Array configurations of oxometallate–Ag cluster hybrid nanocomposites

Abstract

Oxometallate (MO)-modified Ag clusters have attracted considerable research interest due to their aesthetic structures and fascinating properties. MOs can be a key factor in the organization of Ag ions into a complex entity in an ordered way. Enormous effort has been devoted to exploiting the synthetic strategy of MO–Ag cluster hybrid nanocomposites. Herein, we intend to review the recent progress in the design and construction of MO–Ag clusters with daedal architectures and potential applications. Accordingly, six different types of MO–Ag clusters are summarized mainly in terms of array configurations: (1) MOs @ Ag shells; (2) Ag core @ MOs; (3) Ag core @ MOs @Ag shells; (4) coordination polymers; (5) intercluster compounds and (6) mixed array configurations. Besides, the synergistic effects between MOs and Ag clusters are presented. Based on the current development in this area, this minireview is expected to provide a perspective on controllable preparation of MO–Ag clusters, and a basic understanding of the structure/property correlations of Ag nanoparticle-metal oxide materials, thereby assisting in building functional nanomaterials.

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1. Introduction

Controllable synthesis of metal nanocomposites is a forefront research topic in the field of metal nanoparticles. Among them, silver-transition metal oxide composites show unique physicochemical properties and potential catalytic activity, due to the interfacial effects.^1–4 However, it is still a daunting challenge to investigate the metal nanoparticle/support interfacial interactions due to the lack of real molecular models. To overcome these intrinsic drawbacks, atomically precise Ag clusters and oxometallates (MOs) are employed as building units to construct hybrid assemblies. Compared with traditional Ag nanoparticles, Ag clusters have exhibited a variety of advantages in catalytic properties,^5–7 such as quantum size effects, definite atomic arrangements and unique surface structures. MOs are molecular oxides based on high-valence early transition metals, which can be regarded as model compounds of transition metal oxides. According to the classic hard and soft acids and bases (HSAB) theory, Ag atoms as a soft acid would be preferable for soft-base organic ligands like phosphines, thiolates and alkynyls, but have a weaker tendency for inorganic oxo anions. However, the multiple negative valence states of MOs can assist in the effective charge balance of Ag cluster cations. Additionally, compared with commonly used organic molecules serving as terminal or bridging ligands on the surface of the cluster cores, MOs possess multiple oxygen atoms, leading to many more various interfacial bonding configurations and assembly modes. Therefore, despite the presence of HSABs, MOs can act as a substrate and co-assemble with Ag species effectively. Plenty of MO–Ag cluster hybrid composites have been extensively designed with novel physical/chemical properties and potential applications in the fields of optics, electricity, catalysis, etc.^8–10 There are several unique advantages of MO–Ag cluster hybrid composites (Scheme 1): (1) novel and various coordination motifs between MOs and Ag atoms can enrich the structural diversity of hybrid clusters; (2) MOs can participate in the frontier orbitals of Ag clusters and contribute to the electronic structure, endowing them with unique functionalities; (3) Ag cluster–MO hybrid composites serve as a molecular model for silver nanoparticle/metal oxide materials to advance the fundamental study in nanoscience and nanotechnology. However, one question always made researchers confused as to what specific role do MOs play in the construction of hybrid nanocomposites. Therefore, it is essential to categorize these MO–Ag clusters based on their assembly modes.

Scheme 1 The schematic diagram of the assembly modes which are classified into six types, including MO @ Ag shells, Ag core @ MOs, Ag core @ MOs @ Ag shells, coordination polymers, intercluster compounds and mixed array configurations.

So far, some reviews on MO-based Ag clusters have been published. For instance, Wang's group summarized the structures and properties of anion-templated silver clusters.⁸ Recently, Zheng and his partners published an elaborate review on POM-templated high-nuclear Ag clusters.⁹ Moreover, Zhang and his co-workers published a review focusing on Ag clusters protected by oxometallates.¹⁰ These reviews reveal that the arrangements of nanosized silver clusters and MO anions in the crystal structure can account for their synergistic effects. However, there is no comprehensive review summarizing the arrangements of structural configurations in various Ag–MO systems. Therefore, a review focusing on the categories of MO roles in the synthesis of MO-based Ag clusters will assist in understanding the structure–property and structure–functionality correlations. In this review, the assembly modes of Ag cluster–MO hybrid nanocomposites are introduced and the specific contribution of MOs to the construction of hybrid nanostructures is emphatically summarized. Meanwhile, we focused on the decisive effect of MOs on the physicochemical properties of silver clusters. Moreover, some insights into structure–effect relationships will be put forward, in terms of improving the rational design of such nanocomposites with potential applications.

2. The assembly modes of MO–Ag cluster hybrid nanocomposites

Ag clusters modified by metal-oxo module engineering have demonstrated significant effectiveness and potential in recent years. Therein, the geometric symmetry and charge distribution of MOs can influence the assembly process of hybrid clusters, and thus induce the interaction mode of MOs and Ag clusters. According to their structural characteristics, the Ag cluster–MO nanocomposites are basically classified into five categories in Fig. 1: (1) MO @ Ag shells; (2) Ag core @ MOs; (3) Ag core @ MOs @ Ag shells; (4) coordination polymers; and (5) intercluster compounds. Actually, the combination of any two kinds of cluster composites mentioned above can give rise to (6) mixed array configurations, which greatly stimulate the expansion of structural diversity in this field.

Fig. 1 Illustration of five array configurations of the MO–silver cluster kernel.

2.1. MO @ Ag shell mode

Metal-oxo modules are widely employed in an anion templating approach. The cluster nuclearity and shape can be controlled by adjusting the size and geometry of metal-oxo modules. Hence, the reported MO @ silver(I) clusters are classified into the following three subclasses based on the types of MOs: (1) mononuclear oxometallates @ Ag clusters; (2) isopolyoxoanions @ Ag clusters; and (3) heteropolyoxoanions @ Ag clusters.

2.1.1. Mononuclear oxometallates @ Ag clusters. The arrangement of multiple mononuclear oxometallates within silver nanoclusters can tailor skeleton shapes to spheres, anisotropic oblate spheroids or elongated rods. Tetrahedral anions such as CrO₄²⁻ are chosen to induce the generation of spherical Ag cages.¹¹ In 2009, Wang's group used CrO₄²⁻ as the directing agent to prepare [CrO₄@Ag₂₂(C≡C^tBu)₁₈](BF₄)₂. The shortest Ag–O distance is 2.634 Å, indicating that CrO₄²⁻ binds loosely with the Ag(I) cage.¹² By using flat PhC≡C– in place of the bulky ^tBuC≡C ligand, a new peanut-shaped [Ag₃₅(C≡CPh)₂₈(CrO₄)₂(TMEDA)₄]³⁺ cluster was isolated.¹² The double-chromate templated [(CrO₄)₂@Ag₃₅] kernel can be viewed as the fusion of two single cages of CrO₄@Ag₂₂ with five shared silver atoms. Su's group isolated a multi-shell silver cluster [Ag₅₅(MoO₄)₆(C≡C^tBu)₂₄(CH₃COO)₁₈]⁺ having a six-fold symmetry, which is templated by six [MoO₄]²⁻ anions.¹³ Besides silver–alkynyl clusters, the chromate-templating effect on silver–sulfide clusters also deserves to be explored. The reaction of {(HNEt₃)₂[Ag₁₀(SC₆H₄^tBu)₁₂]}_n with CF₃COOAg and K₂Cr₂O₇ yielded tetrahedral [Ag(CrO₄)₄@Ag₄₆(SC₆H₄^tBu)₂₄(CF₃COO)₁₈ (DMF)₄]³⁻,¹⁴ whose shell consists of a silver-centred Ag₁₁ superadamantane capped by four half-cuboctahedra, wrapping four templating CrO₄²⁻. In addition to CrO₄²⁻, MoO₄²⁻ is a tetrahedral oxyanion and widely used in directing the formation of nanosized Ag cages.¹⁵ Sun's group obtained [Ag₄(MoO₄S₄)₅ @Ag₆₆(^tBuC₆H₄S)₄₀(mbc)₁₅(H₂O)₂]³⁻ by the reaction of Ag₂O, Na₂MoO₄, and m-methoxybenzoic acid in CH₃OH/CH₂Cl₂, and found that the outer S₄ square is associated with the inner [(MoO₄)@Ag₄(MoO₄)₄] square array.¹⁶ They also reported [(MoO₄)₅@Ag₅₂S₆(^tBuC₆H₄S)₂₀(dppm)₁₀@(MoO₄)₁₀]¹⁰⁻ with the shape of an anisotropic oblate spheroid, involving templating of five MoO₄²⁻ ions.¹⁷ The above cases indicate that the arrangement of multiple tetrahedral anions inside Ag cages can control the size and shape of cluster products, as shown in Fig. 2.

Fig. 2 Illustration of the inductive effect of mononuclear oxometallates on the nuclearity and geometry of the Ag cages (M = Cr/Mo).

2.1.2. Isopolyoxoanions @ Ag clusters. Polyoxometalates (POMs) are a class of metal-oxo clusters with structural versatility. Thus, the strategy of encapsulation of POMs into silver cages gives rise to a series of fascinating structures. Among them, isopolyoxoanions containing only one kind of early transition metal ion are important family members of POMs and widely employed in directing Ag ions to gather into high-nuclear clusters of variable geometric structures.

Isopolyoxoanions usually isomerize or transform to other forms different from their original compositions and structures when enwrapped into the silver cages. The precursor [Mo₇O₂₄]⁶⁻ is labile and can be transformed into two [Mo₆O₂₂]⁸⁻ configurations with higher charge densities. [(A-Mo₆O₂₂)₂@Ag₆₀(^tBuC≡C)₃₈](OTf)₆ was prepared through the reaction of [^tBuC≡CAg]_n with AgOTf followed by the addition of [Mo₇O₂₄]⁶⁻.¹⁸ The cluster skeleton can be viewed as two [Mo₆O₂₂]⁸⁻ nuts encapsulated within peanut shell-like Ag₆₀. The reaction of the precursor [Mo₇O₂₄]⁶⁻ with AgCF₃COO and {(HNEt₃)₂[Ag₁₀(SC₆H₄^tBu)₁₂]}_n gave rise to a mango-like B-Mo₆O₂₂@Ag₅₈S₂(SC₆H₄^tBu)₃₆(CF₃COO)₁₀ cluster,¹⁹ which can be viewed as two Ag₁₀S₆ bowls capped upside and downside of a Ag₃₈S₂₆ belt in Fig. 3a.

Fig. 3 A summary of the transformation of POMs in the reported Ag clusters: (a) [Mo₇O₂₄]⁶⁻ precursor, (b) Lindqist-type precursor and (c) [α-Mo₈O₂₆]⁴⁻precursor. The C and H atoms are omitted for clarity.

The enclosure of the [A-Mo₆O₂₂]⁸⁻ anion can also be found in C_2h-symmetry Mo₆O₂₂@Ag₄₀(^tBuC≡C)₂₀(CF₃COO)₁₂ with Lindqvist-type [Mo₆O₁₉]²⁻ as the POM precursor.²⁰ As we know, there are only 6 terminal oxygen atoms in the classical Lindqvist-type anion, whereas [Mo₆O₂₂]⁸⁻ possesses 16 terminal oxygen atoms, improving its template effect. In addition to the silver–alkynyl clusters above, the [Mo₆O₁₉]²⁻ polyoxoanion can undergo a reassembly for changing the nuclearity in the formation of POM-based silver thiolate nanoclusters.²¹ Gao et al. isolated a giant peanut-like [(Mo₂₀O₆₆)@Ag₆₂(S^tBu)₄₀]¹⁰⁺ cluster, comprising the Ag₆₂ shell and [Mo₂₀O₆₆]¹²⁻ polyoxoanion core generated from [Mo₆O₁₉]²⁻.²²In situ generated [α-Mo₅O₁₈]⁶⁻ from (ⁿBu₄N)₂[Mo₆O₁₉] can be captured in the interior of the shuttle-shaped Ag₃₈(^tBuS)₁₈(PhCOO)₁₄ shell (Fig. 3b).¹⁵ [(W₆O₂₁)@Ag₃₄(^tBuS)₂₆(CF₃COO)]⁺ was obtained from the reaction of ^tBuSAg, CF₃COOH, (Bu₄N)₂[W₆O₁₉] and Et₃N in methanol,²³ whose central anion [W₆O₂₁]⁶⁻ originated from the isomerization of the [W₆O₁₉]²⁻ precursor.

In addition to Lindqvist-type POMs, [α-Mo₈O₂₆]⁴⁻ is widely employed as a POM percursor to direct Ag ions to aggregate into high-nuclear clusters. However, [α-Mo₈O₂₆]⁴⁻ is easily decomposed into [MoO₄]²⁻.¹³ In 2018, Sun's group realized the isolation of Mo₇O₂₄@Ag₄₁(ⁱPrS)₁₉(p-TOS)₁₆(CH₃OH)₄ and Mo₅O₁₈@Ag₃₆(ⁱPrS)₁₈(PhSO₃)₁₂(DMF)₆.^24,25 With such a synthetic method, they reported [(Mo₈O₂₈)@Ag₄₈(p-MePhS)₂₄(CF₃COO)₁₄(H₂O)₄(DMF)₂]²⁺ encapsulating unprecedented [Mo₈O₂₈]⁸⁻, which is generated from [α-Mo₈O₂₆]⁴⁻ (Fig. 3c).²⁶ These examples further confirm that the POM precursors tend to transform into the corresponding templates with higher negative charge densities, more terminal oxygen atoms and stronger coordination ability to Ag atoms, which promotes the stabilization of the core–shell configuration. However, the flexible polyoxoanion templates often lead to unpredictable structures.

Given the fact that the synthesis of POM-templated Ag clusters always relies on luck, researchers are seeking an approach to control the silver cluster structures at the molecular level. Apart from the serendipitous transformation cases mentioned above, it is notable that some specific stimuli can trigger the structural transformation of POM-based Ag clusters. Gao et al. used polyoxovanadate as the template anion to construct dodecahedrane-like [Ag₃₀(^tBuS)₂₀V^V₁₀V^IV₂O₃₄],²⁷ and found that the thiolate Ag cluster can respond to external acid–base stimuli. With its Ag nanosized cage intact, the polyoxovanadate core undergoes transformation to a D_3d configuration upon acidification, and reverts to its original C_2h structure upon addition of a base (Fig. 4a). As shown in Fig. 4b, Sun et al. found that PhCOOH can induce the synergetic core–shell transformation involving enlargements of the inner anion template (Mo₆O₂₂⁸⁻ → Mo₈O₂₈⁸⁻) and the external silver(I) thiolate cage (Ag₄₄(ⁱPrS)₂₀(PhCOO)₁₆(CH₃CN)₂ → Ag₅₀(ⁱPrS)₂₄(PhCOO)₁₈(CH₃CN)₂). This is the first example of a smaller-to-larger silver nanocluster conversion via anion template growth. The proposed breakage–growth–reassembly mechanism based on time-dependent ESI-MS can provide inspiration for the design of new POM/organosilver(I) functional materials.²⁸

Fig. 4 (a) The transformation of polyoxovanadate cores inside the [Ag₃₀(^tBuS)₂₀]¹⁰⁺ shell. Reproduced with permission from ref. 27. Copyright 2016 Wiley-VCH. (b) Core–shell transformation from Mo₆O₂₂@Ag₄₄ to Mo₈O₂₈@Ag₅₀. Reproduced with permission from ref. 28. Copyright 2018, Nature Publishing Group. (c) The synthesis of V₂O₇@Ag₂₄ and V₁₀O₂₈@Ag₄₀ using the [V₁₀O₂₈]⁶⁻ precursor under basic and acidic conditions.

Some flexible isopolyoxoanions can undergo structural transformation into new forms adapting to the peripheral silver cage, while some isopolyoxoanions retain their original compositions and structures when participating in the formation of silver clusters. In the presence of CF₃COOH in an organic medium, the reaction of [^tBuC≡CAg]_n, CF₃COOAg and [H₃V₁₀O₂₈]³⁻ gave rise to V₁₀O₂₈@Ag₄₀(^tBuC≡C)₂₂(CF₃COO)₁₂ with C_2h symmetry.²⁰ The ellipsoidal skeleton consists of 40 silver atoms and [V₁₀O₂₈]⁶⁻. The acidic conditions can limit the decomposition of [V₁₀O₂₈]⁶⁻ anions into low-nuclearity species, but under basic conditions, [V₁₀O₂₈]⁶⁻ decomposes readily into [V₂O₇]⁴⁻, immediately. Although the same silver salt, [^tBuC≡CAg]_n and polyoxometalate salts were employed as the reactants, V₂O₇@Ag₂₄(C≡C^tBu)₁₄(CF₃COO)₆ was isolated with a smaller oxoanion inside the smaller Ag cage (Fig. 4c). Thus, the presence of acid/base sources may be an important factor to the transformation of isopolyoxoanions during the formation of POM-encapsulated silver clusters.

2.1.3. Heteropolyoxoanions @ Ag clusters. Different from isopolyoxoanions, heteropolyoxoanions are POMs formed from the fusion of different inorganic oxygenates, and its designable synthesis and controllable properties via heteroatom doping are advantageous in catalysis, electrochemistry, etc.²⁹ Hence, the introduction of heteropolyoxoanions into Ag cluster systems can influence the function of the final products through bringing in their intrinsic properties.
Keggin-structured heteropolyoxoanions as templates. In 2014, Wang et al. synthesized a silver alkynyl cluster, namely, [(PW₉O₃₄)₂@Ag₇₀ (^tBuC≡C)₄₄(H₂O)₂]⁸⁺ containing a Ag₇₀ shell and two lacunary [PW₉O₃₄]⁹⁻ templating ions by an ionothermal approach.³⁰ Ten Ag(I) ions are accommodated in the middle of the skeleton and sandwiched by the [PW₉O₃₄]⁹⁻ units. As shown in Fig. 5a, cyclic voltammetric studies on [BMIm]BF₄ (BMIm = 1-butyl-3-methylimidazolium) reveal that there is fast core–shell electronic communication between the inner [PW₉O₃₄]⁹⁻ and outer Ag ions. Using the same POMs, Zang's group prepared [(PW₉O₃₄)₂@Ag₆₇(p-F PhS)₃₆(DMAc)₂(CF₃COO)₆]·(CF₃COO)₇ ((PW₉O₃₄)₂@Ag₆₇). For this silver thiolate nanocluster, the separator is an almost flat plane of Ag₁₂, which is sandwiched by two lacunary Keggin phosphotungstate anions.³¹ The difference in the redox behavior between the CF₃COOAg precursor and (PW₉O₃₄)₂@Ag₆₇ can be observed in Fig. 5b, affected by the central [PW₉O₃₄]⁹⁻. Xie and his partners succeeded in isolating three [SiW₉O₃₄]¹⁰⁻-templated clusters, [(SiW₉O₃₄)@Ag₃₄Cu₆(^tBuC≡C)₁₈(^tBuPO₃)₉(CH₃CN)₂(H₂O)], [(SiW₉O₃₄)@Ag₃₇Cu₆(^tBuC≡C)₁₈(^tBuPO₃)₉(CH₃CN)₆]²⁺, and [(WO₆)(SiW₉O₃₄)@Ag₅₁(^tBuC≡C)₂₇(ⁿBuPO₃)₃(CH₃CO₂)₃(CH₃CN)₃]²⁻ (Fig. 5c).³² Such an observation indicates that Ag clusters with various structures can be successfully constructed through the same POM template.

Fig. 5 (a) Molecular structure (left) and cyclic voltammogram (right) of (PW₉O₃₄)₂@Ag₇₀. Reproduced with permission from ref. 30. Copyright 2012, Royal Society of Chemistry. (b) Molecular structure (left) and cyclic voltammogram (right) of (PW₉O₃₄)₂@Ag₆₇. Reproduced with permission from ref. 31. Copyright 2018 American Chemical Society. (c) Molecular structures of (SiW₉O₃₄)@Ag₃₄Cu₆, (SiW₉O₃₄)@Ag₃₇Cu₆ and (WO₆)(SiW₉O₃₄)@Ag₅₁. Reproduced with permission from ref. 32. Copyright 2018, Wiley-VCH.

Although there have been various reports on the design and synthesis of Keggin-heteropolyacid @ silver clusters, the high-dimensional assembly in this field is still rare. Sun's group achieved face-fusion transformation from single-pod pumpkin-like [(PW₉O₃₄)@Ag₅₁(ⁱPrS)₂₅(CF₃COO)₁₇(DMF)₃(CH₃OH)₃] (PW₉O₃₄@Ag₅₁) to double-pod (PW₉O₃₄)₂@Ag₇₂ by modifying the Ag shell with bridging ligands, which replaced the coordinated DMF ligands and constructed a POM@Ag cluster-based MOF.³³ As illustrated in Fig. 6, the introduction of 4,4′-bipyridine (bipy) and 1,4-bis(4-pyridinylmethyl)piperazine (pi-bipy) into PW₉O₃₄@Ag₅₁ induces the assembly of a 2D 4⁴-sql layered {[(PW₉O₃₄)₂@Ag₇₂S(ⁱPrS)₄₁(CF₃COO)₈(bipy)_5.5(CH₃OH)(H₂O)]CF₃COO}_n and a 3D pcu framework {[(PW₉O₃₄)₂@Ag₇₂S(ⁱPrS)₄₂(CF₃COO)₇(pi-bipy)_4.5(CH₃OH)]₃CF₃COO}_n, respectively. The proposed breakage–fusion conversion is similar to the transformation from Mo₆O₂₂@Ag₄₄ to Mo₈O₂₈@Ag₅₀.²⁸ Notably, the major differences appear in the Ag ligand shells, while [PW₉O₃₄]⁹⁻ remains intact during the cluster transformation and cluster-based network formation, which is distinguished from the templating-anion-conversed cases mentioned in the 2.1.2 section. The new exploration gives a probable strategy to simultaneously induce structural transformation with the POM core intact and the high-dimensional assembly of POM @ Ag via employing bridging ligands.

Fig. 6 Molecular structures of PW₉O₃₄@Ag₅₁ and (PW₉O₃₄)₂@Ag₇₂ (left), and the two simplified networks with (PW₉O₃₄)₂@Ag₇₂ as the node (yellow balls) and bipy as the linker (right). Reproduced with permission from ref. 33. Copyright 2022, Nature Publishing Group.

Waugh-structured heteropolyoxoanions as templates. Wang et al. synthesized a 64-nuclearity silver alkynyl cluster, namely, Mn^IIIMn^IV₂Mo₁₄O₅₆@Ag₆₄(C≡C^tBu)₃₈(CF₃COO)₈ (Mn^IIIMn^IV₂Mo₁₄O₅₆@Ag₆₄).³⁴ As shown in Fig. 7b, a meso POM anion [Mn^IIIMn^IV₂Mo₁₄O₅₆]¹⁷⁻ is constructed by using the Waugh-type racemic-{Mn^IVMo₇} units connected together through a {Mn^III} fragment. Furthermore, the ESR studies confirmed the existence of a mixed valence of Mn(III) and Mn(IV), and the comparison of voltammetric behaviors between Mn^IIIMn^IV₂Mo₁₄O₅₆@Ag₆₄ and [MnMo₉O₃₂]⁶⁻ can reveal that the outer silver shell also participates in the redox process (Fig. 7c and d).

Fig. 7 (a) Single-crystal X-ray structure of Mn^IIIMn^IV₂Mo₁₄O₅₆@Ag₆₄. (b) Illustration of meso-{Mn₃Mo₁₄} from the connection of D/L-{MnMo₉}. (c) The ESR spectrum of Mn^IIIMn^IV₂Mo₁₄O₅₆@Ag₆₄. (d) The voltammetric behaviors of (NH₄)₆[MnMo₉O₃₂] (black) and Mn^IIIMn^IV₂Mo₁₄O₅₆@Ag₆₄ (red). Reproduced with permission from ref. 34. Copyright 2016, American Chemical Society.

Weakley-structured heteropolyoxoanions as templates. Zuo et al. obtained [HoW₁₀O₃₆@Ag₄₂(^tBuC≡C)₂₈Cl₄]⁺ (HoW₁₀O₃₆@Ag₄₂), which has a peanut-like skeleton with HoW₁₀O₃₆ as the nuts and 42 silver(I) ions constructing the shell (Fig. 8a). The magnetic measurements reveal that HoW₁₀O₃₆@Ag₄₂ shows a similar SMM (single-molecule magnetic) behaviour to the free parent cluster [HoW₁₀O₃₆]⁹⁻, which arises from the Ho(III) ion (Fig. 8b and c).³⁵ The encapsulation via silver cages can be an effective means to separate the single SMM molecules from each other and achieve magnetic insulation. Another functional Weakley-POM can also endow the cluster composites with special physicochemical properties. Sun and coworkers employed the red-emissive Weakley-type [EuW₁₀O₃₆]⁹⁻ as the template anion to isolate and characterize rod-like [(EuW₁₀O₃₆)₂@Ag₇₂(^tBuC≡C)₄₈Cl₂]⁴⁺ (Fig. 8d).³⁶ The final cluster showed a higher fluorescence intensity than [EuW₁₀O₃₆]⁹⁻ (Fig. 8e), which can be ascribed to the confinement of [EuW₁₀O₃₆]⁹⁻ inside the Ag–alkynyl cage through Ag–O bonds, thereby reducing the energy loss caused by molecular vibrations.

Fig. 8 (a) Molecular structure of HoW₁₀O₃₆@Ag₄₂. (b) Temperature dependence of the χ_MT values at 2 kOe. (c) Temperature dependence of the in-phase (χ′) and out-of-phase (χ′′) at different frequencies. Reproduced with permission from ref. 35. Copyright 2013, Royal Society of Chemistry. (d) Molecular structure of (EuW₁₀O₃₆)₂@Ag₇₂. (e) The solid-state emission spectra of (EuW₁₀O₃₆)₂@Ag₇₂ (red) and Na₉[EuW₁₀O₃₆] (black). Reproduced with permission from ref. 36. Copyright 2018, Wiley-VCH.

The above investigation underscores the following aspects: (1) both isopolyoxoanions and heteropolyoxoanions can effectively shape the geometries of silver clusters; (2) some flexible isopolyoxoanion precursors tend to transform into template anions with stronger coordination ability to Ag atoms in silver cluster generation, whereas most of the heteropolyoxoanions can retain their original compositions and structures; (3) employing functional MOs is an efficient strategy for introducing unique physical properties into the cluster system. For example, the enhancement in fluorescence intensity is attributed to the confinement of POMs inside the Ag shell; a core–shell electronic communication is observed in the POM-templated cluster; and single-molecule magnetic POMs can be encapsulated by the silver–alkynyl cage. Thus, the rational design of POM @ Ag clusters with the target functionality requires a careful consideration for POMs.

2.2. Ag core @ MO shell mode

As oxygen-donor ligands, MOs can play roles of structure-directing units and peripheral stabilizing layers for metal cores from undesired aggregation. Thus, the universality and versatility of MO ligands in the construction of well-defined Ag clusters highly deserve to be further explored. Based on the recent advancements in Ag core @ MO nanostructures, the reported MO ligands are categorized as mononuclear oxometallate ligands, POM ligands and metalloligands.

2.2.1. Ag core @ mononuclear oxometallates. Tetrahedral MO₄⁻ (CrO₄⁻, MoO₄⁻ and WO₄⁻) can bite {Ag₃} vertex units and further regulate the overall configurations of the Ag clusters. The introduction of CrO₄²⁻ into silver–alkynyl systems gave rise to Ag₄₈(C≡C^tBu)₂₀(CrO₄)₇ with a pseudo-5-fold symmetric metal skeleton.³⁷ As illustrated in Fig. 9a, the Ag₄₈ core can be viewed as a Ag₂₃ cylinder encircled by an outer Ag₂₅ shell. The two CrO₄²⁻ anions are located at the two poles of the Ag₂₃ cylindrical kernel and the additional five CrO₄²⁻ anions are anchored onto the equator region. Lu's group reported mononuclear oxometallates and phosphine ligand co-protected superatomic silver nanoclusters including Ag₂₈(dppb)₆(MoO₄)₄, Ag₂₈(dppb)₆(WO₄)₄ and [Ag₃₂(dppb)₁₂(MoO₄)₄]⁴⁺ (Fig. 9b and c).³⁸ The structure of the Ag₂₈ core possesses a pyramidal geometry with four Ag₆ facets capping the four faces of the Ag₄ tetrahedron to form a two-shell Ag₄@Ag₂₄ core. The tetrahedral MO₄²⁻ anions are mounted over four triangular faces of the inner Ag₄ tetrahedron of the Ag₂₈ core and induce the tetrahedral geometry of the final clusters. In [Ag₃₂(dppb)₁₂(MoO₄)₄]⁴⁺, an additional four Ag atoms are grafted on the O atoms of the MoO₄²⁻ anion, giving rise to four MoO₄Ag(dppb)₃ surface motifs.

Fig. 9 View of the arrangement of MoO₄²⁻ anions on the surfaces of (a) Ag₄₈(C≡C^tBu)₂₀(CrO₄)₇, (b) Ag₂₈(dppb)₆(WO₄)₄ and (c) [Ag₃₂(dppb)₁₂(MoO₄)₄]⁴⁺.

2.2.2. Ag core @ POMs. Compared with mononuclear oxometallates, POMs with polyatomic backbones can enwrap and stabilize the Ag core more effectively. Notably, POMs play a protective role, and meanwhile enrich the physical and chemical properties of the Ag clusters, thus improving the catalytic performance.

Employment of POMs with selective charge distribution can be a pivotal step towards the Ag core @ POM configuration. As shown in Fig. 10a, Jansen and Gruber reported [CO₃@Ag₄₂ (C≡C^tBu)₂₇(CH₃CN)₂(CoW₁₂O₄₀)₂]⁺ (Ag₄₂@{CoW₁₂O₄₀}₂) formed by two Keggin [CoW₁₂O₄₀]⁶⁻ units sandwiching a toroidal shaped Ag₄₂ cluster. The upper and lower ends of the silver toroid are recessed, and it contains an outer cylinder of nine Ag₆ rings joined by their trans edges and a {Ag₆CO₃} unit located at the midpoint height of the Ag₃₆ cylinder.³⁹ Ozeki's group reported [CO₃@Ag₄₂(C≡C^tBu)₂₇(SiW₉Nb₃O₄₀)₂]⁻ (Ag₄₂@{SiW₉Nb₃O₄₀}₂), in which two Nb-substituted Keggin [α-A-SiW₉Nb₃O₄₀]⁷⁻ polyoxotungstates cap both the openings of a toroidal Ag₄₂ cluster (Fig. 10b),⁴⁰ identical to the silver toroid of Ag₄₂@{CoW₁₂O₄₀}₂. The largest structural dissimilarity between the two clusters lies in the binding sites of the Keggin polyoxometalates to the toroidal Ag₄₂ clusters. The Ag₄₂ cluster in Ag₄₂@{SiW₉Nb₃O₄₀}₂ shows affinity to the corner-shared triad of [WO₆] octahedra, while Ag₄₂@{CoW₁₂O₄₀}₂ favors assembling with the corner-shared triad. They amplified the selective doping of less positively charged Nb(V) which increased the surface negative charge of the corner-shared triad, leading to an opposite preference for the binding site compared to the unsubstituted analogue. This result indicates that the binding site of metal-oxo clusters can be modulated by tuning the surface electron density. In the two hybrid compounds, silver alkynyl and polyoxometalate moieties are symmetrically connected. The addition of [ⁿBu₄N]₅H₄[P₂W₁₅Nb₃O₆₂] to a CH₃CN solution containing ^tBuC≡CAg and AgOTf led to the formation of an asymmetrically fused polyoxometalate–silver alkynide composite cluster, [Ag₂₅(C≡C^tBu)₁₆(CH₃CN)₄(P₂W₁₅Nb₃O₆₂)] (Ag₂₅@P₂W₁₅Nb₃O₆₂), where 25 Ag atoms are selectively attached to the Nb-substituted hemisphere of the pedestal Dawson [α-P₂W₁₅Nb₃O₆₂]⁹⁻ (Fig. 10c).⁴¹ Cronin and coworkers isolated [Ag₁₈Cl@(Te₃W₃₈O₁₃₄)₂] with C₂ symmetry, whose Ag₁₈Cl fragment is supported by two superlacunary [Te₃W₃₈O₁₃₄]²⁸⁻ frameworks.⁴²

Fig. 10 Molecular structures of (a) Ag₄₂@{CoW₁₂O₄₀}₂, (b) Ag₄₂@{SiW₉Nb₃O₄₀}₂ and (c) Ag₂₅@P₂W₁₅Nb₃O₆₂.

The lacunary POM clusters have more well-defined vacant sites and higher negative charges compared to the plenary POMs. Hence, lacunary POMs are expected to assemble with electrophilic Ag clusters to generate the “Ag cluster within POM cluster” skeleton. By employing lacunary silicotungstate ligands, a series of Ag clusters encapsulated by two or more silicotungstate ligands have been designed with unique structures. Mizuno and coworkers obtained [Ag₄@(γ-H₂SiW₁₀O₃₆)₂(DMSO)₂]⁸⁻ (Ag₄@{SiW₁₀O₃₆}₂), with diamond-shaped [Ag₄]⁴⁺ stabilized in the lacunary pocket of {SiW₁₀} (Fig. 11a).⁴³ Compared to the AgOAc or POMs, Ag₄@{SiW₁₀O₃₆}₂ showed better catalytic performance in the hydrolytic oxidation of silanes to silanols. With the same Ag salt and silicotungstate ligands, they synthesized [Ag₆(γ-H₂SiW₁₀O₃₆)₂]⁸⁻ (Ag₆@{SiW₁₀O₃₆}₂) using Ph₂SiH₂ as the reducing agent.⁴⁴ The structural analysis indicates that the octahedron-shaped [Ag₆]⁴⁺ core is sandwiched by two lacunary {SiW₁₀} units (Fig. 11b). Unfortunately, Ag₆@{SiW₁₀O₃₆}₂ was quickly decomposed in organic solvents such as acetone, CH₃CN, and DMF. With such an assembly strategy, they continued to report a novel fcc-type [Ag₇]⁵⁺ core stuffed inside an unprecedented triangular hollow [Si₃W₂₇O₉₆]¹⁸⁻ framework (Fig. 11c).⁴⁵ Hence, Ag₇@Si₃W₂₇O₉₆ exhibited much higher stability than Ag₆@{SiW₁₀O₃₆}₂. Recently, Suzuki's group achieved the assembly of trefoil-propeller-shaped {Ag₂₇}¹⁷⁺ surrounded by three open-Dawson-type [Si₂W₁₈O₆₆]¹⁶⁻ units (Fig. 11d),⁴⁶ and they explored the assembly processes of Ag₂₇@{Si₂W₁₈O₃₆}₃ on the basis of mass spectrometry analysis: initially, {Si₂W₁₈} protected Ag₂ possessing large vacant sites can accommodate an additional four Ag atoms to give an octahedral Ag₆ cluster. Subsequently, hcp-type Ag₉ can assemble with such three {Si₂W₁₈-Ag₆} and then aggregate into Ag₂₇@{Si₂W₁₈O₆₆}₃. These results will enhance our current understanding of the formation mechanism of metal-oxo-cluster-protected silver clusters. Further research reveals that Ag₂₇@{Si₂W₁₈O₆₆}₃ exhibits ultrastability in organic solvents and a unique interface between the Ag core and POM shell for efficient H₂ cleavage.⁴⁷ The {Si₂W₁₈} module can provide storage for the generated protons on the negatively charged basic surface, whilst the mixed-valence {Ag₂₇} core can accommodate additional electrons. Recently, they continued to report stimuli-responsive and reversible control of the electronic states of Ag₂₇via addition of an acid or base. Because the protonation/deprotonation of [Si₂W₁₈O₆₆]¹⁶⁻ can cause changes in the electron donation from the POM ligands to the Ag nanocluster, consequently the electron density distribution changed.⁴⁸ These two works provide a new perspective on the unique redox and acid/base properties of the molecular hybrids of Ag nanoclusters and POMs.

Fig. 11 (a) Polyhedral and ball-and-stick representation of Ag₄@(SiW₁₀O₃₆)₂. Reproduced with permission from ref. 43. Copyright 2012, Wiley-VCH. (b) Polyhedral and ball-and-stick representation of Ag₆@(SiW₁₀O₃₆)₂. Reproduced with permission from ref. 44. Copyright 2013, Royal Society of Chemistry. (c) Polyhedral and ball-and-stick representation of Ag₇@Si₃W₂₇O₉₆. Reproduced with permission from ref. 45. Copyright 2020, Wiley-VCH. (d) Polyhedral and ball-and-stick representation of Ag₂₇@(Si₂W₁₈O₃₆)₃. Reproduced with permission from ref. 46. Copyright 2019, American Chemical Society.

In conclusion, in this section, the charge distribution of POMs is uneven on the surface, and the positive Ag cluster cores prefer to bind the partial surface of metal-oxo modules with higher negative charges, thus providing effective protection for the periphery of the silver clusters. However, it still remains a challenge to control the charge distribution of POMs. Therefore, the organic-molecule modification of MOs can be another efficient synthetic strategy to construct Ag core @ MO nanostructures.

2.2.3. Metalloligands. To ensure that oxometallates remain outside the Ag cores, we can construct metalloligands from the interaction of MOs and organic ligands to prevent the oxygen at one side of oxometallates from coordinating with silver atoms. Thus, metalloligands with pre-organized coordination sites can only be used as a protective group on the periphery of Ag clusters.
Mo-based metalloligands. The –OH groups of thiacalixarene can ligate Mo^VI with the assistance of the –S– bridge to form a metalloligand with more terminal O atoms for further binding to Ag sites. Mo-based metalation on thiacalixarenes can construct metalloligands with coordination sites and continuous carbon backbones, which can provide an additional stabilization effect for Ag clusters. Recently, Sun et al. used Mo^VIO₃⁻ anchored p-tertbutylthiacalix[4]arene (H4TC4A) as a metalloligand to obtain [Cl@Ag₄₂(MoO₃–TC4A)₆(RS)₁₈]⁻ (Ag₄₂) nanoclusters (Fig. 12a).⁴⁹ With such a stepwise assembly strategy, they continued to synthesize hexagonal Ag₁₈(Mo₂O₅PTC4A)₆(EtS)₆(Tos)₂ (Ag₁₈-a) and rectangular Ag₁₈S{Mo₂O₅(PTC4A)₂(MoO₂ⁿPrO)[MoO(ⁿPrO)₂]}₂(CyS)₆(Tos)₂(ⁿPrO)₂ (Ag₁₈-b) from the reaction of H4PTC4A, (ⁿBu₄N)₄(α-Mo₈O₂₆) and [RSAg]_n. As shown in Fig. 12b, a [Mo^VI₂O₅PTC4A]¹²⁻ metallamacrocycle is constructed from the ligation of PTC4A⁴⁻ to two {MoO₃} units by using sulfur atoms and phenol oxygen atoms.⁵⁰ The condensation process of the metalloligand is blocked by propan-1-ol ligation, thereby causing the structural dissimilarity between Ag₁₈-a and Ag₁₈-b. More notably, Ag₁₈-a showed high charge separation efficiency and photothermal conversion ability.

Fig. 12 Stepwise synthetic routes for (a) Ag₄₂ and (b) Ag₁₈-a and Ag₁₈-b.

Ti-based metalloligands. One of the most interesting aspects of Ti-based metalloligands lies with the fact that these titanium-oxo modules can be viewed as transferable building blocks or synthons of TiO₂. As shown in Fig. 13a, titanium-based metalloligands with trifurcate TiL₃ moieties (L = salicylate) serve as a cornerstone for the generation of a series of Ag–S clusters possessing a pyramidal geometry, capping the vertices of the {Ag₃} triangular subunits.⁵¹ Zang's group employed such a synthetic strategy to encapsulate all-carboxylate-protected 2e Ag clusters into two novel Ti-based coordination cages based on salicylic acid and its derivatives, which exhibit optical limiting effects in DMF solution and temperature-dependent photoluminescence properties.⁵² The molecular structures of (TBA)₂{Ag₆Ti₆(Sal)_x(L₁)₆(HL₁)_6−x} (Ag₆@Ti₆) and (Ag(CH₃CN)₃){Ag₉Ti₄(Sal)₁₂(CH₃CN)₃} (Ag₉@Ti₅) are presented in Fig. 13b. In addition to the TiL₃ moieties, the hollow-like Ti-oxo ring stabilized by benzoate ligands can provide sufficient oxygen sites inside the cavity for the coordination of silver ions. Zhang et al. constructed triple core–shell Ag₆@Ti₁₆@-(benzoate)₂₆ nanoclusters (Ag₆@Ti₁₆) by wrapping the subvalent Ag₆ octahedron with two macrocyclic Ti₈O₄₀ clusters through Ag–O–Ti coordination interactions (Fig. 13c).⁵³ The Ag₆ cluster inserted inside the cricoid Ti₁₆-oxo shell adopts different geometric configurations, which contribute to different optical limiting activities. However, Ag₆@Ti₁₆ still suffers from poor stability and is easily dissociated in moisture due to the inner Ag core terminated with weak CH₃CN solvents. Subsequently, the Ag₅@Ti₈ core–shell nanocluster, whose Ag₅ is terminated with ^tBuC≡C– and CH₃CN, was obtained through the introduction of alkyne ligands and silver ions into the synthetic reaction system of Ti₈O₄₀. To preclude the frailty of CH₃CN, the reaction conditions were further modulated through a replacement of acetonitrile with toluene. Consequently, the Ag₁₄ nanorod @ Ti₁₆-oxo nanoring was successfully obtained, whose Ag₁₄ core can be viewed as two pyramidal Ag₅ sandwiching a rhombus Ag₄ unit through argentophilic interactions (Fig. 13d and e).⁵⁴Ag₁₄@Ti₁₆ represents the first example of Ag clusters synergistically stabilized by alkynyl and Ti-oxo cluster shells. The robust protecting shell of Ag₁₄@Ti₁₆ accounts for its superhydrophobicity and excellent ambient stability. In summary, anchoring an organic ligand on the partial surface of Ti-oxo clusters can be an effective approach to avoid coordination to additional Ag atoms.

Fig. 13 (a) Molecular structures (top) and illustrations (bottom) of a series of Ag_n@Ti₄. Reproduced with permission from ref. 51. Copyright 2020, American Chemical Society. (b) A single Ag(I) atom or Ag cluster confined in Ti cages. Reproduced with permission from ref. 52. Copyright 2020, Springer. Molecular structures of (c) Ag₆@Ti₈, (d) Ag₅@Ti₈ and (e) Ag₁₄@Ti₁₆. Reproduced with permission from ref. 53 and 54. Copyright 2018 and 2021, Wiley-VCH.

Yb-based metalloligands. Lanthanide-oxo clusters have attracted considerable attention in view of their magnetic, catalytic, and luminescence properties. Nevertheless, little effort has been focused on the combination of Ag clusters and Ln-oxo clusters. In 2018, Xie and his coworkers presented the first Ag–Yb clusters (Fig. 14a), namely, [Cl@Ag₁₆(C≡C^tBu)₉(hfac)(CH₃CO₂)]-[Yb₃(μ₃-OH)₃(^tBuPO₃)₃(hfac)₃(CH₃OH)₃] (Ag₁₆-Yb₃) and [Cl@Ag₁₆(C≡C^tBu)₇(hfac)₂(^tBuPO₃)]₂-[Yb₆(μ₃-OH)₂(μ₃-O)₆(^tBuPO₃)₆(CH₃OH)₆(H₂O)₆] ({Ag₁₆}₂-Yb₆).³² In Ag₁₆-Yb₃, each Yb(III) atom of the cap-shaped trinuclear [Yb₃(μ₃-OH)(μ₃OH)₃]⁶⁺ subunit is coordinated by one hexafluoroacetylacetone (hfac), and each ^tBuPO₃²⁻ binds two Yb and five Ag atoms, thereby leading to the connection of a silver alkynyl cluster with a hydroxoytterbium phosphonate cluster. As shown in Fig. 14b, the octahedral [Yb₆(μ₃-OH)₂(μ₃-O)₆]⁴⁺ subunit is sandwiched by two silver alkynyl clusters, which can be viewed as a fusion of two Ag₁₆-Yb₃.

Fig. 14 Molecular structures of (a) Ag₁₆-Yb₃ and (b) {Ag₁₆}₂-Yb₆.

Based on all of the above points, the construction of such stable Ag core @ MO clusters requires robust MO shells to preclude hydrolysis reactions in moisture and abundant oxo sites to anchor Ag atoms efficiently. Additionally, incorporation of some organic ligands (alkynyls, etc.) and MO shells can prevent the exposure of Ag clusters to the humid environment, thus providing collaborative stabilization of Ag cores.

2.3. Ag core @ MO @ Ag shell mode

We have witnessed the fruitful advancements in POM-templated Ag(I) clusters, but multi-shell POM-templated superatomic Ag clusters are rarely observed in the literature. This is because achieving the fast kinetics of the reduction process by NaBH₄ is a major challenge to control the complicated assembly process of Ag^+1/0 species and metal-oxo modules in multi-shell nanostructures. Tremendous efforts have been made in developing effective and efficient synthetic methods to slow down the reduction process and obtain molecularly pure POM-templated superatomic Ag clusters.

Recently, Sun and coworkers adopted a strategy combining an anion template and solvent–intervention in the assembly of silver nanoclusters and then harvested a series of stable crystalline products. In 2018, they used a binary solvent system (CH₃OH/DMF) to obtain Ag₆@(MoO₄)₇@Ag₅₆(MoO₄)₂(ⁱPrS)₂₈(p-TOS)₁₄(DMF)₄ (Ag₆@(MoO₄)₇@Ag₅₆) possessing a seven-fold symmetric wheel-like topology.²⁴ As shown in Fig. 15a, the cluster-in-cluster structure comprises an octahedral Ag₆⁴⁺ kernel trapped in the centre of the Ag₅₆ nanowheel with seven MoO₄²⁻ anion templates around it. Two additional MoO₄²⁻ anions serve as hubcaps of the wheel. MoO₄²⁻ anions play binary roles of a template to support the outermost Ag shell and inorganic ligands. Subsequently, they adopted the same synthetic method to prepare a hierarchical multi-shell structure Ag₁₀@(MoO₄)₇Ag₆₀(^tBuC₆H₄S)₃₃(mbc)₁₈(DMF)(H₂O)₂ (Ag₁₀@(MoO₄)₇@Ag₆₀), whose Ag₆₀ shell was further protected by m-methoxybenzoic acid (mbc), ^tBuC₆H₄S and solvent molecules.¹⁶ Seven MoO₄²⁻ anions are arranged in the interstice between the Ag₁₀ core and the Ag₆₀ shell. They continued to synthesize a three-layered core–shell Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀(MoO₄)₂(CyhS)₃₆(CF₃SO₃)₁₆(DMF)₆ (Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀) with the outermost shell capped by CyhS⁻, CF₃SO₃⁻, MoO₄²⁻ and solvent molecules.⁵⁵ This work establishes a one-pot synthesis of mixed-valence Ag^0/1+ clusters with in situ-generated POMs in the interlayer. The Ag₁₀ inner core (Ag₁₀-1) of Ag₁₀@(MoO₄)₇@Ag₆₀ is constructed from two bipyramids sharing one edge with the central tetrahedron, whilst the fcc-structured Ag₁₀ core (Ag₁₀-2) captured in Ag₁₀@(Mo₇O₂₆)₂@Ag₇₀ can be seen as a fusion of two single-edge opened Ag₆ octahedra sharing an edge. In addition to the Mo-based POMs, they used WO₄²⁻ as the MO percursor to obtain Ag₁₀@(W₇O₂₆)₂@Ag₇₄S₂(ⁱPrS)₄₀(ⁿPrCOO)₁₈ (Ag₁₀@(W₇O₂₆)₂@Ag₇₄-a) and Ag₁₀@(W₇O₂₆)₂@Ag₇₄S₂(ⁱPrS)₄₀(PhCOO)₁₈ (Ag₁₀@(W₇O₂₆)₂@Ag₇₄-b) displaying silver shell isomerism.⁵⁶ The two clusters share the same innermost Ag₁₀ core (Ag₁₀-3), which can be considered as an Ag₆ octahedron capped by four additional silver tetrahedra. Ag₁₀-3 is symmetrically wrapped by two crescent-shaped [W₇O₂₆]¹⁰⁻units, and the Ag₁₀@(W₇O₂₆)₂ fragment is enclosed in a silver shell shaped as flat-headed (Ag₁₀@(W₇O₂₆)₂@Ag₇₄-a) and cuspidal prolate spheres (Ag₁₀@(W₇O₂₆)₂@Ag₇₄-b), respectively. Interestingly, PhCOOH can induce a one-way transformation from Ag₁₀@(W₇O₂₆)₂@Ag₇₄-a to Ag₁₀@(W₇O₂₆)₂@Ag₇₄-b (Fig. 15b), which represents the first example of carboxylate-stimulated shell isomerism in silver nanoclusters. In the preparation of the five multi-shell nanoclusters mentioned above, the aldehyde group of DMF can not only offer O donors with coordination affinity for Ag atoms, but also function as a mild reductant, thus leading to a kinetics-controlled growth course during assembly. Based on all the above, this solvent-intervention approach yielded a series of complicated multi-shell nanostructures containing reductive silver cores, thus providing a deeper insight into silver nanocluster assembly, flexibility and reactivity.

Fig. 15 (a) A summary of Ag clusters adopting the Ag core @ MO @ Ag shell mode. (b) Structural conversion of Ag₁₀@(W₇O₂₆)₂@Ag₇₄-a and Ag₁₀@(W₇O₂₆)₂@Ag₇₄-b. (c) The inner Ag cores of these Ag clusters. Reproduced with permission from ref. 24. Copyright 2018, Nature Publishing Group. Reproduced with permission from ref. 16. Copyright 2019, Wiley-VCH. Reproduced with permission from ref. 55 and 56. Copyright 2019, Royal Society of Chemistry.

2.4. Coordination polymers

Based on all of the above results, various core–shell MO–Ag composite clusters have been successfully constructed. Besides this, MOs and Ag clusters can be facile candidates for generating coordination polymers through Ag–O bonds. The effort to realize the co-assembly of POMs and Ag clusters into coordination polymers aims to discover new properties due to cooperative effects.^57,58 Unambiguous determination of the structures of such polymers is critically important to understand inherent assembly rules, although it is a big challenge due to the difficulties in obtaining X-ray quality single crystals. As shown in Fig. 16a and b, Jansen and Gruber prepared the first chain-like polymeric compounds {[Ag₁₄(C≡C^tBu)₈(C₃H₇NO)₁₀][Ag₁₂(C≡C^tBu)₆Cl(DMF)₁₀]H[P₂W₁₈O₆₂]₂}_n and {[Ag₁₆(C≡C^tBu)₁₁(CH₃CN)₆][P₂W₁₈O₆₂]}_n in 2011, which are constructed from the linkage of the Wells–Dawson [P₂W₁₈O₆₂]⁻ and the silver alkynyl cluster building blocks through direct Ag–O–W bridges.⁵⁹ These results indicated that the O-donor inorganic ligands can indeed act as bridging ligands in the formation of higher dimensional Ag cluster-based frameworks. By using Lindqvist-type [Mo₆O₁₉]²⁻ in place of [P₂W₁₈O₆₂]⁶⁻, a two-dimensional infinite sheet {[Ag₁₁(ⁱBuS)₉(CH₃CN)](Mo₆O₁₉)}_n was constructed by connecting thiolate-bridged Ag₁₁-based chain-like motifs and [Mo₆O₁₉]²⁻ anions through Ag–O coordination interaction (Fig. 16c).⁶⁰ Zang’ group introduced [Mo₆O₁₉]²⁻ into Ag–S systems to synthesize another coordination polymer [Ag₁₀(S^tBu)₆(CH₃CN)₈(Mo₆O₁₉)₂]_n (Fig. 16d),⁶¹ which exhibits green photoluminescence at room temperature. In these silver–MO aggregations, MOs played a special role similar to an organic linker in extending the discrete Ag clusters to the higher dimensionality.

Fig. 16 Packing diagram of (a) {[Ag₁₄(C≡C^tBu)₈(C₃H₇NO)₁₀][Ag₁₂(C≡C^tBu)₆Cl(DMF)₁₀]H[P₂W₁₈O₆₂]₂}_n, (b) {[Ag₁₆(C≡C^tBu)₁₁(CH₃CN)₆][P₂W₁₈O₆₂]}_n, (c) {[Ag₁₁(ⁱBuS)₉(CH₃CN)](Mo₆O₁₉)}_n and (d) [Ag₁₀(S^tBu)₆(CH₃CN)₈(Mo₆O₁₉)₂]_n.

2.5. Interclusters

When the coordination sites on the Ag cluster surface are occupied by organic ligands, the formation of Ag–O bonds meets with obstruction, resulting in inaccessibility to POM–Ag-based MOFs. Herein, Ag clusters with positive charges can co-assemble with negatively charged POMs through electrostatic attraction to form well-defined intercluster compounds. Well-ordered crystalline arrangements would demand a selection of suitable building blocks with long-range Coulomb interactions and a size matching relationship. Jansen and coworkers obtained a series of [Ag₁₄(C≡C^tBu)₁₂]-POMs (POM = [W₆O₁₉]²⁻, [PW₁₂O₄₀]³⁻, and [SiMo₁₂O₄₀]⁴⁻), in which O_h-symmetry {Ag₁₄(C≡C^tBu)₁₂} fragments are stacked with POM anions into electrostatic force-induced assemblies. POMs can contribute much more than counterions in charge balance. Employing functional POMs is expected to endow the intercluster compounds with special properties. Wang's group chose [Ag₆₂S₁₃(S^tBu)₃₂]⁴⁺ and [Mo₆O₁₉]²⁻ as building units for Ag₆₂-(Mo₆O₁₉)₂ (Fig. 17a).⁶² As depicted in Fig. 17b, the charge-transfer behavior was firstly observed and presented as an emission switching property. Besides Ag(I) cluster-based assemblies, Zhu's group isolated Ag nanocluster-based assemblies exhibiting crystalline-state optical properties through manipulation of [Pt₁Ag₂₈(SAdm)₁₈(PPh₃)₄]²⁺ and POM counterions (Fig. 17c and d).⁶³ It is worth noting that the same [Mo₆O₁₉]²⁻ precursors can connect Ag₁₀(S^tBu)₆(CH₃CN)₈ ⁶¹ through Ag–O bonds to form coordination polymers in section 2.4. It is virtually impossible for {Pt₁Ag₂₈(SAdm)₁₈(PPh₃)₄} to accommodate [Mo₆O₁₉]²⁻ due to the steric hindrance of the bulky thiolate ligands.

Fig. 17 (a) Packing diagram and (b) luminescence change induced by CHCl₃ of [Ag₆₂S₁₃(S^tBu)₃₂][Mo₆O₁₉]₂. Reproduced with permission from ref. 62. Copyright 2012, Wiley-VCH. (c) Structures and the optical absorptions and (d) emissions of Pt₁Ag₂₈–Mo₆O₁₉, Pt₁Ag₂₈–W₆O₁₉ and Pt₁Ag₂₈–PW₁₂O₄₀. Reproduced with permission from ref. 63. Copyright 2021, American Chemical Society.

2.6. Mixed array configurations

Virtually, MOs can play dual roles of a peripheral ligand and templating agent in the synthesis of Ag clusters. As depicted in Fig. 18a, Mak et al. obtained [(O₂)V₂O₆]₃@Ag₄₃(C≡CPh)₁₉@[(^tBuPO₃)₄V₄O₈]₃ ([(O₂)V₂O₆]₃@Ag₄₃@[(^tBuPO₃)₄V₄O₈]₃) through the reaction of polyoxovanadates with AgC≡CPh, AgNO₃, and ^tBuPO₃H₂ in the presence of H₂O₂. It is noteworthy that the surface [(^tBuPO₃)₄(V₄O₈)]⁴⁻ moiety is connected to the peroxo group in [(O₂)V₂O₆]⁴⁻ to construct [(^tBuPO₃)₄(V₄O₈)(O₂)(V₂O₆)]⁸⁻, which penetrates the Ag₄₃ skeleton.⁶⁴ Later, they continued to report (^tBuPO₃)₄V₄O₈-capped {[(O₂)V₂O₆]₂@Ag₃₆(C≡C^tBu)₁₂[(^tBuPO₃)₄V₄O₈]₂(^tBuPO₃)₂(NO₃)₇(Py)(DMF)}³⁺ ([(O₂)V₂O₆]₂@Ag₃₆@[(^tBuPO₃)₄V₄O₈]₂),⁶⁵ encapsulating a pair of [(O₂)V₂O₆]⁴⁻ (Fig. 18b). Shortly, they harvested [(V₂O₇)₂@Ag₄₄(C≡C^tBu)₁₄@(V₃₂O₉₆)] ((V₂O₇)₂@Ag₄₄@(V₃₂O₉₆)),⁶⁶ in which a pyrovanadate-templated globular Ag alkynyl cluster is circumscribed by macrocyclic polyoxovanadate [V₃₂O₉₆]³²⁻ (Fig. 18c). As shown in Fig. 18d, MoO₄²⁻ anions simultaneously act as central templates and exterior ligands in [MoO₄@Ag₂₄(MeC₆H₄S)₁₂(dppf)₆(MoO₄)₄]²⁺ (MoO₄@Ag₂₄@(MoO₄)₄).²¹ Ulteriorly, MO₄²⁻ can aggregate into polyoxoanion templates to stabilize a silver cage. Sun's group obtained a rod-like (Mo₆O₂₂)₂@Ag₇₆(MeOC₆H₄S)₂₈(dppm)₈(MoO₄)₁₆ ((Mo₆O₂₂)₂@Ag₇₆@(MoO₄)₁₆) cluster with an S₄-symmetry kernel.¹⁷ The 76-nucleus nanocluster is templated by two bicubane Mo₆O₂₂⁸⁻ anions in situ generated from MoO₄²⁻ anions, and the outer silver shell is protected by other MoO₄²⁻ anions (Fig. 18e). (MoO₄)₅@Ag₅₂S₆@(MoO₄)₁₀¹⁷ mentioned in section 2.1.1 and Ag₆@(MoO₄)₇@Ag₅₆@(MoO₄)₂ ²⁴ in section 2.3 also display such structural types. Dual roles of MOs can also be observed in high-dimensional frameworks. Sun and his partners isolated enantiomeric chiral 3D frameworks of [CrO₄@Ag₂₀(ⁱSPr)₁₀(Cr₂O₇)₂(COOCF₃)₄(DMF)₄]_n through spontaneous resolution, encapsulating one CrO₄⁻ as a template, and two Cr₂O₇²⁻ anions serve as bridges to connect the neighboring CrO₄@Ag₂₀ node (Fig. 18f).¹⁴

Fig. 18 Molecular structures of (a) [(O₂)V₂O₆]₃@Ag₄₃@[(^tBuPO₃)₄V₄O₈]₃, (b) [(O₂)V₂O₆]₂@Ag₃₆[(^tBuPO₃)₄V₄O₈]₂, (c) (V₂O₇)₂@Ag₄₄@(V₃₂O₉₆), (d) MoO₄@Ag₂₄@(MoO₄)₄ and (e) (Mo₆O₂₂)₂@Ag₇₆@(MoO₄)₁₆. (f) Each CrO₄@Ag₂₀ subunit connected to its adjacent four neighbours by Cr₂O₇²⁻. (g) Molecular structure (left) and cyclic voltammogram (right) of SiW₁₀O₃₇@Ag₄₁-β-SiW₁₂O₄₀.

In addition to the dual roles of ligands and templates, POMs can function simultaneously as templates and counterions for Ag clusters. Wang's group obtained [α-SiW₁₀O₃₇@Ag₄₁(^tBuC≡C)₂₇(CH₃CN)₃][β-SiW₁₂O₄₀] by encapsulation of the dilacunary silicodecatungstate.⁶⁷ In the generation of Ag clusters, the precursor [γ-SiW₁₀(H₂O)₂O₃₄]⁴⁻ is transformed to [α-SiW₁₀O₃₇]¹⁰⁻ and [β-SiW₁₂O₄₀]⁴⁻, which play respective roles of templating agents and counterions. As illustrated in Fig. 19, interesting electrochemical behaviours can be observed with the stable Ag₄₁ cage as an electron transfer mediator and the two silicotungstate clusters as multiple electro-redox centers.

Fig. 19 Molecular structure (left) and cyclic voltammogram (right) of SiW₁₀O₃₇@Ag₄₁-β-SiW₁₂O₄₀. Reproduced with permission from ref. 67. Copyright 2016, Royal Society of Chemistry.

3. Conclusions

Overall, the formation of atomically precise MO–Ag cluster nanocomposites can facilitate the establishment of structure-effect relationships of Ag nanoparticle-transition metal oxide materials at atomic scale. The minireview is concluded by briefly outlining the roles of MOs in the formation of a variety of silver clusters. MOs act as not only templates encapsulated by silver shells but also multidentate inorganic ligands at the periphery of the silver core. Besides, MOs can be embedded into the interspace between the Ag core and Ag shell. In addition to Ag–O coordinative assemblies, the cationic Ag clusters and anionic POMs are basic units for the assembly of intercluster compounds via electrostatic forces. The sizes, charge distribution, and structural topologies of the MO/Ag clusters can influence array configurations. Moreover, ligands, solvents and the pH of the assembly environment can cause structural modulation of the final cluster composites. Furthermore, functional MOs can be an important parameter to control the physicochemical properties including electron communication, luminescence thermochromism, single-molecule magnetism, stability and optical limiting effects of the final composite clusters. Thus, research studies on MO–Ag clusters have been relatively systematic and comprehensive in terms of synthesis and properties.

Nevertheless, the specific contribution and action mechanism of MOs towards the properties of cluster composites still remain unclear to date. The deeper understanding needs further systematic research in the future. Meanwhile, despite the similarities in the physical and chemical properties of coinage metal nanoclusters (Cu, Ag and Au), researchers are still mainly keen on synthesizing MO–Ag clusters. The insights into the cooperative effects of these MO–Ag clusters can trigger the directional design and controllable synthesis strategies of functional metal-oxo-modulated Cu or Ag metal cluster nanocomposites.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (22201038, 22101048, 21971038, 22271046 and 21975044), and the Fujian Provincial Department of Science and Technology (2019H6012, 21019L3004 and 2022J05040).

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Footnote

† These authors contributed equally to this review.

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