Tuning the Accessibility and Activity of Au25(SR)18 Nanocluster Catalysts through Ligand Engineering

In this work, the effects of thiolate ligands (‐SR, e.g., chain length and functional moiety) on the accessibility and catalytic activity of thiolate‐protected gold nanoclusters (e.g., Au₂₅(SR)₁₈) for 4‐nitrophenol hydrogenation is reported. The data suggest that Au₂₅(SR)₁₈ bearing a shorter alkyl chain shows a better accessibility to the substrates (shown by shorter induction time, t₀) and a higher catalytic activity (shown by higher apparent reaction rate constant, k_app). The functional moiety of the ligands is another determinant factor, which clearly suggests that ligand engineering of Au₂₅(SR)₁₈ would be an efficient platform for fine‐tuning its catalytic properties.     

Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate‐Protected Metal Clusters to Synthesize Controlled Metal Clusters

It is now possible to accurately synthesize thiolate (SR)‐protected gold clusters (Au_n(SR)_m) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Au_n(SR)_m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.     

Balancing the Rate of Cluster Growth and Etching for Gram‐Scale Synthesis of Thiolate‐Protected Au25 Nanoclusters with Atomic Precision

We report a NaOH‐mediated NaBH₄ reduction method for the synthesis of mono‐, bi‐, and tri‐thiolate‐protected Au₂₅ nanoclusters (NCs) with precise control of both the Au core and thiolate ligand surface. The key strategy is to use NaOH to tune the formation kinetics of Au NCs, i.e., reduce the reduction ability of NaBH₄ and accelerate the etching ability of free thiolate ligands, leading to a well‐balanced reversible reaction for rapid formation of thermodynamically favorable Au₂₅ NCs. This protocol is facile, rapid (≤3 h), versatile (applicable for various thiolate ligands), and highly scalable (>1 g Au NCs). In addition, bi‐ and tri‐thiolate‐protected Au₂₅ NCs with adjustable ratios of hetero‐thiolate ligands were easily obtained. Such ligand precision in molecular ratios, spatial distribution and uniformity resulted in richly diverse surface landscapes on the Au NCs consisting of multiple functional groups such as carboxyl, amine, and hydroxy. Analysis based on NMR spectroscopy revealed that the hetero‐ligands on the NCs are well distributed with no ligand segregation. The unprecedented synthesis of multi‐thiolate‐protected Au₂₅ NCs may further promote the practical applications of functional metal NCs.     

Basic [Au25(SCH2CH2Py)18]−⋅Na+ Clusters: Synthesis,Layered Crystallographic Arrangement,and Unique Surface Protonation

The synthesis of high‐purity and high‐yield Au₂₅ clusters protected by the basic pyridyl ethanethiol (HSCH₂CH₂Py, 4‐PyET and 2‐PyET) is presented. Single‐crystal X‐ray diffraction of the [Au₂₅(4‐PyET)₁₈]^??Na⁺ clusters has revealed a structure similar to that known for the phenyl ethanethiolate analogue, but with pyridyl‐N coordination to Na⁺, a more relaxed ligand shell, and a profoundly layered arrangement in the solid state. Because of the pendant Py moiety, the [Au₂₅(PyET)₁₈]^? clusters are endowed with unique (de)protonation equilibria, which has been characterized in detail by UV/Vis absorption and ¹H NMR spectroscopy. [Au₂₅(PyET)₁₈]^? clusters showed an unexpectedly H⁺‐dependent solubility that is tunable in aqueous and organic solvents. The successful synthesis of the basic Py‐terminated thiolate‐protected Au₂₅ clusters paves the way to realize a new family of metalloid clusters possessing basic properties.     

Basic [Au25(SCH2CH2Py)18]−⋅Na+ Clusters: Synthesis,Layered Crystallographic Arrangement,and Unique Surface Protonation

The synthesis of high‐purity and high‐yield Au₂₅ clusters protected by the basic pyridyl ethanethiol (HSCH₂CH₂Py, 4‐PyET and 2‐PyET) is presented. Single‐crystal X‐ray diffraction of the [Au₂₅(4‐PyET)₁₈]^??Na⁺ clusters has revealed a structure similar to that known for the phenyl ethanethiolate analogue, but with pyridyl‐N coordination to Na⁺, a more relaxed ligand shell, and a profoundly layered arrangement in the solid state. Because of the pendant Py moiety, the [Au₂₅(PyET)₁₈]^? clusters are endowed with unique (de)protonation equilibria, which has been characterized in detail by UV/Vis absorption and ¹H NMR spectroscopy. [Au₂₅(PyET)₁₈]^? clusters showed an unexpectedly H⁺‐dependent solubility that is tunable in aqueous and organic solvents. The successful synthesis of the basic Py‐terminated thiolate‐protected Au₂₅ clusters paves the way to realize a new family of metalloid clusters possessing basic properties.     

Controlled growth of molecularly pure Au25(SR)18 and Au38(SR)24 nanoclusters from the same polydispersed crude product

We report the controlled growth of Au25(SR)18 and Au38 (SR)24 (where R = CH2CH2Ph) nanoclusters of molecular purity via size-focusing from the same crude product that contains a distribution of nanoclusters. In this method, gold salt was first mixed with tetraoctylammonium bromide (TOAB), and then reacted with excess thiol to form Au(I)-SR polymers in THF (as opposed to toluene in previous work), followed by NaBH 4 reduction. The resultant crude product contains polydisperse nanoclusters and was then used as the common starting material for controlled growth of Au25(SR)18 and Au38(SR)24 , respectively. In Route I, Au25(SR)18 nanoclusters of molecular purify were produced from the crude product after 6 h aging at room temperature. In Route II, the crude product was isolated and further subjected to thermal thiol etching in a toluene solution containing excess thiol, and one obtained pure Au38(SR)24 nanoclusters, instead of Au25(SR)18 . This work not only provides a robust and simple method to prepare both Au25(SR)18 and Au38(SR)24 nanoclusters, but also reveals that these two nanoclusters require different environments for the size-focusing growth process.     

Aurophilic Interactions in the Self‐Assembly of Gold Nanoclusters into Nanoribbons with Enhanced Luminescence

Aurophilic interactions (Au^I???Au^I) are crucial in directing the supramolecular self‐assembly of many gold(I) compounds; however, this intriguing chemistry has been rarely explored for the self‐assembly of nanoscale building blocks. Herein, we report on studies on aurophilic interactions in the structure‐directed self‐assembly of ultrasmall gold nanoparticles or nanoclusters (NCs, <2 nm) using [Au₂₅(SR)₁₈]^? (SR=thiolate ligand) as a model cluster. The self‐assembly of NCs is initiated by surface‐motif reconstruction of [Au₂₅(SR)₁₈]^? from short SR‐[Au^I‐SR]₂ units to long SR‐[Au^I‐SR]_x (x>2) staples accompanied by structure modification of the intrinsic Au₁₃ kernel. Such motif reconstruction increases the content of Au^I species in the protecting shell of Au NCs, providing the structural basis for directed aurophilic interactions, which promote the self‐assembly of Au NCs into well‐defined nanoribbons in solution. More interestingly, the compact structure and effective aurophilic interactions in the nanoribbons significantly enhance the luminescence intensity of Au NCs with an absolute quantum yield of 6.2 % at room temperature.     

One‐Pot Synthesis of Phenylmethanethiolate‐Protected Au20(SR)16 and Au24(SR)20 Nanoclusters and Insight into the Kinetic Control

We report two synthetic routes for concurrent formation of phenylmethanethiolate (‐SCH₂Ph)‐protected Au₂₀(SR)₁₆ and Au₂₄(SR)₂₄ nanoclusters in one‐pot by kinetic control. Unlike the previously reported methods for thiolate‐protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH₄ and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold–thiolate cluster synthesis. We demonstrate that the synthesis of ‐SCH₂Ph‐protected Au₂₀ and Au₂₄ nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH₄ under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH₄ under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au₂₀(SCH₂Ph)₁₆ and Au₂₄(SCH₂Ph)₂₀ clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH₄ solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH₂S‐protected Au₂₀ and Au₂₄ nanoclusters with the PhC₂H₄S‐protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH₂S‐capped nanoclusters when compared to the PhC₂H₄S‐capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.     

Features of the decomposition of the neutral nitrosyl iron complexes with aryl-containing thiolate ligands in various solvents. Reaction with glutathione

The decomposition of two neutral binuclear nitrosyl iron complexes (NICs) of the µ-S structural type and general composition [Fe₂(SR)₂(NO)₄]⁰ was studied in comparison. The exchange reaction of the thiophenol or 2-aminothiophenol thiolate ligand by glutathione (GSH) in neutral NICs was studied. The reaction system was analyzed by spectrophotometry to prove the presence of a new NICs with the GS^– ligand in it. It was found that, unlike the earlier studied binuclear cationic NICs of the µ-S type and general composition [Fe₂(µ-SR)₂(NO)₄]²⁺SO₄?nH₂O with cysteamine and penicillamine ligands in which both thiolate ligands exchange by GS^–, in these neutral complexes both thiolate ligands are de-tached by only one GSH ligand is attached. A water molecule is inserted into the second free site. It is assumed that the antitumor activity of the neutral NICs can be determined not only by their NO-donor activity but also by their ability to exchange the thiolate ligand by GS^–, i.e., "to remove" GSH from the medium as in the case of cationic NICs. The discovered reaction can prevent, most likely, the S-glutathionylation of important metabolites in the presence of GSH and is very significant for metabolism of NICs.     

Au10(TBBT)10: the Begining and the End of Aun(TBBT)m Nanoclusters

Gold(I) thiolate compounds (i.e. Au^I-SR) are important precursors for the synthesis of atomically precise Au_n(SR)_m nanoclusters. However, the nature of the Au^I-SR precursor remains elusive. Here, we report that the Au₁₀(TBBT)₁₀ complex is a universal precursor for the synthesis of Au_n(TBBT)_m nanoclusters (where TBBT=4-tertbutylbenzenethiol/thiolate). Interestingly, the Au₁₀(TBBT)₁₀ complex is also found to be re-generated through extended etching of the Au_n(SR)_m nanoclusters with excess of TBBT thiol and O₂. The formation of well-defined Au₁₀(TBBT)₁₀ complex, instead of polymeric Au^I-SR, is attributed to the bulkiness of the TBBT thiol. Through 1D and 2D NMR characterization, the structure of Au₁₀(TBBT)₁₀ is correlated with the previously reported X-ray structure, which contains two inter-penetrated Au₅(TBBT)₅ rings. The photophysical property of Au₁₀(TBBT)₁₀ complex is further probed by femtosecond transient absorption spectroscopy. The accessibility of the precise Au₁₀(TBBT)₁₀ precursor improves the efficiency of the synthesis of the Au_n(TBBT)_m nanoclusters and is expected to further facilitate excellent control and understanding of the reaction mechanisms of nanocluster synthesis.     

Absolute configuration retention of a configurationally labile ligand during dynamic processes of thiolate protected gold clusters

Monolayer protected metal clusters are dynamic nanoscale objects. For example, the chiral Au₃₈(2-PET)₂₄ cluster (2-PET: 2-phenylethylthiolate) racemizes at moderate temperature. In addition, ligands and metal atoms can easily exchange between clusters. Such processes are important for applications of monolayer protected metal clusters; however, the mechanistic study of such processes turns out to be challenging. Here we use a configurationally labile, axially chiral ligand, biphenyl-2,2′-dithiol (R/S-BiDi), as a probe to study dynamic cluster processes. It is shown that the ligand exchange of free R/S-BiDi on a chiral Au₃₈(2-PET)₂₄ cluster is diastereospecific. Using chiral chromatography, isolated single diastereomers of the type anticlockwise/clockwise-Au₃₈(2-PET)₂₂(R/S-BiDi)₁ could be isolated. Upon heating, the cluster framework racemizes, while the R/S-BiDi ligand does not. These findings demonstrate that during cluster racemization and/or ligand exchange between clusters, the R/S-BiDi ligand is sufficiently confined, thus preventing its racemization, and exclude the possibility that the ligand desorbs from the cluster surface.

The ligand exchange between a configurationally labile BiDi ligand and intrinsically chiral Au₃₈ gold nanoclusters is diastereoselective. More importantly, the adsorbed ligand retains its configuration during dynamic cluster processes.     

双核Au(I)磷硫配合物激发态性质和金属间弱相互作用的从头算研究

采用MP2和CIS方法分别优化双核Au(I)磷硫配合物, [Au₂(SHCH₂SH)₂]^2＋ (1), [Au₂(SHCH₂SH)(PH₂CH₂PH₂)]^2＋(2), [Au₂(PH₂CH₂PH₂)₂]^2＋ (3), [Au₂(SHCH₂SH)(SCH₂S)] (4), [Au₂(PH₂CH₂PH₂)(SCH₂S)] (5)和[Au₂(SCH₂S)₂]^2－ (6), 基态和激发态的结构. 计算结果表明基态时1～6中存在Au(I)-Au(I)弱吸引作用, 激发态时1～5的金属间相互作用明显增强而6则减弱, 这与实验研究结果一致. 单激发组态相互作用计算揭示: 磷硫配体的更替使得Au(I)配合物跃迁性质呈现MC→MMLCT→MLCT的规律性变化(MC: 金属中心; MMLCT: 金属金属到配体电荷转移; MLCT: 金属到配体电荷转移).     

Templated Atom‐Precise Galvanic Synthesis and Structure Elucidation of a [Ag24Au(SR)18]− Nanocluster

Synthesis of atom‐precise alloy nanoclusters with uniform composition is challenging when the alloying atoms are similar in size (for example, Ag and Au). A galvanic exchange strategy has been devised to produce a compositionally uniform [Ag₂₄Au(SR)₁₈]^? cluster (SR: thiolate) using a pure [Ag₂₅(SR)₁₈]^? cluster as a template. Conversely, the direct synthesis of Ag₂₄Au cluster leads to a mixture of [Ag_25?xAu_x(SR)₁₈]^?, x=1–8. Mass spectrometry and crystallography of [Ag₂₄Au(SR)₁₈]^? reveal the presence of the Au heteroatom at the Ag₂₅ center, forming Ag₂₄Au. The successful exchange of the central Ag of Ag₂₅ with Au causes perturbations in the Ag₂₅ crystal structure, which are reflected in the absorption, luminescence, and ambient stability of the particle. These properties are compared with those of Ag₂₅ and Ag₂₄Pd clusters with same ligand and structural framework, providing new insights into the modulation of cluster properties with dopants at the single‐atom level.     

Unraveling the Impact of Gold(I)–Thiolate Motifs on the Aggregation‐Induced Emission of Gold Nanoclusters

Aggregation‐induced emission (AIE) provides an efficient strategy to synthesize highly luminescent metal nanoclusters (NCs), however, rational control of emission energy and intensity of metal NCs is still challenging. This communication reveals the impact of surface Au^I‐thiolate motifs on the AIE properties of Au NCs, by employing a series of water‐soluble glutathione (GSH)‐coordinated Au complexes and NCs as a model ([Au₁₀SR₁₀], [Au₁₅SR₁₃], [Au₁₈SR₁₄], and [Au₂₅SR₁₈]^?, SR=thiolate ligand). Spectroscopic investigations show that the emission wavelength of Au NCs is adjustable from visible to the near‐infrared II (NIR‐II) region by controlling the length of the Au^I‐SR motifs on the NC surface. Decreasing the length of Au^I‐SR motifs also changes the origin of cluster luminescence from AIE‐type phosphorescence to Au⁰‐core‐dictated fluorescence. This effect becomes more prominent when the degree of aggregation of Au NCs increases in solution.     

Ligand‐Stabilized and Atomically Precise Gold Nanocluster Catalysis: A Case Study for Correlating Fundamental Electronic Properties with Catalysis

We present results from our investigations into correlating the styrene‐oxidation catalysis of atomically precise mixed‐ligand biicosahedral‐structure [Au₂₅(PPh₃)₁₀(SC₁₂H₂₅)₅Cl₂]²⁺ (Au₂₅‐bi) and thiol‐stabilized icosahedral core–shell‐structure [Au₂₅(SCH₂CH₂Ph)₁₈]^? (Au₂₅‐i) clusters with their electronic and atomic structure by using a combination of synchrotron radiation‐based X‐ray absorption fine‐structure spectroscopy (XAFS) and ultraviolet photoemission spectroscopy (UPS). Compared to bulk Au, XAFS revealed low Au–Au coordination, Au? Au bond contraction and higher d‐band vacancies in both the ligand‐stabilized Au clusters. The ligands were found not only to act as colloidal stabilizers, but also as d‐band electron acceptor for Au atoms. Au₂₅‐bi clusters have a higher first‐shell Au coordination number than Au₂₅‐i, whereas Au₂₅‐bi and Au₂₅‐i clusters have the same number of Au atoms. The UPS revealed a trend of narrower d‐band width, with apparent d‐band spin–orbit splitting and higher binding energy of d‐band center position for Au₂₅‐bi and Au₂₅‐i. We propose that the differences in their d‐band unoccupied state population are likely to be responsible for differences in their catalytic activity and selectivity. The findings reported herein help to understand the catalysis of atomically precise ligand‐stabilized metal clusters by correlating their atomic or electronic properties with catalytic activity.     

Chiral Gold Nanoclusters: Atomic Level Origins of Chirality

Chiral nanomaterials have received wide interest in many areas, but the exact origin of chirality at the atomic level remains elusive in many cases. With recent significant progress in atomically precise gold nanoclusters (e.g., thiolate‐protected Au_n (SR)_m ), several origins of chirality have been unveiled based upon atomic structures determined by using single‐crystal X‐ray crystallography. The reported chiral Au_n (SR)_m structures explicitly reveal a predominant origin of chirality that arises from the Au–S chiral patterns at the metal–ligand interface, as opposed to the chiral arrangement of metal atoms in the inner core (i.e. kernel). In addition, chirality can also be introduced by a chiral ligand, manifested in the circular dichroism response from metal‐based electronic transitions other than the ligand's own transition(s). Lastly, the chiral arrangement of carbon tails of the ligands has also been discovered in a very recent work on chiral Au₁₃₃(SR)₅₂ and Au₂₄₆(SR)₈₀ nanoclusters. Overall, the origins of chirality discovered in Au_n (SR)_m nanoclusters may provide models for the understanding of chirality origins in other types of nanomaterials and also constitute the basis for the development of various applications of chiral nanoparticles.     

Fluorescent Gold Nanoclusters with Interlocked Staples and a Fully Thiolate‐Bound Kernel

The structural features that render gold nanoclusters intrinsically fluorescent are currently not well understood. To address this issue, highly fluorescent gold nanoclusters have to be synthesized, and their structures must be determined. We herein report the synthesis of three fluorescent Au₂₄(SR)₂₀ nanoclusters (R=C₂H₄Ph, CH₂Ph, or CH₂C₆H₄^tBu). According to UV/Vis/NIR, differential pulse voltammetry (DPV), and X‐ray absorption fine structure (XAFS) analysis, these three nanoclusters adopt similar structures that feature a bi‐tetrahedral Au₈ kernel protected by four tetrameric Au₄(SR)₅ motifs. At least two structural features are responsible for the unusual fluorescence of the Au₂₄(SR)₂₀ nanoclusters: Two pairs of interlocked Au₄(SR)₅ staples reduce the vibration loss, and the interactions between the kernel and the thiolate motifs enhance electron transfer from the ligand to the kernel moiety through the Au?S bonds, thereby enhancing the fluorescence. This work provides some clarification of the structure–fluorescence relationship of such clusters.     

Same Magic Number but Different Arrangement: Alkynyl‐Protected Au25 with D3 Symmetry

Two homoleptic alkynyl‐protected gold clusters with compositions of Na[Au₂₅(C≡CAr)₁₈] and (Ph₄P)[Au₂₅(C≡CAr)₁₈] (Na? 1 and Ph₄P? 1 , Ar=3,5‐bis(trifluoromethyl)phenyl) were synthesized via a direct reduction method. 1 is a magic cluster analogous to [Au₂₅(SR)₁₈]^? in terms of electron counts and metal‐to‐ligand ratio. Single‐crystal structure analysis reveals that 1 has an identical Au₁₃ kernel to [Au₂₅(SR)₁₈]^?, but adopts a distinctly different arrangement of the six peripheral dimer staple motifs. The steric hindrance of alkynyl ligands is responsible for the D₃ arrangement of Au₂₅. The introduction of alkynyl also significantly changes the optical absorption features of the nanocluster as supported by DFT calculations. This magic cluster confirms that there is a similar but quite different parallel alkynyl‐protected metal cluster universe in comparison to the thiolated one.     

Solvent‐Induced Cluster‐to‐Cluster Transformation of Homoleptic Gold(I) Thiolates between Catenane and Ring‐in‐Ring Structures

Supramolecular ensembles adopting ring‐in‐ring structures are less developed compared with catenanes featuring interlocked rings. While catenanes with inter‐ring closed‐shell metallophilic interactions, such as d¹⁰–d¹⁰ Au^I–Au^I interactions, have been well‐documented, the ring‐in‐ring complexes featuring such metallophilic interactions remain underdeveloped. Herein is described an unprecedented ring‐in‐ring structure of a Au^I‐thiolate Au₁₂ cluster formed by recrystallization of a Au^I‐thiolate Au₁₀ [2]catenane from alkane solvents such as hexane, with use of a bulky dibutylfluorene‐2‐thiolate ligand. The ring‐in‐ring Au^I‐thiolate Au₁₂ cluster features inter‐ring Au^I–Au^I interactions and underwent cluster core change to form the thermodynamically more stable Au₁₀ [2]catenane structure upon dissolving in, or recrystallization from, other solvents such as CH₂Cl₂, CHCl₃, and CH₂Cl₂/MeCN. The cluster‐to‐cluster transformation process was monitored by ¹H NMR and ESI‐MS measurements. Density functional theory (DFT) calculations were performed to provide insight into the mechanism of the “ring‐in‐ring? [2]catenane” interconversions.     

钯掺杂的25-原子和38-原子金纳米团簇

In this work, we describe two synthetic procedures for preparing palladium doped 25-atom nanoclusters (referred to as Pd₁Au₂₄(SR)₁₈, where ―SR represents thiolate, R=C₂H₄Ph). Pure Pd₁Au₂₄(SC₂H₄Ph)₁₈ nanoclusters are isolated by solvent extraction and size exclusion chromatography. Mass spectrometry and optical spectroscopy analyses demonstrate that the Pd₁Au₂₄(SC₂H₄Ph)₁₈ nanocluster adopts the same core-shell structure as that of the homogold Au₂₅(SC₂H₄Ph)₁₈ nanocluster, that is, a Pd- or Au-centered icosahedron surrounded by six Au₂(SR)₃ “staple”-like motifs. Similar doping behavior has also been observed in 38-atom M₃₈(SR)₂₄ (M: metal) nanoclusters, indicating the unique behavior of Pd dopant being preferentially located in the icosahedral center. The catalytic activity of Pd₁Au₂₄(SC₂H₄Ph)₁₈ has also been evaluated for the selective hydrogenation of α,β-unsaturated ketone (e.g., benzalacetone) to α,β- unsaturated alcohol, and a 42% conversion of benzalacetone is attained.