Au57 Ag53 (C≡CPh)40 Br12 : A Large Nanocluster with C 1 Symmetry

Alkynyl-Protected Bimetallic Nanoclusters with a Hybrid Mackay Icosahedral Ag42 Cu12 Cl Kernel and an Octahedral Ag22 Cu12 Kernel

A facile strategy that directly reduces alkynyl-silver precursors and copper salts for the synthesis of bimetallic nanoclusters using the weak reducing agent Ph2 SiH2 is demonstrated. Two alkynyl-protected concentric-shell nanoclusters, (Ph4 P)2 [Ag22 Cu12 (C≡CR)28 ] and (Ph4 P)3 [Ag42 Cu12 Cl(C≡CR)36 ] (Ag22 Cu12 and Ag42 Cu12 Cl, R=bis(trifluoromethyl)phenyl), were successfully obtained and characterized by single-crystal X-ray diffraction and electro-spray ionization mass spectrometry. For the first time, a hybrid 55-atom two-shell Mackay icosahedron was found in Ag42 Cu12 Cl, which is icosahedral M54 Cl instead of M55 . The incorporation of a chloride in the metal icosahedron contributes to the stability of the cluster from both electronic and geometric aspects. Alkynyl ligands show various binding-modes including linear "RC≡C-Cu-C≡CR" staple motifs.

Unveiling the Remarkable Stability and Catalytic Activity of a 6-Electron Superatomic Ag30 Nanocluster for CO2 Electroreduction

Nanocluster catalysts face a significant challenge in striking the right balance between stability and catalytic activity. Here, we present a thiacalix[4]arene-protected 6-electron [Ag30(TC4A)4(iPrS)8] nanocluster that demonstrates both high stability and catalytic activity. The Ag30 nanocluster features a metallic core, Ag104+, consisting of two Ag3 triangles and one Ag4 square, shielded by four {Ag5@(TC4A)4} staple motifs. Based on DFT calculations, the Ag104+ metallic kernel can be viewed as a trimer comprising 2-electron superatomic units, exhibiting a valence electron structure similar to that of the Be3 molecule. Notably, this is the first crystallographic evidence of the trimerization of 2-electron superatomic units. Ag30 can reduce CO2 into CO with a Faraday efficiency of 93.4% at -0.9 V versus RHE along with excellent long-term stability. Its catalytic activity is far superior to that of the chain-like AgI polymer ∞1{[H2Ag5(TC4A)(iPrS)3]} (∞1Agn), with the composition similar to Ag30. DFT calculations elucidated the catalytic mechanism to clarify the contrasting catalytic performances of the Ag30 and ∞1Agn polymers and disclosed that the intrinsically higher activity of Ag30 may be due to the greater stability of the dual adsorption mode of the *COOH intermediate on the metallic core.

Recent progress in dichalcophosphate coinage metal clusters and superatoms

Atomically precise clusters of group 11 metals (Cu, Ag, and Au) attract considerable attention owing to their remarkable structure and fascinating properties. One of the unique subclasses of these clusters is based on dichalcophosphate ligands of [(RO)₂PE₂]^- type (E = S or Se, and R = alkyl). These ligands successfully stabilise the most diverse Cu, Ag, and Au clusters and superatoms, spanning from simple ones to amazing assemblies featuring unusual structural and bonding patterns. It is noteworthy that such complicated clusters are assembled directly from cheap and simple reagents, metal(I) salts and dichalcophosphate anions. This reaction, when performed in the presence of a hydride or other anion sources, or foreign metal ions, results in hydrido- or anion-templated homo- or heteronuclear structures. In this feature article, we survey the recent advances in this exciting field, highlighting the powerful synthetic capabilities of the system "a metal(I) salt - [(RO)₂PX₂]^- ligands - a templating anion or borohydride" as an inexhaustible platform for the creation of new atomically precise clusters, superatoms, and nanoalloys.

Alkynyl-Protected Chiral Bimetallic Ag22 Cu7 Superatom with Multiple Chirality Origins

Understanding the origin of chirality in the nanostructured materials is essential for chiroptical and catalytic applications. Here we report a chiral AgCu superatomic cluster, [Ag₂₂ Cu₇ (C≡CR)₁₆ (PPh₃ )₅ Cl₆ ](PPh₄ ), Ag₂₂ Cu₇ , protected by an achiral alkynyl ligand (HC≡CR: 3,5-bis(trifluoromethyl)phenylacetylene). Its crystal structure comprises a rare interpenetrating biicosahedral Ag₁₇ Cu₂ core, which is stabilized by four different types of motifs: one Cu(C≡CR)₂ , four -C≡CR, two chlorides and one helical Ag₅ Cu₄ (C≡CR)₁₀ (PPh₃ )₅ Cl₄ . Structural analysis reveals that Ag₂₂ Cu₇ exhibits multiple chirality origins, including the metal core, the metal-ligand interface and the ligand layer. Furthermore, the circular dichroism spectra of R/S-Ag₂₂ Cu₇ are obtained by employing appropriate chiral molecules as optical enrichment agents. DFT calculations show that Ag₂₂ Cu₇ is an eight-electron superatom, confirm that the cluster is chirally active, and help to analyze the origins of the circular dichroism.

Title: Advances of Gold Nanoclusters for Bioimaging

Gold nanoclusters (AuNCs) have become a promising material for bioimaging detection because of their tunable photoluminescence, large Stokes shift, low photobleaching, and good biocompatibility. Last decade, great efforts have been made to develop AuNCs for enhanced imaging contrast and multimodal imaging. Herein, an updated overview of recent advances in AuNCs was present for visible fluorescence (FL) imaging, near-infrared fluorescence (NIR-FL) imaging, two-photon near-infrared fluorescence (TP-NIR-FL) imaging, computed tomography (CT) imaging, positron emission tomography (PET) imaging, magnetic resonance imaging (MRI), and photoacoustic (PA) imaging. The justification of AuNCs applied in bioimaging mentioned above applications was discussed, the performance location of different AuNCs were summarized and highlighted in an unified parameter coordinate system of corresponding bioimaging, and the current challenges, research frontiers, and prospects of AuNCs in bioimaging were discussed. This review will bring new insights into the future development of AuNCs in bio-diagnostic imaging.

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.

Directional Doping and Cocrystallizing an Open-Shell Ag39 Superatom via Precursor Engineering

Metal precursors employed in the bottom-up synthesis of metal nanoclusters (NCs) are of great importance in directing their composition and geometrical structure. In this work, a silver nanocluster co-protected by phosphine and thiolate, namely, [Ag₃₉(PFBT)₂₄(TPP)₈]^2- (Ag₃₉, PFBT = pentafluorobenzenethiol, TPP = triphenylphosphine), was isolated and structurally characterized. It adopts a three-layered Ag₁₃@Ag₁₈@Ag₈S₂₄P₈ core-shell structure. The Ag₁₃@Ag₁₈ kernel is unusual in multilayer noble metal NCs. By introducing a copper precursor in the synthesis, a bimetallic nanocluster [Ag₃₇Cu₂(PFBT)₂₄(TPP)₈]^2- (Ag₃₇Cu₂) with an identical structure to Ag₃₉ apart from two outer Ag atoms being substituted by Cu atoms was obtained. Astoundingly, the Cu precursor used in the synthesis was found to be critical in determining the final structure. The alteration of the Cu precursor led to the cocrystallization of the above alloy nanocluster with a Ag₁₄ nanocluster, namely, [Ag₃₇Cu₂(PFBT)₂₄(TPP)₈]^2-·[Ag₁₄(PFBT)₆(TPP)₈] (Ag₃₇Cu₂·Ag₁₄). The electronic structure analyzed by theoretical calculation reveals that Ag₃₉ is a 17-electron open-shell superatom. The optical absorption of Ag₃₉, Ag₃₇Cu₂, and Ag₃₇Cu₂·Ag₁₄ was compared and studied in detail. This work not only enriches the family of alloy metallic nanoclusters but also provides a metal NC-based cocrystal platform for in-depth study of its crystal growth and photophysical property.

Optical Activity from Anisotropic-Nanocluster-Assembled Supercrystals in Achiral Crystallographic Point Groups

Accomplishing optical activity in achiral materials has long been a challenge. Achiral nanomaterials that crystallize in achiral point groups are generally optically inactive. Herein we report the surprising observation of optical activity in several achiral point groups for supercrystals assembled from anisotropic metal nanoclusters with atomic precision. By analyzing multiple achiral nanoclusters with different molecular structures and symmetry space groups, we have identified that the molecular anisotropy of nanocluster entities and their asymmetric arrangement in point groups of supercrystals are the two key factors for the realization of optical activity in such supercrystals. We have further exploited the polarization effect of the nanocluster supercrystals as a polarization switch that can alter the polarized state of the linearly polarized light. Our findings have broadened the fundamental principles for producing nanomaterial-based optical activity and devices with polarization effects.

Ligand Engineering toward the Trade-Off between Stability and Activity in Cluster Catalysis

We report the structures, stability and catalysis properties of two Ag₂₁ nanoclusters, namely [Ag₂₁ (H₂ BTCA)₃ (O₂ PPh₂ )₆ ]SbF₆ (1) and [Ag₂₁ (C≡CC₆ H₃ -3,5-R₂ )₆ (O₂ PPh₂ )₁₀ ]SbF₆ (2) (H₄ BTCA=p-tert-butylthiacalix[4]arene, R=OMe). Both Ag₂₁ structures possess an identical icosahedral kernel that is surrounded by eight peripheral Ag atoms. Single-crystal structural analysis and ESI-MS revealed that 1 is an 8-electron cluster and 2 has four free electrons. Theoretical results show that the P-symmetry orbitals are found as HOMO-1 and HOMO states in 1, and the frontier unoccupied molecular orbitals (LUMO, LUMO+1 and LUMO+2) show D-character, indicating 1 is a superatomic cluster with an electronically closed shell 1S² 1P⁶ , while 2 has an incomplete shell configuration 1S² 1P² . These two Ag₂₁ clusters show superior stability under ambient conditions, and 1 is robust even at 90 °C in toluene and under oxidative conditions (30 % H₂ O₂ ). Significantly, 2 exhibits much higher activity than 1 as catalyst in the reduction of 4-nitrophenol. This work demonstrates that ligands can influence the electronic structures of silver clusters, and further affect their stability and catalytic performance.

A chloride-doped silver-sulfide cluster [Ag148S26Cl30(CCBut)60]6+: hierarchical assembly, enhanced luminescence and cytotoxicity to cancer cells

The formation of high-nuclear silver(I) clusters remains elusive and their potential applications are still underdeveloped. Herein, we report an unprecedented gigantic Ag₁₄₈ ([Ag₁₄₈S₂₆Cl₃₀(CCBu^t)₆₀](SbF₆)₆) cluster co-templated by Cl^- and S^2-, which was well-defined by single-crystal X-ray diffraction and high-resolution mass spectrometry. The cluster exhibits a hierarchical structure consisting of fused Ag₂₄X₁₆ kernel, Ag₆₀X₂₀ shell and "cluster of clusters assembling" of four pentagonal concave polyhedral {Ag₁₆X₅} units. Furthermore, the silver cluster emits red light at room temperature with a prominent 39.6% QY. The cellular uptake and cytotoxicity indicate that Ag₁₄₈ induces apoptosis of cancer cells in a dose-dependent manner.

Site-Specified Doping of Silver Atoms into Au25 Nanocluster as Directed by Ligand Binding Preference

For the first time site-specific doping of silver into a spherical Au₂₅ nanocluster has been achieved in [Au₁₉Ag₆(MeOPhS)₁₇(PPh₃)₆] (BF₄)₂ (Au₁₉Ag₆) through a dual-ligand coordination strategy. Single crystal X-ray structural analysis shows that the cluster has a distorted centered icosahedral Au@Au₆Ag₆ core of D₃ symmetry, in contrast to the I_h Au@Au₁₂ kernel in the well-known [Au₂₅(SR)₁₈]⁻ (R = CH₂CH₂Ph). An interesting feature is the coexistence of [Au₂(SPhOMe)₃] dimeric staples and [P–Au–SPhOMe] semi-staples in the title cluster, due to the incorporation of PPh₃. The observation of only one double-charged peak in ESI-TOF-MS confirms the ordered doping of silver atoms. Au₁₉Ag₆ is a 6e system showing a distinct absorption spectrum from [Au₂₅(SR)₁₈]⁻, that is, the HOMO–LUMO transition of Au₁₉Ag₆ is optically forbidden due to the P character of the superatomic frontier orbitals.

For the first time site-specific doping of silver into a spherical Au₂₅ nanocluster has been achieved in [Au₁₉Ag₆(MeOPhS)₁₇(PPh₃)₆] (BF₄)₂. It is a 6e system showing quite a different absorption spectrum from [Au₂₅(SR)₁₈]⁻.

A homoleptic alkynyl-protected [Ag9Cu6( t BuC[triple bond, length as m-dash]C)12]+ superatom with free electrons: synthesis, structure analysis, and different properties compared with the Au7Ag8 cluster in the M15 + series

We report the first homoleptic alkynyl-protected AgCu superatomic nanocluster [Ag9Cu6( t BuC[triple bond, length as m-dash]C)12]+ (NC 1, also Ag9Cu6 in short), which has a body-centered-cubic structure with a Ag1@Ag8@Cu6 metal core. Such a configuration is reminiscent of the reported AuAg bimetallic nanocluster [Au1@Ag8@Au6( t BuC[triple bond, length as m-dash]C)12]+ (NC 2, also Au7Ag8 in short), which is also synthesized by an anti-galvanic reaction (AGR) approach with a very high yield for the first time in this study. Despite a similar Ag8 cube for both NCs, structural anatomy reveals that there are some subtle differences between NCs 1 and 2. Such differences, plus the different M1 kernel and M6 octahedron, lead to significantly different optical absorbance features for NCs 1 and 2. Density functional theory calculations revealed the LUMO and HOMO energy levels of NCs 1 and 2, where the characteristic absorbance peaks can be correlated with the discrete molecular orbital transitions. Finally, the stability of NCs 1 and 2 at different temperatures, in the presence of an oxidant or Lewis base, was investigated. This study not only enriches the M15 + series, but also sets an example for correlating the structure-property relationship in alkynyl-protected bimetallic superatomic clusters.

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.

A Nanocluster [Ag307Cl62(SPhtBu)110]: Chloride Intercalation, Specific Electronic State, and Superstability

The controlling synthesis of novel nanoclusters of noble metals (Au, Ag) and the determination of their atomically precise structures provide opportunities for investigating their specific properties and applications. Here we report a novel silver nanocluster [Ag₃₀₇Cl₆₂(SPh^tBu)₁₁₀] (Ag₃₀₇) whose structure is determined by X-ray single crystal diffraction. The structure analysis shows that nanocluster Ag₃₀₇ contains a Ag₁₆₇ core, a surface shell of [Ag₁₄₀Cl₂S₁₁₀], and a Cl₆₀ intermediate layer located between Ag₁₆₇ and [Ag₁₄₀Cl₂S₁₁₀]. It is a first example that such many chlorides are intercalated into a Ag nanocluster. Chlorides are released in situ from solvent CHCl₃. Nanocluster Ag₃₀₇ exhibits superstability. Differential pulse voltammetry experiment reveals that Ag₃₀₇ has continuous charging/discharging behavior with a capacitance value of 1.39 aF, while the Ag₃₀₇ has a surface plasmonic feature. These characteristics show that Ag₃₀₇ is of metallic behavior. However, its electron paramagnetic resonance (EPR) spectra display a spin magnetic behavior which could be originated from the unpassivated dangling bonds of surface atoms. The direct capture of EPR signals can be attributed to the Cl^- intercalating layer which partly suppresses the electronic interactions between core and surface atoms, resulting in the relatively independent electronic states for core and surface atoms.

Construction of Silver Clusters Capped by Zwitterionic Ethynide Ligands

A zwitterionic ligand 3-(triethylammonio)propyne (TAP) has been employed to construct nine silver ethynide compounds for the first time. Single-crystal X-ray analyses reveal that compounds 1 and 2 are silver ethynide assemblies based on the Ag₃ subunits and clusters 3-8 are small discrete clusters of Ag₃, Ag₆, Ag₈, and Ag₁₂, respectively, ligated by the bulky TAP ligand with different auxiliary ligands. In addition, upon acquiring the tripod-like ^tBuPO₃^2-, a unprecedented 80 nuclei silver ethynide cluster was isolated and determined to be [(CF₃CO₂)₅@Ag₈₀(TAP)₁₄(^tBuPO₃)₁₆(CF₃CO₂)₂₄]¹⁹⁺ by crystallography and thermogravimetric analysis. The C₁ symmetry of Ag₈₀ was deconstructed to be two [Ag₄₀(TAP)₇(^tBuPO₃)₈(CF₃CO₂)₁₂]¹²⁺ secondary building subunits arranged in a cross way, with five CF₃CO₂^- trapped in the center. These results highlight that the elaborate selection of ethynide ligands is of great importance in the synthesis of novel silver ethynide clusters.

Atomically Precise Alkynyl- and Halide-Protected AuAg Nanoclusters Au78Ag66(C≡CPh)48Cl8 and Au74Ag60(C≡CPh)40Br12: The Ligation Effects of Halides

Reported herein are the synthesis and structures of two high-nuclearity AuAg nanoclusters, namely, [Au₇₈Ag₆₆(C≡CPh)₄₈Cl₈]^q- and [Au₇₄Ag₆₀(C≡CPh)₄₀Br₁₂]^2-. Both clusters possess a three-concentric-shell Au₁₂@Au₄₂@Ag₆₀ structure. However, the dispositions of the metal atoms, and the ligand coordination modes, of the outermost shells of these clusters are distinctly different. These structural differences reflect the bonding characteristics of the halide ligands. As revealed by density functional theory analysis, these clusters exhibit superatomic electron shell closings at magic numbers of 92 (for q = 4) and 84, respectively, consistent with their spherical shapes. Both clusters exhibit unusual multivalent redox properties.

Enriching Structural Diversity of Alkynyl-Protected Gold Nanoclusters with Chlorides

The synthesis and isolation of alkynyl/chloride-protected gold nanoclusters is described. Silica gel column chromatography is effective in isolating gold nanoclusters from the as-synthesized cluster mixture to give the clusters Na[Au₂₅ L₁₈ ] (Au₂₅ ), [HNEt₃ ]₃ [Au₆₇ L₃₂ Cl₄ ] (Au₆₇ ), [HNEt₃ ]₄ [Au₁₀₆ L₄₀ Cl₁₂ ] (Au₁₀₆ ), L=3,5-bis(trifluoromethyl)-phenylacetylide. Au₆₇ and Au₁₀₆ are new clusters; the structures were determined by X-ray single-crystal diffraction. Au₆₇ contains a distorted Au₁₈ Marks decahedron shelled by an irregular Au₃₂ and further protected with two V-shaped Au₂ L₃ , 13 linear AuL₂ staples and 4 chlorides. Au₆₇ is the first structurally determined 34e superatomic gold nanocluster. Au₁₀₆ is composed of 106 Au atoms co-protected by alkynyls and chlorides. It has a Au₇₉ kernel, like in Au₁₀₂ (p-MBA)₄₄ . The surface structure of Au₁₀₆ includes 20 linear Au-alkynyl staples, 5 Cl-Au-Cl and 2 Cl-Au motifs. These three gold nanoclusters show size-dependent electrochemical properties.

First-principles exploration of the versatile configurations at an alkynyl-protected coinage metal(111) interface

Alkynyl groups (R-C[triple bond, length as m-dash]C-) have attracted intense interest recently as alternative ligands to thiolates to protect atomically precise coinage metal nanoclusters, and more than two dozen compositions have been structurally resolved. However, structure determinations indicated that the interface shows strong metal sensitivity, where a staple motif is the common structural feature at the interface of Au-alkynyl nanoclusters, while the bridging motif dominates at the RC[triple bond, length as m-dash]C-/Ag and RC[triple bond, length as m-dash]C-/Cu interface. To understand their interfacial differences, we employed density functional theory (DFT) calculations to examine the versatile interfacial structures between CH3C[triple bond, length as m-dash]C- and the coinage metal surface; both the (111) surface as well as a surface with a metal adatom are investigated. We find that the alkynyl/gold(111) interface does prefer to form the staple motifs, and a linear flat-lying staple motif is preferred. The adatom occupies the bridge position and two CH3C[triple bond, length as m-dash]C- ligands lie diagonally at the fcc hollow sites with the C[triple bond, length as m-dash]C bond interacting with the surface Au by both σ- and π-coordination modes. In contrast, the bridging motif is energetically more favored on Ag(111) and Cu(111). The alkynyl carbons form strong σ, π- or σ-only bonds with the surface Ag/Cu, forming μ3-bridge coordination over the fcc hollow site. The binding strengths have the order of Cu(111) > Ag(111) > Au(111). The difference in structural preference is attributed to the intrinsic metal attributes with the different gaps of energetic penalty for surface energy, adatom creation and vacancy formation. The other two reasons are the differences of alkynyl-metal bond character and vdW interaction. We further show that this structure preference is also the preferred bonding mode of CH3C[triple bond, length as m-dash]C- on M55 clusters and the CH3S-/M(111) interface. Our insights greatly facilitate the structural elucidation and provide useful guidelines for future structure predictions in alkynyl-protected metal nanosystems.

A construction guide for high-nuclearity (≥50 metal atoms) coinage metal clusters at the nanoscale: bridging molecular precise constructs with the bulk material phase

Synthesis remains a major strength in chemistry and materials science and relies on the formation of new molecules and diverse forms of matter. The construction and identification of large molecules poses specific challenges and has historically lain in the realm of biological (organic)-type molecules with evolved synthesis methods to support such endeavours. But with the development of analytical tools such as X-ray crystallography, new synthesis methods toward large metal-based (inorganic) molecules and clusters have come to the fore, making it possible to accurately determine the precise distribution of hundreds of atoms in large clusters. In this review, we focus on different synthesis protocols used to form new metal clusters such as templating, alloying and size-focusing strategies. A specific focus is on group 11 metals (Cu, Ag, Au) as they currently predominate large metal cluster investigations and related Au and Ag bulk surface phenomena. This review focuses on metal clusters that have very high-nuclearity, i.e. with 50 or more metal centers within the isolated cluster. This size domain, it is believed, will become increasingly important for a variety of applications as these metal clusters are positioned at the interface between the molecular and bulk phases, whilst remaining a classic nanomaterial and retaining unique nano-sized properties.

Not Completely Innocent: How Argon Binding Perturbs Cationic Copper Clusters

Argon is often considered as an innocent probe that can be attached and detached to study the structure of a particular species without perturbing the species too much. We have investigated whether this assumption also holds for small copper cationic clusters and demonstrated that small but significant charge transfer from argon to metal changes the remaining binding positions, leading in general, to weaker binding of other argon atoms. The exception is binding to just one copper ion, where the binding of the first argon facilitates the binding of the second.

From understanding the roles of tetraoctylammonium bromide in the two-phase Brust-Schiffrin method to tuning the size of gold nanoclusters

The two-phase Brust-Schiffrin (B-S) method has been widely used for synthesizing small-sized Au nanoparticles (NPs) of size 2-6 nm, as well as Au nanoclusters (NCs) of size <2 nm. However, size tuning of Au NCs at the atomic level by this method is challenging probably due to a lack of in-depth understanding of its mechanism. Herein, we report the identification of two roles of tetraoctylammonium bromide (TOAB) in the two-phase B-S method: TOAB not only transfers Au(iii) precursors but also transfers the reducing agent NaBH₄ from the aqueous to the organic phase. On this basis, we developed a novel two-phase synthetic strategy by decoupling the roles of the TOAB: (1) using the hydrophobic selenolate ligand to transfer Au(iii) precursors from the aqueous to the organic phase via the formation of selenolate-Au(i) complexes and (2) deploying a small amount of TOAB as "shuttles" to transfer NaBH₄ into the organic phase for controlled reduction of selenolate-Au(i) complexes in organic phase. Using this strategy, size tuning of Au NCs at the atomic level could be achieved by simply varying the amount of TOAB. The high yields of Au NCs (≥76%) together with the short synthetic time (≤3 h) and size-tuning capability further illustrate the attractiveness of this synthetic strategy. These advantages also present the classical B-S method with greater strength and flexibility towards NC synthesis.

Total Structure Determination of the Largest Alkynyl-Protected fcc Gold Nanocluster Au110 and the Study on Its Ultrafast Excited-State Dynamics

Great attention has been paid to nanoclusters having face-centered-cubic (fcc) metal kernels, because of the similarity of metal packing to that of bulk gold. So far, there is no precedent example of an all-alkynyl-protected fcc gold nanocluster with more than 100 gold atoms. We report the synthesis and total structure determination of an alkynyl-protected gold nanocluster [NEt₃H]₂[Au₁₁₀(p-CF₃C₆H₄C≡C)₄₈] (Au₁₁₀). It has an fcc Au₈₆ kernel with 24 peripheral Au(C≡CR)₂ staples. The Au₈₆ kernel consists of six close packing layers in the pattern of Au₆:Au₁₆:Au₂₁:Au₂₁:Au₁₆:Au₆. Electronic absorption spectroscopy shows Au₁₁₀ has a molecular-like discrete electronic structure, and transient absorption experiments reveal its nonmetallic nature.

Chirality and Surface Bonding Correlation in Atomically Precise Metal Nanoclusters

Chirality is ubiquitous in nature and occurs at all length scales. The development of applications for chiral nanostructures is rising rapidly. With the recent achievements of atomically precise nanochemistry, total structures of ligand-protected Au and other metal nanoclusters (NCs) are successfully obtained, and the origins of chirality are discovered to be associated with different parts of the cluster, including the surface ligands (e.g., swirl patterns), the organic-inorganic interface (e.g., helical stripes), and the kernel. Herein, a unified picture of metal-ligand surface bonding-induced chirality for the nanoclusters is proposed. The different bonding modes of M-X (where M = metal and X = the binding atom of ligand) lead to different surface structures on nanoclusters, which in turn give rise to various characteristic features of chirality. A comparison of Au-thiolate NCs with Au-phosphine ones further reveals the important roles of surface bonding. Compared to the Au-thiolate NCs, the Ag/Cu/Cd-thiolate systems exhibit different coordination modes between the metal and the thiolate. Other than thiolate and phosphine ligands, alkynyls are also briefly discussed. Several methods of obtaining chiroptically active nanoclusters are introduced, such as enantioseparation by high-performance liquid chromatography and enantioselective synthesis. Future perspectives on chiral NCs are also proposed.

Composition-Dependent Antimicrobial Ability of Full-Spectrum AuxAg25-x Alloy Nanoclusters

Alloying is an efficient chemistry to diversify the properties of metal nanoparticles; however, the atomic-level understandings of the composition-dependent physicochemical properties and their related biological performance are presently lacking. Here, we developed a full spectrum of alloy metal nanoclusters (NCs), Au_xAg_25-x(MHA)₁₈ (MHA = 6-mercaptohexanoic acid) with x = 0-25, and investigated their composition-dependent antimicrobial performance. Interestingly, we observed a U-shape antimicrobial behavior of Au_xAg_25-x(MHA)₁₈ NCs, where the alloy NCs showed decreased antimicrobial ability instead of the common trend of increasing. Detailed atomic-level characterizations of the AuAg NCs suggest that the decreased performance of alloy NCs is due to their enhanced stability after alloying, which can deactivate their capability in generating reactive oxygen species (ROS) that can kill the bacteria. More interestingly, the transition point of the antimicrobial performance was only obtained with our full-spectrum Au_xAg_25-x(MHA)₁₈ NCs, which indicates the importance of exploring the composition-dependent properties and application performance in a full-spectrum composition range. A library of full-spectrum alloy NCs also provides a good platform to investigate other composition-dependent physicochemical and biological properties of metal NCs.

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.

Homoleptic alkynyl-protected gold nanoclusters with unusual compositions and structures

We report two novel homoleptic alkynyl-protected gold nanoclusters, which were synthesized by direct reduction of AuC[triple bond, length as m-dash]CR. Single-crystal X-ray structural analysis reveals that they have compositions of Au42(C[triple bond, length as m-dash]CC6H4-2-CF3)22 (1) and Au50(C[triple bond, length as m-dash]C6H4-3-F)26 (2), respectively. Cluster 2 is the first Au50 nanocluster, and the metal-to-ligand ratios of 1 and 2 are different from those of known Aun(SR)m or Aux(C[triple bond, length as m-dash]CR)y nanoclusters. In addition, the metal kernels of these two clusters are built up unprecedented units. This work offers further insights into the synthesis of all-alkynyl-protected gold nanoclusters via a direct reduction method.

A novel geometric structure of a nanocluster with an irregular kernel: Ag30Cu14(TPP)4(SR)28

Here, we report a bi-ligand protected bimetallic nanocluster Ag₃₀Cu₁₄(TPP)₄(SR)₂₈. It is composed of an Ag₂₇ kernel and Ag₃Cu₁₄(TPP)₄(SR)₂₈ shell. Typically, the metal atoms are arranged irregularly. Both the core and shell exhibit the characteristics of C₂-symmetry.

A one-dimensional infinite silver alkynyl assembly [Ag8(C[triple bond, length as m-dash]C t Bu)5(CF3COO)3(CH3CN)] n : synthesis, crystal structure and properties

A high-yield silver alkynyl assembly [Ag₈(CC^tBu)₅(CF₃COO)₃(CH₃CN)]_n (1) constructed from [AgCC^tBu]_n ligand, CF₃COOAg and CH₃CN auxiliary ligands with a one-dimensional infinite chain structure has been obtained in one pot. Compound 1 has been well-defined and characterized. The photocurrent properties and the temperature-sensitive luminescent properties of 1 have been investigated.

A high-yield one-dimensional infinite silver alkynyl assembly [Ag₈(CC^tBu)₅(CF₃COO)₃(CH₃CN)]_n (1) displays excellent photocurrent properties and temperature-sensitive luminescence 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.

Unravelling the formation mechanism of alkynyl protected gold clusters: a case study of phenylacetylene stabilized Au144 molecules

Despite recent progress in the preparation of alkynyl protected Au clusters with molecular purity (e.g., Na[Au₂₅(C[triple bond, length as m-dash]CAr)₁₈, Ar = 3,5-(CF₃)₂C₆H₃^-, Au₃₆(C[triple bond, length as m-dash]CPh)₂₄, Au₄₄(C[triple bond, length as m-dash]CPh)₂₈, and Au₁₄₄(C[triple bond, length as m-dash]CAr)₆₀, Ar = 2-F-C₆H₄^-), the formation mechanism still remains elusive. Herein, a new molecule-like alkynyl Au cluster was successfully prepared, and its formula was determined as Au₁₄₄(PA)₆₀ (PA = PhC[triple bond, length as m-dash]C-, phenylacetylene). In the formation of Au₁₄₄(PA)₆₀, the introduction of ethanol in post-synthesis treatment to manipulate the aggregation state of the precursor was found to play a critical role in producing the Au144 clusters. During the Au₁₄₄(PA)₆₀ formation process, the contents of PA, (PA)₂ and (PA)₄ were monitored by absorbance and gas chromatography-mass spectrometry (GC-MS), disclosing that Au₁₄₄(PA)₆₀ molecules were generated in sync with (PA)₄. Finally, the formation mechanism of Au₁₄₄(PA)₆₀ molecules has been tentatively proposed, of which three major stages are involved. This study can shed light on the formation mechanism that may be exploited for the precise control of the synthesis of alkynyl protected coinage metal clusters.

Isomerization in Alkynyl-Protected Gold Nanoclusters

We report the controlled synthesis and structures of two isomeric gold nanoclusters, whose compositions are determined to be Au₂₃(C≡CBu^t)₁₅ (denoted as Au₂₃-1 and Au₂₃-2) by single-crystal X-ray diffraction and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. This is the first time isomerism is discovered in alkynyl-protected gold nanoclusters. The metal-to-ligand ratios in these two clusters are different from known Au_n(SR)_m systems and have not been observed in the Au_x(C≡CPh)_y family. This pair of isomers exhibits different optical properties, although they have similar structures and identical components. For both Au₂₃ clusters, time-dependent density functional theory calculations revealed the frontier orbitals highest occupied molecular orbital (HOMO)-1, HOMO, and lowest unoccupied molecular orbital (LUMO) are mainly constructed from the Au₁₅ kernel and V-shaped alkynyl-gold motifs. The HOMO → LUMO transition of Au₂₃-1 is optically forbidden, whereas it is allowed in Au₂₃-2. It is also found that Au₂₃-2 cluster can be transformed to Au₂₃-1 spontaneously under ambient conditions. This work offers further insight into the synthesis and isomerism of all-alkynyl-protected gold nanoclusters and will stimulate more investigation of isomeric metal nanoclusters.

A stably discrete 31-nuclearity silver(i) thiolate nanocluster luminescent thermometer supported by DMF auxiliary ligands

The stably discrete [Ag₃₁S₃(S^tBu)₁₇(CF₃COO)₇(CO₃)_0.5(CF₃COOH)_0.5(DMF)₄] nanocluster in Ag31S20-DMF (1) shaped in a turtle-like structure exhibits temperature-sensitive luminescence properties.

[Au7Ag9(dppf)3(CF3CO2)7BF4]n: a linear nanocluster polymer from molecular Au7Ag8 clusters covalently linked by silver atoms

We report the total structure determination of a covalently bonded nanocluster polymer [Au7Ag9(dppf)3(CF3CO2)7BF4]n (1). This features a one-dimensional linear chain, consisting of unique molecular building blocks, Au7Ag8(dppf)3(CF3CO2)7 clusters, which are linked together by Ag-O bonds rather than Au-Au interactions or bidentate organic bridges as previously reported.

Alternating Array Stacking of Ag26 Au and Ag24 Au Nanoclusters

An assembly strategy for metal nanoclusters using electrostatic interactions with weak interactions, such as C-H⋅⋅⋅π and π⋅⋅⋅π interactions in which cationic [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ and anionic [Ag₂₄ Au(2-EBT)₁₈ ]^- nanoclusters gather and assemble in an unusual alternating array stacking structure is presented. [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- is a new compound type, a double nanocluster ion compound (DNIC). A single nanocluster ion compound (SNIC) [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- was also synthesized, having a k-vector-differential crystallographic arrangement. [PPh₄ ]⁺ [Ag₂₄ Au(2,4-DMBT)₁₈ ]^- adopts a different assembly mode from both [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- and [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- . Thus, the striking packing differences of [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- , [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- and the existing [PPh₄ ]⁺ [Ag₂₄ Au(2,4-DMBT)₁₈ ]^- from each other indicate the notable influence of ligands and counterions on the self-assembly of nanoclusters.

The stability enhancement factor beyond eight-electron shell closure in thiacalix[4]arene-protected silver clusters

Destroying coordination open sites may significantly enhance the stability of metal nanoclusters.

We report the synthesis and structures of two 34-atom metal nanoclusters, namely [Ag₃₄(BTCA)₃(C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆ and [AuAg₃₃(BTCA)₃(C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆, where H₄BTCA is p-tert-butylthiacalix[4]arene and tfa is trifluoroacetate. Their compositions and structures have been determined by single-crystal X-ray structural analysis and ESI-MS. The cationic cluster consists of a centered icosahedron M@Ag₁₂ (M = Ag or Au) core that is surrounded by 21 peripheral silver atoms. Surrounding protection is provided by four kinds of ligands, including three BTCA, nine ^tBuC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C, four tfa, and three methanol solvent ligands. It was found that the Ag₅@BTCA μ₅-coordination motif of thiacalixarene is critical for high stability of the title clusters, and extra stability enhancement can be achieved by doping a gold atom at the center of the silver cluster. This work suggests that coordination saturation should be taken into account in addition to electronic and geometric factors for analyzing metal nanocluster stabilities.

Alkynyl Approach toward the Protection of Metal Nanoclusters

The past decades have witnessed great advances in the synthesis, structure determination, and properties investigation of coinage metal nanoclusters. These monodisperse clusters have well-defined molecular structures, which is advantageous in correlating structures and properties. Metal nanoclusters are large molecules consisting of many components, so it is a big challenge to prepare them in a rational way. Strenuous efforts have been made to control their geometric and electronic structures, in order to optimize their various properties. A metal nanocluster normally contains a metal core and a peripheral ligand shell. The ligands do not only function as simple stabilizing agents. It has been revealed that these ligands are able to influence the formation processes of the nanoclusters, and they may also dictate the sizes, shapes, and properties of nanoclusters. There are mainly three types of ligands that are widely used as surface anchors on coinage metal nanoclusters: thiolates, phosphines, and halides. Recent ligand engineering has extended the scope to alkynyl ligands. As alkynyl ligands are versatile in interacting with metal atoms, interesting alkynyl-metal interfacial structures including linear, L-shaped, and V-shaped staple motifs can be generated, as well as a series of novel coinage metal nanoclusters that exhibit intriguing molecular geometries. The staple motifs do not simply resemble the surface structures of thiolate-protected nanoclusters, because the incorporation of alkynyl ligands may significantly alter diverse properties of nanoclusters. Compared with thiolate-protected gold nanoclusters, alkynyl-protected ones with identical metal cores exhibit distinctly different absorption profiles and show much improved catalytic activities for semihydrogenation of alkynes. In addition, the participation of alkynyl ligands could profoundly affect the luminescent properties of nanoclusters. These "ligand effects" are mainly attributed to the different nature of alkynyl ligands, as electronic perturbation through π-conjugated units may largely modulate the electronic structure of the whole cluster. In this Account, we describe the development of coinage metal nanoclusters protected with alkynyl ligands. We will first briefly bring up the emergence of alkynyl ligands as anchoring groups on the surfaces of nanoclusters. Then we present the direct reduction method for the synthesis of the following four categories of nanoclusters: (a) gold nanoclusters with mixed-ligand shells, (b) all alkynyl-protected gold nanoclusters, (c) heterobimetallic gold nanoclusters, and (d) silver nanoclusters. Their molecular structures are described, and their various alkynyl-metal interfacial structures are compared with thiolate-metal staples. Finally, ligand effects on the properties of the clusters, including optical absorption, luminescence, and catalysis, are discussed. The alkynyl ligands play an important role in terms of both structural and property aspects. We believe this Account will attract increasing attention to alkynyl ligands, which have shown promising potential in generating new structures and properties of coinage metal nanoclusters.

Kinetics of Spontaneous Bimetallization between Silver and Noble Metal Nanoparticles

A physical mixture of polymer-protected Ag nanoparticles and Rh, Pd, or Pt nanoparticles spontaneously forms Ag-core bimetallic nanoparticles. The formed nanoparticles were smaller than the parent Ag nanoparticles. In the initial process of this reaction, the surface plasmon absorption of Ag nanoparticles diminished and then almost ceased within one hour. Within several minutes, the decrease in Ag surface plasmon absorption could be analyzed by second-order reaction. This reaction was accelerated with an increase of temperature and the energy gap in the Fermi level between Ag and the other metals. The activation energy (Ea ) of this reaction could be determined. An electron transfer reaction from Ag to other metal nanoparticles was proposed as the initial interaction between these metal nanoparticles because the Fermi level of Ag is relatively high, and the electron transfer is possible in terms of energy. The Marcus plot between the rate constant and the driving force, roughly estimated from the work function of metals, and the observed Ea values reasonably explained the proposed electron transfer mechanism.