Isomerism in Au–Ag Alloy Nanoclusters: Structure Determination and

1 day ago - Synopsis. Herein, we have obtained a pair of structural isomers, [Au9Ag12(SR)4(dppm)6X6]3+-C and [Au9Ag12(SR)4(dppm)6X6]3+-Ac [dppm = bis(...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Isomerism in Au−Ag Alloy Nanoclusters: Structure Determination and Enantioseparation of [Au9Ag12(SR)4(dppm)6X6]3+ Shan Jin,† Fengqing Xu,† Wenjun Du, Xi Kang, Shuang Chen, Jun Zhang, Xiaowu Li, Daqiao Hu, Shuxin Wang,* 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 S Supporting Information *

ABSTRACT: Revealing structural isomerism in a nanocluster remains significant but challenging. Herein, we have obtained a pair of structural isomers, [Au9Ag12(SR)4(dppm)6X6]3+-C and [Au9Ag12(SR)4(dppm)6X6]3+-Ac [dppm = bis(diphenyphosphino)methane; HSR = 1-adamantanethiol/tertbutylmercaptan; X = Br/Cl; C stands for one of the structural isomers being chiral; Ac stands for another being achiral], that show different structures as well as different chiralities. These structures are determined by single-crystal X-ray diffraction and further confirmed by high-resolution electrospray ionization mass spectrometry. On the basis of the isomeric structures, the most important finding is the different arrangements of the Au5Ag8@Au4 metal core, leading to changes in the overall shape of the cluster, which is responsible for structural isomerism. Meanwhile, the two enantiomers of [Au9Ag12(SR)4(dppm)6X6]3+-C are separated by high-performance liquid chromatography. Our work will contribute to a deeper understanding of the structural isomerism in noble-metal nanoclusters and enrich the chiral nanocluster.

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nanoscale engineering.42 Pradeep and co-workers have recently reported other instances of isomerism in monolayer-protected silver cluster ions, based on gas-phase (ion-mobility) measurements.43 Besides, with regard to phosphine-protected Au9 clusters, isomerization can be observed in which [Au 9 (PPh 3 ) 8 ] 3 + with a crown motif changes into [Au9(PPh3)8]3+ with a butterfly motif during the evaporation process in the presence of different counterions.26c,44 A detailed analysis of these isomers provides an ideal way to understand the structure−property correlations. Meanwhile, chirality has recently become an intensively studied field because it opens new possibilities in catalysis applications.45,46 Further, information from the total X-ray structure determination,47−49 CD50,51 and NMR52,53 spectroscopies, as well as theoretical calculations,54,55 confirmed the existence of chirality in the nanocluster and gave deep insight into its origin. With regard to the metal cluster, the arrangement of chiral ligands on the achiral metal surface can lead to chiral patterns. Also, achiral ligands can also form local chiral patterns on an achiral surface. Structurally, a series of nanoclusters have been found to have chiral arrangements, such as Au28(TBBT)20,38 Au30S(S-tBu)18,34 Au36(SPh)24,47 Au38(SR)24,40,41 Au40(o-MBT)24,36 Au130(p-MBT)20,48 Au133(SPhp-But)52,49 [Au20(PP3)4]Cl4,23c [Ag78(Dppp)6(SR)42],27 [Ag23(PPh3)8(SC2H4Ph)18],28 [Ag32(dppm)5(SAdm)13Cl8]3+, and

INTRODUCTION Significant progress on the controlled synthesis and structural characterization of ligand-protected atomic precise metal clusters has promoted its application in catalysis, biological, electrochemistry, and solar cells.1−8 Furthermore, enormous progress on structure−property correlation was made after clusters protected by thiol and phosphine or coprotected by phosphine and thiol (halogen) were confirmed by single-crystal X-ray diffraction (SCXRD) along with density functional theory.9−29 Among the reported clusters, the experimental observation of the structural isomerism remains a mystery because of the challenge of unraveling the intrinsic structure at the atomic level. So far, a few structural isomers in the nanocluster have been reported, such as [Au 13 (PMe 2 Ph) 10 Cl 2 ] 3+ and [Au 13 (dppmH) 6 ](NO 3 ) n ], 24 [Au11(Ph2P(CH2)2PPh2)6]3+ and [Au11(Ph2P(CH2)3PPh2)5]3+,25 Au21(StBu)15 and Au21(SAdm)15,30,31 Au24(SCH2PhtBu)20 and Au24(SePh)20,32,33 Au30S(S-tBu)18 and Au30(SAdm)18,34,35 Au52(PET)32 and Au52(TBBT)32,36,37 and so on. Au28(SR)20 clusters show ligand-induced isomerization, as revealed by their crystal structures.38,39 Recently, a pair of structural isomers of the Au38(SCH2CH2Ph)24 cluster were discovered by the Wu and Jin groups,40,41 by modification of the synthetic protocol, in which the two isomers exhibit distinctly different optical, stability, and catalytic properties. Meanwhile, the discovery of polymorphism in magic-sized Au144(SR)60 clusters using atomic pair distribution function analysis of X-ray diffraction opens a new dimension in © XXXX American Chemical Society

Received: January 19, 2018

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DOI: 10.1021/acs.inorgchem.8b00183 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry [Ag45(dppm)4(S-But)16Br12]3+.56 These well-determined nanoclusters will hopefully provide opportunities to further explore the origin of the chirality at the atomic level and shed light on the controllable synthesis of new nanoclusters with chiral properties using multiligands.24c,27,28,56 Herein, we report other novel alloy structural isomers, [Au9Ag12(SR)4(dppm)6X6]3+-C and [Au9Ag12(SR)4(dppm)6X6]3+-Ac (where SR = SAdm/S-tBu; X = Cl/Br; C stands for one of the structural isomers being chiral; Ac stands for another being achiral) in alloy nanoclusters. Importantly, the novel structural isomeric Au−Ag bimetallic nanoclusters show different chiral properties, in which one is chiral and the other is achiral. Furthermore, the optical isomers of the alloy nanocluster [Au9Ag12(SR)4(dppm)6X6]3+-C are separated by high-performance liquid chromatography (HPLC). These nanoclusters have similar Au5Ag8 kernels; however, the arrangement of the four extra Au atoms on these kernels leads to differences in the structure, which results in the chirality differences.

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EXPERIMENTAL SECTION

The synthetic procedures are shown in Scheme 1 (for details, see the Supporting Information). Briefly, HAuCl4·3H2O and AgNO3 dissolved

Scheme 1. Synthetic Procedure for the Structural Isomer Au9Ag12(SR)4(dppm)6X6]3+ (X = Br/Cl)

Figure 1. ESI-MS spectra of (A) [Au9Ag12(TBM)4(dppm)6Br6]3+-Ac, (B) [Au9Ag12(TBM)4(dppm)6Br 6] 3+-C, (C) [Au9Ag12(SAdm)4(dppm)6Br6]3+-Ac, and (D) [Au9Ag12(SAdm)4(dppm)6Br6]3+-C and optical absorption spectra of the (E) [Au9Ag12(SR)4(dppm)6Br6]3+-Ac and (F) [Au9Ag12(SR)4(dppm)6Br6]3+-C nanoclusters in dichloromethane as well as the spectra on the energy scale of the (G) [Au9Ag12(SR)4(dppm)6Br6]3+-Ac and (H) [Au9Ag12(SR)4(dppm)6Br6]3+-C nanoclusters. The insets of parts G and H are zooms of the spectra.

in methanol are mixed in toluene with tetra-n-octylammonium bromide/chloride). Then, the bis(diphenylphosphino)methane (dppm) and thiol ligands are added to the above solution. After that, a reducing agent (i.e., NaBH4) is rapidly added. Alloy nanoclusters are then precipitated out of the solution. The precipitates are washed with hexane and collected by centrifugation. The asobtained products consist of structural isomers. Then the structural isomers can be separated by methanol, in which one is chiral and the other is achiral. The final nanoclusters in CH2Cl2 are crystallized by layering hexane. SCXRD reveals that the composition of the synthesized product is [Au9Ag12(SR)4(dppm)6X6]3+ (SR = SAdm/ TBM; X = Cl/Br). After careful analysis of the differences of the crystal structure, the [Au9Ag12(SR)4(dppm)6X6]3+ nanocluster can be divided into structural isomers. Meanwhile, more important findings are that the structural isomers have different chiral properties, in which one is chiral (Au9Ag12-C for short hereafter) and the other is achiral (Au9Ag12-Ac for short hereafter). Further, the optical enantiomers of Au9Ag12-C are separated by HPLC. High-resolution electrospray ionization mass spectrometry (ESIMS) is performed subsequently to determine the molecular formula and valence state. The mass spectra of the structural isomer [Au9Ag12(TBM)4(dppm)6Br6]3+ are shown in Figure 1A,B; the intense peak centered at m/z 2069.94 corresponds to the 3+ charge and can be perfectly assigned by the calculated result (m/z 2069.98). Analogous analysis is also carried on the structural isomer [Au9Ag12(SAdm)4(dppm)6Br6]3+. The intense peak centered at m/z 2174.01 (calculated as m/z 2174.04; Figure 1C,D, insets) corresponds to the 3+ charge. The 3+ charges of all samples correspond well with the X-ray crystal data. It is worth noting that using Cl instead of Br has

the same result in both chiral and achiral cases; the ESI-MS and UV− vis spectra of [Au9Ag12(SR)4(dppm)6Cl6]3+ can be found in Figure S1. Note that the peaks labeled by asterisks in Figures 1A−D and S1A−D correspond to [Au9Ag12(SR)4(dppm)6X6(SbF6)]2+ and have also been observed in the mass spectra of [Ag19(dppm)3(PhCC)14](SbF6)3.57 The experimentally isotopic peaks are in perfect agreement with the simulated ones (Figure S2). As illustrated in Figure 1E, both SAdm- and TBM-ligand-protected [Au9Ag12(SR)4(dppm)6X6]3+-Ac nanoclusters show six similar peaks at 322, 380, 410, 487, 522, and 625 nm in the optical absorption spectra. Nonetheless, a small difference for the absorption spectra is found with regard to the [Au9Ag12(SR)4(dppm)6X6]3+-C nanoclusters. Aside from the similar peaks at 322, 380, 410, and 487 nm, the last optical peak of the SAdm section shows a 30 nm red shift compared with the TBM section. Subtle differences might originate from the different steric hindrances of S-tBu and SAdm. Furthermore, parts G and H of Figure 1 show the energy-scale absorption spectra of [Au9Ag12(SR)4(dppm)6Br6]3+-Ac and [Au9Ag12(SR)4(dppm)6Br6]3+C nanoclusters, with the optical energy gaps of [Au9Ag12(SR)4(dppm)6Br6]3+-Ac being 1.56 eV and those of [Au9Ag12(SR)4(dppm)6Br6]3+-C being 1.51/1.58 eV, respectively. Because of their close energy levels, the structural isomerism of the alloy nanoclusters can be stable to obtain the crystals. B

DOI: 10.1021/acs.inorgchem.8b00183 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

icosahedron. The locations of the Au and Ag atoms are fixed, and the five Au atoms are almost in the same plane (Figure 2). The Au5Ag8 icosahedron and four Au atoms constitute the Au5Ag8@Au4 metallic kernel of [Au9Ag12(SR)4(dppm)6X6]3+. Then the surface shells protect the nucleus perfectly. Considering that the icosahedral structures of the structural isomer [Au9Ag12(SR)4(dppm)6X6](SbF6)3 nanocluster are similar, the arrangement of the extra four Au atoms on these kernels leads to different structures, resulting in the chirality differences. With respect to the differences in the structural isomers consisting of chiral Au9Ag12-C and achiral Au9Ag12-Ac, we split the structure step by step. As shown in Figure 3A, the icosahedral Ag8Au5 cores are similar within the two nanocluster isomers. The nuclear average distance for both nanoclusters is 2.89 Å, which is slightly longer than that in bulk (2.80 Å). For each nanocluster, with continued growth of an extra Au atom, the Au5Ag8@Au1 structure is formed without obvious differences (Figure 3B). Significant variation occurs with the growth of the second Au atom (Figure 3C,D), which prompts the Au5Ag8@Au2 kernel to assume two forms: growth along the same or opposite sides of the first Au atoms. When two Au atoms are on the same side, the two dppm ligands (bonding the added Au atoms and the icosahedral Ag8Au5 core) increase the steric hindrance, meaning that the other Au-dppm section (Figure 3E, highlighted in blue) cannot be added to the same side and the achiral Au5Ag8@Au4 metallic kernel forms (Figure 3E). If the two added Au atoms are on opposite sides, the combination of the twisted dppm ligands leads to an asymmetric, chiral Au5Ag8@Au4 5 metallic kernel (Figure 3F). In both Au9Ag12-Ac and Au9Ag12-C, the peripheral Ag and Au atoms are all connected by thiol ligands to form the final total structure. The peripheral four Ag atoms are capped by phosphine ligands and connected with the kernel Ag atoms by μ3-X (X = Br/Cl) atoms;

The structures of the obtained structural isomer nanoclusters are determined by SCXRD. Because of the low signal-to-noise ratio of the crystal diffraction data for [Au9Ag12(S-But)4(dppm)6Cl6](SbF6)2.5-C and the high residual electron density generated from the tail effect of heavy-metal atoms like Au and Ag, the corresponding half missing anion cannot be determined. However, through ESI-MS and other characterizations, the charge state of such a nanocluster can be confirmed to be +3. The unit cell of chiral Au9Ag12-C contains a pair of enantiomers (Figures 2A and S3), while achiral Au9Ag12-Ac does not

Figure 2. Structures of the structural isomer [Au9Ag12(SR)4(dppm)6X6]3+: (A) [Au9Ag12(SR)4(dppm)6X6]3+-C; (B) [Au9Ag12(SR)4(dppm)6X6]3+-Ac. Color code: sky blue, Ag; orange, Au; red, S; green, Br or Cl; pink, P; gray, C. (Figures 2B and S4). The detailed overall structural analysis is as follows: five Au atoms and eight Ag atoms constitute the informal

Figure 3. Divergence in the formation of the structural isomers: Au9Ag12-Ac and Au9Ag12-C. (A) formation of the icosahedron Ag5Au8 nucleus; (B) addition of a first extra Au atom to give the structure Au5Ag8@Au1, which is similar in both chiral and achiral nanoclusters; (C and D) addition of a second extra Au atom to give different Au5Ag8@Au2 arrangements capped by dppm ligands (These are shown in space-filled and line modes, respectively); (E and F) formation of the chiral and achiral Au5Ag8@Au4 structures; (G and H) total structures of achiral and chiral alloy nanoclusters, respectively. Color code: sky blue, Ag; orange/yellow, Au; red, S; green, Br or Cl; gray/blue, C; pink, P. C

DOI: 10.1021/acs.inorgchem.8b00183 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry meanwhile, the four Ag atoms on the edge of the icosahedron are capped by two μ 2 -X (X = Br/Cl) atoms (Figure 3G,H). Comprehensive analysis of the two kinds of growth patterns indicates that the arrangement of the extra four Au atoms on these kernels leads to the structural isomerism, which then results in the chirality differences. Further, the two enantiomers of the Au9Ag12-C nanoparticle are shown in Figure 2A. The chirality can be viewed from the left- and right-handed arrangements of the Au5Ag8@Au4 kernel. We define the left and right isomers by the rotation of the Au4 units on the M13 kernel. The outside ligands also impact the chirality of the Au9Ag12-C particles. Further, the chiral HPLC is used to separate the enantiomers according to the reported method.58 Circular dichroism (CD) spectroscopy is used to characterize the as-separated enantiomers (Figure 4). The mirror-imaged CD spectra show three distinct bands at 325, 363, 428, and 483 nm, weak peaks at 340, 400, and 448 nm, and a broad peak at ∼590 nm.

ing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Shuxin Wang: 0000-0003-0403-3953 Manzhou Zhu: 0000-0002-3068-7160 Author Contributions †

S.J. and F.X. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support by the National Natural Science Foundation of China (Grants 21372006, U1532141, 21602002, and 21631001), Ministry of Education and Education Department of Anhui Province, and 211 Project of Anhui University. We are grateful to Dr. Jiangwei Zhang for his great help on single-crystal analysis.

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Figure 4. CD spectra of enantiomers 1 and 2 for the Au9Ag12-C nanocluster.

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CONCLUSIONS In summary, we have discovered for the first time structural isomerism in the alloy nanocluster [Au9Ag12(SR)4(dppm)6X6]3+, which contains a pair of chiral Au9Ag12-C and achiral Au9Ag12-Ac, and separated the two enantiomers of Au9Ag12-C by HPLC. The structures are determined by SCXRD and further confirmed by ESI-MS. Although these chiral and achiral nanoclusters have similar icosahedral Ag8Au5 cores, the differences in the growth model of the outside Au-dppm section contribute to the structural isomerism of the Au9Ag12 nanoclusters. Our work might hopefully shed new light on the structural isomerism in the nanocluster range and may also motivate more studies on chirality.

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REFERENCES

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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.8b00183. Details of the synthesis process, characterization, HPLC separation, X-ray analysis, graphic information, and Figures S1−S4 and Tables S1−S3 offering more details on the nanocluster [Au9Ag12(SR)4(dppm)6X6]3+ (PDF) Accession Codes

CCDC 1580344−1580346 contain 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 emailD

DOI: 10.1021/acs.inorgchem.8b00183 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00183 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00183 Inorg. Chem. XXXX, XXX, XXX−XXX