'Passivated Precursor' Approach to All-Alkynyl-Protected Gold Nanoclusters and Total Structure Determination of Au130

A 'passivated precursor' approach is developed for the efficient synthesis and isolation of all-alkynyl-protected gold nanoclusters. Direct reduction of dpa-passivated precursor Au-dpa (Hdpa=2,2'-dipyridylamine) in one-pot under ambient conditions gives a series of clusters including Au22(C≡CR)18 (R=-C6H4-2-F), Au36(C≡CR)24, Au44(C≡CR)28, Au130(C≡CR)50, and Au144(C≡CR)60. These clusters can be well separated via column chromatography. The overall isolation yield of this series of clusters is 40 % (based on gold), which is much improved in comparison with previous approaches. It is notable that the molecular structure of the giant cluster Au130(C≡CR)50 is revealed, which presents important information for understanding the structure of the mysterious Au130 nanoclusters. Theoretical calculations indicated Au130(C≡CR)50 has a smaller HOMO-LUMO gap than Au130(S-C6H4-4-CH3)50. This facile and reliable synthetic approach will greatly accelerate further studies on all-alkynyl-protected gold nanoclusters.

Structural Engineering toward Gold Nanocluster Catalysis

Atomically precise gold nanoclusters provide great opportunities to explore the relationship between the structure and properties of nanogold catalysts. A nanocluster consists of a metal core and a surface ligand shell, and both the core and shell have significant effects on the catalytic properties. Thanks to their precise structures, the active metal site of the clusters can be readily identified and the effects of ligands on catalysis can be disclosed. In this Minireview, we summarize recent advances in catalytic research of gold nanoclusters, emphasizing four strategies for constructing open metal sites, including by post-treatment, the bulky ligands strategy, the surface geometric mismatch method, and heteroatom doping procedures. We also discuss the effects of ligands on the catalytic activity, selectivity, and stability of gold cluster catalysts. Finally, we present future challenges relating to gold cluster catalysis.

Ligand-Shell Engineering of a Au28 Nanocluster Boosts Electrocatalytic CO2 Reduction

Subtle tailoring of gold nanoclusters (NCs) could significantly change their physicochemical properties. However, direct comparison of the catalytic performance of gold NCs with identical metal cores but distinct ligand shells is rarely elucidated. In this work, a novel gold NC, Au₂₈ (C₂ B₁₀ H₁₁ S)₁₂ (tht)₄ Cl₄ (Au₂₈ -S), was isolated by a facile self-reducing synthesis. Au₂₈ -S adopts an identical Au₂₈ metal framework to that of the reported alkynyl-protected Au₂₈ -C. The different protective layers lead to distinctions in their electronic structure and optical properties. Furthermore, Au₂₈ -S shows better catalytic activity for the electrochemical reduction of CO₂ to CO. Theoretical calculations identified the active sites and shed light on the catalytic mechanism to elucidate the different catalytic performances. This work provides an ideal platform to study the protective layer-activity relationship of gold NCs, and may also provide guidance in the design of metal NC-based catalysts.

A classical [V10O28]6- anion templated high-nuclearity silver thiolate cluster

Polyoxovanadates (POVs) as templates are still scarcely observed in silver clusters. Herein, the largest known POV-based silver cluster (Ag50) was synthesized, which is a core-shell conformation composed of the in situ generated classical [V₁₀O₂₈]^6- core and Ag₅₀ shell, constrained by the S- and O-donor ligands with a specific distribution. Such {V₁₀O₂₈@Ag₅₀} structure displays geometric inheritance from the D_2h symmetric decavanadate to the silver skeleton. The solution behavior, solid-state stability and photoelectric properties are discussed in detail. This work provides enlightenment for the further construction of POV-templated high-nuclearity silver clusters.

Nitrogen Donor Protection for Atomically Precise Metal Nanoclusters

Surface organic ligands are critical in dictating the structures and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, nitrogen donor ligands have not been used in the protection for well-defined metal nanoclusters until recently. This review focuses on recent developments in atomically precise metal nanoclusters stabilized by different types of nitrogen donor ligands, in which the synthesis, total structure determination and various properties are covered. We hope that this review will provide insights into the rational design of N donor-protected metal nanoclusters in terms of structural and functional modulation.

Ligand-Protected Au55 with a Novel Structure and Remarkable CO2 Electroreduction Performance

A Au₅₅ nanocluster with the composition of [Au₅₅ (p-MBT)₂₄ (Ph₃ P)₆ ](SbF₆ )₃ (p-MBT=4-methylbenzenethiolate) is synthesized via direct reduction of gold-phosphine and gold-thiolate precursors. Single-crystal X-ray diffraction reveals that this Au₅₅ nanocluster features a face-centered cubic (fcc) Au₅₅ kernel, different from the well-known two-shell cuboctahedral arrangement in Au₅₅ (Ph₃ P)₁₂ Cl₆ . The Au₅₅ cluster shows a wide optical absorption band with optical energy gap (E_g =1.28 eV). It is found that the exclusion of chloride is crucial for the formation of the title cluster, otherwise rod-like [Au₂₅ (SR)₅ (PPh₃ )₁₀ Cl₂ ]²⁺ is obtained. The strategy to run synthetic reaction in the absence of halide leads to new members of phosphine/thiolate co-protected metal nanoclusters. The Au₅₅ nanocluster exhibits high catalytic activity and selectivity for electrochemical reduction of CO₂ to CO; the Faradaic efficiency (FE) reaches 94.1 % at -0.6 V vs. reversible hydrogen electrode (RHE).

Robust Gold Nanocluster Protected with Amidinates for Electrocatalytic CO2 Reduction

The first all-amidinate-protected gold nanocluster [Au₂₈ (Ph-form)₁₂ ](OTf)₂ (Ph-form=N,N'-diphenylformamidinate) (Au₂₈ ) has been synthesized and structurally resolved. Single crystal X-ray diffraction reveals that Au₂₈ has a compact Au₄ @Au₂₄ tetrahedral core-shell structure of T symmetry, which is fully protected by 12 bridging formamidinate ligands. This cluster is quite robust as indicated by the fact that it can stay intact in solution at 80 °C for 6 d. It exhibits excellent catalytic performance for the electroreduction of CO₂ with 96.5 % Faradaic efficiency (FE) at -0.57 V and a maximum TOF of 1731 h^-1 at -0.87 V. Its superior stability is also manifested in the fact that the supported catalyst Au₂₈ /CNTs maintains stable potentials at ca. -0.69 V for 40 h with FE(CO)s>91 %. A superatomic electron configuration of 1S² 1P⁶ 2S² 1D⁴ has been clarified by DFT computations, and the strong gold-ligand binding and geometric shell closure account for the superior stability of Au₂₈ .

Carboranealkynyl-Protected Gold Nanoclusters: Size Conversion and UV/Vis-NIR Optical Properties

Structure evolution has become an effective way to assemble novel monolayer-protected metal nanomolecules. However, evolution with alkynyl-stabilized metal clusters still remains rarely explored. Herein, we present a carboranealkynyl-protected gold nanocluster [Au₂₈ (C₄ B₁₀ H₁₁ )₁₂ (tht)₈ ]³⁺ (Au₂₈ , tht=tetrahydrothiophene) possessing an open-shell electronic structure with 13 free electrons, which was isolated by a facile self-reduction method with 9-HC≡C-closo-1,2-C₂ B₁₀ H₁₁ as the two-in-one reducing and protecting agent. Notably, Au₂₈ undergoes a complete transformation in methanol into a stable and smaller-sized nanocluster [Au₂₃ (C₄ B₁₀ H₁₁ )₉ (tht)₆ ]²⁺ (Au₂₃ ) bearing 12 valence electrons and crystal-field-like split superatomic 1D orbitals. The transformation process was systematically monitored with ESI-MS and UV/Vis absorption spectra. Au₂₈ and Au₂₃ both display optical absorption covering the UV/Vis-NIR range and NIR emission, which facilitates their potential application in the biomedical and photocatalytic fields.

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.

Face-Centered-Cubic Ag Nanoclusters: Origins and Consequences of the High Structural Regularity Elucidated by Density Functional Theory Calculations

Face-centered-cubic (FCC) silver nanoclusters (NCs) adopting either cubic or half-cubic growth modes have been recently reported, but the origin of these atomic assembly patterns and how they are achieved, which would inform our understanding of larger FCC silver nanomaterials, are both unknown. In this study, the cubic and half-cubic growth modes have been unified based on common structural characteristics, and differentiated depending on the starting blocks (cubic vs. half cubic). In both categories, the silver atoms adopt octahedral Ag₆ , linear AgS₂ (in projection drawing), or tetrahedral AgS₃ P binding modes, and the sulfur atoms adopt T-shaped SAg₃ and orthogonal SAg₄ modes. An additional T-shaped AgS₃ mode is oriented on the surface edge in cubic NCs to complete the cubic framework. Density functional theory calculations indicated that the high structural regularity originates from the strong diffusing capacity of the Ag(5d) and S(3p) orbitals, and the angular momentum distribution of the formed superatomic orbitals. The equatorial orientation of μ⁴ -S or μ⁴ -Ag determines whether growth stops or continues. In particular, a density-of-states analysis indicated that the octahedral silver atoms are chemically more reactive than the silver atoms in the AgS₃ P motif, regardless of whether the parent NC functions as an electron donor or acceptor.

Reversible Switching of Catalytic Activity by Shuttling an Atom into and out of Gold Nanoclusters

It is challenging to control the catalyst activation and deactivation by removal and addition of only one central atom, as it is almost impossible to precisely abstract an atom from a conventional catalyst and analyze its catalysis. Here we report that the loss of one central atom in Au₂₅ (resulting in Au₂₄ ) enhances the catalytic activity in the oxidation of methane compared to the original Au₂₅ . More importantly, the activity can be readily switched through shuttling the central atom into Au₂₄ and out of Au₂₅ . This work will serve as a starting point for design rules on how to control catalytic performance of a catalyst by an atom alteration.

Atomically Tailored Gold Nanoclusters for Catalytic Application

Recent advances in the synthetic chemistry of atomically precise metal nanoclusters (NCs) have significantly broadened the accessible sizes and structures. Such particles are well defined and have intriguing properties, thus, they are attractive for catalysis. Especially, those NCs with identical size but different core (or surface) structure provide unique opportunities that allow the specific role of the core and the surface to be mapped out without complication by the size effect. Herein, we summarize recent work with isomeric Au_n NCs protected by ligands and isostructural NCs but with different surface ligands. The highlighted work includes catalysis by spherical and rod-shaped Au₂₅ (with different ligands), quasi-isomeric Au₂₈ (SR)₂₀ with different R groups, structural isomers of Au₃₈ (SR)₂₄ (with identical R) and Au₃₈ S₂ (SR)₂₀ with body-centred cubic (bcc) structure, and isostructural [Au₃₈ L₂₀ (PPh₃ )₄ ]²⁺ (different L). These isomeric and/or isostructural NCs have provided valuable insights into the respective roles of the kernel, surface staples, and the type of ligands on catalysis. Future studies will lead to fundamental advances and development of tailor-made catalysts.

Fcc versus Non-fcc Structural Isomerism of Gold Nanoparticles with Kernel Atom Packing Dependent Photoluminescence

Structural isomerism allows the correlation between structures and properties to be investigated. Unfortunately, the structural isomers of metal nanoparticles are rare and genuine structural isomerism with distinctly different kernel atom packing (e.g., face-centered cubic (fcc) vs. non-fcc) has not been reported until now. Herein we introduce a novel ion-induction method to synthesize a unique gold nanocluster with a twist mirror symmetry structure. The as-synthesized nanocluster has the same composition but different kernel atom packing to an existing gold nanocluster Au₄₂ (TBBT)₂₆ (TBBT=4-tert-butylbenzenethiolate). The fcc-structured Au₄₂ (TBBT)₂₆ nanocluster shows more enhanced photoluminescence than the non-fcc-structured Au₄₂ (TBBT)₂₆ nanocluster, indicating that the fcc-structure is more beneficial for emission than the non-fcc structure. This idea was supported by comparison of the emission intensity of another three pairs of gold nanoclusters with similar compositions and sizes but with different kernel atom packings (fcc vs. non-fcc).

Insights into Interfaces, Stability, Electronic Properties, and Catalytic Activities of Atomically Precise Metal Nanoclusters from First Principles

Atomically precise, ligand-protected metal nanoclusters are of great interest for their well-defined structures, intriguing physicochemical properties, and potential applications in catalysis, biology, and nanotechnology. Their structure precision provides many opportunities to correlate their geometries, stability, electronic properties, and catalytic activities by closely integrating theory and experiment. In this Account, we highlight recent theoretical advances from our efforts to understand the metal-ligand interfaces, the energy landscape, the electronic structure and optical absorption, and the catalytic applications of atomically precise metal nanoclusters. We mainly focus on gold nanoclusters. The bonding motifs and energetics at the gold-ligand interfaces are two main interests from a computational perspective. For the gold-thiolate interface, the -RS-Au-SR- staple motif is not always preferred; in fact, the bridging motif (-SR-) is preferred at the more open facets such as Au(100) and Au(110). This finding helps understand the diversity of the gold-thiolate motifs for different core geometries and sizes. A great similarity is demonstrated between gold-thiolate and gold-alkynyl interfaces, regarding formation of the staple-type motifs with PhC≡C- as an example. In addition, N-heterocyclic carbenes (NHCs) without bulky groups also form the staple-type motif. Alkynyls and bulky NHCs have the strongest binding with the gold surface from comparing 27 ligands of six types, suggesting a potential to synthesize NHC-protected gold clusters. The energy landscape of nanosystems is usually complex, but experimental progress in synthesizing clusters of the same Au-S composition with different R groups and isomers of the same Au _n(SR) _m formula have made detailed theoretical analyses of energetic contributions possible. Ligand-ligand interactions turn out to play an important role in the cluster stability, while metastable isomers can be obtained via kinetic control. Although the superatom-complex theory is the starting point to understand the electronic structure of atomically precise gold clusters, other factors also greatly affect the orbital levels that manifest themselves in the experimental optical absorption spectra. For example, spin-orbit coupling needs to be included to reproduce the splitting of the HOMO-LUMO transition observed experimentally for Au₂₅(SR)₁₈^-, the poster child of the family. In addition, doping can lead to structural changes and charge states that do not follow the superatomic electron count. Atomically precise metal nanoclusters are an ideal system for understanding nanocatalysis due to their well-defined structures. Active sites and catalytic mechanisms are explored for selective hydrogenation and hydrogen evolution on thiolate-protected gold nanoclusters with and without dopants. The behavior of H in nanogold is analyzed in detail, and the most promising site to attract H is found to be coordinately unsaturated Au atoms. Many insights have been gained from first-principles studies of atomically precise, ligand-protected gold nanoclusters. Interesting and important questions remaining to be addressed are pointed out in the end.