to the well-known (Au₁₁)³⁺ by electrophilic addition of two Au(I) units whereas (HPdAu₈)⁺ was converted to a new hydride-doped (HPdAu₁₀)³⁺. A two-step mechanism was proposed for hydride-mediated growth of Au-based superatoms: closure of the electronic structures by adsorption of a hydride, followed by the addition of two Au(I) units. The selective formation of Au₃₄ superatoms in PVP is also explained by assuming that hydride-doped Au clusters with 34 electrons were involved as key intermediates. The Au₃₄ superatom exhibited the localized surface plasmon resonance (LSPR) band by reacting with BH₄^- due to the electron donation by multiply adsorbed hydrides. The LSPR band disappeared by exposing hydride-doped Au₃₄ to dissolved O₂, but reappeared by reaction with BH₄^-. Catalysis for hydrogenation of C═C bonds was generated by doping a single Pd or Rh atom to Au₃₄. The results reported here demonstrate that the hydride doped to chemically modified Au superatoms mimics Au^- in terms of electron count. The hydride-mediated growth processes observed will contribute to the development of an atomically precise, bottom-up method of synthesizing new artificial elements in a periodic table for nanoscale materials. The interaction of hydride(s) with Au superatoms will find application in hydrogenation catalysis and hydrogen sensing.

Isomerism in Au-Ag Alloy Nanoclusters: Structure Determination and Enantioseparation of [Au9Ag12(SR)4(dppm)6X6]3

Revealing structural isomerism in a nanocluster remains significant but challenging. Herein, we have obtained a pair of structural isomers, [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C and [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-Ac [dppm = bis(diphenyphosphino)methane; HSR = 1-adamantanethiol/ tert-butylmercaptan; X = Br/Cl; C stands for one of the structural isomers being chiral; Ac stands for another being achiral], that show different structures as well as different chiralities. These structures are determined by single-crystal X-ray diffraction and further confirmed by high-resolution electrospray ionization mass spectrometry. On the basis of the isomeric structures, the most important finding is the different arrangements of the Au₅Ag₈@Au₄ metal core, leading to changes in the overall shape of the cluster, which is responsible for structural isomerism. Meanwhile, the two enantiomers of [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C are separated by high-performance liquid chromatography. Our work will contribute to a deeper understanding of the structural isomerism in noble-metal nanoclusters and enrich the chiral nanocluster.

Cluster assemblies as superatomic solids: a first principles study of bonding & electronic structure

The synthesis of cluster based materials poses an exciting challenge for experimental chemistry. The main advantage of these materials compared to conventional bulk compounds is the simple tunability of the chemical and physical characteristics of individual clusters. As a consequence, cluster assemblies can theoretically be used for the creation of designer materials exhibiting specifically desired properties. Since superatoms reveal a large intrinsic thermodynamic stability and often very interesting tunable electronic characteristics, they seem to be an excellent choice as building blocks for the bulk. Here, we present a detailed first principles analysis of carefully chosen superatomic cluster binary and bulk assemblies, in order to determine which forces control the attractive interaction in superatomic solids, and how the individual cluster properties affect these assemblies. This study uses the highly tunable and stable Au₁₃(RS(AuSR)₂)₆ cluster with a variety of dopants as a model system, while the principles are likely transferable to other ligand protected systems with a straightforward superatomic electron count, such as aluminum or sodium clusters. Three different superatomic materials based on doped gold clusters, boranes and C₆₀s are constructed and evaluated. Beyond the verification that superatoms can be used to create materials that reveal emergent atom-based solid like properties, various factors influencing superatomic materials, such as the EA, IP and relative sizes of the clusters, have been identified and critically evaluated.

Polyoxometalate-Assisted, One-Pot Synthesis of a Pentakis[(triphenylphosphane)gold]ammonium(2+) Cation Containing Regular Trigonal-Bipyramidal Geometries of Five Bonds to Nitrogen

Novel intercluster compounds consisting of pentakis[(triphenylphosphane)gold]ammonium(2+) cation (1) and Keggin polyoxometalate (POM) anions, i.e., {[Au(PPh₃)]₅(μ₅-N)}₃[α-PM₁₂O₄₀]₂ (1-PW for M = W; 1-PMo for M = Mo), were synthesized in 30-36% yield by one-pot reaction of the protonic acid form of the Keggin POMs, H₃[α-PM₁₂O₄₀]·nH₂O (n = 13 for M = W; n = 15 for M = Mo) with monomeric (triphenylphosphane)gold(I) carboxylate [Au(RS-pyrrld)(PPh₃)] [RS-Hpyrrld = (RS)-2-pyrrolidone-5-carboxylic acid] in the presence of aqueous NH₃ at a molar ratio of 2:15:x (x = 3 for 1-PW; x = 7.5 for 1-PMo). These compounds resulted from the nitrogen-centered phosphanegold(I) clusterization of in situ generated monomeric phosphanegold(I) units, [Au(PPh₃)]⁺ or [Au(L)(PPh₃)]⁺ (L = NH₃ or solvent), during the carboxylate elimination of [Au(RS-pyrrld)(PPh₃)] in the presence of the Keggin POMs and aqueous NH₃. The products 1-PW and 1-PMo were characterized by elemental analysis, Fourier transform infrared, thermogravimetric and differential thermal analyses (TGA/DTA), X-ray crystallography, and solid-state cross-polarization magic-angle-spinning (CPMAS) (³¹P and ¹⁵N) and solution (³¹P{¹H} and ¹H) NMR spectroscopy. The lattice contained three independent {[Au(PPh₃)]₅(μ₅-N)}²⁺ cations, of which two took regular trigonal-bipyramidal (TBP) geometries and the third took a distorted, square-pyramidal (SP) geometry. These geometries are in contrast to those reported by Schmidbaur's group for {[Au(PPh₃)]₅(μ₅-N)}²⁺ cations as BF₄ salts. Density functional theory and ONIOM calculations for {[(L₃P)Au]_nN}^(n-3)+ (L = H or Ph; n = 4-6) showed that the pentacoordinate cluster is energetically most stable and its TBP structure is only 1.6 kcal mol^-1 more stable than its SP structure, in accordance with the experimental facts.

From aqueous speciation to supramolecular assembly in alkaline earth-uranyl polyoxometalates

The interplay between aqueous alkaline earth (Ca, Sr, Ba) polycationic speciation and uranyl-peroxide polyoxometalate self-assembly and evolution is described here using solution (Raman and X-ray scattering) and solid-state (microscopy, X-ray diffraction) characterization. Supramolecular assembly of Sr-encapsulated and decorated polyanions and polycations yields the fourth largest inorganic unit cell reported from single-crystal X-ray diffraction.

Observing the real time formation of phosphine-ligated gold clusters by electrospray ionization mass spectrometry

Real-time monitoring of the gold cluster synthesis by electrospray ionization mass spectrometry reveals distinct formation pathways for Au₈, Au₉ and Au₁₀ clusters.

Ligand induced structural isomerism in phosphine coordinated gold clusters revealed by ion mobility mass spectrometry

Structural isomerism in ligated gold clusters is revealed using electrospray ionization ion mobility spectrometry mass spectrometry.

Assembly of titanium-oxo cations with copper-halide anions to form supersalt-type cluster-based materials

A facile approach for the synthesis of binary titanium-oxo and copper-halide cluster-based crystalline materials with salt-like packing has been successfully established.

First-principles calculations of the electronic structure and bonding in metal cluster-fullerene materials considered within the superatomic framework

Inspired by recent success of synthesizing cluster assembled compounds we address the question to what extent the three new materials [Co₆Se₈(PEt₃)₆][C₆₀]₂, [Cr₆Te₈(PEt₃)₆][C₆₀]₂, and [Ni₉Te₆(PEt₃)₈]C₆₀, upon forming bulk compounds, imitate atomic analogues. Although experimental results suggest the latter, a theoretical approach is the method of choice for offering a conclusive answer and for studying the actual superatomic character. The concept of superatoms for describing atom-imitating clusters is very intriguing since it allows chemists to apply their chemical intuition - a useful tool for predicting new materials - when it comes to inter-cluster reactions. Thus, we systematically study the lattice structure, the intercluster binding, and the electronic structure by density functional theory and assess them in terms of their superatomic features. We show that collective properties arise upon bulk formation, which promotes arguments for the formation of solids in which the constituent clusters have a superatomic character that determines some form of chemical bonding. Additionally, we find evidence for the formation of superatomic states. Unfortunately, however, due to the mixing of electronic states of transition metals and chalcogen atoms, no typical electronic shell closing in the cluster cores can be identified.

The effect of counteranions on the molecular structures of phosphanegold(i) cluster cations formed by polyoxometalate (POM)-mediated clusterization

The effect of counteranions on the molecular structures of phosphanegold(i) cluster cations formed by polyoxometalate (POM)-mediated clusterization was investigated. A novel intercluster compound, [{(AuLCl)2(μ-OH)}2]3[α-PMo12O40]2·3EtOH (1-PMo12), was obtained as orange-yellow plate crystals in 12.0% yield from a 6 : 1 molar ratio reaction of the monomeric phosphanegold(i) carboxylato complex [Au(RS-pyrrld)(LCl)] (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid; LCl = tris(4-chlorophenyl)phosphane) in CH2Cl2 with the free acid-form of Keggin polyoxometalate (POM), H3[α-PMo12O40]·14H2O. An EtOH/H2O (5 : 1, v/v) solvent mixture was used. The dimeric cation [{(AuLCl)2(μ-OH)}2](2+) in 1-PMo12 was in a parallel-edge arrangement that was formed by self-assembly through the inter-cationic aurophilic interactions of the μ-OH-bridged dinuclear phosphanegold(i) cation. The POM anion in 1-PMo12 was successfully exchanged with a smaller PF6(-) anion by the use of an anion-exchange resin. POM-free, colorless block crystals of [{(AuLCl)3(μ3-O)}2](PF6)2·4CH2Cl2 (2-PF6) were obtained by vapor diffusion in 14.1% yield. During the synthesis of 2-PF6, a compound with mixed counteranions (one POM and one PF6(-) anion), i.e. [{(AuLCl)4(μ4-O)}]2[α-PMo12O40]PF6 (3-PMo12PF6), was obtained in 66.4% yield. Both products were characterized by elemental analysis, TG/DTA, FT-IR, (31)P{(1)H} NMR, (1)H NMR, and X-ray crystallography. X-ray crystallography revealed that the countercation in 2-PF6 was the dimeric cation of the μ3-O-bridged tris{phosphanegold(i)} species, whereas that in 3-PMo12PF6 consisted of an unusual μ4-O-bridged tetragonal-pyramidal tetrakis{phosphanegold(i)} cation. Therefore, we concluded that the POM anion significantly contributed to the stabilization of these countercations (parallel-edged arrangement in 1-PMo12 and μ4-O-bridged tetragonal-pyramid in 3-PMo12PF6). Moreover, the previously reported yellow crystals of [{(AuLF)2(μ-OH)}2]3[PMo12O40]2·3EtOH (4-PMo12: LF = tris(4-fluoro phenyl)phosphane) were successfully converted to the POM-free crystalline OTf(-) salt [{(AuLF)2(μ-OH)}2](OTf)2·0.5Et2O (4-OTf) by the use of an anion-exchange resin. X-ray crystallography also revealed that the parallel-edge arrangement of the dimeric cation in 4-PMo12 was converted to the crossed-edge arrangement of that in 4-OTf. These results illustrate that the AuOPOM and hydrogen-bonding (C-HOPOM and O-HOPOM) interactions between the phosphanegold(i) cluster cation and the Keggin POM anion in the solid state significantly contribute to the structure, composition, and stability of the phosphane gold(i) cluster cations in 4-PMo12.

Crystal structure of [Ag3(DMF)7][W12O40P], a 2D-coordinated system, C21H49Ag3N7O47PW12

C21H49Ag3N7O47PW12, monoclinic, P21/c (no. 14), a = 17.329(4) Å, b = 17.098(3) Å, c = 22.479(5) Å, β = 105.19(3)°, V = 6427.7 Å3, Z = 4, Rgt(F) = 0.0664, wRref(F2) = 0.1543, T = 293 K.

Reaction mechanism governing formation of 1,3-bis(diphenylphosphino)propane-protected gold nanoclusters

This report outlines the determination of a reaction mechanism that can be manipulated to develop directed syntheses of gold monolayer-protected clusters (MPCs) prepared by reduction of solutions containing 1,3-bis(diphenylphosphino)propane (L(3)) ligand and Au(PPh(3))Cl. Nanocluster synthesis was initiated by reduction of two-coordinate phosphine-ligated [Au(I)LL'](+) complexes (L, L' = PPh(3), L(3)), resulting in free radical complexes. The [Au(0)LL'](•) free radicals nucleated, forming a broad size distribution of ligated clusters. Timed UV-vis spectroscopy and electrospray ionization mass spectrometry monitored the ligated Au(x), 6 ≤ x ≤ 13, clusters, which comprise reaction intermediates and final products. By employing different solvents and reducing agents, reaction conditions were varied to highlight the largest portion of the reaction mechanism. We identified several solution-phase reaction classes, including dissolution of the gold precursor, reduction, continuous nucleation/core growth, ligand exchange, ion-molecule reactions, and etching of colloids and larger clusters. Simple theories can account for the reaction intermediates and final products. The initial distribution of the nucleation products contains mainly neutral clusters. However, the rate of reduction controls the amount of reaction overlap occurring in the system, allowing a clear distinction between reduction/nucleation and subsequent solution-phase processing. During solution-phase processing, the complexes undergo core etching and core growth reactions, including reactions that convert neutral clusters to cations, in a cyclic process that promotes formation of stable clusters of specific metal nuclearity. These processes comprise "size-selective" processing that can narrow a broad distribution into specific nuclearities, enabling development of tunable syntheses.