Anion-templated silver nanoclusters: precise synthesis and geometric structure

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.

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][127][128][129][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 aniontemplated 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.

Synthesis methods
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, PR 3 , 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@Cu 14 (SC(CH 3 ) 2 CH 2 NH 2 ) 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][132][133][134]. When CO 3 2is used as a template source, atmospheric CO 2 can also be used, where CO 2 is converted to CO 3 2- 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 CO 3 2from H 2 CO 3 formed by hydration of CO 2 [125]. Solvothermal methods are also frequently used in the synthesis of aniontemplated Ag NCs [135][136][137][138][139][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][142][143][144].

X@Ag 8 NCs
There are numerous reports on X@Ag 8 NCs with a diverse range of central anions, including fluoride ions (F -), Cl -, and bromide ions (Br - Br) were used, and the central X anion is formed by their dissociation in the reaction process.
These eleven X@Ag 8 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 Ag 8 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  Black ones are methods without any anion sources. Data are taken from ref [133]. Reprinted with permission from [133], copyright (2014, the Royal Society of Chemistry).

X@Ag 14 NCs
The synthesis of X@Ag 14 NC was reported by Rais and colleagues in 2001 [147] and 2002 [148]. These X@Ag 14 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@Ag 14 NCs, most of which were synthesized by the stirring method. In the synthesis of 21 (Cl@Ag14a), Ag-containing polymer compound [Ag(C≡C t Bu)] n was used as an Ag precursor, whereas silver tetrafluoroborate (AgBF 4 ) was used as an Ag precursor in the synthesis of 20 (F@Ag14), 22 (Cl@Ag14b), and 23 (Br@Ag14). The Fin 20 (F@Ag14) was generated by the hydrolysis reaction [164,165] of the Ag precursor (AgBF 4 ). For 21 (Cl@Ag14a) and 22 (Cl@Ag14b), the Clwas generated by the dissociation of the chloroform (the reaction solvent) and Bu 4 NCl added as the Clsource, respectively. In the synthesis of 23 (Br@Ag14), Bu 4 NBr was added as the Brsource. 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@Ag 14 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@Ag 8 NCs, the size of the Ag framework in X@Ag 14 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@ Ag 14 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 Ag 14 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.

X@Ag 16 NCs
The synthesis of X@Ag 16 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@Ag 16 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@Ag 16 core has ellipsoid structure. There structures are largely different from the   [133,145,157]. Reprinted with permission from [133], copyright (2014, the Royal Society of Chemistry).
above-described X@Ag 8 and X@Ag 14 NCs. Thus, the structure of X@Ag n core strongly changes depending on the nymber of Ag atoms.

Other X@Ag n NCs
At the end of this section, we introduce the syntheses of X@Ag n NCs that are different from the Ag NCs described in Sections 3.1.1 and 3.1.2.
The first example is the I@Ag n NC, containing an iodide ion (I -). Representative examples of I@Ag n NCs include 13 (I@Ag10) [133], 14 (I@Ag11a), 15 (I@Ag11b) [157], and 17 (I@Ag12) [145] ( Figure 7A). All of these I@Ag n NCs have been synthesized by Liu and colleagues. They used tetrabutylammonium iodide (Bu 4 NI) as the Iprecursor 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 ( 1 H and 31 P) magnetic resonance spectroscopy ( 31 P { 1 H} 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 Ag 11 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 X 2 @Ag n NC, in which two X anions are encapsulated within one Ag framework. Liu and colleagues reported the syntheses of 16 (Br 2 @Ag12), 18 (I 2 @Ag12a), and 19 (I 2 @Ag12b) [146] in 2013, and Xie and Mak reported the synthesis of 36 (Cl 2 @Ag21) [152] in 2012 (Table 2). These X 2 @Ag n 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 (Br 2 @Ag12) is 2.80 Å, which is longer than the average X-Ag distance (2.98 Å) in 18 (I 2 @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 X 2 @Ag n NCs, similar to X@Ag 8 and X@Ag 14 NCs.

X'@Ag 9 NCs
For X'@Ag 9 NCs, the synthesis of 42 (S@Ag9) and 43 (Se@Ag9) with S 2or selenide ion (Se 2-) at the central position has been reported by Liu and colleagues in 2017 [134] (Table 3). Both X'@Ag 9 NCs were synthesized by mixing the Ag precursor, tetrakis(acetonitrile) copper(I) hexafluorophosphate (Ag(MeCN) 4 PF 6 ), and the ligand precursor, chalcogen-containing ammonium salt (NH 4 X' 2 P(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 ([Ag 5 (S 2 P(OEt) 2 ) 4 (PF 6 )] 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'@Ag 9 NCs have a geometric structure with three Ag atoms surrounding an hourglassshaped Ag 6 core. However, there are also differences between them. For example, focusing on the hourglass-shaped Ag 6 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)   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.

Oxoanion-templated Ag NCs
In the following section, we describe the Ox@Ag n NCs that encapsulate Ox ions at the central position. The representative Ox@Ag n NCs are summarized in Table 4 [125,[135][136][137][141][142][143]158,159,172,177,181,. 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@Ag n NCs (Section 3.3.3).
Pioneering examples for CrO 4 @Ag n NCs are 118 (CrO 4 @Ag 22 , Figure 11(a)) and 139 ((CrO 4 ) 2 @Ag35, Figure 11(b)), reported by Wang and colleagues in 2009 [187]. The former, 118 (CrO 4 @Ag22), was synthesized by adding TMEDA to a methanol solution containing [AgC≡C t Bu] n and AgBF 4 and then adding potassium dichromate (K 2 Cr 2 O 7 ). The latter, 139 ((CrO 4 ) 2 @Ag35, Figure 11(b)), was synthesized using [AgC≡CPh] n instead of [AgC≡C t Bu] n . In addition to these reports, K 2 Cr 2 O 7 has often been used as a precursor for CrO 4 2- [136,185,187]. Notably, for 118 (CrO 4 @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 ((CrO 4 ) 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 CrO 4 2anions. Because AgC≡CPh is less bulky than AgC≡C t Bu, large NCs can form in the synthesis of the latter. These observations agree with those for SR-protected Au n NCs [207].

NMOx@Ag n NCs
Anions in the form of NMOx include CO 3 2-, NO 3 -, X'O 3 2-, X'O 4 2-(X' = S, Se, Te), and ClO 4 -. Representative NMOx@Ag n 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 (CO 3 @Ag17) and 94  (CO 3 @Ag19) [125], the central anion was produced by the conversion of atmospheric CO 2 to CO 3 2by TMEDA. The use of atmospheric CO 2 was also adapted in the synthesis of 99 (CO 3 @Ag20a) and 100 (CO 3 @Ag20b).

Polyoxometalate-templated Ag NCs
The POM@Ag n NCs are characterized by a higher number of constituent Ag atoms than other aniontemplated Ag n NCs, such as X@Ag n NCs, X'@Ag n NCs, and Ox@Ag n NCs. This is because the encapsulated POMs are generally larger than X, X', and Ox. The smallest POM@Ag n NC reported is 148 (POV@Ag22) [204], which encapsulates [V 2 O 7 ] 4and 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  2 ). Data are taken from ref [191].
somewhat larger frameworks. The history of research on POM@Ag n NCs is short, and currently, the largest anion-templated Ag NCs are 39 (Cl@Ag216) [155] and 147 (SO 4 @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][230][231][232][233][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 CF 3 CO 2 H was added as a stabilizer for POM to make the solution acidic and when AgCF 3 CO 2 was added as a stabilizer for POM to make the solution neutral. In the former experimental condition, 165 (POV@Ag40a) or 167 (POMo@Ag40) (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  2 ). Data are taken from ref [192].  [127], and Hu, Shi, Ji, and colleagues [129]. Therefore, we look at the differences between POM@Ag n 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 (MoO 4 @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 (NO 3 @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.

Hydride/deuteride-templated Ag NCs
Finally, we describe Ag NCs that contain H − and D − as central anions. Table 6 summarizes the representative H/D@Ag n NCs [132,[235][236][237][238][239][240]. The number of reports on H/D@Ag n NCs is smaller than that on other aniontemplated Ag NCs, and the first report of H/D@Ag n NCs was in 2010. In H/D@Ag n NCs, the Ag 11 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@Ag n 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     (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 NaBH 4 to solutions of X@Ag 8 NCs (2 (F@Ag8a) or 7 (Cl@Ag8d), table 2) and monitored the reaction process by 31 P 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@Ag 8 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 Ag 8 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@Ag 8 NCs (Figure 3) [132].
Liu and colleagues also reported other H/D@Ag n NCs, including H/D@Ag 7 NCs and H/D@Ag 11 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@Ag 8 NCs. Specifically, for 216 (H@Ag11) and 217 (D@Ag11), AgNO 3 was used as the Ag salt and NaS 2 CNPr 2 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(CH 3 CN) 4 PF 6 was used as the Ag salt and NH 4 Se 2 P(O i Pr) 2 or NH 4 S 2 P(OEt) 2 as the ligand precursor. The resulting DCM solution was reduced with NaBH 4 or NaBD 4 at room temperature to obtain the H/ D@Ag 7 NCs. H@Ag 7 NCs can also be synthesized by the reaction of X'@Ag 10 (E 2 P(OR) 2 ) 8 (X' = Se, R = i Pr or X' = S, R = Et) with two equivalent amounts of [BH 4 ] -. Reaction tracking by 31 P NMR revealed that 1) H@Ag 8 NCs, such as 207 (H@Ag8a), are formed as reaction intermediates in these reactions (Figure 21(a)), and 2) H@Ag 8 NCs are obtained (regenerated) by the reaction of the obtained H@Ag 7 NCs with an equal amount of Ag salt (Figure 21(b)). Figure 20(a) shows the geometric structure of 216 (H@Ag11). The Ag 11 framework is formed by capping six Ag atoms against the three-way bipyramidal Ag 5 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 threeway bipyramidal molecular structure capped by six Ag atoms [237].
In 201 (H@Ag7a, Figure 20(b)), H is encapsulated in the tetrahedral Ag 4 framework, and three of the four triangles in the tetrahedron are capped by Ag atoms to form the Ag 7 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 Ag 8 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 (μ 2 , μ 2 ) occurs between Ag and the two Se atoms in Se 2 P(O i Pr) 2 , whereas in 201 (H@Ag7a), two coordination modes (μ 2 , μ 2 and μ 2 , μ 1 ) occur between Ag and the two Se atoms. Removing one capped Ag atom cleaves the three Ag-Se bonds. This   [240]. Reprinted with permission from [240], copyright (2021, American Chemical Society). results in a new coordination mode (μ 2 , μ 1 ) between the three Se and Ag atoms, changing the coordination pattern between the Ag(I) and ligand [235].
In the H/D@Ag n NCs described above, the H/ D@Ag 4 core structure is included. For a long time, there has been no report on the isolation of H/ D@Ag 4 NCs without capping by Ag atoms. In 2021, Konno and colleagues succeeded in isolating two types of H/D@Ag 4 NCs, 199 (H@Ag4Rh4) and 200 (D@Ag4Rh4) [240]. Their synthesis methods differ significantly from the syntheses of H/D@Ag n NCs described above. Specifically, 199 (H@Ag4Rh4) and 200 (D@Ag4Rh4) were formed by inserting H or D into [Ag 4 {Rh(L-Cys) 3 } 4 ][Na] 8 (Ag4Rh4, Figure 22A (a)), which has an empty Ag framework. They reacted Ag4Rh4 with NaBH 4 or NaBD 4 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 NaBH 4 but caused the formation of [Ag (I) 4 H] 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 1 H NMR analysis ( Figure 22B). The overall geometric structures of Ag4Rh4 and 199 (H@Ag4Rh4) are similar, both having a slightly distorted tetrahedral Ag 4 framework. These geometric structures are also similar to (Δ)4-[Zn 4 O(Rh(L-Cys) 3 ) 4 ] [244][245][246][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 Cu 8 NCs with H − inclusion [247,248].
For 199 (H@Ag4Rh4, J HÀ 107 Ag = 68.75 Hz, J HÀ 108 Ag = 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, J AgÀ H = 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).

Conclusion
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: (1) 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. (2) The reported central anion species are generally grouped into X (halide ion), X' (chalcogenide ion), Ox (oxoanion), POM (polyoxometalate), and H/D (hydride/deuteride). (3) 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. (4) 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. (5) 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. (6) 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. Considering this breadth of knowledge, many anion-templated Ag NCs with novel geometric structures may be created in the future.

Outlook
As described in this review, a large number of aniontemplated 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: (1) Establishment of clear synthesis design guidelines. For the synthesis of Ag NCs with relatively simple geometric structures, such as X@Ag 8 NCs, some reports have described synthetic design guidelines based on stoichiometry [133]. However, the syntheses of most aniontemplated 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. (2) 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 aniontemplated 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][249][250]. Future studies are expected to establish methods of improving the stability of anion-templated Ag NCs and provide insights into their practical application. (3) 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][252][253][254] and anion-templated lanthanide NCs [255][256][257][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.

Disclosure statement
No potential conflict of interest was reported by the author(s).