English

• 1. INTRODUCTION

  Nanoscale metal particles have been a hot research topic due to their prominent catalytic, optical, magnetic and electrochemical properties^[1]. However, polydispersity restricts a deep understanding of the structure-function relationships of nanomaterials in many aspects. Metal nanoclusters (NCs) with well-defined structures are advantageous in correlating structures and properties. Significant progress has been made in the synthesis, structural determination and property investigation of NCs over the past decade^[2-4]. A series of ligand-protected nanoclusters with various sizes and morphologies have been reported. Different synthetic methods and various reaction conditions (e.g., reducing agent, solvent, reactant ratio and temperature) have been used in the preparation of a large number metal NCs^[5-8]. As a rare case, it is found that the isomerization of alkynyl-protected gold NCs can be realized under slightly different synthetic conditions^[9]. This pair of Au₂₃(^tBuC≡C)₁₅ isomers displays different stability and optical properties due to their different arrangements of the surface motifs. Earlier cases were also found in thiolated family, i.e., Au₃₈(PET)₂₄^{[10, 11]}, Au₄₂(TBBT)₂₆^{[12, 13]} and Au₂₈(CHT)₂₀^[14] (PET = phenylethanethiolate, TBBT = 4-tertbutyl benzenethiol, CHT = cyclohexanethiolate).

  A ligand-protected metal nanocluster is composed of an internal metal kernel and a surface ligand shell (Fig. 1). Various ligands have been employed in the synthesis of NCs^[2], including phosphines, thiolates, alkynyls, amines, and macrocyclic ligands. As indispensable components of NCs, surface ligands not only provide a protecting shell around the metal core, but also play an important role in determining the structures and properties of the clusters^{[2, 4]}. Ligand engineering is the organization of surface ligands on metal nanoclusters in order to modulate their physicochemical properties. The approach involves structural control through utilizing different types of ligands with preorganized functional groups and steric hindrance. For instance, water-soluble clusters may be synthesized by introducing hydrophilic ligands^[15], and chiral clusters can be obtained by using chiral ligands^[16]. Many issues remain challenging in terms of studying ligand effects on various aspects, such as how ligands influence atomic packing in the cluster structure and what is the underlying basis of ligand effects on the catalytic performance. In this perspective, we describe recent advances in ligand engineering in metal NCs regarding the following three aspects: (1) Ligand-induced structural control of NCs; (2) Ligand effects on optical properties of NCs; (3) Ligand effects in catalysis by NCs. Furthermore, we present an outlook on the prospects of ligand engineering in terms of the rational synthesis of functional NCs.

  Figure 1

  

  Figure 1.  Schematic diagram of nanocluster containing metal core and peripheral ligand shell

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  2. LIGAND-INDUCED STRUCTURAL CONTROL OF NCS

  The first scientific report on the structurally well-defined NCs can be traced back as early as in 1969^[17]. Shortly after this work, a number of NCs have been synthesized by Mingos et al. in the 1970s^[18]. Later, great advances have been witnessed in thiolate-protected NCs in the new century^[19-21]. Recently, protecting ligand scope has been extended to alkynyl^[22], calixarene^[23] and amido ligands^[24]. It has been demonstrated that the ligands have a great impact on the geometric structures of metal clusters. The type, steric hindrance and coordination preference of the ligands are the main factors influencing the structures of NCs.

  2.1 Effect of the type of ligand on the structure of metal NCs: the case of thiolate and alkynyl

  Benefited from the development of synthetic methodology and the advance of instrumentation, a number of ligand-protected NCs have been identified, especially those thiolate-^[19] and alkynyl-protected^[22] ones. Thiolate is the most extensively used in NCs as it is a soft base binding strongly with soft coinage metals. Similarly, negatively-charged alkynyl forms strong metal-carbon interfaces with coinage metals. A significant difference is their binding preference: an alkynyl ligand can adopt either σ- or/and π-coordination modes, while a thiolate takes σ-binding only.

  Thiolate-protected and alkynyl-protected gold NCs with an identical metal-to-ligand ratio were found to have the same metal cores, such as in the cases of Au₃₆L₂₄^{[25, 26]}, Au₄₄L₂₈^{[25, 27]} and Au₁₄₄L₆₀^{[28, 29]} (L is a thiolate or alkynyl ligand), respectively. On the contrary, this is not what found in the case of [Au₂₅(C≡CR)₁₈]^–[30]. [Au₂₅(C≡CR)₁₈]^– has identical metal-to-ligand ratio as [Au₂₅(SR)₁₈]^– (Fig. 2a, b)^{[30, 31]}, but their structures are not the same. Both Au₂₅ clusters consist of an icosahedral Au₁₃ kernel and six dimeric Au₂L₃ (L = thiolate or alkynyl) staples, but the arrangement of peripheral motifs in [Au₂₅(C≡CR)₁₈]^– is different from that of [Au₂₅(SR)₁₈]^–, resulting in the change of the overall structure as well as the symmetry reduction from S₆ to D₃. Structural simulation shows that this arrangement difference comes from the steric hindrance caused by the rigid nature of the alkynyl ligand.

  Figure 2

  

  Figure 2.  Structures of (a) thiolated Au₂₅ and (b) alkynyl-protected Au₂₅. (c) Plot of compositions of Au_x(C≡CR)_y and Au_n(SR)_m NCs. From refs. ^[30] and ^[32]

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  Recent examples Au₄₂(C≡CR)₂₂ and Au₅₀(C≡CR)₂₆ also demonstrate the effect of the rigid nature of the alkynyl on the structures^[32]. Fig. 2c shows a plot of the compositions of Au_x(C≡CR)_y and Au_n(SR)_m with the number of gold against the number of ligands. It is noted that Au_x(C≡CR)_y usually has smaller or identical ligand-to-gold ratios in comparison with Au_n(SR)_m. These results suggest that a variety of alkynyl-protected gold NCs different from Au_n(SR)_m are yet to be discovered.

  2.2 Effect of the steric hindrance of ligand on the size and structure of clusters

  Steric hindrance of the ligand is also a key factor in tuning the size and structure of NCs. Tracy et al.^[33] reported that the core size could be altered by controlling the bulkiness of ligands in the synthesis of Au NCs, i.e., the cluster size decreases as the bulkiness of the ligand increases. Chen et al.^[34] synthesized three Au NCs by employing isomeric methylbenzenethiols (para-/meta-/ortho-MBT) under similar synthetic conditions, and found that the size of Au_n(SR)_m increases with the decrease of steric hindrance of the ligands (Fig. 3). Tsukuda and co-workers^[35] reported that the metal-to-ligand ratio of NCs could be raised by using bulkier ligands. It's worth noting that the hindrance of the thiolate ligand also plays a key role in the transformation from Au₃₈(SC₂H₄Ph)₂₄ to Au₃₆(SPh-t-Bu)₂₄^[36]. Moreover, steric effects of phosphines in affecting cluster size were also observed^[18]. These facts indicate that the size, composition and structure of the clusters can be tuned by controlling the ligand bulkiness.

  Figure 3

  

  Figure 3.  Structure-controlled synthesis of Au_n(SR)_m NCs by p-/m-/o-methylbenzenethiols. From ref. ^[34]

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  2.3 Effect of the coordination motif of ligands on the structure

  Wang et al.^[23] reported the first structurally determined calixarene-protected metal nanocluster in 2016. This silver nanocluster [Ag₃₅(H₂BTCA)₂(BTCA)(C≡CBu^t)₁₆](SbF₆)₃, (H₄BTCA, p-tert-butylthiacalix^[4]-arene), consists of 35 Ag atoms, 16 alkynyls and three thiacalixarene ligands. Notably, there are two types of coordination motifs of thiacalixarene with silver atoms in the structure, Ag₅@BTCA and Ag₄@H₂BTCA (Fig. 4). It is found later that the stability of the cluster can be enhanced by realizing coordination saturation of ligands, i.e. transformation of Ag₄@H₂BTCA to Ag₅@BTCA in [Ag₃₄(BTCA)₃(C≡CBu^t)₉(tfa)₄(CH₃OH)₃]-SbF₆^[37]. Dipyridylamide, a tridentate ligand, its multiple bridging ability can facilitate the formation of various types of binding motifs with metal atoms, which favors the generation of structure diversity^[38].

  Figure 4

  

  Figure 4.  Two types of coordination motifs of the thiacalix^[4]arene with silver atoms: (a) Ag₄@H₂BTCA. (b) Ag₅@BTCA. From ref. ^[23]

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  3. ROLES OF LIGANDS TO THE OPTICAL PROPERTIES OF NCs

  Metal NCs consist of less than 200 metal atoms exhibiting discrete energy levels in contrast to the case of large-size nanoparticles, because of quantum size effect^[19]. Strong quantum confinement effect in NCs results in intriguing molecular-like properties, such as multiple absorption bands, magnetism, enhanced photoluminescence and redox behavior^[39].

  The electronic structures of NCs are affected by ligands, leading to the change of absorption spectra^{[25, 28, 40]}. Recently, alkynyl-protected Au NCs as the counterpart of Au_n(SR)_m have been reported, which provides an ideal platform to investigate ligand effects on the optical properties of NCs. As shown in Fig. 5a-b, significant differences were observed between the alkynyl-protected Au₃₆, Au₄₄, and their thiolated-counterparts^[25-27], respectively. These findings strongly suggest that the electronic structures of Au NCs can be influenced by surface ligands of different nature. Thiolated-Au₂₅ as the most extensively studied gold nanocluster, its UV-vis spectrum displays three absorption bands in visible region (Fig. 5c)^[31]. Alkynyl-stabilized Au₂₅ shows similar optical features to those of [Au₂₅(SR)₁₈]^–, but red shift was observed in [Au₂₅(C≡CR)₁₈]^– (Fig. 5c)^[30]. Theoretical calculation shows that the deep occupied (HOMO-5) and unoccupied (LUMO+7) orbitals of [Au₂₅(C≡CR)₁₈]^- are significantly constituted by C(p) atomic orbitals of alkynyl ligands (Fig. 5d). Therefore, the electronic structure is disturbed by surface ligands in [Au₂₅(C≡CR)₁₈]^–, which leads to the observation of different absorption spectrum.

  Figure 5

  

  Figure 5.  Absorption spectra of (a) [Au₃₆(C≡CPh)₁₈] vs [Au₃₆(SR)₁₈]^–, (b) [Au₄₄(C≡CPh)₂₈] and [Au₄₄(SR)₂₈] (c) [Au₂₅(C≡CR)₁₈]^– vs [Au₂₅(SR)₁₈]^–. (d) Kohn-Sham molecular orbital energy levels diagram and the associated populations of atomic orbitals in each KS molecular orbital for a model compound [Au₂₅(C≡CR)₁₈^–. From refs. ^[25] and ^[30]

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  In addition, the electronic structures of NCs can be influenced by changing the functional groups of the ligands. It was found that the optical properties of aromatic ligand-protected NCs are different from those protected by aliphatic ligands. Das et al.^[41] reported that cyclopean-tane-thiolato-protected Au₃₆ has a blue-shift in the absorption spectrum in comparison with 4-tertbutylbenzene thiolated one. Similar results were also observed in thiolate-protected Au₂₅^[42]. Xie and co-workers^[43] found that the optical properties of thiolated Au₂₅ can be affected by ligands with different charges (positive, negative or neutral group).

  In addition, the photoluminescence properties of NCs were also strongly influenced by protecting ligands. Wu et al.^[44] found that the fluorescence of thiolated Au₂₅ clusters can be enhanced by employing ligands with electron-rich atoms (e. g., N, O) and groups (e.g., -COOH, NH₂). Similarly, the luminescence intensity of Au₃₆(SR)₂₄ (SR = SC₂H₄Ph, SPhF, SPhBr, cyclopentanethiol, respectively) was observed to increase with the decrease of electronegativity of thiols^[45]. Tsukuda et al.^[46] also reported an interesting result of the PLQY on Au₂₂(C≡CR)₁₈NCs depended significantly on the R group of ligands.

  4. LIGAND EFFECTS IN CATALYSIS BY METAL NCS

  The successes in the synthesis of well-defined metal nanoclusters with atomic precision provide great opportunities to investigate catalytic properties of NCs in terms of understanding the structure-property relationships. Extensive efforts have been made in studying size effects and doping effects in catalytic reactivity of NCs.

  Surface ligands not only provide protection for metal core, but also play a crucial role in regulating catalytic properties of clusters. Zheng et al.^[47] reported an interesting result that the ligand-capped cluster Au₃₄Ag₂₈(PhC≡C)₃₄ shows better activity in the hydrolytic oxidation of organosilanes to silanols than the bare cluster Au₃₄Ag₂₈. Zhao et al.^[48] also found that the chemoselectivity can be tuned in transfer hydrogenation of nitrobenzaldehyde with removal or retention of ligands of NCs. In addition, investigations in the Ullmann hetero-coupling reaction by Jin et al.^[42] indicated that aromatic ligand-protected Au₂₅ gives the higher coversion and selectivity than those of nonaromatic thiolate-protected ones.

  Wang et al.^[49] reported a unique case, where ligand effects have been clearly addressed in terms of metal cluster catalysis. Because the only variable between the two clusters [Au₃₈(L)₂₀(Ph₃P)₄]²⁺ (L is alkynyl or thiolate) is the protecting ligand, their significantly different catalytic performance can be attributed to the ligand effects. The semihydrogenation reactions of alkynes were studied with supported alkynyl-protected and thiolate-protected Au₃₈ NCs as catalysts. As shown in Fig. 6, the alkynyl-protected Au₃₈ gives a > 97% conversion while thiolated one exhibits negligible catalytic activities (< 2%) under similar conditions. Although the underlying mechanism of ligand effects on cluster catalysis remains unclear, it is believed that the electronic structures are disturbed by alkynyl ligands, which account for the excellent catalytic performance. It highlights the great opportunity to optimize cluster catalytic performance through ligand engineering.

  Figure 6

  

  Figure 6.  Catalytic performance of the hydrogenation reaction of alkynes using alkynyl-protected and thiolate-protected Au₃₈ serve as catalysts. From ref. ^[22]

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  5. PERSPECTIVE

  Metal nanoclusters are structurally well-defined, which present ideal models for studying the structure-property relationships of related materials. Ligands not only act as protecting agents, but also play an important role in tailoring the structure, optical and catalytic properties. Ligand effects on the structures and properties of metal NCs have attracted recent attention, future efforts should be made to modulate the function of NCs through ligand engineering in a designed way. The first challenge will effectively control the atomic arrangement of metal clusters (e.g., fcc, hcp and icosahedral) through ligand design. It can be anticipated that the linear arrangement of coordination donors in polydentate ligands may facilitate the formation of the fcc-structure (small surface curvature). Secondly, it will be interesting to check the underlying driving force for structural transformation mechanism of metal NCs through the use of ligands with medium binding ability. Thirdly, it is worthwhile to assemble metal NCs into poly-dimensional cluster-based framework materials by ligand-exchange strategy. The key is to preorganize loosely coordinated ligands on the cluster precursors. Finally, for catalytic purpose it is demanded to achieve surface open sites through rational organization of surface ligands. This may be realized by employing ligands of different bulkiness (steric hindrance) to leave uncoordinated metals on the cluster surface. In summary, we believe that ligand engineering will continue to play an important role in metal cluster chemistry, and it will attract more chemists to do ligand engineering in a rational way.

  
  
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  Figure 1  Schematic diagram of nanocluster containing metal core and peripheral ligand shell

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  Figure 2  Structures of (a) thiolated Au₂₅ and (b) alkynyl-protected Au₂₅. (c) Plot of compositions of Au_x(C≡CR)_y and Au_n(SR)_m NCs. From refs. ^[30] and ^[32]

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  Figure 3  Structure-controlled synthesis of Au_n(SR)_m NCs by p-/m-/o-methylbenzenethiols. From ref. ^[34]

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  Figure 4  Two types of coordination motifs of the thiacalix^[4]arene with silver atoms: (a) Ag₄@H₂BTCA. (b) Ag₅@BTCA. From ref. ^[23]

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  Figure 5  Absorption spectra of (a) [Au₃₆(C≡CPh)₁₈] vs [Au₃₆(SR)₁₈]^–, (b) [Au₄₄(C≡CPh)₂₈] and [Au₄₄(SR)₂₈] (c) [Au₂₅(C≡CR)₁₈]^– vs [Au₂₅(SR)₁₈]^–. (d) Kohn-Sham molecular orbital energy levels diagram and the associated populations of atomic orbitals in each KS molecular orbital for a model compound [Au₂₅(C≡CR)₁₈^–. From refs. ^[25] and ^[30]

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  Figure 6  Catalytic performance of the hydrogenation reaction of alkynes using alkynyl-protected and thiolate-protected Au₃₈ serve as catalysts. From ref. ^[22]

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