Yusuke Horita,Mai Ishimi,Yuichi Negishi
Anion-templated silver nanoclusters: precise synthesis and geometric structure Y. HORITA et al.
Metal cluster,silver cluster,anion-templated cluster,geometric structure,synthesis method
ABSTRACT Metal nanoclusters (NCs) are gaining much attention in nanoscale materials research because they exhibit size-specific physicochemical properties that are not observed in the corresponding bulk metals. Among them, silver (Ag) NCs can be precisely synthesized not only as pure Ag NCs but also as anion-templated Ag NCs. For anion-templated Ag NCs, we can expect the following capabilities: 1) size and shape control by regulating the central anion (anion template); 2) stabilization by adjusting the charge interaction between the central anion and surrounding Ag atoms; and 3) functionalization by selecting the type of central anion. In this review, we summarize the synthesis methods and influences of the central anion on the geometric structure of anion-templated Ag NCs, which include halide ions, chalcogenide ions, oxoanions, polyoxometalate, or hydride/deuteride as the central anion. This summary provides a reference for the current state of anion-templated Ag NCs, which may promote the development of anion-templated Ag NCs with novel geometric structures and physicochemical properties.
Anion-templated silver nanoclusters: precise synthesis and geometric structure Y. HORITA et al.

Metal nanoclusters (NCs) are gaining much attention in nanoscale materials research because they exhibit size-specific physicochemical properties that are not observed in the corresponding bulk metals. Among them, silver (Ag) NCs can be precisely synthesized not only as pure Ag NCs but also as anion-templated Ag NCs. For anion-templated Ag NCs, we can expect the following capabilities: 1) size and shape control by regulating the central anion (anion template); 2) stabilization by adjusting the charge interaction between the central anion and surrounding Ag atoms; and 3) functionalization by selecting the type of central anion. In this review, we summarize the synthesis methods and influences of the central anion on the geometric structure of anion-templated Ag NCs, which include halide ions, chalcogenide ions, oxoanions, polyoxometalate, or hydride/deuteride as the central anion. This summary provides a reference for the current state of anion-templated Ag NCs, which may promote the development of anion-templated Ag NCs with novel geometric structures and physicochemical properties.

Aggregates of a few to several hundred metal atoms are called metal nanoclusters (NCs). Metal NCs are very small, with a particle size of 1–2 nm, which can give rise to novel geometric structures and discretization of electronic levels [1–13]. Related to these factors, size-specific physicochemical properties can emerge in metal NCs that are not observed in the corresponding bulk metals [14–17]. Accordingly, metal NCs have attracted much attention in academic fields and industries related to materials chemistry [18–22]. Among the metal NCs, those composed of coinage metals, such as gold (Au) [23–85], silver (Ag) [86–103], and copper (Cu) [104–107], are particularly promising as novel functional materials because of their high stability. In particular, Ag NCs have been increasingly studied in recent years because of their luminescence with high quantum yield [108] and the specific catalytic ability for carbon dioxide (CO2) reduction [109]. These Ag NCs can be synthesized with atomical precision using ligands, such as thiolate (SR) [110–115], phosphine (PR3) [116–118], and acetylide (C≡CR) [119,120]. For ligand-protected Ag NCs containing anions at the central position (hereafter, anion-templated Ag NCs), synthesis of new NCs has been continuously reported since 1996. The anion-templated metal NCs were discovered in the synthesis of Cu NCs. In 1976, Freeman et al. [121] and Schugar et al. [122] both synthesized Cl@Cu14Cl(SC(CH3)2CH2NH2)12, where SC(CH3)2CH2NH2 is penicillamine, and the element or compound before ‘@’ is the central anion. In this case, chloride ions (Cl–), which were included in the reagents, served as the template anion. In 1993, Tang and colleagues reported on S@Cu14(SPh)12(PPh3)6 (S = sulfide ion, SPh = benzenethiolate, PPh3 = triphenylphosphine, Table 1) [123], where carbon disulfide (CS2) was used as the source of a template anion (S2−), and thus, anion-templated Cu NCs were strategically synthesized. Later, they successfully determined the geometric structure of S@Ag14(SPh)12(PPh3)8·4MeOH·13H2O (MeOH = methanol, H2O = water) [124], which is the first report on anion-templated Ag NCs. In 2009, Wang and colleagues reported the synthesis of [CO3@Ag17(C≡CtBu)14]+ (CO3 = carbonate ion, C≡CtBu = tert-butylacetylene, Table 1) [125], which demonstrated that oxoanions (Ox) can also function as a template. Recently, Ag NCs encapsulating polyoxometalate (POM), which is formed by the condensation of Ox, hydride (H), or deuteride (D), have also been reported, and the types of anion species in anion-templated Ag NCs are diversifying (Figure 1). In the synthesis of Ag NCs, the anion-template method is effective for generating new structures and new functions. Specifically, 1) the size and shape of Ag NCs can be controlled, 2) the charge interactions within Ag NCs can be well stabilized, and 3) novel functions can be added to Ag NCs, all depending on the selection of the central anion. Notably, several reviews have focused on anion-templated Ag NCs [126–130], but these reviews are limited and do not comprehensively summarize the synthesis methods and geometric structures obtained using all of the aforementioned anions (halide (X), chalcogenide (X’), Ox, POM, and H/D). In this review, we categorize anions into five types, namely X, X’, Ox, POM, and H/D, and describe the synthesis and geometric structures of anion-templated Ag NCs encapsulating these anions. The paper discusses the influences of central anions on the geometric structure of anion-templated Ag NCs by comparing representative anion-templated Ag NCs. Owing to space limitations, all the chemical compositions of the reported anion-templated Ag NCs that are not described in detail are summarized in Tables 2–6. This summary aims to provide readers with the latest knowledge on anion-templated Ag NCs, serving as design guidelines for the creation of novel structures. In Section 2, we describe the synthesis methods for anion-templated Ag NCs. In Section 3, we describe the results of the synthesis and geometric structures of the anion-templated Ag NCs while categorizing them by anion species. After a short conclusion in Section 4, the challenges and future avenues for the development of anion-templated Ag NCs are described in Section 5.

Anion-templated Ag NCs are generally synthesized by mixing Ag salts and ligand compounds in solution and adding a reducing agent or other agents to the solution. The addition of a reducing agent is not essential, and in some cases, anions can be formed without a reducing agent [131]. SR, PR3, and C≡CR are frequently used as ligands because of their high affinity to Ag. During synthesis, a precursor, which induces the formation of a template anion, is generally added. However, the addition of precursors is also not essential, and in some cases, anions can be formed without adding precursors, as in the case of Cl@Cu14(SC(CH3)2CH2NH2)12 described in Section 1. In both cases, anion-templated Ag NCs cannot form unless the conditions are conducive to the generation of a central anion. In most cases, anions are formed by stirring the mixture to induce a chemical reaction (stirring method, Figure 2(a)). The solvothermal (Figure 2(b)) and ultrasonication methods (Figure 2(c)) are also often used. The stirring method is the most frequently used, and the majority of anion-templated Ag NCs are synthesized by this method. In this method, specific central anions are produced by simple stirring and the addition of reaction accelerators. Stirring is generally performed at temperatures from −20 to 25 °C [131–134]. When CO32– is used as a template source, atmospheric CO2 can also be used, where CO2 is converted to CO32– by adding N,N,N’,N’-tetramethylethylenediamine (TMEDA, Table 1) to the reaction system: in this reaction, the basicity of the solvent is important to produce CO32– from H2CO3 formed by hydration of CO2 [125]. Solvothermal methods are also frequently used in the synthesis of anion-templated Ag NCs [135–140]. In these methods, the chemical species in the reaction system are thermally decomposed to produce central anions. Typically, the heating is conducted at lower temperatures (60–80 °C) than those used for the synthesis of metal nanoparticles. The reason is that high temperatures might result in the decomposition of Ox and POM. In the ultrasonication method, a chemical source of the template anion is dissolved in a solvent, which is then sonicated to induce the formation of anions. Using the ultrasonication method, the reaction solution can be heated and cooled more rapidly compared with the solvothermal method, allowing different reactions to proceed [141–144].

In the following sections, we describe the syntheses and geometric structures of anion-templated Ag NCs containing X (X@Agn NCs, Section 3.1, Figure 1(a)), X’ (X’@Agn NCs, Section 3.2, Figure 1(b)), Ox (Ox@Agn NCs, Section 3.3, Figure 1(c)), POM (POM@Agn NCs, Section 3.4, Figure 1(d)), and H/D (H/D@Agn NCs, Section 3.5, Figure 1(e)). The abbreviations for the ligands, functional groups, and solvents used in this review are summarized in Table 1.

Table 2 summarizes the reported X@Agn NCs and includes their abbreviations [131–133,145–163]. In the following section, we describe their syntheses and the effects of the different X species on the geometric structures of Ag NCs.

There are numerous reports on X@Ag8 NCs with a diverse range of central anions, including fluoride ions (F–), Cl–, and bromide ions (Br–). All these X@Ag8 NCs have been synthesized by Liu and colleagues. They reported the syntheses of 2 (F@Ag8a), 3 (F@Ag8b), 4 (Cl@Ag8a), 5 (Cl@Ag8b), 6 (Cl@Ag8c), 7 (Cl@Ag8d), 8 (Cl@Ag8e), 9 (Br@Ag8a), 10 (Br@Ag8b), 11 (Br@Ag8c), and 12 (Br@Ag8d) from 2004 to 2014 [131–133]. All of these X@Ag8 NCs were synthesized using the stirring method. For the precursors of each X, tetrabutylammonium fluoride (Bu4NF), tetrabutylammonium chloride (Bu4NCl), tetrabutylammonium bromide (Bu4NBr), benzyltriethylammonium chloride ((PhCH2)Et3NCl), or tetraphenylphosphonium bromide (PPh4Br) were used, and the central X anion is formed by their dissociation in the reaction process. These eleven X@Ag8 NCs all contain the same number of Ag atoms, and their geometric structures are similar; they all have a distorted cubic framework composed of eight Ag atoms, with X located at the central position (Figure 3). However, the X–Ag distance differs slightly depending on the central X anion. For example, in 2 (F@Ag8a, Figure 3(a)), the average X–Ag distance from the central X anion to the surrounding core Ag is 2.70 Å, whereas the distance is 2.89 Å in 7 (Cl@Ag8d, Figure 3(b)). There is also a difference in the average Ag–Ag distance within the Ag8 core between 2 and 7; it is 3.13 Å for 2 (F@Ag8a, Figure 3(a)) and 3.34 Å for 7 (Cl@Ag8d, Figure 3(b)). These results indicate that a smaller central X anion results in a more contracted X@Ag8 NCs framework. Furthermore, only four Ag atoms form bonds with the central F– in 2 (F@Ag8a), whereas eight Ag atoms bond to the central Cl– in 7 (Cl@Ag8d). Thus, a different type of central X anion induces a slight difference in the size of the X@Ag8 NC framework and the number of bonds within the framework. Moreover, in the synthesis performed by Liu and colleagues, under conditions without an anion source, one-dimensional structures of [Ag5(S2P(OiPr)2)4]n(PF6)n were formed instead of X@Ag8 NCs. However, it has been shown that the addition of an anion source to the resulting [Ag5(S2P(OiPr)2)4]n(PF6)n leads to the formation of 3 (F@Ag8b), 8 (Cl@Ag8e), and 12 (Br@Ag8d) (Figure 4).

The synthesis of X@Ag14 NC was reported by Rais and colleagues in 2001 [147] and 2002 [148]. These X@Ag14 NCs are pioneering examples of Ag NCs containing X. In Table 2, 20 (F@Ag14), 21 (Cl@Ag14a), 22 (Cl@Ag14b), and 23 (Br@Ag14) correspond to these X@Ag14 NCs, most of which were synthesized by the stirring method. In the synthesis of 21 (Cl@Ag14a), Ag-containing polymer compound [Ag(C≡CtBu)]n was used as an Ag precursor, whereas silver tetrafluoroborate (AgBF4) was used as an Ag precursor in the synthesis of 20 (F@Ag14), 22 (Cl@Ag14b), and 23 (Br@Ag14). The F– in 20 (F@Ag14) was generated by the hydrolysis reaction [164,165] of the Ag precursor (AgBF4). For 21 (Cl@Ag14a) and 22 (Cl@Ag14b), the Cl– was generated by the dissociation of the chloroform (the reaction solvent) and Bu4NCl added as the Cl– source, respectively. In the synthesis of 23 (Br@Ag14), Bu4NBr was added as the Br– source. Figure 5A shows the geometric structures of 20 (F@Ag14, Figure 5A(a)), 21 (Cl@Ag14a, Figure 5A(b)), and 23 (Br@Ag14, Figure 5A(c)), as determined by single-crystal X-ray diffraction (SC-XRD). All of the X@Ag14 NCs have a cubic framework consisting of eight Ag atoms, with an additional Ag atom on each of the six faces of the cube. This results in the construction of a rhombic dodecahedral Ag framework. Similar to X@Ag8 NCs, the size of the Ag framework in X@Ag14 NCs varies depending on the X at the central position. For example, for 20 (F@Ag14), 21 (Cl@Ag14a), and 23 (Br@Ag14), the average X–Ag distances are 2.91, 2.97, and 2.99 Å, respectively, and the average Ag–Ag distances are 3.14, 3.22, and 3.24 Å, respectively. The X–Ag distances in these X@ Ag14 NCs (X = F, Cl, and Br) are longer than those in the AgF (2.46 Å) [166], AgCl (2.77 Å) [167], and AgBr (2.89 Å) salts [167], indicating that the free volume exists in the Ag14 cage. The free volume decreases with the increasing ionic radius of X. For 20 (F@Ag14), 22 (Cl@Ag14b), and 23 (Br@Ag14), electrospray ionization mass spectrometry (ESI-MS) has been performed, in addition to SC-XRD. Characteristic peaks appear in the spectra of 22 (Cl@Ag14b) and 23 (Br@Ag14) (Figure 5B), which means that these NCs can maintain their chemical composition in solution. In contrast, no peak has been observed at the position that can be attributed to 20 (F@Ag14), implying that 20 (F@Ag14) degrades in solution.

The synthesis of X@Ag16 NC was also often reported. In Table 2, 27 (Cl@Ag16a), 28 (Cl@Ag16b), 29 (Cl@Ag16c), 30 (Cl@Ag16d), and 31 (Cl@Ag16e) correspond to these X@Ag16 NCs. Among them, 27, 28, 30, and 31 were prepared by the stirring method, whereas 29 was prepared by the ultrasonication method. Figure 6 shows the geometrical structure of 29 (Cl@Ag16c). This NC encapsulates a template chloride anion, with argentophilic Ag⋯Ag bond distances in the range of 2.703–3.286 Å. The chloride ion coordinates to three silver atoms in this structure. The X@Ag16 core has ellipsoid structure. There structures are largely different from the above-described X@Ag8 and X@Ag14 NCs. Thus, the structure of X@Agn core strongly changes depending on the nymber of Ag atoms.

At the end of this section, we introduce the syntheses of X@Agn NCs that are different from the Ag NCs described in Sections 3.1.1 and 3.1.2. The first example is the I@Agn NC, containing an iodide ion (I–). Representative examples of I@Agn NCs include 13 (I@Ag10) [133], 14 (I@Ag11a), 15 (I@Ag11b) [157], and 17 (I@Ag12) [145] (Figure 7A). All of these I@Agn NCs have been synthesized by Liu and colleagues. They used tetrabutylammonium iodide (Bu4NI) as the I– precursor and the stirring method for synthesis. In the syntheses of 13 (I@Ag10) and 14 (I@Ag11a), although the same ligands were used, an excess of one equivalent of the Ag precursor was added in the synthesis of 14 (I@Ag11a) over the case of 13 (I@Ag10). They tracked the effect of this excess Ag precursor on the chemical composition of the product by multinuclear (1H and 31P) magnetic resonance spectroscopy (31P {1H} NMR). The results demonstrated that 14 (I@Ag11a) begins to form approximately 5 minutes after initiation of the reaction (Figure 7B). Comparing the geometric structures, 13 (I@Ag10) has a cage structure with one Ag atom missing from the Ag11 cage of 14 (I@Ag11a). The addition of an extra Ag precursor caused the addition of an Ag atom to the missing site, resulting in the conversion of 13 (I@Ag10) to 14 (I@Ag11a). From this observation, they suggested that NCs with one Au atom added to 13 (I@Ag10) could be obtained by adding the compound that would serve as an Au precursor to the solution of 13 (I@Ag10). The second example is the X2@Agn NC, in which two X anions are encapsulated within one Ag framework. Liu and colleagues reported the syntheses of 16 (Br2@Ag12), 18 (I2@Ag12a), and 19 (I2@Ag12b) [146] in 2013, and Xie and Mak reported the synthesis of 36 (Cl2@Ag21) [152] in 2012 (Table 2). These X2@Agn NCs are generally similar in shape, and all of them have peanut- or butterfly-shaped frameworks composed of Ag and S atoms (Figure 8). The average X–Ag distance in 16 (Br2@Ag12) is 2.80 Å, which is longer than the average X–Ag distance (2.98 Å) in 18 (I2@Ag12a), protected by the same ligand. This indicates that the X–Ag distance also varies with the size of the central X anion in the case of X2@Agn NCs, similar to X@Ag8 and X@Ag14 NCs.

In this section, we describe the X’@Agn NCs that encapsulate X’ at the central position. Table 3 summarizes the representative X’@Agn NCs [124,133–135,137,143,144,168–182]. Again, to discuss the effects of the central X’ species on the geometric structure of the X’@Agn NCs, we limit the examples to the X’@Ag9 NCs and X’@Ag14 NCs.

For X’@Ag9 NCs, the synthesis of 42 (S@Ag9) and 43 (Se@Ag9) with S2– or selenide ion (Se2–) at the central position has been reported by Liu and colleagues in 2017 [134] (Table 3). Both X’@Ag9 NCs were synthesized by mixing the Ag precursor, tetrakis(acetonitrile)copper(I) hexafluorophosphate (Ag(MeCN)4PF6), and the ligand precursor, chalcogen-containing ammonium salt (NH4X’2P(OEt)2, X’ = S or Se), in acetone, followed by the addition of NaX’H (X’ = S or Se), which was then stirred at a low temperature (−20 or 0 °C). For the synthesis of 42 (S@Ag9), a polymer compound ([Ag5(S2P(OEt)2)4(PF6)]n) can be used as a precursor, instead of the ligand precursor. Figure 9A shows the geometric structures of 42 (S@Ag9) and 43 (Se@Ag9), as determined by SC-XRD [134] Both X’@Ag9 NCs have a geometric structure with three Ag atoms surrounding an hourglass-shaped Ag6 core. However, there are also differences between them. For example, focusing on the hourglass-shaped Ag6 core, the average Ag–Ag distance is slightly shorter in 42 (S@Ag9) than in 43 (Se@Ag9), with 3.40 and 3.66 Å, respectively. The average X’–Ag distance is also slightly shorter for 42 (S@Ag9) compared with 43 (Se@Ag9), exhibiting 2.62 and 2.83 Å, respectively. These factors lead to a slightly different distortion of the Ag9 framework (Figure 9B) The different central X’ anions also affect the electronic structures of Ag NCs [134]. As shown in Figure 9C, a peak is observed at 370 nm in the optical absorption spectrum of 42 (S@Ag9), whereas peaks appear around 400 and 450 nm in the optical absorption spectrum of 43 (Se@Ag9) (Figure 9C) [134]. These differences are due to the varying degrees of charge transfer between the central X’ anion and the Ag framework.

For X’@Ag14 NCs, 54 (S@Ag14a), 55 (S@Ag14b), 56 (S@Ag14c), 57 (S@Ag14d), and 60 (Se@Ag14) have been reported (Table 3). The former examples, 54 (S@Ag14a), 55 (S@Ag14b), and 56 (S@Ag14c) and 57 (S@Ag14d), which encapsulate S2–, were reported by Jin and colleagues [124], Fuhr and colleagues [174], and Corrigan and colleagues [156], respectively, and 60 (Se@Ag14), which encapsulates Se2–, was synthesized by Dehnen and colleagues [175]. Notably, 54 (S@Ag14a) and 60 (Se@Ag14) are protected by the same ligand, and both have been synthesized using Ag(I)–PR3 compounds as an Ag precursor. In the case of 54 (S@Ag14a), it was synthesized simply by dissolving the Ag precursor ((PPh3)2Ag(S2CSPh)) in DCM, whereas 60 (Se@Ag14) was synthesized by slowly dropping the Se2– precursor (PhSeSiMe3) into a THF solution containing the Ag precursor ((PPh3)3AgNO3) and 1,4-bis(trichlorostannyl)butane (Cl3Sn(CH2)4SnCl3). Based on these results, it can be inferred that the synthesis of Se@Agn NCs requires more complex steps than that of S@Agn NCs. Figure 10A shows the geometric structures of 54 (S@Ag14a) and 60 (Se@Ag14). Both frameworks have a geometric structure with one Ag atom on each facet of a four-sided bipyramidal core formed by the bonding of the central X’ anion with six Ag atoms (Figure 10B). In these cage structures, consisting of fourteen Ag atoms, chalcogen atoms are bridging the Ag atoms. Thus, these cage structures are different from those of 20 (F@Ag14), 21 (Cl@Ag14a), 22 (Cl@Ag14b), and 23 (Br@Ag14), described in Section 3.1.2 (Figure 5). The average X’–Ag distance in 54 (S@Ag14a) is 2.91 Å, and that in 60 (Se@Ag14) is 2.67 Å. Accordingly, 54 (S@Ag14a) has a larger free volume in the cage structure. The longer average X’–Ag bond length in 54 (S@Ag14a) indicates that the smaller ionic radii of the central S2– anion (1.91 ± 0.07 Å) [183] is responsible, compared with the larger ionic radii of Se2– (2.09 ± 0.04 Å) [183], which is unlike the X@Agn and X’@Ag9 NCs. This probably occurs because the Ag–X’ bond is relatively strong and X’@Ag14 NCs cannot easily change their cage structure with the changing ionic radius of the central anion. This also appears to be the reason why the average X’–Ag length in 54 (S@Ag14a) is relatively long.

In the following section, we describe the Ox@Agn NCs that encapsulate Ox ions at the central position. The representative Ox@Agn NCs are summarized in Table 4 [125,135–137,141–143,158,159,172,177,181,184–206]. Among them, we first describe the syntheses and geometric structures of Ag NCs that encapsulate metallic oxide ions (MOx) (Section 3.3.1) or nonmetallic oxide ions (NMOx) (Section 3.3.2). Then, we discuss how the differences in the central anions affect the geometric structures of Ox@Agn NCs (Section 3.3.3).

Typical examples of central MOx anions are chromate (CrO42–), molybdate (MoO42–), and vanadate (VO43–) (Table 4). Pioneering examples for CrO4@Agn NCs are 118 (CrO4@Ag22, Figure 11(a)) and 139 ((CrO4)2@Ag35, Figure 11(b)), reported by Wang and colleagues in 2009 [187]. The former, 118 (CrO4@Ag22), was synthesized by adding TMEDA to a methanol solution containing [AgC≡CtBu]n and AgBF4 and then adding potassium dichromate (K2Cr2O7). The latter, 139 ((CrO4)2@Ag35, Figure 11(b)), was synthesized using [AgC≡CPh]n instead of [AgC≡CtBu]n. In addition to these reports, K2Cr2O7 has often been used as a precursor for CrO42– [136,185,187]. Notably, for 118 (CrO4@Ag22), the products did not contain TMEDA. However, even these NCs cannot be synthesized without the addition of TMEDA during synthesis. On the contrary, 139 ((CrO4)2@Ag35) has a geometric structure in which TMEDA is chelated to Ag. Unlike the former, the latter forms a giant peanut-like structure because of the inclusion of two CrO42– anions. Because AgC≡CPh is less bulky than AgC≡CtBu, large NCs can form in the synthesis of the latter. These observations agree with those for SR-protected Aun NCs [207]. Regarding MoO4@Agn NCs, 108 (MoO4@Ag20, Figure 12) [186] was synthesized by adding the POM complex [Mo6O19](nBu4N)2, the precursor of MoO42–, and tetraethylammonium hydroxide (Et4NOH) to a methanol solution containing AgC≡CtBu and AgCF3CO2 (pH = 6.7, stirring method). For MoO4@Agn NCs, unlike the case of CrO4@Agn NCs, (NH4)6Mo7O24·4 H2O, MoO2(acac)2 (acac = acetylacetone), and Na2MoO4 can also be used as precursors of the central anion (MoO42–) [135,172,191]. When trifluoroacetic acid (CF3CO2H) was used instead of AgCF3CO2 to increase the acidity of the solution (pH = 3.5), Ag NCs encapsulating Mo6O19 (POM) were synthesized instead of MoO42– (MOx). These results indicate that controlling the solution pH is required for synthesizing the MoO4@Agn NCs using POM as a precursor [208]. Regarding VO4@Agn NCs, 132 (VO4@Ag26, Figure 13) was synthesized by adding ammonium metavanadate (NH4VO3) to a MeOH/MeCN mixture containing [AgC≡CtBu]n and AgCF3CO2 and heating the solution to 70 °C (solvothermal method). It has been experimentally shown that larger Ag NCs can be formed when trivalent VO43– is used as a template source than when divalent CrO42– and MoO42– are used, although the reason remains unclear.

Anions in the form of NMOx include CO32–, NO3–, X’O32–, X’O42– (X’ = S, Se, Te), and ClO4–. Representative NMOx@Agn NCs are described in Table 4. Salts containing the respective oxides are generally used as precursors for the central NMOx anion. However, there are exceptions. For example, in the synthesis of 91 (CO3@Ag17) and 94 (CO3@Ag19) [125], the central anion was produced by the conversion of atmospheric CO2 to CO32– by TMEDA. The use of atmospheric CO2 was also adapted in the synthesis of 99 (CO3@Ag20a) and 100 (CO3@Ag20b). The geometric structures of 94 (CO3@Ag19) [125] and 113 (SO4@Ag21) [187], protected by the same ligand, are shown in Figure 14. Both NMOx@Agn NCs were reported by Wang and colleagues in 2009 and were synthesized using similar compounds and methods, except for the central anion precursor. Despite the similarities in their syntheses, these NMOx@Agn NCs differ in the number of constituent Ag atoms, with 113 (SO4@Ag21) exhibiting a larger Ag cage structure than 94 (CO3@Ag19). The NMOx anions encapsulated in 94 (CO3@Ag19) and 113 (SO4@Ag21) contain three and four O atoms, respectively. Because Ag atoms form bonds with O atoms when NMOx acts as a central anion, SO42– can form bonds with a larger number of Ag atoms than CO32–. The SO42– also requires a larger ionic volume than CO32–. These factors contribute to the formation of Ag cage structures that are larger in SO4@Agn NCs than in CO3@Agn NCs.

In 2014, Liu and colleagues reported the synthesis of 87 ([SO4@Ag16]2), 88 ([SeO4@Ag16]2), 89 ([CrO4@Ag16]2), and 90 ([MoO4@Ag16]2). Although these Ox@Agn NCs include the same ligand, they have different central Ox anions. Figure 15 shows the geometric structures of 87 ([SO4@Ag16]2), 88 ([SeO4@Ag16]2), 89 ([CrO4@Ag16]2), and 90 ([MoO4@Ag16]2) [191]. These Ox@Agn NCs have similar Ag cage structures, and Ox is surrounded by sixteen Ag atoms in all of them, which consists of an icosahedral Ag12 framework with four surrounding Ag atoms. The structures of these Ox@Agn NCs are dimerized via S and Ag bonding. However, several differences also exist in the coordination mode between the central Ox anion and Ag atoms, and in the degree of distortion in the Ag cage structures (Figure 15). Specifically, differences occur in the Ag–O coordination mode, with µ4; η2, η1, η1 for 87 ([SO4@Ag16]2), µ6; η2, η2, η1, η1 for 88 ([SeO4@Ag16]2), µ6; η2, η2, η2 for 89 ([CrO4@Ag16]2), and µ8; η3, η3, η2 for 90 ([MoO4@Ag16]2). In addition, 87 ([SO4@Ag16]2), 88 ([SeO4@Ag16]2), and 89 ([CrO4@Ag16]2) have a uniform orientation of tetrahedral Ox against the Ag framework, whereas the orientation of tetrahedral Ox in 90 ([MoO4@Ag16]2) is different (Figure 15). The sizes of the central anions also differ slightly among 87 ([SO4@Ag16]2), 88 ([SeO4@Ag16]2), and 89 ([CrO4@Ag16]2), which affects their overall structure. For example, the average Ag–Ag distances in the icosahedral Ag12 framework surrounding the central anion Ox are estimated to be 4.016 Å, 4.051 Å, and 4.033 Å, respectively. The order of the average Ag–Ag distances is consistent with that of the average central atom–O distances in the central Ox anion (SO42–, 1.464 Å; SeO42–, 1.632 Å; CrO42–, 1.628 Å). Thus, the size of the Ag framework in Ox@Agn NCs depends on the size of the central Ox anion. For 90 ([MoO4@Ag16]2), the average Mo–O distance for the central MoO42– anion is 1.721 Å, which is longer than the central atom–O distance in the other Ox systems. This may be the reason why 90 ([MoO4@Ag16]2) encapsulates the central MoO42– anion in a different orientation compared with the other Ox@Agn NCs. The average Ag–Ag distance (4.052 Å) in the icosahedral Ag12 framework is also longer in 90 ([MoO4@Ag16]2) than in 87 ([SO4@Ag16]2), 88 ([SeO4@Ag16]2), and 89 ([CrO4@Ag16]2). However, the degree of extension in the icosahedral Ag12 framework in 90 ([MoO4@Ag16]2) is not significant (0.001 Å) when accounting for the difference in size between SeO42– and MoO42– anions (0.089 Å). In 90 ([MoO4@Ag16]2), the central MoO42– anion is encapsulated in a different orientation compared with the others, which appears to induce a relaxation of the extension in the icosahedral Ag12 framework. In addition, 82 ([SO3@Ag16]2), 83 ([SeO3@Ag16]2), and 84 ([TeO3@Ag16]2) were reported by Liu and colleagues in 2015 (Table 4) [192]. The geometric structures of these Ox@Agn NCs (Figure 16) are similar to 87 ([SO4@Ag16]2) and 88 ([SeO4@Ag16]2) (Figure 15). In Section 3.3.2, we explained that the central Ox anion can induce different geometric structures (e.g. planar triangular or tetrahedral) depending on the number of O atoms, and Ox@Agn NCs with different numbers of Ag atoms can be formed depending on the central Ox anion. However, 82 ([SO3@Ag16]2), 83 ([SeO3@Ag16]2), and 84 ([TeO3@Ag16]2) have the same number of Ag atoms and the same shape, despite having a different number of O atoms in the central Ox anion compared with 87 ([SO4@Ag16]2) and 88 ([SeO4@Ag16]2). This is likely because the central X’O32– anion (X’ = S, Se, or Te) has a triangular pyramidal shape, instead of a triangular shape (Figure 16). In 82 ([SO3@Ag16]2), 83 ([SeO3@Ag16]2), and 84 ([TeO3@Ag16]2), there are only three O atoms bonded to the triangular pyramid, but there are lone pairs at the apex (X’) of the pyramid, which maintains the surrounding icosahedral Ag12 framework. These results indicate that the geometric structure of Ox@Agn NCs is greatly affected not only by the number of O atoms in the Ox anion but also by the shape of the Ox anion.

Table 5 summarizes the representative POM@Agn NCs [135,137–140,152,158,159,184,186,202,204,209–228]. Many of them contain polyoxovanadate (POV@Agn NCs), polyoxomolybdate (POMo@Agn NCs), and polyoxotungstate (POW@Agn NCs), and the number of reported examples decreases in the order of POMo@Agn NCs > POV@Agn NCs > POW@Agn NCs. More than half of the POM@Agn NCs were reported after 2018, demonstrating that these anion-templated Agn NCs have been receiving attention more recently. The POM@Agn NCs are characterized by a higher number of constituent Ag atoms than other anion-templated Agn NCs, such as X@Agn NCs, X’@Agn NCs, and Ox@Agn NCs. This is because the encapsulated POMs are generally larger than X, X’, and Ox. The smallest POM@Agn NC reported is 148 (POV@Ag22) [204], which encapsulates [V2O7]4– and was found in 2022 by Jin and colleagues. Despite having the smallest size, 148 (POV@Ag22) is larger than most Ag NCs that encapsulate X or X’. Therefore, POM is effective in synthesizing Ag NCs with somewhat larger frameworks. The history of research on POM@Agn NCs is short, and currently, the largest anion-templated Ag NCs are 39 (Cl@Ag216) [155] and 147 (SO4@Ag78) [189]. However, it is expected that larger anion-templated Ag NCs will be created in the future using POM as an anion template. Mak and colleagues first reported on 149 (POV@Ag24a), 165 (POV@Ag40a), and 167 (POMo@Ag40) in 2009 [186]. POM had already attracted attention as a functional material at that time, and because the O atom of POM has a high affinity for Ag(I) ions, there had been several reports on the synthesis of POM–Ag(I) complexes before then [229–234]. However, there were no reported examples of compounds that encapsulate POM in the Ag framework until the report of Mak and colleagues. In that report, they also found that the chemical compositions of the products vary depending on the presence or absence of stabilizers in the organic solvent during the reaction. Specifically, they compared the chemical composition of the products when CF3CO2H was added as a stabilizer for POM to make the solution acidic and when AgCF3CO2 was added as a stabilizer for POM to make the solution neutral. In the former experimental condition, 165 (POV@Ag40a) or 167 (POMo@Ag40) was formed, encapsulating [V10O28]6– or [Mo6O22]8–, whereas in the latter experimental condition, 149 (POV@Ag24a) or 108 (MoO4@Ag20) was formed, encapsulating smaller POMs such as [V2O7]4– or Ox, [MoO4]2– (Table 4). Regarding 165 (POV@Ag40a) and 167 (POMo@Ag40), although they contain the same number of Ag atoms, there is a large difference in the edge-to-edge Ag–Ag distances between them (17.665 Å for 165 (POV@ Ag40a) vs. 14.309 Å for 167 (POMo@Ag40), Figure 17). This is because the Ag framework in 167 (POMo@Ag40) is regularly and compactly surrounding the central [Mo6O22]8– anion, resulting in a more spherical shape compared with that of 165 (POV@Ag40a). Many studies on the synthesis of novel POM@Agn NCs and their mechanisms have been reported since 2009. Reviews of these POM@Agn NCs have recently been reported by Li, Zheng, and colleagues [127], and Hu, Shi, Ji, and colleagues [129]. Therefore, we look at the differences between POM@Agn NCs and other anion-templated Ag NCs, which have not been described in previous reviews. For example, 29 (Cl@Ag16c, Table 2, Figure 18(a)), 98 (MoO4@Ag19, Table 4, Figure 18(b)), and 150 (POV@Ag24b, Table 5, Figure 18(c)) have increasing numbers of Ag atoms in this order [159]. This result demonstrates the trend described at the beginning of this section (Section 3.4), stating that POM encapsulation tends to yield larger-sized Ag NCs. This trend is also observed in the Ag NCs linked by phosphonic acid in the Ag framework (117 (NO3@Ag22, Figure 18(d)) and 166 (POV@Ag40b, figure 18(e))). In addition, Jin and colleagues reported the synthesis of Ag NCs (Ag@Ag16, Figure 18(f)) in which the central Cl− anion in 29 (Cl@Ag16c) is replaced by an Ag atom [159]. Ag@Ag16 has the same number of Ag atoms, but a different cage structure, compared with 29 (Cl@Ag16c). This result indicates that Ag NCs without the central anion can be synthesized by making slight modifications to the synthesis methods of anion-templated Ag NCs described in this review.

Finally, we describe Ag NCs that contain H− and D− as central anions. Table 6 summarizes the representative H/D@Agn NCs [132,235–240]. The number of reports on H/D@Agn NCs is smaller than that on other anion-templated Ag NCs, and the first report of H/D@Agn NCs was in 2010. In H/D@Agn NCs, the Ag11 framework has the highest number of constituent Ag atoms. Thus, H and D tend to yield smaller Ag NCs than those obtained with other anions. The earliest H/D@Agn NCs were 207 (H@Ag8a), 208 (H@Ag8b), 209 (H@Ag8c), 212 (D@Ag8a), 213 (D@Ag8b), and 214 (D@Ag8c), all reported in 2010 by Liu and colleagues [132,236]. Although the formation of [Ag4H]+ complexes had been previously observed in the gas phase [241], the isolation of ligand-protected H/D@Agn NCs had not been reported. Regarding the syntheses, 207 (H@Ag8a) and 208 (H@Ag8b) were obtained by stirring Ag(MeCN)4PF6 and NH4Se2P(OR)2 (R = iPr or Et) in THF for 1 h, then cooling the resulting solution to −20 °C, to which NaBH4 was added. Similarly, 212 (D@Ag8a) and 213 (D@Ag8b) were obtained, but using NaBD4 instead of NaBH4. The synthesis of 209 (H@Ag8c) and 214 (D@Ag8c) was conducted in a similar manner. Interestingly, these synthesis methods are similar to those of H-protected Ag nanoclusters [242,243], but the products were different. This reason is expected to be clarified in the future work. In addition to such direct synthesis methods, they also added NaBH4 to solutions of X@Ag8 NCs (2 (F@Ag8a) or 7 (Cl@Ag8d), table 2) and monitored the reaction process by 31P NMR spectroscopy. In both cases, the central anion was replaced by H and D within a few minutes, inducing the formation of 209 (H@Ag8c) and 214 (D@Ag8c). As an example of the H/D@Ag8 NCs, Figure 19 shows the geometric structure of 207 (H@Ag8a) [236]. The Ag atoms that form the Ag framework are classified into two categories: 1) Ag atoms that form a tetrahedron surrounding H− and 2) Ag atoms that surround the tetrahedron. Thus, the Ag8 framework is formed by four Ag atoms capping each triangular facet of a tetrahedron composed of four Ag, which is different from the distorted cubic framework of X@Ag8 NCs (Figure 3) [132]. Liu and colleagues also reported other H/D@Agn NCs, including H/D@Ag7 NCs and H/D@Ag11 NCs. For example, 216 (H@Ag11, Figure 20(a)) and 217 (D@Ag11) were reported in 2011 [237], and 201 (H@Ag7a, Figure 20(b)), 202 (H@Ag7b), 204 (D@Ag7a), and 205 (D@Ag7b) were reported in 2013 [235]. Overall, the synthetic methods are similar to those for H/D@Ag8 NCs. Specifically, for 216 (H@Ag11) and 217 (D@Ag11), AgNO3 was used as the Ag salt and NaS2CNPr2 as the ligand precursor, which were mixed according to the stoichiometric ratio, and the resulting solutions were then reduced in the liquid phase at −20 °C for 3 hours. For 201 (H@Ag7a), 202 (H@Ag7b), 204 (D@Ag7a), and 205 (D@Ag7b), Ag(CH3CN)4PF6 was used as the Ag salt and NH4Se2P(OiPr)2 or NH4S2P(OEt)2 as the ligand precursor. The resulting DCM solution was reduced with NaBH4 or NaBD4 at room temperature to obtain the H/D@Ag7 NCs. H@Ag7 NCs can also be synthesized by the reaction of X’@Ag10(E2P(OR)2)8 (X’ = Se, R = iPr or X’ = S, R = Et) with two equivalent amounts of [BH4]–. Reaction tracking by 31P NMR revealed that 1) H@Ag8 NCs, such as 207 (H@Ag8a), are formed as reaction intermediates in these reactions (Figure 21(a)), and 2) H@Ag8 NCs are obtained (regenerated) by the reaction of the obtained H@Ag7 NCs with an equal amount of Ag salt (Figure 21(b)). Figure 20(a) shows the geometric structure of 216 (H@Ag11). The Ag11 framework is formed by capping six Ag atoms against the three-way bipyramidal Ag5 structure, with the central H− anion encapsulated in only one of the triangular pyramids. Because the H is difficult to observe by SC-XRD, the location of the H is concluded by predicting the geometric structure with the minimum energy using density functional theory calculations. This is the first example of such a three-way bipyramidal molecular structure capped by six Ag atoms [237]. In 201 (H@Ag7a, Figure 20(b)), H is encapsulated in the tetrahedral Ag4 framework, and three of the four triangles in the tetrahedron are capped by Ag atoms to form the Ag7 framework. The geometric structure of 201 (H@Ag7a) is the same as that of 207 (H@Ag8a), except one capped Ag atom is removed from the Ag8 framework. The difference in the number of Ag atoms in both cases leads to a difference in the coordination between the Ag(I) and ligand. For example, in 207 (H@Ag8a), one coordination mode (μ2, μ2) occurs between Ag and the two Se atoms in Se2P(OiPr)2, whereas in 201 (H@Ag7a), two coordination modes (μ2, μ2 and μ2, μ1) occur between Ag and the two Se atoms. Removing one capped Ag atom cleaves the three Ag–Se bonds. This results in a new coordination mode (μ2, μ1) between the three Se and Ag atoms, changing the coordination pattern between the Ag(I) and ligand [235]. In the H/D@Agn NCs described above, the H/D@Ag4 core structure is included. For a long time, there has been no report on the isolation of H/D@Ag4 NCs without capping by Ag atoms. In 2021, Konno and colleagues succeeded in isolating two types of H/D@Ag4 NCs, 199 (H@Ag4Rh4) and 200 (D@Ag4Rh4) [240]. Their synthesis methods differ significantly from the syntheses of H/D@Agn NCs described above. Specifically, 199 (H@Ag4Rh4) and 200 (D@Ag4Rh4) were formed by inserting H or D into [Ag4{Rh(L-Cys)3}4][Na]8 (Ag4Rh4, Figure 22A(a)), which has an empty Ag framework. They reacted Ag4Rh4 with NaBH4 or NaBD4 in an aqueous NaOH solution, followed by the addition of an excess amount of ethanol to obtain powders of 199 (H@Ag4Rh4, Figure 22A(b)) or 200 (D@Ag4Rh4). They also examined the reaction mechanism for this reaction by X-ray absorption spectroscopy and magnetic measurements. The results suggested that this reaction did not cause the reduction of Ag(I) in [Ag(I)4]4+ by NaBH4 but caused the formation of [Ag(I)4H]3+, in which H is included at the central position. Because it is difficult to definitively conclude the position of H in the product using SC-XRD, they confirmed the presence and position of the central H anion by 1H NMR analysis (Figure 22B). The overall geometric structures of Ag4Rh4 and 199 (H@Ag4Rh4) are similar, both having a slightly distorted tetrahedral Ag4 framework. These geometric structures are also similar to (Δ)4-[Zn4O(Rh(L-Cys)3)4] [244–247]. However, closer inspection reveals that the structure is slightly more contracted in 199 (H@Ag4Rh4) than that in Ag4Rh4, which indicates that the insertion of H may contract the structure of the Ag NCs. The contraction of the framework is also observed in Cu8 NCs with H− inclusion [247, 248]. For 199 (H@Ag4Rh4, = 68.75 Hz, = 79.25 Hz), it has also been shown that the coupling constant between the Ag atom and central H− anion is approximately twice as large as that of 207 (H@Ag8a, = 33 Hz). The higher coupling constant is explained by the average bond distance between the Ag atom and central H− anion, which is much shorter in 199 (H@Ag4Rh4) (1.86 Å) compared with that in 207 (H@Ag8a) (2.50 Å).

This review summarizes the synthesis and geometric structure of a wide variety of anion-templated Ag NCs. Through this summary, the following points were discussed: The current synthesis methods are broadly categorized into the stirring method, solvothermal method, and ultrasonication method, with the stirring method being the most widely used. The reported central anion species are generally grouped into X (halide ion), X’ (chalcogenide ion), Ox (oxoanion), POM (polyoxometalate), and H/D (hydride/deuteride). The anion species affect the Ag NC volume, where POM is more effective for synthesizing larger anion-templated Ag NCs and H/D utilization is more effective for synthesizing smaller anion-templated Ag NCs. When X is used as an anion species, it is possible to synthesize Ag NCs with the same number of Ag atoms but different central anions. In this case, the X–Ag and Ag–Ag distances tend to extend with the increasing size of the central X anion. When X is used as the anion species, if X’ is present in the system, the cage structure can contain both Ag and X’. Because these cage structures are tough, they can also contain large X’ anions such as I–. Additionally, under these conditions, anion-templated Ag NCs can be synthesized in which two X anions are encapsulated within one Ag framework. When X’ is used as the anion species, the cage structure can be formed with Ag only if the number of Ag atoms is small. However, under these conditions, both Ag and X’ are often included in the cage structure. When Ox or POM is used as the anion species, the number of Ag atoms in the cage structure can easily be varied because of the diversity of these anions. The chemical composition and geometric structure of the resulting anion-templated Ag NCs vary depending on the number of O atoms in the central anions and the geometric structure of the central anions. Moreover, the chemical compositions of the central Ox and POM anions significantly depend on the synthesis conditions. For Ag NCs using H/D as anion species, unlike other anion-templated Ag NCs, they can be synthesized not only by liquid-phase reduction but also by the reaction of X@Agn NCs with [BH4]−, or by the insertion of H− or D− into empty Ag NCs. Considering this breadth of knowledge, many anion-templated Ag NCs with novel geometric structures may be created in the future.

As described in this review, a large number of anion-templated Ag NCs have been reported, which indicates that the use of the anion-template method is quite effective in creating a wide variety of metal NCs. To further develop this research field, the following points should be investigated in the future: Establishment of clear synthesis design guidelines. For the synthesis of Ag NCs with relatively simple geometric structures, such as X@Ag8 NCs, some reports have described synthetic design guidelines based on stoichiometry [133]. However, the syntheses of most anion-templated Ag NCs appear to be a matter of coincidence. Future research is expected to be conducted on the formation mechanism so that anion-templated Ag NCs with the desired chemical compositions and geometric structures can be synthesized. Development of applied research. Although it was not addressed in this review, it is possible to add new functions to anion-templated Ag NCs using functional anions. However, there is currently little research on the application of anion-templated Ag NCs. This may be because of the low stability of anion-templated Ag NCs. As discussed in previous studies on SR-protected metal NCs, the stability of metal NCs can be overcome to some extent by selecting the ligand functional group [248–250]. Future studies are expected to establish methods of improving the stability of anion-templated Ag NCs and provide insights into their practical application. Utilization of other metal elements. This review describes the current status in the field of anion-templated Ag NCs, but more research is expected to conducted on anion-templated NCs with metal elements other than Ag. Because there have been reports of anion-templated Cu NCs [251–254] and anion-templated lanthanide NCs [255–258,260], it is likely that the anion-template method can be used for the synthesis of other metal NCs. Considering the bonding between the anion species and the cage metal, the use of Ag facilitates the synthesis259, but more active use of other metal elements is necessary to increase the diversity of their functions and applications.