Introduction

High-nuclearity clusters of silver ions continue to fascinate chemists because of their intriguing geometrical characteristics such as high symmetry, large dimension and architectural beauty as well as some promising applications1,2,3. Recent advances in the silver cluster field clearly show the subdivisions: type-I, the silver atoms exclusively aggregate to the cluster core and show an average oxidation state between 0 and 14,5,6,7,8,9; type-II, the formation of metal core is not exclusively from the aggregation of monovalent silver but with the participation of small anions or/and organic ligands10,11,12,13,14,15,16. The representatives of type-I and -II clusters are [Ag44(p-MBA)30]4− 17 and [Ag490S188(StC5H11)114]18, respectively. These two kinds of silver clusters may be seen as molecular embryos of the bulk phases of silver metal and binary silver sulfides, respectively, and thus emerged as a hot frontier in chemistry and nanoscience research.

We have been working on the development of new strategies to access silver clusters with some level of control on the metal nuclearity and geometry for a given ligand type19,20,21,22,23,24. However, we must admit that almost all silver clusters were originally isolated serendipitously and the cases of deliberate modulation of nuclearity and geometry are very few, with most based on an anion template method. Among diverse anions, polyoxometalates (POMs) as a family of inorganic cluster have very rich structures and interesting properties25,26,27,28,29,30,31. Recent work showed the encapsulation of POMs into silver-thiolate/alkynyl clusters could effectively direct the nuclearity and geometry of the final products depending on the size and shape of the POMs  32,33,34,35,36,37,38. Moreover, POMs usually isomerize or transform to other forms different from their original compositions and structures when enwrapped into the silver clusters39, which should be controlled by the pH of the assembly environment and influenced by polarity of solvents40. On the other hand, solvents such as DMF (N,N-Dimethylformamide) contain aldehyde group that not only could endow electrons from O atom to form coordination bond with metal centre, but also has the ability to reduce Ag(I) to Ag(0)41, which favours the formation of type-I silver clusters.

With these points in mind, herein, we show that it is quite effective to follow the solvent–intervention strategy to combine two types of silver clusters into one compound. They are sevenfold symmetry cluster-in-cluster silver nanowheels with an octahedral Ag64+ kernel trapped in the centre. The family of SD/Ag7–SD/Ag10 (SD = SunDi) is the highest-nuclearity silver clusters possessing wheel-like topology and trapping the maximum MoO42− ions as template in one wheel. To justify the unique solvent effect in this assembly system, another two high-nuclearity silver-thiolate clusters (SD/Ag11 and SD/Ag12) are isolated with the Ag64+ kernel template replaced by two novel POMs in the absence of DMF. This work pioneers a solvent–intervention approach to construct high-nuclearity silver nanowheels incorporating both an octahedral Ag64+ kernel and a sevenfold symmetric silver shell.

Results

Structures of SD/Ag7–SD/Ag10

Although p-TOS−, CF3SO3− and NO3− were employed as anions in the construction of SD/Ag7–SD/Ag10 from CH3OH/DMF (v:v = 4:1) as a binary solvent (Fig. 1), single-crystal X-ray diffraction (SCXRD) analyses showed that their sevenfold symmetric wheel-like topology was conserved, suggesting these anions have little effects on the cluster skeletons. Due to different molecules packing in the lattice (Supplementary Figure 1), they crystallize in different space groups, monoclinic P21/c, monoclinic C2/c, tetragonal P42/ncm and monoclinic I2/m space groups, respectively. Their formulae were determined as [Ag6@(MoO4)7@Ag56(MoO4)2(iPrS)28(p-TOS)14(DMF)4] (SD/Ag7), [Ag6@(MoO4)7@Ag56(MoO4)2(iPrS)28(p-TOS)14·3DMF] (SD/Ag8), [Ag6@(MoO4)7@Ag56(MoO4)2(iPrS)28(CF3SO3)14(DMF)4] (SD/Ag9), [Ag6@(MoO4)7@Ag56(MoO4)2(iPrS)28(NO3)14] (SD/Ag10). Because of the striking similarities of the molecules in SD/Ag7–SD/Ag10, only that of SD/Ag7 is described in detail here. Selected details of the data collection and structure refinements are listed in Supplementary Table 1.

Fig. 1
figure 1

Schematic representation of the synthesis of clusters SD/Ag7-SD/Ag/12. These clusters are formed by a solvent-controlled method

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The centrosymmetric silver wheel of SD/Ag7, as shown in Fig. 2a, like a motorcycle tyre, comprises of an Ag56 shell and an octahedral Ag64+ kernel in the centre with up to seven MoO42− as anion templates around it. Two additional MoO42− anions serve as hubcaps of the wheel. This wheel has a large dimension even without considering the organic ligands, measuring ~1.7 nm across its circular face (Fig. 2b) and 0.8 nm across the rectangular rim (Fig. 2c). It resembles the Preyssler-type polyoxometalate. In an asymmetric unit, a half of a wheel was identified and other half could be generated by inversion symmetry. The Ag56 shell is covered by 28 iPrS− and 14 p-TOS− ligands and consolidated by abundant Ag⋯Ag interactions (2.926–3.367 Å), forming 14 silver trigons and 42 silver tetragons fused together in an edge-sharing manner. In detail, on the circular face of the wheel, the silver trigons are isolated by silver tetragons and share their edges; whereas all silver tetragons are fused together to form the rectangular rim of the wheel (Supplementary Figure 2). All ligands exclusively capped on the tetragonal faces, where µ4 iPrS−, µ4−η2:η1:η1 p-TOS− and µ3−η1:η1:η1 p-TOS− were observed with the Ag–S and Ag–O distances ranging 2.374–2.552 and 2.400–2.770 Å, respectively.

Fig. 2
figure 2

Single-crystal structure of SD/Ag7. a The total structure of SD/Ag7 showed in ball-and-stick model. Ball-and-stick model of SD/Ag7 showing the front (b) and side (c) views with all C atoms omitted for clarity. (Colour legend: purple, Ag on the shell; cyan, Ag in the kernel; yellow, S; red, O; green, Mo)

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Alternatively, we can also imagine this wheel as five concentric metallocycles of different diameters linked by µ4 iPrS− ligands. The five metallocycles, nearly perpendicular to the pseudo sevenfold axis, contain one large Ag14 ring at the equator (ϕ = 17.14 Å; dAg⋯Ag = 3.554–4.032 Å) with two Ag7 rings (ϕ = 7.68 Å; dAg⋯Ag = 3.366–3.797 Å) and two small Ag14 rings (ϕ = 13.66 Å; dAg⋯Ag = 2.926–3.331 Å) lying above and below this ring (Fig. 3a). Two Ag7 rings are not face-to-face stacking but rotated by an angle of 27.6° with respect to each other, whereas two small Ag14 rings are almost face-to-face stacking (Supplementary Figure 3). Interestingly, we found that up to seven MoO42− anions were arranged along the equatorial Ag14 ring inside (Fig. 3b). All seven MoO42− ions adopt the same μ8−η2:η2:η2:η2 bonding fashion to all Ag14 rings as well as the inner octahedral Ag64+ kernel with an average Ag–O distance of 2.444 Å (Fig. 3c). This is the maximum number of MoO42− ions as template trapped in one cluster.  The previous record was a 55-nuclearity silver-alkynyl cluster with six MoO42− ions inside42. Not only the number of Ag atoms in each ring, but also the number of MoO42− ions are 7 or multiples of 7, which ultimately leads to, albeit not rigorous, a sevenfold symmetry of the overall wheel. The overall geometry of such cluster shows typical anisotropy with rare high-order odd symmetry which should be dictated by initially prearranged seven MoO42− anions around Ag64+ kernel, as justified by a series of [Ag6@(MoO4)7@Ag n ] (n = 28–32) intermediates observed in the electrospray ionization mass spectrometry (ESI-MS) of an early-stage reaction mixture during the synthesis of SD/Ag7 (Supplementary Figure 4 and Supplementary Table 2). Generally, the orders of symmetry with a prime number equal to or higher than 5 are disfavoured43, thus artificial molecular clusters with five- or sevenfold symmetry remain a rarity44,45,46, although such symmetries usually exist in biomacromolecules such as HslV (heat shock locus V) and chaperone GroEL47, 48. The sevenfold symmetric characteristic is the first observed in silver clusters as exemplified by SD/Ag7–SD/Ag10.

Fig. 3
figure 3

Disassembled skeletal structure of SD/Ag7. a Five concentric metallocycles: two Ag7 (purple); two small Ag14 (cyan) and one large Ag14 (red). b Seven MoO42− anions arranged along the equatorial Ag14 ring. c The bonding of MoO42− with Ag14 rings and inner Ag6 kernel with Ag–O bonding highlighted by black. d The polyhedral mode showing octahedral Ag6 kernel. e The Ag6 kernel protected by nine MoO42− anions with seven in the equator and two at the poles. (Colour legend: purple, Ag; red, O; green, Mo)

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Another structural feature is a centremost octahedral Ag64+ kernel (Ag1, Ag2, Ag3 and their symmetry equivalents) which is formed purely by Ag⋯Ag interactions ranging from 2.659 to 2.852 Å (Fig. 3d). These short contacts between silver atoms are even shorter than those in bulk Ag metal (2.886 Å), indicating the significant argentophilicity49,50,51. The Ag6 kernel is stabilized by a total of nine MoO42− anions through the Ag–O bonding (Fig. 3e) at the exposed [111] facets or edges of octahedron. This Ag64+ kernel can be deemed as the smallest single-crystal octahedral silver nanocrystal52 cut from the face-centred cubic (fcc) lattice of bulk silver but with a slight shrinkage due to the addition of two more electrons to the bonding orbitals53. Although several inorganic compounds, such as Ag6Ge10P1254, Ag6O255, Ag5SiO456 and Ag5GeO457, have been identified to contain such subvalent cluster unit, it is still rarely found in ligand-capped silver clusters58, 59. The formation of such partially reduced species should be caused by the reductive ability of DMF, which is widely used in the controlled synthesis of multiple-twin decahedral and icosahedral silver nanocrystals with special favourable [111] faces by reducing Ag+ to Ag041. During this assembly process, DMF was partially oxidized to Me2NCOOH, which was unambiguously verified by the 13C NMR (nuclear magnetic resonance) of HCl-digested reaction mother solution (Supplementary Figure 5). Based on above analyses and discussions, we found the reducing ability of DMF dictates the formation of the innermost Ag64+ kernel, which then attracts seven MoO42− anions around it, thus forming the final sevenfold symmetric silver nanowheel. All these results clearly demonstrate that the reducibility of DMF is the key to such unique silver nanowheel.

Molecular wheels caught chemists’ eyes as far back as 20-year ago since the giant POM wheels (Mo154, Mo176 and even larger Mo248) were first characterized by Müller using X-ray diffraction60. Following their work, other giant wheel-like clusters, such as Mn8461, Ni2462 and Pd8463, were also successfully synthesized, suggesting such clusters were possible beyond Mo-based POMs. Among them, Mo154 and Pd84 are two limited sevenfold symmetric wheels. The wheel-like topology in silver clusters has not been reported up to now.

A careful examination of the packing of the wheels in the crystal lattice of SD/Ag7 reveals that there are intermolecular C–H⋯O hydrogen bonding and van der Waals force between neighbours; the nearest Ag⋯Ag distance between neighbouring units is ~8.3 Å, with nearest neighbours oriented parallel to one another, that is, packing face to face (Supplementary Figure 1). However, the wheels packing showed an inclined rim-to-face orientation for SD/Ag8 and SD/Ag9, and a displaced face-to-face orientation for SD/Ag10.

Structures of SD/Ag11 and SD/Ag12

The comparative experiment to justify the reduction-induced formation of subvalent Ag64+ kernel has resulted in the isolation of another two huge silver clusters SD/Ag11 and SD/Ag12 in the absence of DMF ([Mo7O24@Ag41(iPrS)19(p-TOS)16(CH3OH)4·4CH3OH] (SD/Ag11) and (n-Bu4N)1.5[Mo5O18@Ag36(iPrS)18(p-TOS)13.5(CH3CN)·1.5CH3CN] (SD/Ag12)). In their centres, two novel POMs anions are trapped instead of the Ag64+ kernel, suggesting the decisive role of DMF on the formation of subvalent Ag64+ kernel.

When the solvent was changed to methanol, SD/Ag11 crystallized in the orthorhombic space group Pccn and was isolated as a 41-nuclearity ellipsoidal silver shell loaded with a C2v-Mo7O246− in the centre as template (Fig. 4a). There are 19 μ4 iPrS−, 16 p-TOS− (2 × μ4−η2:η1:η1, 6 × μ3−η1:η1:η1, 6 × μ2−η1:η1 and 2 × μ4−η2:η2), four CH3OH (2 × μ2 and 2 × μ1) molecules coordinated on this irregular Ag41 shell with the Ag–S and Ag–O distances of 2.388–2.532 and 2.338–2.592 Å. The abundant Ag⋯Ag interactions (2.886–3.374 Å) finally reinforce the overall shell (Fig. 4b). As shown in Fig. 4c, an Mo7O246− ion was clearly resolved and the bond valence sum (BVS) calculations64 indicate that all Mo atoms have an oxidation state of +6 (Mo1: 5.811; Mo2: 5.716; Mo3: 6.002 and Mo4: 6.030). All Mo atoms are coordinated to six O atoms, forming seven MoO6 octahedra, which are fused to Mo7O246− by exclusively edge-sharing fashion. The Mo–O bond lengths vary from 1.711 to 2.511 Å. The 41 silver atoms arranged around the surface of Mo7O246− forming the core–shell structure. Among 41 silver atoms, 26 are coordinated to both terminal and bridge O atoms (Fig. 4d) with Ag–O distances in the range of 2.378–2.783 Å. The Mo7O246– should be in situ generated from α-Mo8O264−.

Fig. 4
figure 4

Single-crystal structure of SD/Ag11. a The total structure of SD/Ag11 with Mo7O246− shown in space filling mode. b The Ag41 skeleton viewed along different directions. c Polyhedral representation of Mo7O246−. d The bonding between Mo7O246− and Ag atoms at the shell. (Colour legend: purple, Ag; yellow, S; red, O; green, Mo)

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When using CH3CN as solvent, we isolated SD/Ag12 as a 36-nuclearity silver cluster with an Mo5O186− as core as revealed by SCXRD structural analysis (Fig. 5a). This anionic Ag36 cluster can be shaped as a rugby-like skeleton (Fig. 5b) built from an equatorial Ag18S12 barrel adding two half-cuboctahedral Ag9S3 caps above and below, and they are joined together by argentophilic interactions and through Ag–S bonds (Fig. 5c). The Ag36 shell is peripherally bridged by 18 μ4 iPrS−, 13.5 p-TOS− (9 × μ3−η1:η1:η1, 1 × μ3−η2:η1 and 3.5 × μ2−η1:η1) and one CH3CN molecule. The Ag–S and Ag–O distances are located in the range of 2.430–2.637 and 2.293–2.569 Å, respectively. The Ag36 skeleton consists of 8 trigons and 18 tetragons, which are edge-fused together to form the Ag36 shell. The μ4-iPrS− exclusively bonded to tetragons, whereas p-TOS− coordinated to both trigons on Ag9S3 caps and partial tetragons on Ag18S12 barrel. In SD/Ag12, Ag⋯Ag distances between 2.926 and 2.354 Å; are observed. The dimensions of the Ag36 shell account for ~1.4 nm in length and 0.9 nm in width. The most fascinating feature of SD/Ag12 is the inner Mo5O186− core, which is built from two octahedral MoO6, two square-pyramidal MoO5 and one tetrahedral MoO4 units (Fig. 5d). The 36 silver atoms aggregated around the surface of Mo5O186− forming the core–shell structure. Among 36 silver atoms, 25 are coordinated to both terminal and bridge O atoms (Fig. 5e). Such Mo5O186− may be transient and can only be stabilized in the void of silver cluster through the rich Ag–O bonding.

Fig. 5
figure 5

Single-crystal structure of SD/Ag12. a The total structure of SD/Ag12 with Mo5O186− shown in polyhedral mode. b The Ag36 skeleton comprised of Ag3 trigons and Ag4 tetragons. c Schematic showing one Mo5O186− enwrapped by Ag18 barrel, then closed by two half-cuboctahedral Ag9S3 caps. d Polyhedral representation of Mo5O186− with two octahedral MoO6, two square-pyramidal MoO5 and one tetrahedral MoO4 units shown in green, yellow and cyan, respectively. e The bonding between Mo5O186− and Ag atoms at the shell. (Colour legend: purple, Ag; yellow, S; red, O; green, Mo)

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From the above structural analyses, we found that (i) the same POM precursor can transform to different POMs in different solvents (MeOH vs MeCN); (ii) Mo-based anions with rich geometries and compositions templated diverse silver clusters; (iii) DMF plays an important role in the reductive formation of subvalent silver kernel; (iv) multiple simple and small anion templates (MoO42−) can also induce the formation of large silver clusters through special arrangement of the anions. The multiple roles of solvents promise rational access to more complex and diverse silver clusters with special geometries or symmetries.

Electrospray ionization mass spectrometry

To detect the stability of the cluster, the ESI-MS of SD/Ag8, SD/Ag9 and SD/Ag11 dissolved in acetonitrile were performed in positive-ion mode. As depicted in Fig. 6a, b, SD/Ag8 displays five identifiable species (2a–2e), whereas SD/Ag9 shows six species (3a–3f) with the charge state of +4 in the range of m/z = 1800–2400. After carefully analyzing these peaks in the ESI-MS of SD/Ag8 and SD/Ag9, we did not find any parent [Ag6@(MoO4)7@Ag56] species, which means neither SD/Ag8 nor SD/Ag9 are stable in acetonitrile, however, fragments with the [Ag6@(MoO4)7] core were unambiguously detected. The dominating species (2b and 3e), centred at m/z = 1952.2731 and 2010.2228, can be assigned to [Ag6@(MoO4)7@Ag35(iPrS)11(p-TOS)6Cl(OH)3(CH3CN)4(H2O)9]4+ (Calc. m/z = 1952.2102) and [Ag6@(MoO4)7@Ag36(iPrS)13(CF3SO3)9(H2O)4]4+ (Calc. m/z = 2010.2902), respectively. The other species in the ESI-MS of SD/Ag8 and SD/Ag9 were identified based on the experimental and theoretical isotope distributions and presented in the Supplementary Figures 6, 7 and Supplementary Tables 3, 4. All species 2a–2e and 3a–3f have cores of [Ag6@(MoO4)7@Ag n ] (n2a–2e = 32 – 36 and n3a–3f = 32 – 37), which indicates that the innermost [Ag6@(MoO4)7] core in SD/Ag8 and SD/Ag9 is stable in solution, despite the absence of the complete Ag56 parent shell. Considering the charge state of the assigned species, the subvalent nature of Ag64+ was further justified by the ESI-MS results. With respect to outer ligand shell, the Δm/z between 3a and 3b is 45.48, which is similar to the Δm/z (45.98) between 3b and 3c, and the mass difference corresponds to one AgiPrS, indicating the coordination–dissociation equilibrium with losing or adding AgiPrS one by one in solution. Based on the above results, the presence of seven MoO42− anions enwrapping an Ag6 kernel should be precursor species, which is then closed by the outer Ag56 wheel at a second step in the growth process. The overall hierarchical cluster-in-cluster growth process was thus established for Ag62 cluster family, which was further justified by analyzing the reaction mixture by ESI-MS before crystallization (Supplementary Figure 4 and Supplementary Table 2).

Fig. 6
figure 6

The solution behaviours of SD/Ag8 and SD/Ag9. Positive-mode ESI-MS of SD/Ag8 (a) and SD/Ag9 (b) dissolved in acetonitrile. Insets: Comparison of the experimental (black spectrum) and simulated (superimposed red spectrum) isotopic envelopes for 2b and 3e

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As shown in Fig. 7, the species found in ESI-MS of SD/Ag11 dissolved in acetonitrile could be divided into two groups based on their charge states, +3 (5a–5s) and +2 (5t–5z), respectively, in the range of m/z = 2700–5100. The most abundant peak in the triply-charged species (see the inset of Fig. 7) located at m/z = 2910.2894 (5 g), which can be assigned to [Mo7O24@Ag41(iPrS)18(p-TOS)9Cl5(CH3CN)4(H2O)]3+ (Calc. m/z = 2910.2880) and the most abundant peak in doubly-charged species centred at m/z = 4592.8767 (5w), which can be attributed to [Mo7O24@Ag41(iPrS)10(p-TOS)15Cl6(OH)2(CH3CN)3(H2O)]2+ (Calc. m/z = 4592.8338). The other 24 species were also identified and are shown in the Supplementary Figure 8 and Supplementary Table 5. Compared to the crystallographic result of SD/Ag11, we discovered that all species have the identical [Mo7O24@Ag41] skeleton, which indicates the core of SD/Ag11 is intact in acetonitrile.

Fig. 7
figure 7

Positive-ion ESI-MS of SD/Ag11 dissolved in acetonitrile. Insets: The enlarged MS of triply-charged species (blue line) and comparison of the experimental (black spectrum) and simulated (superimposed red spectrum) isotopic envelopes for 5g

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UV–Vis absorption spectra and luminescence properties

The solid-state optical diffuse reflectance spectra of SD/Ag9, SD/Ag11 and SD/Ag12 were investigated at room temperature. As shown in Fig. 8a, compound SD/Ag9 exhibits one main peak centred at 338 nm and weak shoulder peak at 483 nm, which should be assigned to the n → π* transition of iPrS−and ligand-to-metal charge transfer (LMCT) transition, respectively. However, compounds SD/Ag11 and SD/Ag12 show only one peak at 335 and 328 nm, respectively, both belongs to the n → π* transition of iPrS−.

Fig. 8
figure 8

The spectral properties of SD/Ag9, SD/Ag11 and SD/Ag12. The solid-state UV–Vis (a) and emission spectra (b) of the clusters SD/Ag9, SD/Ag11 and SD/Ag12

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Furthermore, the photoluminescence properties of SD/Ag9, SD/Ag11 and SD/Ag12 were also studied in the solid state. As shown in Fig. 8b, compound SD/Ag9 is emission silent at room temperature, however, it emits yellow light at 77 K with a maximum at 587 nm (λex = 400 nm), which is probably due to a ligand-to-metal charge transfer (LMCT, charge transfer from the S 3p to Ag 5s orbital) transition disturbed by Ag⋯Ag interactions65. This temperature-sensitive emission behaviours should be related to the variations of molecule rigidity and argentophilicity under different temperatures and have been previously observed in other silver-thiolate clusters such as [(CO3)@Ag20(SBut)10(DMF)6(NO3)8]66 and [S@Ag56S12(SBut)20](CH3COO)1067. Compounds SD/Ag11 and SD/Ag12 emit green light at room temperature with a similar maximum emission peak at ca. 493 nm upon the excitation at 330 nm, which should be attributed to the LMCT transition, or mixed with Ag⋯Ag interactions.

Discussion

In summary, we discovered a family of sevenfold symmetric silver nanoclusters featuring the anisotropic wheel-like geometry encapsulating an octahedral nanofragment of metallic silver inside. The formation of such nanoclusters was realized by solvent modulation, which not only dictates the formation of octahedral Ag64+ kernel, but also influence the in-situ-generated Mo-based templates. Different anions, p-TOS−, CF3SO3− and NO3−, were involved in four wheel-like silver-thiolate clusters but without breaking the basic wheel-like backbones. The overall Ag62 nanocluster features a hierarchical cluster-in-cluster structure comprising an octahedral Ag64+ kernel in the centre with up to seven MoO42− as templates around it, supporting the outermost Ag56 shell. The formation of an inner octahedral Ag64+ kernel is closely dependent on the DMF as reducing agent. The unique sevenfold odd symmetry should be dictated by the prearranged seven MoO42− templates around the Ag64+ kernel. In the absence of DMF, two different high-nuclear silver-thiolate clusters were identified without the Ag64+ kernel trapped in the inner space but instead two POM templates were found and in situ generated from the same starting POM precursor. The present study launches a more rational approach to construct high-nuclearity silver clusters in which both the formation of octahedral Ag64+ kernel and in situ generation of various Mo-based anion templates is controlled by the solvents.

Methods

Synthesis of the (AgiPrS) n

The precursor of (AgiPrS) n was prepared by the following reported procedure20. The solution of AgNO3 (30 mmol, 5.1 g) in 75 mL acetonitrile was mixed with 100 mL ethanol containing iPrSH (30 mmol, 2.8 mL) and 5 mL Et3N under stirring for 3 h in the dark at room temperature, then the yellow powder of (AgSiPr) n was isolated by filtration and washed with 10 mL ethanol and 20 mL ether, then dried in the ambient environment (yield: 97%).

Synthesis of SD/Ag7 and SD/Ag8

p-TOSAg (0.1 mmol, 27.9 mg) and (AgiPrS) n (0.05 mmol, 9.2 mg) together with [(n-C4H9)4N]4[α-Mo8O26] (0.0002 mmol, 4.2 mg) were dissolved in a mixed solvent of methanol: N,N′-dimethylformamide (5 mL, v/v = 4/1). The reaction mixture was sealed and heated at 65 °C for 2000 min, and then cooled to room temperature for 800 min. Then, the brown solution was filtered and the filtrate left to evaporate slowly for 2 weeks in the dark at room temperature. Brown block crystals of SD/Ag7 and diamond crystals of SD/Ag8 were crystallized in the yields of 10% and 13%, respectively.

Elemental analyses calc. (found) for SD/Ag7: C194H322Ag62Mo9N4O82S42: C, 18.03 (18.12); H, 2.51 (2.59); N, 0.43 (0.39)%. Selected IR peaks (cm−1): 2910 (w), 1441 (w), 1374 (w), 1248 (w), 1144 (m), 1115 (m), 1003 (m), 781 (m), 677 (s), 595 (s), 555 (s).

Elemental analyses calc. (found) for SD/Ag8: C191H315Ag62Mo9N3O81S42: C, 17.86 (17.79); H, 2.47 (2.51); N, 0.33 (0.31)%. Selected IR peaks (cm−1): 2934 (w), 1621 (w), 1486 (w), 1451 (w), 1373 (w), 1245 (w), 1153 (s), 1117 (s), 1004 (s), 784 (s), 677 (s), 600 (w), 564 (s).

Synthesis of SD/Ag9

The synthetic condition was similar to that described for SD/Ag7 and SD/Ag8, except that the p-TOSAg was substituted by CF3SO3Ag (0.1 mmol, 25.6 mg), brown block crystals of SD/Ag9 were crystallized in a yield of 35% after 3 days.

Elemental analyses calc. (found) for SD/Ag9: C110H224Ag62F42Mo9N4O82S42: C, 17.86 (17.90); H, 2.47 (2.55); N, 0.32 (0.29)%. Selected IR peaks (cm−1): 2952 (w), 1643 (w), 1448 (w), 1373 (w), 1224 (m), 1167 (m), 1011 (m), 784 (m), 635 (s), 511 (m).

Synthesis of SD/Ag10

The synthetic condition was similar to that described for SD/Ag7 and SD/Ag8, except that the p-TOSAg was replaced by AgNO3 (0.1 mmol, 17 mg), brown crystals of compound SD/Ag10 were crystallized in a yield of 20% after 3 weeks.

Elemental analyses calc. (found) for SD/Ag10: C84H196Ag62Mo9N14O78S28: C, 9.09 (9.13); H, 1.78 (1.69); N, 1.77 (1.79)%. Selected IR peaks (cm−1): 2948 (w), 1650 (w), 1526 (w), 1451 (w), 1387 (m), 1267 (m), 1146 (m), 1047 (m), 776 (s), 600 (m).

Synthesis of SD/Ag11

The synthetic condition was similar to that described for SD/Ag7 and SD/Ag8 but using MeOH (5 mL) instead, yellow rod crystals of SD/Ag11 were isolated by filtration, washed with EtOH and dried in air (yield: 35%).

Elemental analyses calc. (found) for SD/Ag11: C177H277Ag41Mo7O80S35: C, 21.47 (21.51); H, 2.82 (2.79) %. Selected IR peaks (cm−1): 2927 (w), 1600 (w), 1500 (w), 1444 (w), 1380 (w), 1249 (w), 1110 (s), 1004 (s), 869 (m), 841 (m), 805 (m), 677 (s), 613 (m), 550 (s).

Synthesis of SD/Ag12

The synthetic condition was similar to that described for SD/Ag7 and SD/Ag8 but using MeCN (5 mL) instead, yellow block crystals of compound SD/Ag12 were crystallized in a yield of 20% after 10 days.

Elemental analyses calc. (found) for SD/Ag12: C177.5H283Ag36Mo5N4O58.5S31.5: C, 24.28 (24.19); H, 3.25 (3.28); N, 0.64 (0.61) %. Selected IR peaks (cm−1): 2934 (w), 1600 (w), 1495 (w), 1448 (w), 1382 (w), 1241 (w), 1117 (m), 1004 (s), 876 (w), 812 (m), 677 (s), 600 (m), 556 (s).

Data availability

The X-ray crystallographic coordinates for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition number CCDC: 1815301-1815304, 1815357 and 1815305 for SD/Ag7-SD/Ag12. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.