From 8- to 18-Cluster Electrons Superatoms: Evaluation via DFT Calculations of the Ligand-Protected W@Au12(dppm)6 Cluster Displaying Distinctive Electronic and Optical Properties

The iconic W@Au₁₂ icosahedral bare cluster reaches the favorable closed-shell superatomic electron configuration 1S² 1P⁶ 1D¹⁰, making it an 18-cluster electron (18-ce) superatom. Here, we pursue the evaluation of a ligand-protected counterpart based on the construction of a fully phosphine-protected [W@Au₁₂(dppm)₆] cluster strongly related to the characterized [Au₁₃(dppm)₆]⁵⁺ homometallic counterpart. The later cluster has the same total number of valence electrons as the former but is considered an 8-ce superatom with 1S² 1P⁶ configuration. The fundamental differences between 8- and 18-ce species are investigated. The character of the frontier orbitals varies from 1P/1D in the 8-ce case to a 1D/ligand for 18-ce species, enabling an efficient charge transfer toward the ligands upon irradiation, being interesting for electron injection in optoelectronic devices and black absorbers applications. Excited-state properties are also revisited, showing different geometrical and electronic structure variations between 8- and 18-ce species. Moreover, the continuum between the 8- and 18-ce limits has been explored by varying the nature of the encapsulated dopant between group 6 and group 11. The transition between the 8- and 18-ce counts can be formally situated between Pt (8-ce) and Ir (18-ce). Thus, 18-ce derivatives obtained as doped counterparts of homometallic gold clusters can introduce useful alternatives to achieve different properties in related structural motifs, which can be further explored owing to their extension of the well-established versatility of current gold nanoclusters.

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.

Controlled Synthesis of Diphosphine-Protected Gold Cluster Cations Using Magnetron Sputtering Method

We demonstrated, for the first time, atomically precise synthesis of gold cluster cations by magnetron sputtering of a gold target onto a polyethylene glycol (PEG) solution of 1,3-bis(diphenylphosphino)propane (Ph₂PCH₂CH₂CH₂PPh₂, dppp). UV-vis absorption spectroscopy and electrospray ionization mass spectrometry revealed the formation of cationic species, such as [Au(dppp)_n]⁺ (n = 1, 2), [Au₂(dppp)_n]²⁺ (n = 3, 4), [Au₆(dppp)_n]²⁺ (n = 3, 4), and [Au₁₁(dppp)₅]³⁺. The formation of [Au(dppp)₂]⁺ was ascribed to ionization of Au(dppp)₂ by the reaction with PEG, based on its low ionization energy, theoretically predicted, mass spectrometric detection of deprotonated anions of PEG. We proposed that [Au(dppp)₂]⁺ cations thus formed are involved as key components in the formation of the cluster cations.

Insight Into the Stability and Electronic and Optical Properties of N-Heterocyclic Carbene Analogues of Halogen/Phosphine-Protected Au13 Superatomic Clusters

Atomically precise gold nanoclusters (AuNCs) belong to a relevant area offering useful templates with tunable properties toward functional nanostructures. In this work, we explored the feasible incorporation of N-heterocyclic carbenes (NHCs), as part of the protecting-ligand shell in AuNCs. Our results, which are based on the substitution of phosphine ligands in experimentally characterized AuNCs by NHCs in various eight-electron superatoms Au₁₃ and M₄Au₉ (M = Cu, Ag), indicate similar electronic structure and stability but somewhat different optical properties. These findings support the feasible obtention of novel targets for explorative synthetic efforts featuring NHC ligands on medium-sized species based on the recurrent Au₁₃ icosahedral core. The hypothetical species appear to be interesting templates for building blocks in nanostructured materials with tuned properties, which encourage experimental exploration of ligand versatility in homo- and heterometallic superatomic clusters.

Ligand-Core Interaction in Ligand-Protected Ag25(XR)18 (X= S, Se, Te) Superatoms. Evaluation of Anchor Atom Role via Relativistic DFT Calculations

The isostructural and isoelectronic silver [Ag25(SR)18]- (R=Ligand) cluster to [Au25(SR)18]- gold clusters allows to further understand the fundamental similarities between Au and Ag, at the ultrasmall nanoscale (< 2 nm)...

Evaluation of ultrasmall coinage metal M13(dppe)6 M = Cu, Ag, and Au clusters. Bonding, structural and optical properties from relativistic DFT calculations

Ultrasmall ligand-protected clusters are prototypical species for evaluating the variation at the bottom of the nanoscale range. Here we explored the ultrasmall gold-phosphine M₁₃(dppe)₆ cluster, as a prototypical framework to gain insights into the fundamental similarities and differences between Au, Ag, and Cu, in the 1-3 nm size range, via relativistic DFT calculations. Different charge states involving 8- and 10-cluster electron (ce) species with a 1S²1P⁶ and 1S²1P⁶1D² configuration, leading to structural modification in the Au species between Au₁₃(dppm)₆⁵⁺ and Au₁₃(dppm)₆³⁺, respectively. Furthermore, this structural distortion of the M₁₃ core is found to occur to a lower degree for the calculated Ag and Cu counterparts. Interestingly, optical properties exhibit similar main patterns along with the series, inducing a blue-shift for silver and copper, in comparison to the gold parent cluster. For 10-ce species, the main features of 8-ce are retained with the appearance of several weak transitions in the range. The ligand-core interaction is enhanced for gold counterparts and decreased for lighter counterparts resulting in the Au > Cu > Ag trend for the interaction stabilization. Hence, the Ag and Cu counterparts of the Au₁₃(dppm)₆ cluster appear as useful alternatives, which can be further explored towards different cluster alternatives for building blocks for nanostructured materials.

Ligand Design in Ligand-Protected Gold Nanoclusters

The design of surface ligands is crucial for ligand-protected gold nanoclusters (Au NCs). Besides providing good protection for Au NCs, the surface ligands also play the following two important roles: i) as the outermost layer of Au NCs, the ligands will directly interact with the exterior environment (e.g., solvents, molecules and cells) influencing Au NCs in various applications; and ii) the interfacial chemistry between ligands and gold atoms can determine the structures, as well as the physical and chemical properties of Au NCs. A delicate ligand design in Au NCs (or other metal NCs) needs to consider the covalent bonds between ligands and gold atoms (e.g., gold-sulfur (Au-S) and gold-phosphorus (Au-P) bond), the physics forces between ligands (e.g., hydrophobic and van der Waals forces), and the ionic forces between the functional groups of ligands (e.g., carboxylic (COOH) and amine group (NH₂ )); which form the underlying chemistry and discussion focus of this review article. Here, detailed discussions on the effects of surface ligands (e.g., thiolate, phosphine, and alkynyl ligands; or hydrophobic and hydrophilic ligands) on the synthesis, structures, and properties of Au NCs; highlighting the design principles in the surface engineering of Au NCs for diverse emerging applications, are provided.

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.

Gold nanoclusters: An ultrasmall platform for multifaceted applications

Gold nanoclusters (Au NCs) with a core size below 2 nm form an exciting class of functional nano-materials with characteristic physical and chemical properties. The properties of Au NCs are more prominent and extremely different from their bulk counterparts. The synthesis of Au NCs is generally assisted by template or ligand, which impart excellent cluster stability and high quantum yield. The tunable and sensitive physicochemical properties of Au NCs open horizons for their advanced applications in various interdisciplinary fields. In this review, we briefly summarize the solution phase synthesis and origin of the characteristic properties of Au NCs. A vast review of recent research work introducing biosensors based on Au NCs has been presented along with their specifications and detection limits. This review also highlights recent progress in the use of Au NCs as bio-imaging probe, enzyme mimic, temperature sensing probe and catalysts. A speculation on present challenges and certain future prospects have also been provided to enlighten the path for advancement of multifaceted applications of Au NCs.

The positional isomerism in bimetal nanoclusters

Enriching the variety of isomerism in the nanocluster field is exciting but challenging.

Role of sterics in phosphine-ligated gold clusters

This study examined the solution-phase exchange reactions of triphenylphosphine (PPh₃) ligands on Au₈L₇²⁺ (L = PPh₃) gold clusters with three different tolyl ligands using electrospray ionization mass spectrometry to provide insight into how steric differences in the phosphines influence the extent of ligand exchange and the stability of the resulting mixed-phosphine clusters.

Stibine-protected Au13 nanoclusters: syntheses, properties and facile conversion to GSH-protected Au25 nanocluster

Monostibine-protected ionic Au13 nanoclusters, namely, [Au13(L)8(Cl)4][Cl] (L= SbPh3, 2a·Cl; Sb(p-tolyl)3, 2b·Cl) were prepared by the direct reduction of Au(L)Cl with NaBH4 in dichloromethane. Anion exchange with 2a·Cl afforded [Au13(SbPh3)8(Cl)4][X] (X = PF6, 2a·PF6; BPh4, 2a·BPh4). All these have been characterized by multinuclear NMR, ESI-MS and UV-Vis spectroscopy. Crystallographic analysis of 2a·BPh4 reveals that the cation possesses C 2v symmetry and the tridecagold core adopts a closed icosahedron configuration. The weaker coordinating ability of the stibine ligands leads to the ready reaction of 2b·Cl with PPh3 or glutathione (GSH) to form the smaller phosphine-protected cluster [Au11(PPh3)8Cl2][Cl] or larger thiolate-protected cluster Au25(SG)18, respectively. In the latter reaction, the addition of a small amount (0.5 to 3.5 equivalents) of a suitable oxidant such as K3(Fe(CN)6 accelerates the conversion rate significantly.

Synthesis and characterization of size-controlled atomically precise gold clusters

In this article, synthetic strategies and characterization methodologies of atomically precise gold clusters have been summarized. The typical and effective synthetic strategies including a systematic “size-focusing” methodology has been developed for attaining atomically precise gold clusters with size control. Another universal synthetic methodology is ligand exchange-induced size/structure transformation (LEIST) based on from one stable size to another. These two methodologies have largely expanded the “universe” of atomically precise gold clusters. Elite of typical synthetic case studies of ligand protected gold clusters are presented. Important characterization techniques of these atomically precise gold clusters also are included. The identification and characterization of gold clusters have been achieved in terms of nuclearity (size), molecular formulation, and geometrical structures by the combination of these techniques. The determination of gold cluster structure based on single crystals is of paramount importance in understanding the relationship of structure–property. The criterion and selection of these typical gold clusters are all “strictly” atomically precise that all have been determined ubiquitously by single crystal diffraction. These related crystallographic data are retrieved from Cambridge Crystallographic Data Centre (CCDC) up to 30th November 2017. Meanwhile, the cutting edge and other important characterization methodologies including electron diffraction (ED), extended X-ray absorption fine structure (EXFAS), and synchrotron sources are briefly reviewed. The new techniques hold the promise of pushing the limits of crystallization of gold clusters. This article is not just an exhaustive and up to date review, generally summarized synthetic strategies, but also a practical guide regarding gold cluster synthesis. We called it a “Cookbook” of ligand protected gold clusters, including synthetic recipes and characterization details.

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Ultrasmall Noble Metal Nanoparticles: Breakthroughs and Biomedical Implications

As a bridge between individual atoms and large plasmonic nanoparticles, ultrasmall (core size <3 nm) noble metal nanoparticles (UNMNPs) have been serving as model for us to fundamentally understand many unique properties of noble metals that can only be observed at an extremely small size scale. With decades'efforts, many significant breakthroughs in the synthesis, characterization and functionalization of UNMNPs have laid down a solid foundation for their future applications in the healthcare. In this review, we aim to tightly correlate these breakthroughs with their biomedical applications and illustrate how to utilize these breakthroughs to address long-standing challenges in the clinical translation of nanomedicines. In the end, we offer our perspective on the remaining challenges and opportunities at the frontier of biomedical-related UNMNPs research.

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.

On AunAt clusters as potential astatine carriers

To understand interactions between astatine atoms with gold clusters the Au_nAt and Au_nX clusters, n = 12 or 13, X = F, Cl, Br, and I, were calculated at the DFT level using basis sets with a quasi-relativistic pseudopotential.

Understanding ligand effects in gold clusters using mass spectrometry

This review summarizes recent research on the influence of phosphine ligands on the size, stability, and reactivity of gold clusters synthesized in solution. Sub-nanometer clusters exhibit size- and composition-dependent properties that are unique from those of larger nanoparticles. The highly tunable properties of clusters and their high surface-to-volume ratio make them promising candidates for a variety of technological applications. However, because "each-atom-counts" toward defining cluster properties it is critically important to develop robust synthesis methods to efficiently prepare clusters of predetermined size. For decades phosphines have been known to direct the size-selected synthesis of gold clusters. Despite the preparation of numerous species it is still not understood how different functional groups at phosphine centers affect the size and properties of gold clusters. Using electrospray ionization mass spectrometry (ESI-MS) it is possible to characterize the effect of ligand substitution on the distribution of clusters formed in solution at defined reaction conditions. In addition, ligand exchange reactions on preformed clusters may be monitored using ESI-MS. Collision induced dissociation (CID) may also be employed to obtain qualitative insight into the fragmentation of mixed ligand clusters and the relative binding energies of differently substituted phosphines. Quantitative ligand binding energies and cluster stability may be determined employing surface induced dissociation (SID) in a custom-built Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Rice-Ramsperger-Kassel-Marcus (RRKM) based modeling of the SID data allows dissociation energies and entropy values to be extracted. The charge reduction and reactivity of atomically precise gold clusters, including partially ligated species generated in the gas-phase by in source CID, on well-defined surfaces may be explored using ion soft landing (SL) in a custom-built instrument combined with in situ time of flight secondary ion mass spectrometry (TOF-SIMS). Jointly, this multipronged experimental approach allows characterization of the full spectrum of relevant phenomena including cluster synthesis, ligand exchange, thermochemistry, surface immobilization, and reactivity. The fundamental insights obtained from this work will facilitate the directed synthesis of gold clusters with predetermined size and properties for specific applications.

The structure and optical properties of the [Au18(SR)14] nanocluster

Decreasing the core size is one of the best ways to study the evolution from Au(I) complexes into Au nanoclusters. Toward this goal, we successfully synthesized the [Au18(SC6H11)14] nanocluster using the [Au18(SG)14] (SG=L-glutathione) nanocluster as the starting material to react with cyclohexylthiol, and determined the X-ray structure of the cyclohexylthiol-protected [Au18(C6H11S)14] nanocluster. The [Au18(SR)14] cluster has a Au9 bi-octahedral kernel (or inner core). This Au9 inner core is built by two octahedral Au6 cores sharing one triangular face. One transitional gold atom is found in the Au9 core, which can also be considered as part of the Au4(SR)5 staple motif. These findings offer new insight in terms of understanding the evolution from [Au(I)(SR)] complexes into Au nanoclusters.

Facile synthesis and optical properties of magic-number Au13 clusters

Synthesis of molecular gold clusters through a post-synthetic scheme involving HCl-promoted nuclearity convergence was examined with various phosphine ligands. Systematic studies with a series of bis(diphenylphosphino) ligands (Ph(2)P-(CH(2))(m)-PPh(2)) using electrospray ionization mass spectrometry (ESI-MS) and electronic absorption spectroscopy demonstrated that the use of dppp (m = 3), dppb (m = 4) and dpppe (m = 5) as the ligands resulted in the formation of [Au(13)P(8)Cl(4)](+) type clusters, whereas the [Au(13)P(10)Cl(2)](3+) type cluster was formed with dppe (m = 2). The cluster species did not survive the HCl treatment step when monophosphines PPh(3), PMe(2)Ph, and POct(3) were employed, but [Au(13)(POct(3))(8)Cl(4)](+) was isolated as a minor product in the NaBH(4) reduction of Au(POct(3))Cl in aqueous THF. Electronic absorption and photoluminescence studies of a series of Au(13) clusters revealed that their optical properties are highly dependent on the phosphine/chloride composition ratio, but are far less so on the phosphine structure.

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.