Bulky Thiolate-Protected Silver Nanocluster Ag ₂₁₃ (Adm-S) ₄₄ Cl ₃₃ with Excellent Electrocatalytic Performance toward Oxygen Reduction

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE10 Jun 2022Bulky Thiolate-Protected Silver Nanocluster Ag213(Adm-S)44Cl33 with Excellent Electrocatalytic Performance toward Oxygen Reduction Chen-Guang Shi, Jian-Hua Jia, Yaling Jia, Guangqin Li and Ming-Liang Tong Chen-Guang Shi Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510006 , Jian-Hua Jia *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510006 , Yaling Jia Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510006 , Guangqin Li Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510006 and Ming-Liang Tong *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou, Guangdong 510006 https://doi.org/10.31635/ccschem.022.202201960 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Atomically precise gold and/or silver nanoclusters play a key role in crystallography and coordination chemistry. Compared with gold nanoclusters, silver nanoclusters become unstable and difficult to crystallize due to the high reactivity of metal silver. Herein, we report a silver nanocluster Ag213(Adm-S)44Cl33 ( Ag213) coprotected by bulky thiolates and chlorides. The low surface thiolate coverage (about 45%) endows Ag213 with high catalytic activity. Supported on activated carbon, Ag213 nanoclusters exhibit excellent electrocatalytic oxygen reduction performance with Eonset and E1/2 values of 0.89 and 0.72 V, respectively, close to the values of commercial Pt/C catalyst. This is the first report on the electrocatalytic oxygen reduction reaction of nanoclusters with more than 100 silver atoms. Ag213 with the diameter of 2.75 nm comprises a core–shell structure Ag7@Ag32@Ag77@Ag97. The strong plasmonic absorption band at 454 nm reveals the metallic nature of Ag213. Interestingly, halide is of importance. Chloride facilitates the formation of Ag213 and Ag56(Adm-S)33Cl16 ( Ag56 Cl) while bromide can promote the formation of Ag56(Adm-S)33Br16 ( Ag56 Br). This work provides an example for the study of large-sized metal nanoclusters and nanocluster-based electrocatalysts. Download figure Download PowerPoint Introduction Atomically precise gold and/or silver (hereinafter referred to as Au/Ag) nanoclusters protected by organic ligands have been widely studied in the last few decades because of their unique atomic arrangement, reactivity, and various applications.1–11 With the help of single-crystal X-ray diffraction (SCXRD) and crystallography, the total structures of many metal nanoclusters and/or clusters with hundreds of Au/Ag atoms have been determined12–21 in order to deeply understand their structure as well as their optical, electronic, magnetic, and catalytic properties.22–30 These nanoclusters are usually composed of metal cores with defined geometries and metal–organic protective layers. Much work focuses on Au/Ag nanoclusters protected by thiolate,31–33 phosphine,34,35 alkynyl,36–38 amide,39,40 selenolate,41,42N-heterocyclic carbene,43,44 and the mixed ligands.45–49 Large-sized metal nanoclusters play a key role in crystallography and coordination chemistry. The strong quantum-size effects are manifested in their physicochemical properties. So far, a few of the atomically precise metal nanoclusters with more than 200 Au/Ag atoms have been reported, such as Ag206, Ag210-211, Au246, (AuAg)267, Au279, Ag307, and Ag374.50–56 However, the synthesis and separation of large-sized (>2 nm) monodisperse metal nanoclusters is still a challenge. Compared with gold nanoclusters, silver nanoclusters become unstable and difficult to crystallize due to the high reactivity of metal silver.57–59 To further improve the stability of silver nanoclusters, it is desirable to introduce the second protective group. For example, Yu and Zhu et al. obtained a nanocluster Ag100 featured with a face-centered cube using thiolate/phosphine mixed ligands.60 Per the analyses of silver nanoclusters with more than 100 metal atoms, including Ag112,61 Ag136,56 Ag141,62 Ag146,63 Ag206,50 and Ag210-211,51 it has been found that halide has an important influence on the synthesis of these nanoclusters due to the strong binding between halide and silver.64 The two largest known silver nanoclusters, Ag307 and Ag374,55,56 for instance, are both coprotected by the 4-tert-butylbenzenethiolate/halide mixed ligands. The difference is that the former has 60 chlorides in the intermediate layer while the latter has all halides in the surface shell, which facilitates the formation of a larger metal core Ag207 as well as the unique surface ligand distribution. Inspired by the bulky thiolates employed in nanocluster Ag141 and its low surface thiolate coverage (∼57%),62 we chose 1-adamantanethiolate (Adm-S) to achieve low surface coverage of thiolates and chose halide as the coprotective group to enhance the stability of silver nanoclusters. Fortunately, we were able to isolate a giant nanocluster, Ag213(Adm-S)44Cl33 ( Ag213), by using Adm-S and chloride. Ag213 with the diameter of 2.75 nm comprises a core–shell structure of Ag7@Ag32@Ag77@Ag97. As far as we know, it is one of the three largest silver nanoclusters. The surface thiolate coverage of Ag213 is only about 45%, suggesting the possibility of high surface reactivity. There have been a number of reports on the electrocatalytic oxygen reduction reaction (ORR) involving Pd and/or Au nanoclusters,65 but few report on the electrocatalyst based on Ag nanoclusters.66 Supported on activated carbon, Ag213 nanoclusters exhibit excellent performance in electrocatalytic ORR. This is the first report on the electrocatalytic ORR of nanoclusters with more than 100 silver atoms. Experimental Methods Materials Silver trifluoromethanesulfonate (AgOTf) was purchased from Adamas Reagent Co., Ltd. (Shanghai, China). Adm-SH was purchased from Beijing HWRK Chemical Co., Ltd. (Beijing, China). Sodium borohydride (NaBH4) was purchased from Beijing Innochem Science & Technology Co., Ltd. (Beijing, China). All the chemical reagents were used as received without further purification. Synthesis of Ag213(Adm-S)44Cl33·4CH2Cl2 (Ag213·4CH2Cl2) In a 20 mL glass bottle, AgOTf (0.25 mmol, 64.2 mg) and Adm-SH (0.12 mmol, 20 mg) were dissolved in a mixture of methanol/dichloromethane (MeOH/DCM, 4 mL, v/v = 1:1), then added to 6 mL DCM. To the mixture after stirring for 30 min, Ph4PCl (0.1 mmol, 37.4 mg) and Et3N (100 μL) were successively added within a few minutes. After another 15 min, the freshly prepared NaBH4 aqueous solution (2 mL, 30 mg/mL) was added under vigorous stirring. The color of the suspension mixture immediately turned light yellow, then brown, and finally black. The reaction was continued for 3 h at room temperature and then aged in the dark for 2 days. The dark precipitate was obtained by adding 10 mL MeOH and washed with MeOH and water. The separated precipitate was again dissolved in 10 mL DCM and filtered. Black block-like crystals were obtained by diffusion of n-hexane into DCM solution after 1 month with a yield of ∼13.8% (based on Ag). Synthesis of Ag56(Adm-S)33Cl16 (Ag56Cl) The synthesis process for Ag56Cl is similar to that for Ag213. To a solution of AgOTf (0.125 mmol, 32.1 mg) and Adm-SH (0.06 mmol, 10 mg), Ph4PCl (0.025 mmol, 9.36 mg) in 6 mL DCM was added. After stirring for 10 min, Et3N (50 μL) and freshly prepared NaBH4 aqueous solution (2 mL, 15 mg/mL) were added successively. The mixture was stirred for 10 h and then washed with water. The organic phase was aged in the dark for 2 days. The dark red filtrate was collected and diffused with the mixture of ether and n-hexane (v/v = 1:1). Red plate crystals ( Ag56 Cl) and black block-like crystals ( Ag213) were obtained after several days. Synthesis of Ag56(Adm-S)33Br16 (Ag56Br) To a solution of AgOTf (0.125 mmol, 32.1 mg) and Adm-SH (0.06 mmol, 10 mg), Ph4PBr (0.025 mmol, 10.5 mg) in 10 mL DCM was added. After stirring for 30 min, Et3N (50 μL) and freshly prepared NaBH4 aqueous solution (2 mL, 15 mg/mL) were added successively. The mixture was stirred for 10 h and then washed with water. The organic phase was aged in the dark for 2 days. The red filtrate was collected and diffused with n-hexane. Red rhombic crystals were separated after several days in a yield of 37.0% (based on Ag). Electrochemistry Crystalline Ag 213 (4 mg) and activated carbon (1 mg) were mixed with naflon (40 μL) and 60% aqueous ethanol (960 μL), giving the Ag213/C ink after 30 min in an ultrasonic bath. Ag213/C ink (10 μL) was added dropwise on a glassy carbon rotating disk electrode (RDE) as the working electrode. Similarly, commercial Pt/C (5 mg) was mixed with naflon (40 μL) and 60% aqueous ethanol (960 μL), giving the Pt/C ink after 30 min in ultrasonic bath. Pt/C ink (10 μL) was added dropwise on glassy carbon RDE as the working electrode. After drying overnight in air, working electrodes were both prepared. A Ag/AgCl electrode and carbon rod were used as reference and counter electrodes, respectively. The electrolyte was 0.1 M KOH (pH 13) solution saturated with pure O2. Scan rates of 10 mV were conducted in all experiments. Results and Discussion Synthesis, characterization, and crystallography The preparation of the Ag213 nanoclusters involved the reaction of silver salt and Adm-S in a MeOH/DCM solvent mixture with the stoichiometric ratio of about 2:1. After the addition of Ph4PCl and Et3N, the mixture was reduced with fresh NaBH4 aqueous solution to form silver nanoclusters. The precipitate was obtained by adding MeOH and then was extracted with DCM. Black crystals were separated by vapor diffusion of n-hexane into the DCM solution. The composition of Ag213 was clarified by the data analyses of SCXRD (see Supporting Information Table S1), electrospray ionization mass spectrometry (ESI-MS), and energy-dispersive X-ray spectrometer (EDS). SCXRD analyses revealed that Ag213 comprised a huge silver kernel peripherally protected by Adm-S groups and chlorides (Figures 1a and 1b). The average diameter of the whole molecule was 2.75 nm and 1.80 nm excluding the organic shell. Ag213 contained a Ag116 core and a surface shell of Ag97(Adm-S)44Cl33, which was different from two much larger silver nanoclusters, Ag307 and Ag374 respectively with the core/shell of Ag167/Cl60/Ag140(RS)110Cl2 or Ag207/Ag167(RS)113Cl2Br2 (RS refers to thiolates).55,56 The Ag116 core showed pseudo-fivefold symmetry (see Supporting Information Figures S1 and S2), the same as the cores of Ag206 and Ag210-211.50,51 However, their peripheral ligand layers were completely different. The Ag116 core was surrounded by a shell consisting of 97 Ag, 44 Adm-S, and 21 Cl. 116 Ag were arranged as a three-shell Russian nesting doll architecture as Ag7@Ag32@Ag77 (Figures 1c–1e). The center was an almost ideal Ag7 decahedron, differing from Ag307 and Ag374 both with a centerd Ag13 core. The Ag–Ag bond lengths ranged from 2.797 to 2.878 Å with an average value of 2.834 Å. The second shell Ag32 was a slightly distorted Ino decadedron. The Ag–Ag bond lengths in this shell range from 2.779 to 2.907 Å with an average of 2.844 Å, which was slightly shorter than the Ag–Ag bond length of bulk silver (ca. 2.889 Å), indicating the characteristics of metallic bonds. The third shell Ag77 was also an Ino decadedron. It is worth noting that each vertex of Ag77 Ino decadedron constructs a decahedron with the corresponding vertex of Ag32 (see Supporting Information Figure S3), which is different from the reported Ag206∼211 clusters.50,51 Due to the coordination of chlorides, it was found that silver atoms have a special arrangement between icosahedron and decahedron (Figure 1f and Supporting Information Figures S4–S6). It formed some short and strong Ag–Ag bonds between the second and third shells. The average value (2.834 Å) is shorter than that between Ag32 and Ag77 Ino decadedrons observed in Ag206 and Ag211 with the average values of 2.879 and 2.848 Å, respectively. The 12 rest of the 33 Cl were respectively located at the poles of pseudodecahedrons or pseudooctahedrons (Figure 1g), connecting the core Ag116 and the shell Ag97(Adm-S)44Cl21. For the outermost shell, the Ag–Ag distances are between 2.786 and 3.821 Å. Due to the coordination of the organic layer, the average value was relatively larger, reaching 3.160 Å. The bond distance details of shell-by-shell in Ag213 are shown in Supporting Information Figure S7, as well as the summary information listed in Supporting Information Table S2. Figure 1 | Structure anatomy of Ag213. (a) Top view and (b) side view of overall structure with H atoms omitted for clarity. (c) Inner core Ag7 decahedron and (d) two-shell Ag7@Ag32. (e) The three-shell Ag7@Ag32@Ag77 Ino decadedrons and (f) the third shell Ag77. (g) 12 Cl located on the pentagonal surface. (h) The organic layer containing 44 S and 33 Cl. Color code: Ag, blue, pink, orange, dark green, or red; S, yellow; Cl, green; C, gray. Download figure Download PowerPoint 44 Adm-S and 33 Cl on the surface of Ag213 were located in different coordination environments (Figure 1h). The S of Adm-S had two coordination modes, namely μ3 and μ4. In addition to 8 μ3-SAg3 on the equatorial plane, there were 36 μ4-SAg4 distributed on the surface (Figure 2a and Supporting Information Figures S8 and S9). The average distances of Ag–S are 2.481 and 2.611 Å, respectively. Cl was coordinated with Ag in μ2 or μ3 mode (Figure 2b and Supporting Information Figure S10), and the average Ag–Cl distance was 2.653 Å. In addition, it was found that ClAg5 and ClAg6 formed the pseudooctahedron and pseudodecahedron, respectively (Figure 2c and Supporting Information Figure S11). The vertical bond lengths of these polyhedrons ranged from 2.508 to 2.589 Å while the distances of other weak interactions were between 2.731 and 3.858 Å, which are much longer than the normal Ag–Cl bond lengths. (The summary of Ag-S and Ag-Cl distances is listed in Supporting Information Tables S3 and S4.) Figure 2 | The types of S/Cl-Ag motifs of Ag213, (a) μ3 and μ4 coordination modes of Adm-S group, (b) μ2 and μ3 coordination modes of Cl atom, (c) ClAg5 pseudooctahedron and ClAg6 pseudodecahedron. Color code: Ag, blue; S, yellow; Cl, green. (d) ESI-MS of Ag213 in DCM, and (e) the enlarged illustrations with experimental (black line) and simulated (red line) isotopic patterns for [Ag213(Adm-S)43Cl34+5CH2Cl2+3H]3+ (m/z = 10,600.88, calcd 10,600.77), [Ag213+5CH2Cl2+3H]3+ (m/z = 10,644.81, calcd 10,644.80), [Ag213(Adm-S)45Cl32+5CH2Cl2+3H]3+ (m/z = 10,688.45, calcd 10,688.50) and corresponding Na+-adducts [Ag213(Adm-S)43Cl34+5CH2Cl2+2H+Na]3+ (m/z = 10,607.15, calcd 10,607.10), [Ag213+5CH2Cl2+2H+Na]3+ (m/z = 10,651.15, calcd 10,651.14), [Ag213(Adm-S)45Cl32+5CH2Cl2+2H+Na]3+ (m/z = 10,695.13, calcd 10,695.18). Download figure Download PowerPoint The composition of Ag213 nanoclusters was further confirmed by ESI-MS measurements using DCM as the solvent (Figure 2d). Two mass peaks were observed at m/z = 7983.86 (calcd 7983.86) and 10644.81 and attributed to [ Ag213+5CH2Cl2+nH]n+ (n = 3, 4). The isotopic distribution was fully consistent with the simulation results (Figure 2e and Supporting Information Figure S12). There were two sets of mass peaks at m/z = 10,600.88 and 10,688.45 on either side of 10,644.81. Interestingly, the difference values between them were almost equal, that is about 43.95 ( Supporting Information Figure S13). This means that the ligand exchange between thiolate and chloride occurred.50,61 Moreover, each of the three peak groups in Figure 2e had a set of overlapping peaks, which were assigned to the corresponding Na+-adducts [ Ag213+5CH2Cl2+2H+Na]3+ (Figure 2e and Supporting Information Figure S13). The other two sets of mass peaks at m/z = 5346.83 (calcd 5346.84) and 6186.42 (calcd 6186.46) are respectively attributed to the fragments [Ag50(Adm-S)30Cl8]2+ and [Ag58(Adm-S)33Cl17+H]2+ of Ag213, which are unstable under electron spray ionization, possibly due to the weak coordination ability of chlorides. EDS analyses further confirmed the ratio of Ag/S/Cl atoms in Ag213 crystals, which was consistent with the results of SCXRD (see Supporting Information Figure S14). Unexpectedly, besides the black crystals of Ag213, a few red crystals were obtained in one pot. The composition was determined as Ag56(Adm-S)33Cl16 ( Ag56 Cl) by SCXRD (see Supporting Information Table S5) and ESI-MS. Ag56 Cl had a core–shell structure with a Ag13Cl4 core encapsulated by Ag43(Adm-S)33Cl12 shell (Figure 3a and Supporting Information Figure S15). Connecting the core to the shell were 15 Cl. This was different from that distributed on the surface of Ag213 because these chlorides lead to the disorder of the Ag13 core (see Supporting Information Figures S16 and S17). A set of mass peaks at m/z = 6065.08 observed in ESI-MS spectra were assigned to [ Ag56 Cl+2H]2+ (Figures 3b and 3c and Supporting Information Figure S18), consistent with the results of SCXRD analyses. By increasing the reaction time of silver salt and Adm-S and controlling the amount of Ph4PCl, the pure product Ag213 can be obtained. Figure 3 | (a) Crystal structure of Ag56 Cl with the shell of 50% transparent and C/H atoms omitted for clarity. Color code: Ag, purple or blue; S, yellow; Cl, green. (b) ESI-MS of Ag56 Cl in DCM, with (c) the experimental (black line) and simulated (red line) isotopic patterns for [Ag56 Cl+2H]2+ (peak a, m/z = 6065.08, calcd 6065.07), [Ag56(Adm-S)33Cl17+2H]2+ (peak b, m/z = 6083.05, calcd 6083.05), [Ag57(Adm-S)33Cl16+2H]2+ (peak c, m/z = 6119.02, calcd 6119.02), [Ag57(Adm-S)33Cl17+2H]2+ (peak d, m/z = 6137.00, calcd 6137.00). Download figure Download PowerPoint Electrocatalysis It was calculated that the reported surface coverage of thiolates on Ag136 and Ag211 was ∼78% and ∼80%, respectively.51,56 Compared with those protected by small-sized thiolates, nanoclusters with bulky Adm-S are apt to provide lower surface thiolate coverage, for example Ag141 was ∼57%.62 The value of Ag213 was smaller, only about 45%, suggesting some remaining surface sites can be accessible by smaller-sized ligands such as chlorides. The particular surface structure would make Ag213 the high surface reactivity. Ag213 nanoclusters show excellent electrocatalytic performance for ORR under an alkaline environment. Ag213 crystals were mixed with activated carbon in an ultrasonic bath to form a catalyst Ag213/C loaded with 20% C. Commercial Pt/C (20 wt %) was conducted to compare with Ag213/C of the same weight. Potential values of the reversible hydrogen electrode (RHE) scale were calibrated according to the following equation: E RHE = E ( Ag / AgCl ) + 0.197 + 0.059 * pH The ORR catalytic performance was recorded by linear sweep voltammetry (LSV) measurements (Figure 4a). The onset potential (Eonset) of Ag213/C catalyst was 0.89 V, and the half-wave potential (E1/2) was 0.72 V while the values of commercial Pt/C catalyst were 1.09 and 0.90 V, respectively. The E1/2 of Ag213/C catalyst was higher than that of Ag22 (0.63 V) and AuAg21 (0.66 V) reported by Zhu et al.66 but lower than that of carbon-supported monoclinic Pd5Bi2 nanocrystals (0.93 V).67 In addition, the gold nanomolecule composites Au279/SWNT (SWNT = single-walled carbon nanotube) also showed catalytic activity with a comparable Eonset value of 0.89 V.68 The observed excellent catalytic performance of Ag213/C mentioned above can be attributed to the high surface reactivity and the unique electronic structure of Ag213.50 Figure 4 | LSV curves and UV–vis absorption spectra. (a) LSV curves of Ag213/C and Pt/C with scan rates of 10 mV/s. ORR tests were performed with RDE (geometric area = 0.196 cm2, 1600 rpm). Ag/AgCl electrode and carbon rod were used as reference and counter electrodes, respectively. The electrolyte is 0.1 M KOH (pH 13) solution saturated with pure O2. UV–vis absorption spectra of (b) Ag213 (blue line), Ag56 Cl (orange line), Ag56 Br (red line), and (c) Ag213 respectively mixed with PPh4Cl (blue line) and PPh4Br (cyan line) in DCM. Download figure Download PowerPoint UV–vis absorption The DCM solution of Ag213 showed a dominant absorption band centered at 454 nm (Figure 4b), which was similar to the surface plasmon resonance absorption band of metallic silver nanoparticles. Compared with Ag141 (460 nm), Ag210-211 (464 nm), Ag374 (465 nm), and Ag307 (473 nm),51,55,56,62 however, the absorption wavelength of Ag213 in solution was shorter. The Ag213 solution was stable at room temperature for at least 2 weeks whether or not exposed to light (see Supporting Information Figures S19 and S20). Distinct from Ag213, the Ag56 Cl had a molecule-like multiband absorption in solution, including a prominent peak at 425 nm and three shoulders at 336, 396, and 513 nm (Figure 4b). The low-energy absorption peak (513 nm) can be attributed to charge transfer within the metal core, while the high-energy absorption shoulders (336, 396, and 425 nm) mainly came from the mixing of metal-to-metal and/or metal-to-ligand charge transfer processes (MMCT and/or MLCT).61 The number of free electrons for Ag213 was calculated to be 136e (213-44-33), which is very close to, but not yet achieving, the magic shell electron count of 138e.69 The role of halide To study the role of halide in the reaction, Ph4PBr was used instead of Ph4PCl in parallel experiments. Instead of Ag213 homologue, the Ag56 Cl homologue was obtained with a formula of Ag56(Adm-S)33Br16 ( Ag56 Br) determined by SCXRD (see Supporting Information Table It is to Cl a with Adm-S to the formation of Ag213. Ph4PBr to Ag213 solution, the recorded UV–vis was similar to Ag56 Br (Figure indicating that Ag213 can be into Ag56 which also that bromide can promote the formation of Ag56 The core–shell structure of Ag56 Br core and (see Supporting Information Figures and was the same as Ag56 Cl. The average of Ag56 Cl and Ag56 Br were both reported clusters with similar nm), nm), and of the three largest silver nanoclusters, Ag213, coprotected by bulky thiolates and chlorides, was separated the core–shell structure with a Ag77 shell. This is the first report on electrocatalytic oxygen reduction activity of nanoclusters with more than 100 silver atoms. Supported on activated carbon Ag213 nanoclusters exhibit excellent electrocatalytic oxygen reduction due to the low surface coverage of thiolates on nanoclusters. The Eonset and E1/2 of the Ag213/C catalyst are 0.89 and 0.72 V, respectively, close to the values and 0.90 V) of commercial Pt/C catalyst. In addition, it is worth that halide is important in Chloride facilitates the formation of Ag213 and Ag56 Cl, while bromide can promote the formation of Ag56 The Ag213 solution the surface plasmon resonance absorption band of metallic silver and (see Supporting Information Figure as well while the Ag56 Cl solution has absorption This work provides a example for the study of large-sized metal nanoclusters and nanocluster-based electrocatalysts. Supporting Information Supporting Information is and SCXRD details of synthesis and ESI-MS and UV–vis and and The data were in the with of ( ( Ag56 and ( Ag56 Br). 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