Unexpected Observation of Heavy Monomeric Motifs in a Basket-like

Dec 17, 2018 - Herein, a new basket-like Au26Ag22(TBBT)30 nanocluster (where TBBT = 4-tert-butylthiophenol) was synthesized by a coreduction method...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Unexpected Observation of Heavy Monomeric Motifs in a Basketlike Au26Ag22 Nanocluster Hao Li, Yongbo Song,* Ying Lv, Yapei Yun, Xinrou Lv, Haizhu Yu, and Manzhou Zhu* Department of Chemistry and Center for Atomic Engineering of Advanced Materials & AnHui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui 230601, China

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S Supporting Information *

Au12@AuAg19 kernel protected by 13 monomeric motifs (−S− Au−S−) and a handle-like Ag3S4 motif. It is worth noting that heavy monomeric motifs are observed for the first time in thiolate-capped metal nanoclusters with smaller size (n < 60). Also, the composition of Au26Ag22 was further testified by electrospray ionization mass spectrometry (ESI-MS), thermogravimetric analysis (TGA), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS). A detailed synthesis is provided in the Supporting Information. Briefly, the as-prepared Au(I)-TBBT complexes were resolved in tetrahydrofuran and then mixed with a methanol solution of AgNO3. After 30 min, the color of the reaction system was light yellow, which showed a smooth curve in the UV−vis absorption spectrum (Figure S1). Then, an aqueous solution of NaBH4 was rapidly added to the above solution. Then the color of the solution changed from light yellow to black, indicating that the polydisperse metal nanoclusters were being generated. After 12 h, the reaction was stopped, and the organic solution was dried by rotary evaporation and washed several times with methanol to remove the residual ligands and byproducts. Finally, the pure product was obtained, which showed a main peak at 515 nm and a shoulder peak at 400 nm in the UV−vis absorption spectrum (Figure 1A). Furthermore, the chemical composition product was characterized by ESI-MS. As shown in Figure 1B, the mass spectrum shows a domain peak at 12585.03 Da, which corresponds to [M + Cs]+ (M = Au26Ag22(TBBT)30; theoretical

ABSTRACT: Herein, a new basket-like Au26Ag22(TBBT)30 nanocluster (where TBBT = 4-tertbutylthiophenol) was synthesized by a coreduction method. Also, the precise structure was characterized by single-crystal X-ray diffraction and displays a shell-by-shell construction with a Au12@AuAg19 kernel protected by a Au13S26@Ag3S4 surface motif. Interestingly, a handle-like Ag3S4 motif was observed that composes the outermost shell. Except for this Ag3S4 motif, the other motifs are all monomeric “−S−Au−S−” motifs, which are rarely reported in thiolate-capped metal (Au or Ag) nanoclusters with small size (n < 60). This work will provide new insight into the growth pattern of thiolate-capped metal nanoclusters.

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etal nanoclusters with precise atoms have attracted much attention1−3 because of their potential application in catalysis,4,5 chemical sensing,6,7 biolabeling,8,9 and drug carriers.10,11 Among these, the thiolate-protected metal (Au and Ag) nanoclusters have been widely studied, and great progress has been achieved in their preparation and structural determination.12−27 It is well-known that the structure of the metal nanocluster usually contains a metal kernel capped by surface motifs.1 Recently, the surface motifs have aroused wide interest because of their significant role in metal nanoclusters. For example, Jiang et al. proposed that the surface motifs play a critical role for the stability of metal nanoclusters.28,29 In addition, some scientists found that tailoring the construction of surface motifs can effectively regulate the properties or performance of metal nanoclusters.30−32 For instance, Chen and co-workers reported a Au20 nanocluster with chirality induced by the arrangement of surface motifs.31 Finally, the surface motifs also can offer insight for understanding the growth mechanism of metal nanoclusters (e.g., the proposition of the force-field-based divide-and-protect approach by Pei)33 because the metal nanoclusters are usually generated from the metal−ligand complex.34 Even so, some fundamental problems are still unclear, such as the structural evolution and kernel− motif relationship. Thus, as a fundamental science, preparing more metal nanoclusters with different sizes and atom-packing modes is still a major issue to be addressed. Herein, we report a new basket-like Au−Ag bimetallic nanocluster: [Au26Ag22(TBBT)30] (TBBT = 4-tert-butylthiophenol, abbreviated as Au26Ag22). Its precise structure has been determined by single-crystal X-ray diffraction, which exhibits a © XXXX American Chemical Society

Figure 1. (A) UV−vis absorption spectrum of Au26Ag22. (B) ESI-MS spectrum of Au26Ag22 in positive mode with the addition of CsOAc. Inset: Comparison of the simulated (red) and experimental (black) isotope distribution patterns. Received: July 16, 2018

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DOI: 10.1021/acs.inorgchem.8b01990 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry value, 12585.16 Da). Also, the observed isotopic pattern in the experiment is in good agreement with the simulated one (Figure 1B, inset). In addition, TGA was employed to analyze the purity of the as-prepared product. As can be seen from Figure S2, a 39.75 wt % loss is observed in the TGA curve, which is consistent with the theoretical value (39.81 wt %) based on the chemical formula. This indicates that the as-prepared products possess high purity. ICP-MS was also performed, which shows that the element ratio of Au and Ag is 25.83:22.03, which is very close to 26:22 determined by single-crystal X-ray analysis. Black crystals were obtained by crystallization in toluene/ methanol for 2 weeks at 6 °C. The crystals were collected and then subjected to the single-crystal X-ray diffraction, which revealed that Au26Ag22 is crystallized in the triclinic space group P1̅ (Table S1). After structural analysis (see the Supporting Information for details), the precise structure of Au26Ag22 was determined, which contains a Au26Ag22 core protected by 30 thiolate ligands (TBBT; Figure 2A). Upon removal of the

Figure 3. Shell-by-shell construction of Au26Ag22: (A) the icosahedral Au12 shell; (B) the AuAg19 shell; (C) the Au12@AuAg19 kernel; (D) the Au13S26 shell; (E) the Au12@AuAg19@Au13S26 unit; (F) the Ag3S4 shell; (G) the Au12@AuAg19@Au13S26@Ag3S4 framework; (H) the total structure of Au26Ag22. Color labels: yellow, Au; green and blue, Ag; red and purple, S; gray, C. All of the H atoms are omitted for clarity.

2.78 Å, respectively, indicating that strong metal−metal bonding exists herein.38 Except for the Au12@AuAg19 kernel, the external protecting motifs also present some fascinating features, which are divided into two shells for better analysis. As shown in Figure 3D, the third shell contains 13 Au atoms and 26 thiolate ligands, which structures into 13 monomeric −S−Au−S− motifs. Interestingly, the 13 monomeric motifs locate on the surface of the AuAg19 shell in the one (motif)-on-one (pentagon) manner, except one pentagon contains two monomeric motifs (Figure S3; 13 = 11 + 2), which may be induced by the single doped Au atom in the AuAg19 shell. Also, more remarkable, the heavy monomeric motifs are first observed in small-sized metal nanoclusters. In previous work, Jin and Jiang et al. has suggested that metal nanoclusters with small size typically have more curved surfaces and thus require longer staple motifs to accommodate the increasing surface curvature, whereas in larger spherical nanoclusters, shorter staple motifs are found to be more common.1,39,40 For clarity, a summary of the surface motifs in some reported Au nanoclusters with precise structures is given in Table S2. So, herein, the observation of heavy monomeric motifs is unexpected, which not only offers new insight into the atompacking mode of the surface structure but also provides more possibilities for structural prediction in theory. Furthermore, the fourth shell is composed of only a Ag3S4 motif, which looks like the handle of a basket. The terminal S atoms exist with the coordination of μ3-S-Ag3 (Figure S4; two Ag atoms are from the AuAg19 shell). Also, the Ag atoms (highlighted with blue) in the Ag3S4 motif also exist with the coordination of μ3-Ag-S3 [two S atoms come from the Ag3S4 motif and one S atom comes from one “−S−Au−S−” (highlighted with purple)]. This coordination mode (μ3-S or μ3-Ag) will enhance the stability in construction. From a growth perspective, the Ag3S4 motif can be viewed as the beginning of the formation of the fourth shell. The stability of the Au26Ag22 nanocluster was monitored by UV−vis spectroscopy at room temperature in toluene. As shown in Figure S5, the UV−vis absorption spectra show no obvious changes within 2 months, indicating that the Au26Ag 22 nanocluster possesses good stability at room temperature. On the basis of the exact chemical formula, the free-valence electron of the Au26Ag22 nanocluster is determined as 18e (18 = 26 + 22 − 30),41 which follows the closed-shell electronic structure. This may be the key factor to account for its stability.

Figure 2. Total crystal structure (A) and framework (B) of the Au26Ag22(TBBT)30 nanocluster. Color labels: yellow, Au; green, Ag; red, S; gray, C. All of the H atoms are omitted for clarity.

terminal alkyl, a basket-like construction with a handle-like Ag3S4 motif is observed (Figure 2B). Furthermore, the framework of Au26Ag22 also shows a shell-by-shell construction, in which four shells are distinguished: an icosahedral Au12 shell, a dodecahedral AuAg19 shell, a Au13S26 shell, and a Ag3S4 shell. Thus, this nanocluster can also be written as Au12@AuAg19@ Au13S26@Ag3S4. To find details of the atom-packing structure, each of the four shells is displayed in Figure 3. As shown in Figure 3A, the average distance of Au−Au in the icosahedral Au12 shell is 2.80 Å, which is shorter than that in bulk gold (2.88 Å),20 as well as that of the icosahedral Au12 shell in rodlike Au25 (∼2.90 Å)35 and spherical Au25 nanoclusters (∼2.93 Å).36 This icosahedral Au12 shell is enclosed by a dodecahedral AuAg19 shell, in which 12 twisted pentagons are observed (Figure 3B), and the average distances of the Ag−Ag and Au−Ag are 3.22 and 3.23 Å, respectively. Combining these two shells, a Au13Ag19 (M32) kernel is formed (Figure 3C). It is worth noting that the M32 core has been observed in other thiolate-capped metal nanoclusters, such as [Ag44(SR)30]4−,13 [Au12Ag32(SR)30]4−,20 and [Au12Cu32(SR)30]4−,37 in which the Au atoms locate at the first shell and only the doping atoms (Ag or Cu) locate at the second shell. However, in Au26Ag22, one Ag atom is replaced by one Au atom in the second shell (AuAg19). It is a remarkable fact that each vertex in the Au12 shell corresponds to a pentagon in the AuAg19 shell (Figure 3C). The average distances of Au−Ag and Au−Au between the Au12 and AuAg19 shells are 2.89 and B

DOI: 10.1021/acs.inorgchem.8b01990 Inorg. Chem. XXXX, XXX, XXX−XXX

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In addition, XPS was also employed to analyze the valence states of the Au and Ag atoms in the Au26Ag22 nanocluster. Also, the spectra of Au 4f and Ag 3d are shown in Figure 4. As shown

Communication

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S.). *E-mail: [email protected] (M.Z.). ORCID

Haizhu Yu: 0000-0003-3010-1331 Manzhou Zhu: 0000-0002-3068-7160 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Z. acknowledges financial support by the NSFC (Grants 21372006, U1532141, and 21631001), the Ministry of Education, and the Education Department of Anhui Province. Y.S. acknowledges financial support by the NSFC (Grant 21801001).

Figure 4. XPS spectra of (A) Au 4f and (B) Ag 3d of the Au26Ag22(TBBT)30 nanocluster.

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in Figure 4A, the peak of Au 4f7/2 locates at 84.45 eV, which is higher than that of Au(0) (84.0 eV), indicating that the average valence state of the Au atoms in Au26Ag22 is between 0 and 1+.42 As for the binding energy of Ag 3d5/2, there are several different viewpoints. For example, Negishi et al. reported that Ag 3d5/2 with lower binding is in the oxidation side relative to that of Ag(0) (367.9 eV).42 Liu et al. reported that the binding energy of Ag 3d5/2 at 368.28 and 368.68 eV is corresponding to Ag(I) and Ag(0), respectively.43 However, Guo and Wang et al. reported the opposite.44,45 In this work, the binding energy of Ag 3d5/2 is 368.6 eV (Figure 4B), but it is difficult to determine the valence state of Ag atoms in Au26Ag22. For a better comparison, XPS of Ag(0) and Ag(I)-SR was performed. As shown in Figure S6, the binding energies of Ag 3d5/2 in Au26Ag22, Ag(0), and Ag(I)-SR are 368.60, 368.89, and 368.35 eV, respectively. These results demonstrate that the average valence state of the Ag atoms in Au26Ag22 is between 0 and 1+. In summary, we have directly synthesized a new basket-like Au26Ag22(TBBT)30 bimetallic nanocluster. Single-crystal X-ray diffraction reveals that this bimetallic nanocluster possesses a shell-by-shell construction, which can be written as Au12@ AuAg19@Au13S26@Ag3S4. Interestingly, heavy monomeric staple motifs are found in this Au26Ag22, which is unexpected in previous works. Furthermore, a handle-like Ag3S4 motif as the outermost shell is observed in this Au26Ag22. This work will offer new insight into the atom-peaking mode of the metal nanoclusters protected by thiolate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01990. Detailed synthesis method, characterization, crystal structure refinements, Figures S1−S6, and Tables S1 and S2 (PDF) Accession Codes

CCDC 1855055 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. C

DOI: 10.1021/acs.inorgchem.8b01990 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b01990 Inorg. Chem. XXXX, XXX, XXX−XXX