Surface Chemistry of Atomically Precise Coinage–Metal Nanoclusters

Article Cite This: Acc. Chem. Res. 2018, 51, 3084−3093

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Surface Chemistry of Atomically Precise Coinage−Metal Nanoclusters: From Structural Control to Surface Reactivity and Catalysis Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”.

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Juanzhu Yan, Boon K. Teo, and Nanfeng Zheng* State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and National & Local Joint Engineering Research Center for Preparation Technology of Nanomaterials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

CONSPECTUS: A comprehensive understanding of chemical bonding and reactions at the surface of nanomaterials is of great importance in the rational design of their functional properties and applications. With the rapid development in cluster science, it has become clear that atomically precise metal clusters represent ideal models for resolving various important and/or unsolved issues related to surface science. This Account highlights our recent efforts on the fabrication of ligand-stabilized coinage nanoclusters with atomic precision from the viewpoint of surface coordination chemistry in particular. The successful synthesis of a large variety of metal clusters in our group has greatly benefitted from the development of an effective amineassisted NaBH4 reduction method. First discussed in this Account is how the introduction of amines in the synthetic protocol enhances the long-term stability and high-yield production of Ag/Cu-based metals in air. Such a method allows the utilization of different organic ligands as surface stabilizing agents to manipulate both the core and surface structures of metal nanoclusters, helping to understand the role of surface ligands in determining the structures of metal nanoclusters. The coordination chemistry of ligands used in the synthesis of metal nanoclusters is crucial in determining their overall shape, metal arrangement, surface ligand binding structure, chirality and also metal exposure. Detailed discussions are given in the following four different systems: (1) The co-use of phosphines and thiolates with rich coordination structures (2 to 4-coordinated) helps to control the formation of a sequence of Ag nanoclusters with a near-perfectly cubic shape; (2) The metal arrangements and surface structures of AuCu clusters highly depend on metal precursors and counter cations used in the synthesis; (3) Metal clusters with intrinsic chirality are readily prepared by introducing chiral ligands or counterions, making it possible to obtain optically active enantiomers and understand the origin of chirality of metal nanoclusters; (4) The variation of metal exposure of the inner metal core of metal nanocluster can be controlled by the surface ligand coordination structure. Such capabilities to manipulate the surface structure of metal nanoclusters allow the creation of model systems for investigating the structure−reactivity relationship of metal nanomaterials. Several important examples are then discussed to highlight the importance of ligand coordination chemistry in tuning the surface reactivity and catalysis of metal nanoclusters. For example, bulky thiolates on Ag are demonstrated to be more labile than small thiolates for making metal nanoclusters with both enhanced ligand exchange capability and catalysis. Alkynyl ligands can be thermally released from metal nanoclusters more easily than thiolates and halides while maintaining the overall structure, thereby serving as ideal systems for understanding the promoting effect of surface stabilizers on catalysis. Finally, we provide a perspective on the principles of surface coordination chemistry of metal nanoclusters and their potential applications with regards to catalysis of protected metal clusters.

Received: July 29, 2018 Published: November 15, 2018 © 2018 American Chemical Society

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Figure 1. (a−c) Structural analysis of [(MAg)44SR]304− (M = Au, Ag): (a) overall structures, (b) M12@Ag20 core, (c) surface Ag2(SR)5 motif. (d,e) Crystal structures of [Ag136(SR)65Br2]− (d) and Ag374(SR)115Br2 (e), and the μ2, μ3, μ4, and μ5 binding modes of thiolated ligands in both nanoparticles (f). Color codes: orange, Au/Ag; green, Ag; yellow, S; brown, halogen; gray, C. All hydrogen and fluorine atoms are omitted for clarity. Reproduced with permission from refs 20 and 24. Copyright 2013, 2016 Springer Nature.

2. HIGH-YIELD SYNTHESIS OF Ag/Cu-BASED NANOCLUSTERS IN AIR The Brust−Schiffrin biphasic method, reported in 1994,17 is widely adopted to synthesize stable protected metal clusters owing to its convenience and simplicity. In the two-phase systems, sodium borohydride (NaBH4), as a strong reducing agent, reacts quickly with water in the presence of metals, which is undesirable for creating a sustained reduction process during the syntheses of Ag/Cu clusters. However, keeping the syntheses in a reductive environment is of great importance to fully reduce Ag/Cu precursors and also to prevent air oxidation of the resulting clusters in the subsequent size-focusing aging process, allowing them to grow and mature into single-sized nanoclusters. Our strategy is to adopt the biphasic synthetic protocol in the preparation of protected Ag/Cu nanoclusters. The key here is to slow the reduction rate of the metal precursors and to offer a sustainable reduction in solution by introducing amines. Synthetically, the metal ions or their precursors, together with ligands (in some cases also with counterions) are chemically reduced by reducing agents such as NaBH4 in the presence of triethylamine in a diphasic solvent system at a low temperature (say, 0 °C).18−20 The function of the organic amine is to form borane−amine complexes with better stability, thereby weakening the reducing power and decomposition rate of reducing agent. This facilitates dispersed nanoclusters formed in the nucleation stage, and with aging, the nuclei undergo selfoptimization and size-focusing processes. Simultaneously, this scenario effectively prevents air oxidation of low-valence Ag and/or Cu, which is detrimental to cluster formation. Finally, counterions of appropriate structure can promote crystallization of the resulting clusters for structure determination as well as separation from other products and impurities (a concurrent and highly effective purification process). This amine-assisted reduction approach has allowed us to produce a number of ultrahigh-nuclearity organic-ligand-protected Ag, Cu, and other noble nanoclusters with atomic precision and novel surface properties, a few of which are highlighted below. The first set of 44 metal−atom Ag and Au−Ag nanoclusters, namely [Ag44(SR)30]4− and [(AuAg)44(SR)30]4− clusters

1. INTRODUCTION The fabrication of nanomaterials has progressed rapidly over the past two decades.1,2 These nanomaterials exhibit promising properties for various research areas, including catalysis,3 sensing,4 medicine,5 energy conversion,6 and optoelectronics.7 A comprehensive understanding of chemical bonding and reactions at the nanostructured surface, as well as the analogies between various disciplines, are of importance in the rational design of multiple functional properties and their applications.8−10 Nevertheless, it is very challenging to fully characterize the surface structures of conventional nanoparticles at the atomic level owing to their extreme complexity and intrinsic heterogeneity.11,12 With rapid advances in cluster science, it has become clear that structurally well-defined protected metal nanoclusters, determined by X-ray crystallographic and mass spectrometric techniques at the atomic scale, represent ideal models to resolve various important and/or unsolved issues related to surface science.9,13,14 Among the coinage metals (Cu, Ag, Au), molecular structures of Au nanoclusters stabilized by thiolates and phosphines are extensively studied and constitute a well-known and significant family in cluster chemistry.13−15 Staple [Aun(SR)n+1] (n = 1, 2) units have been well-documented as common surface structure motifs of thiolated Au nanoparticles.16 In contrast to Au clusters, Ag/Cu clusters often have different but more complicated geometrical motifs, thereby offering great potential for generating a wide variety of cluster species. Unfortunately, silver and copper are vulnerable to air oxidization. Despite several decades of synthetic efforts, it is still nontrivial to prepare Ag/Cu nanoparticles with long-term stability and high yield in air. In this Account, we will present our efforts in the synthesis and characterization of ligand-stabilized coinage nanoclusters with atomic precision, including developing a simply but versatile amine-modified two-phase route toward atomically precise nanoclusters and controlling their surfaces with ligand combinations. These controls on the morphology, metal arrangement, chirality, and surface reactivity allow the design and synthesis of metal nanoparticles with desirable properties, providing a perspective with regards to surface coordination chemistry of nanoclusters in general. 3085

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documented in the literature.31−33 However, fundamental understanding of how these capping agents work in the process is still challenging. Systematic structural investigations of small metal nanoclusters can provide insights on the atomic details of the early or embryonic stages of the growth of metal particles, in particular how capping agents bind to the surface metal atoms and control the shape evolution process of metal nanocrystals. Recently, we presented that the preparation and crystallographic characterization of a sequence of near-perfectly cubic structures tuned by specific chemisorption of the different ligands, namely [Ag14(SR2)12(PR′3)8] (1), [Ag38(SR)26(PR′3)8] (22) and [Ag63(SR)36(PR′3)8]+ (23, SR = SPhF2, R′ = alkyl).18,34,35 Owing to their specific binding capabilities and selectivity toward a given metal surface, capping agents play a critical role in controlling the size and shape of the resulting nanocrystals with atomic-level precision. Each thiolate on the (100) faces is bound at the center of the four Ag atoms of rhombic Ag4 face, whereas thiolates on the edges are three-coordinated to the three Ag atoms of half-rhombic face. Each of the eight Ag atoms situated at the corners of cubic metal framework of clusters is stabilized by one phosphine. More importantly, these molecular nanosized metal clusters are embryonic members of a magic growth sequence, from 1 to 22 to 23 to 33 to n3 (bulk), based on the fcc cluster 1 building block (Figure 2). Furthermore, theoretical

[abbreviated (MAg)44, M = Au and/or Ag] stabilized by fluorinated arylthiols (SR = SPhF, SPhF2 or SPhCF3), are depicted in Figure 1a. Each cluster can be structurally described as a two-shell Keplerate M12@Ag20 core (Figure 1a) enclosed by six Ag2(SR)5 units (Figure 1c). The structure of the Ag2(SR)5 units on the surface of this series of Ag-containing clusters is significantly different from the surface staple motifs, [Aun(SR)n+1] (n = 1, 2) on thiolated Au clusters. Although theoretical analysis confirmed that the stability of these (MAg)44 nanoclusters is due to their 18-electron superatom shell closure, experimental studies clearly revealed that their overall stabilities highly rely on their surface thiolated species and counterions as well.20,21 It should be noted that [Ag44(SPhF)30]4− nanoclusters with multiple absorption bands were initially synthesized by Bakr et al.22 The single-crystal structure of [Ag44(SR)30]4− cluster with p-mercaptobenzoic acid as surface ligands was characterized independently by Bigoni and co-workers.23 The amine-assisted reduction method also allowed us to synthesize a number of giant silver nanoparticles greater than 2 nm. The notable examples are [Ag136(SR)65Br2]− (abbreviated Ag136) and Ag374(SR)115Br2 (abbreviated Ag374), measuring up to 3 nm in core diameter (Figure 1d,e).24 In fact, the Ag374 cluster is the current “world-record” size of single-crystal structures involving zero-valvent metal elements. Synthetically, the success in making the largest atomically precise metal nanoparticle can be traced to the oil-soluble nature and the bulky substituent of the 4-tert-butylbenzenethiolated ligand. Structurally, a regular decahedral Ag57 core in the Ag136 cluster is covered by ten (111) facets, whereas the inner Ino decahedral Ag207 cores in Ag374 are enclosed by five (100) facets at the waist and ten (111) facets at the poles. Thiolates in various binding modes (μ2, μ3, μ4, even μ5) are distributed on surface coordination spheres of these Ag nanoclusters (Figure 1f). The analysis of the electronic structures of Ag136 and Ag374 suggests an even lower bulk limit for the plasmonic behavior than the classical Mie energy for silver, whereby the π electron densities from the electron-rich 4-tert-butylbenzenethiolated sphere contributed a collective dipole oscillation. These two Ag nanoparticles serve as excellent models in the understanding of not only how metal molecular precursors develop from molecules to clusters to nanoparticles but also the structure distortion within twinned metal nanostructures. In addition to these examples, the amine-assisted reduction approach has also been employed in the synthesis of thiolated Ag-based mixed-metal clusters, [MAg24(SR)18]2− (M = Pd, Pt),25 where the metal framework is analogous to that of Au25SR18.26,27 As a variant of the amine-assisted reduction approach mentioned above, a facile reaction scheme was recently devised by our group in the preparation of alkynyl-stabilized AuAg bimetallic nanoclusters. In this protocol, the NaBH4 and triethylamine were replaced by an amine-borane complex (as a reducing agent), and the reaction was conducted in a single phase.28−30 A strong organic alkali, such as sodium methoxide, was simultaneously added to deprotonate the alkyne, thereby producing the alkynyl-protected nanoclusters.

Figure 2. Molecular structures of cube-shaped Ag nanoclusters: (a) [Ag14(SPhF2)12(PPh3)8] (1), (b) [Ag38(SPhF2)26(PnBu3)8] (22), and (c) [Ag63(SPhF2)36(PnBu3)8]+ clusters (23). (d−g) Iidealized fcc growth sequence of corresponding cubes. Color codes: blue, Ag; red, P; yellow, S; green, F; gray, C. Adapted with permission from ref 34. Copyright 2017 American Chemical Society.

calculations of the 23 and 33 clusters were also performed to study their ground-state electronic structure and excited-state absorption. It was found, electronically, that 22 is more stable cluster than 23. In particular, a new level of detailed understanding of the patterned metal growth of cubic metal nanoclusters dominated by the nature of chemisorbed ligands was achieved. Meanwhile, similar ligand coordination specificities are also well documented in the [Ag67(SPhMe2)32(PPh3)8]3+ nanocluster reported by Bakr et al.36 It should be noted that recent studies by Jin and co-workers also discovered periodicities in a box-shaped “magic series” of Au nanoclusters stabilized by 4-tert-butyl-benzenethiolate with a formula of Au8n+4(TBBT)4n+8 (n = 3−6).37 Future work is expected to identify more regular Ag- and/or Au-based periodic

3. SURFACE STRUCTURE CONTROL OF METAL NANOCLUSTERS 3.1. Shape Control by Ligands

Using specific capping ligands to control the size, shape, and exposure surface of metal nanocrystals has been well3086

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observation attests to the instabilities of thiolated Au−Cu bimetallic nanoclusters in general.

3.2. Precursor-Dependent Surface Structure Control

3.3. Chirality Control by Ligands and Counterions

A new series of thiolated AuCu alloy clusters, namely [Au12+nCu32(SR)30+n]4− (n = 0, 2, 4, 6) were recently prepared by our group.38 The structures of these clusters are based on that of (MAg)44.20,23 However, the metal arrangements and surface structures of these AuCu clusters were highly sensitive to the Au(I) precursors and counter cations (i.e., tetraphenylphosphonium, tetrabutylammonium) used in the syntheses. Among them, the structure of [Au12Cu32(SR)30]4− [abbreviated (AuCu)44, n = 0] cluster is essentially the same as that of [M12Ag32(SR)30]4−, wherein the outer-shell Ag atoms are replaced by Cu atoms (Figure 3a).

Chiral nanoparticles are of great importance in nanoscience and nanotechnology owing to their potential applications.39 Because bulk materials often have an achiral structure (e.g., fcc structure), their chiral origin is often difficult to identify.40 In principle, chiral metal nanoparticles can be original from three different sources: (a) the intrinsically chiral metal core, (b) the chiral surface organic ligand, and (c) the asymmetric arrangement of achiral ligands on the metal nanoparticle surface.41−43 Systematic studies over the origin of chirality of chiral molecules have allowed us to manipulate their chirality-related physical, chemical, or biological properties by controlling their chirality.43−48 The prerequisite to such understanding is to devise an effective general method for making chiral metal nanoparticles. To this end, a couple of examples are discussed here to illustrate how ligand binding impacts the structural engineering of chiral metal clusters, in this case, co-stabilized by thiolate and diphosphine. Furthermore, direct synthesis of optically active metal nanoclusters can be achieved by introducing chiral diphosphine ligands and chiral quaternary ammonium counterions. In 2003, two Ag clusters, Ag16(DPPE)4(SPhF2)14 and [Ag32(DPPE)5(SPhCF3)24]2− (abbreviated Ag16 and Ag32; DPPE = 1,2-bis(diphenylphosphino)ethane), co-stabilized by diphosphine and thiolate were prepared and structurally characterized by X-ray crystallographic method.19 In these structures, three thiolates with alternative μ2, μ3, or μ4 coordination modes and a P atom from phosphine preferentially constitute a tetrahedral [AgS3P] motif as a primary surface structure domain for the Ag clusters. Linear [Ag(SR)2] staples and μ3-coordinated standalone thiolates also occur in these Ag clusters as well. As shown in Figure 4, both Ag16 and Ag32 conform to C2 symmetry wherein the asymmetric arrangements of the tetrahedral [AgS3P] coordination units on the surface induce the chirality that can transfer from the ligand shell to the inner metal core, thereby resulting in intrinsic chiral structure. As a further development, the synthetic recipe of Ag16 and Ag32 was modified by replacing DPPE with DPPP, 1,3bis(diphenyphosphino)propane. Bigger intrinsically chiral nanoclusters, [Ag78(DPPP)6(SPhCF3)42] (abbreviated Ag78), were synthesized. Each Ag78 cluster comprises a two-shell Ag66 core enclosed by a Ag-PR-SR complex shell with a twisted trigonal prism shape. On the three vertical edges of the trigonal prismatic framework there are three [Ag4(DPPP)2(SPhCF3)8]4− units twistedly arranged in either left- or right-handed direction. Because of the achiral nature of DPPP, the left- and right-twisted clusters are equally present in the obtained product as racemic pairs. Therefore, the predetermined chirality in the Ag78 cluster can be manipulated by asymmetric construction of quaternary carbon centers in alkyl C−C−C chain in diphosphine to immobilize the orientation of asymmetric [PAgS3] tetrahedra, finally giving rise to its chiral analogues (Figure 5a). A general strategy for the synthesis of optically pure chiral metal nanoparticles with atomic precision was thus developed by using chiral diphosphines to control the chirality.47 Another example of this strategy is our recent work on symmetry breaking of an achiral Au13Cu2 cluster by employing mixed ligands, turning it into a pair of optically pure enantiomers. Specifically, (2R,4R)/(2S,4S)-2,4-bis-

Figure 3. (a−d) Molecular structures of [Au12+nCu32(SR)30+n]4− (n = 0, 2, 4, 6). (e) Surface structure change from Cu2(SR)5 to the Cu2Au(SR)6 unit among [Au12+nCu32(SR)30+n]4− clusters. Color codes: gold, Au; blue, Cu; yellow, S. Adapted with permission from ref 38. Copyright 2014 American Chemical Society.

Owing to the smaller atomic radius of Cu in comparison to that of Ag and Au, there are much weaker Cu···Cu metal interactions in the surface Cu2(SR)5 units, making it possible to have one Au-SR unit inserted into the two Cu atoms. A subtle tuning in the synthetic condition of [Au12Cu32(SR)30]4− readily led to the formation of a series of [Au12+nCu32(SR)30+n]4− clusters, wherein the (44+n) metal atoms were orderly arranged in the Au12@Cu32@Aun shell-by-shell framework without any distortions of the innermost icosahedron (Figure 3b,c). Moreover, on the basis of oxidation susceptibility of coinage elements (that follows the trend of Au > Ag > Cu), it can be concluded that the order of stabilities among these thiolated coinage metal nanoanalogs follows [(AuAg)44(SR)30+n]4− > [Ag44(SR)30+n]4− > [Au12+nCu32(SR)30+n]4−. Experimentally, this stability ordering is supported by the observation of a degradation product, formulated as [Au13Cu12(SR)20]4− in the recrystallization of [Au12+nCu32(SR)30+n]4− clusters. This 3087

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Figure 4. (a,b) Molecular structures of Ag16(DPPE)4(SPhF2)14 (a) and [Ag32(DPPE)5(SPhCF3)24]2− (b) clusters with C2 symmetry. (c,d) Two enantiomers of the Ag16 (c) and Ag32 (d) clusters. Color codes: green, Ag; yellow, S; pink, P. Reproduced with permission from ref 19. Copyright 2013 Royal Society of Chemistry.

Figure 5. Overview of R/S-Ag78 (a) and R/S-Au13Cu2 (b). Color codes: orange, Au; green, Ag or Cu; pink, P; yellow, S; blue, N; light green, F; gray, C. Panel (a) is adapted with permission from ref 47. Copyright 2017 American Chemical Society. Panel (b) is adapted with permission from ref 48. Copyright 2018 Wiley-VCH.

interlayer is arranged in a hexagonal close packing (hcp) manner. The slight distortion lowers the symmetry of its Ag28Cu12 metal skeleton from Td to T, placing it in a chiral entity (Figure 6c−e). As encouraged by the observation of ion-pairing between the anionic (AgCu)40 cluster and ammonium cations under the mass spectrometric condition, the as-prepared racemic mixture was separated into enantiomers by using chiral quaternary ammonium salts as chiral resolving agents (Figure 6f). Subsequent asymmetric synthesis of this chiral cluster using chiral ammonium salts led to the formation of optically active enantiomers (Figure 6g). These simple strategies, ion-pairing induced chiral separation and direct enantioselective synthesis with the unitization of chiral counterions, may be adopted to synthesize chiral metal nanoparticles.

(diphenylphosphino)pentane [abbreviated (2r,4r)/(2s,4s)BDPP] were used as the chiral diphosphines to achieve the one-pot enantioselective synthesis of an optically pure, enantiomeric pair of [Au13Cu2(BDPP)3(SPy)6]+ (SPy = 2pyridylthiolate) without the need of enantioseparation (Figure 5b).48 The chiral arrangement of the SPy ligands resulted from the steric hindrance between SPy and their neighboring phosphine ligands. Such an arrangement led to the breaking symmetry of the achiral icosahedral core. We also demonstrated a facile ion-pairing approach for synthesizing optically active chiral [Ag28Cu12(SR)24]4− [abbre. (AgCu)40] bimetallic nanoclusters (Figure 6a,b) which was controlled by chiral ammonium cations.46 The thiolated (AgCu)40 clusters were first prepared as a racemate and crystallographically characterized. While the 28 Ag atoms in the cluster are arrayed in a distorted fcc pattern, the Ag/Cu 3088

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4. SURFACE REACTIVITIES AND CATALYTIC ACTIVITIES OF METAL NANOCLUSTERS 4.1. Enhanced Surface Reactivities and Catalytic Activities by Bulky Ligands

Although the coordination capability of an organic ligand is primarily related to its coordinately active atoms and the associated functional group, its coordination strength and spatial requirement can often be tuned by its substituent R group, as in the thiolate RS−. We shall discuss two examples whereby bulky substituents greatly impact the surface reactivities and catalytic activities of the metal nanoclusters. As the first example, [Ag141X12(S-Adm)40]3+ (X = Cl, Br, I) was synthesized with a bulky thiol (1-adamantanethiol, HS-Adm) as a protective ligand in the presence of halide ions in the solution. The bulky nature of the adamantyl substituent on the ligand led to low ligand coverage on Ag nanoparticles, leaving surface sites accessible to halides (Figure 8). The existence of some longer-than-normal Ag−S bonds on the waist of the Ag141 nanoparticle indicates that the binding ability of thiolates on the nanoclusters is weakened. The presence of these relatively weak coordination ligands increases the surface chemical reactivity of the Ag141 nanoclusters. In ligand-exchange reactions, the relatively poorbinding bulky thiolates are easily replaced by phenylacetylene or other thiols with relatively weak coordination capacities. Through ligand exchange with water-soluble thiols, these structurally defined silver nanoclusters readily become watersoluble, providing an important basis for their biological applications. This work provides an important research idea to synthesize metal nanoparticles of high surface reactivity by introducing bulky ligands.50 In addition, we also replaced 1-adamantanethiol by another bulky thiol, cyclohexanethiol in cluster synthesis and successfully obtained a novel metallic [Ag206L72]q (Ag206, L = thiolate, halide; q = charge) nanoparticle. The most important characteristic of the reported nanoparticle is that many of the surface ligands are labile due to their low-coordination binding structure. Furthermore, the nanoparticle was found to be a multivalent redox species on account of differential pulse voltammetric and DFT calculation studies. The combination of these two characteristics/properties led to the high surface reactivity and catalysis of the Ag206 nanoparticle. The latter was modeled by cycloisomerization of alkynyl amines (Figure 9).51 These two examples serve as an indirect but specific substratesubstituted process to expose the catalytic metal center on metal nanocluster surfaces at the atomically precise scale by means of X-ray crystallographic techniques.

Figure 6. (a−e) Crystal structure of (nBu4N)4[Ag28Cu12(SR)24]: (AgCu)40 with T symmetry (a−c, e) and ideal fcc/hcp Ag28Cu12 in Td symmetry (d). (f) Chemical structures of BCNC and BCDC. (g) CD spectra of the (AgCu)40 enantiomeric pair. Color codes: orange and green, Ag; blue, Cu; yellow, S; light green, Cl; Pink, N; gray, C. Adapted with permission from ref 46. Copyright 2016 American Chemical Society.

3.4. Ligand-Controlled Exposure of Catalytic Metal Atoms

The ability to control the accessibility of catalytically active metal sites on a metal nanoparticle at the atomic scale is a key issue in the field of nanocatalysis.12 It is desirable to design structurally well-defined exposed metal surface to control their surface reactions.3,13,14 This has been proven feasible by way of precise ligand engineering in the atomic nanocluster system. For instance, by introducing appropriate thiol and phosphine ligands, the surface structures of three Au13Cux bimetallic nanoclusters (x = 2, 4, 8) can be controlled, resulting in the variation of metal exposure in their inner Au12 icosahedral shells (Figure 7a).49 Experimentally, nitrophenol reduction to aminophenol with NaBH4 was chosen as a model reaction to evaluate the corresponding catalytic behavior. The latter is believed to be intimately related to the accessibility of Au sites on these cluster surfaces. Using Au13Cu8 as the catalyst, 4-nitrophenol was fully converted to 4-aminophenol within 10 min (Figure 7b,c). However, when Au13Cu2 or Au13Cu4 was used as the catalyst, the reaction did not proceed owing to the inaccessibility of their inner Au sites. In this case, adopting suitable surface organic ligands can effectively expose the internal metal atoms as well as exhibit superior catalytic performance.

4.2. Binding Strengths of Different Ligands

Generally speaking, the ligand binding strength on coinage metals follows the order of thiolates ≈ phosphines > acetylides > amine as per conventional Hard−Soft Acid−Base (HSAB) theory.52 In this order, the latter ligands can readily be replaced by the former ligands or removed upon heating or other treatments. Recently, terminal alkynes (RCCH) have been used as a new type of protective ligand of coinage metal nanoclusters/nanoparticles.28,53−56 In this connection, a bimetallic nanocluster containing 44 metal atoms, Au24Ag20(SPy)4(PA)20Cl2 (PA = phenylalkynyl), was successfully prepared and structurally characterized by single-crystal analysis.28 Three different types of anionic ligands (i.e., PA, SPy, and Cl−) are present on the cluster surface. Similar to thiolates, alkynyls can bind linearly to surface Au atoms with their σ-bonds, resulting in two kinds of surface staple 3089

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Figure 7. (a) Crystal structures of Au13Cu2 and Au13Cu4 in stick-and-ball model and Au13Cu8 with space filling. (b) UV−vis spectra showing the reduction of 4-nitrophenol with Au13Cu8 as catalyst. (c) Plot of ln(Ct/C0) versus time for Au13Cu8. Color codes: gold, Au; green, Cu; yellow, S; pink, P; gray, C; blue, N. Adapted with permission from ref 49. Copyright 2013 American Chemical Society.

Figure 9. Schematic diagram of thiolated atomically precise, superatomic silver nanoparticle-catalyzed reaction of cycloisomerization of alkynyl amines. Color codes: green, Ag; yellow, S; red, Cl; pink, F; gray, C. Adapted with permission from ref 51. Copyright 2018 Oxford University Press. Figure 8. Structural analysis of [Ag141Br12(S-Adm)40]3+ (top) and ESIMS and images of the solution (bottom) of the clusters after being ligand-exchanged with phenylacetylene and mercaptosuccinic acid. Color codes: pink, blue and green, Ag; yellow, S; brown, Br; gray, C. Adapted with permission from ref 50. Copyright 2017 American Chemical Society.

are arranged in the Keplerate Au12@Ag20@Au12 structure. More important is the fact that the coexistence of three different types of ligands makes it possible to assess the relative coordinative power of the ligands by selectively removing them from the surface. In this context, temperature-programmed decomposition/mass spectrometric (TPD-MS) and thermogravimetric (TGA) studies of Au24Ag20(SPy)4(PA)20Cl2 clearly revealed that the removal of PA occurs readily below 100 °C (Figure 10,

units (PA-Au-PA and PA-Au-SPy) on Au24Ag20(SPy)4(PA)20Cl2. The 44 metal atoms in the cluster 3090

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11).30 On the basis of the observation that surface ligands can promote the catalytic performances of metal nanoclusters, the catalytic activity of gold nanoparticles can also be greatly enhanced by simple phenylacetylene modifications. In this case, the presence of surface ligands introduces a promoting effect on catalysis of metal nanoclusters, which might be attributed to the synergetic activation of silanes and H2O at the Au−PA interface.

5. CONCLUSIONS AND FUTURE PROSPECTS In summary, metal clusters with atomically precise structures readily serve as an excellent model system for investigating the surface coordination chemistry of metal nanomaterials. The system helps to elucidate the role of ligands in dictating the structures of metal nanoclusters (e.g., metal arrangement, surface ligand binding structure, chirality, metal exposure) and the structure−property correlation (e.g., surface reactivity, catalysis) of metal nanoclusters. On the basis of this understanding, the surface modification of metal nanoparticles with certain ligands should help to build desirable surface coordination motifs for optimizing their surface-related properties, such as catalysis. More importantly, the well-defined atomic structure of atomically precise metal nanoclusters allows the use of techniques such as mass spectrometry and nuclear magnetic resonance spectroscopy to track the whole catalytic process and also helps to simplify the model building for theoretical calculations toward molecular understanding of their mechanisms. In an attempt to investigate the catalytic mechanism, highly desirable is the design and synthesis of structurally welldefined metal nanoclusters with combinations of both strong and labile surface ligands that exhibit catalytic metal nanoclusters. The steric effect from the surface ligand layer is expected to play an important role in manipulating the catalytic selectivity as well as the stability. Nonetheless, further systematic research is required to unravel the details of the dynamics and thermodynamics of metal nanoclusters during catalysis. Furthermore, atomically precise organic-stabilized metal nanoclusters are in many respects of interest in the development of functional materials. They are appealing from a structural point of view, providing means to obtain insights into surface structures of nanoparticles at the atomic level. They incorporate an inorganic metal core with distinct physical and chemical properties (e.g., redox, fluorescence, magnetic and electronic states) that incite further developments in diverse fields, such as nanoelectronics, bioimaging, sensing or specific-site heteroge-

Figure 10. Crystal structure (left) of and TPD-MS curve (PA, right top) and ligand exchange (right bottom) of the Au24Ag20(SPy)4(PA)20Cl2 cluster. Color codes: orange, Au; green, Ag; yellow, S; gray, C. Adapted with permission from ref 28. Copyright 2015 American Chemical Society.

top). The amazing result is that the metal core remains intact upon partial removal of the surface PA ligands. Furthermore, the lability of PA ligands on the cluster allows site-specific ligand exchange with other capping ligands to obtain metal clusters with the same core but different surface functionality (Figure 10, bottom). 4.3. Ligand-Promoted Catalysis

As mentioned in the previous section, alkynyl ligands on alkynylstabilized metal nanoclusters are labile enough to be readily released under mild conditions while keeping their metal framework intact. On the basis of this observation, these alkynylstabilized AuAg nanoclusters may serve as ideal models for disclosing the role of surface ligands in catalysis. In the system of Au34Ag28(PA)34 clusters, all the phenylacetylide ligands form staple-like “PA-Au-PA” units in linear two-coordinated mode with surface gold atoms, and the 62 metal atoms in the cluster are arranged into a four-shell Ag@Au17@Ag27@Au17 structure. TPD-MS study also showed that all phenylacetylide ligands on the cluster surface can be selectively removed at rather low temperatures (