Thiol-Induced Synthesis of Phosphine-Protected Gold Nanoclusters

Sep 5, 2017 - Herein, we successfully utilize the thiol-induced synthesis method and obtain three atomically precise phosphine-protected gold nanoclus...
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Thiol-Induced Synthesis of Phosphine-Protected Gold Nanoclusters with Atomic Precision and Controlling the Structure by Ligand/Metal Engineering Shan Jin,‡ Wenjun Du,‡ Shuxin Wang,* Xi Kang, Man Chen, Daqiao Hu, Shuang Chen, Xuejuan Zou, Guodong Sun, 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, People’s Republic of China S Supporting Information *

ABSTRACT: Efficient synthesis of atomically precise phosphinecapped gold nanocluster (with >10 metal atoms) is important to deeply understand the relationship between structure and properties. Herein, we successfully utilize the thiol-induced synthesis method and obtain three atomically precise phosphine-protected gold nanoclusters. Single-crystal X-ray structural analysis reveals that the nanoclusters are formulated as [Au 1 3 (Dppm) 6 ](BPh 4 ) 3 , [Au18 (Dppm)6 Br4 ](BPh 4) 2, and [Au20 (Dppm)6 (CN) 6] (where Dppm stands for bis(diphenylphosphino)methane), which are further confirmed by electrospray ionization mass spectrometry, thermogravimetric analysis, and X-ray photoelectron spectroscopy. Meanwhile, [Au18(Dppm)6Br4](BPh4)2 could be converted into [Au13(Dppm)6](BPh4)3 and [Au20(Dppm)6(CN)6] by engineering the surface ligands under excess PPh3 or moderate NaBH3CN, respectively. Furthermore, according to the different binding ability of silver with halogen, we successfully achieved target metal exchange on [Au18(Dppm)6Br4](BPh4)2 with Ag-SAdm (where HS-Adm stands for 1-adamantane mercaptan) complex and obtained [AgxAu18−x(Dppm)6Br4](BPh4)2 (x = 1, 2) alloy nanoclusters. Our work will contribute to more intensive understanding on synthesizing phosphine-protected nanoclusters as well as shedding light on the structure−property correlations in the nanocluster range.

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INTRODUCTION Organic-capped metal nanoclusters represent an important class of nanomaterials that are widely used for catalysis,1,2 molecular sensing,3−5 electronics,6,7 and biological applications.8 Capping agents can help stabilize nanoclusters against aggregation9−18,23−26 and alter the structure of metal nanoclusters.19−22,27−29 The past few decades have witnessed significant progress in controlling metal nanoclusters with atomic precision and well-determined structures capped by thiol/phosphine ligands.9−29 These well-determined structures demonstrate the detailed information on outside ligands on the metal nanoclusters. Specifically, on the one hand, thiolates often protect metallic core with the formation of staple or oligomer motifs (i.e., RS-(Au/Ag-SR)n). On the other hand, phosphine ligands are applied to stabilize gold nanoclusters in a different form.30−37 Until now, a series of gold nanoclusters capped by phosphine ligands and dual ligands (such as halide) have been synthesized.30−37 Among these nanoclusters, a few larger phosphine-protected nanoclusters have been structurally determined, such as Au13,35 Au14,34 Au18,33 Au20,32 Au22,31 and Au39.30 However, compared with the mild synthesis methods of a series of thiol-protected nanoclusters, the synthesis of the © XXXX American Chemical Society

phosphine-protected gold nanocluster with determined structure remains challenging. Meanwhile, because of the strong interaction of S−Au bonds, thiol ligands can induce the restructuring and functionalization of phosphine−gold nanocluster. For example, the reaction of Au55(PPh3)12Cl6 with thiols yields thiolate-protected Au75 clusters, and 1.5 nm phosphine-stabilized nanocluster with ωfunctionalized thiols yields functionalized gold nanoparticles.38,39 In addition, by using water instead of organic solvent, the phosphine can be completely exchanged by thiolate to form Au25(SR)18 nanocluster.40 So it is uncommon if we find an ultrastable phosphine nanocluster that cannot be ligandetched and structure-altered by thiol ligands, and this nanocluster is easily enriched, while other less stable phosphine−gold nanoclusters in solution would be destroyed. Furthermore, templated doping method has been considered as an effective strategy to synthesize thiolated alloy nanocluster.41−45 But for reported phosphine-protected alloy nanoclusters, most of them are synthesized by the coreduction Received: June 11, 2017

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DOI: 10.1021/acs.inorgchem.7b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of two metal precursors in one-pot reactions.46−49 It raises a question whether templated doping could be used to prepare phosphine-protected alloy clusters or not? Herein, we utilized thiol ligands to induce the synthesis of phosphine-protected gold nanoclusters. The polydisperse gold nanoclusters are size-focused into atomically precise gold nanocluster with the presence of thiol ligands. With this strategy, three gold nanoclusters including[Au13(Dppm)6](BPh4)3, [Au18(Dppm)6Br4](BPh4)2, and Au20(Dppm)6(CN)6 (where Dppm stands for bis(diphenylphosphino)methane) were successfully synthesized. These three nanoclusters are determined by single-crystal X-ray crystallography (SCXC) and further verified by electrospray ionization mass spectra (ESIMS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Note that Au13, Au18, and Au20 nanoclusters with different charge and/or structure have been reported.32,33,35 Interestingly, the Au18 nanocluster can be transferred to Au13 or Au20 nanoclusters under excess PPh3 or moderate NaBH3CN, respectively. Meanwhile, according to the different binding ability of metal with halogen, we successfully achieved target metal exchange on [Au18(Dppm)6Br4](BPh4)2 (which serves as a template) with Ag-SAdm (where HS-Adm stand for 1-admantanethiol) complex and obtained [AgxAu18−x(Dppm)6Br4](BPh4)2 (x = 1, 2) alloy nanoclusters. The thiolate-induced strategy together with the surface/metal engineering will contribute insight to the understanding of synthesizing phosphine-protected gold/alloy nanoclusters with determined structure and on the ligand’s role in the structure transformation.

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colorless to dark. The reaction lasts for 5 h at room temperature. Precipitate is then collected by centrifugation (5 min at ∼7000 rpm), washed with excess methanol, and collected by centrifugation. Later, the obtained product is dissovled in a mixture of 2 mL of CH2Cl2 and 1 mL of CH3OH, and NaBPh4 (20 mg, 0.06 mmol) in 2 mL of CH3OH is added into the solution to replace the anions for crystallization. Green block crystals are crystallized from CH2Cl2/ hexane at room temperature after 4 d. The yield of [Au18(Dppm)6Br4](BPh4)2 is ∼30% (Au atom basis). The solubility of [Au18(Dppm)6Br4](BPh4)2 in the dichloromethane is not high but significantly higher when mixed with a little bit of methanol. Synthesis of [Au20(Dppm)6(CN)6] Nanoclusters. The method for synthesis of [Au20(Dppm)6(CN)6] is similar to that for [Au13(Dppm)6](BPh4)3 nanoclusters. The only difference is that NaBH3CN (200 mg, 3.18 mmol) is dissolved in methanol solution and then added to the colorless solution. The color of the solution instantaneously changs from colorless to dark, different from the gradual color change from colorless to dark over ∼1 min with the NaBH3CN dissolved in H2O. Orange block crystals are crystallized from CH2Cl2/hexane at room temperature after 4 d. The yield of [Au20(Dppm)6(CN)6] is ∼40% (Au atom basis). The [Au13(Dppm)6](BPh4)3, [Au18(Dppm)6Br4](BPh4)2, and [Au20(Dppm)6(CN)6] nanocluster are all synthesized by the thiolinduced size focusing method. To deeply understand experimental process of the thiol-induced size focusing method, the synthesis of Au18(Dppm)6Br4](BPh4)2 is enacted as an example. The synthetic process was monitored by high-resolution electrospray ionization mass spectrometry (ESI-MS) and X-ray photoelectron spectroscopy (XPS). Synthesis of the AgISR Complexes (SR = AdmSH). Aqueous solution (1 mL) of AgNO3 containing 33.8 mg is added to an ethanol solution (7 mL) containing 38 mg of AdmSH under vigorous stirring. After 5 min of stir-mixing, the products are centrifuged at 6500 rpm. The precipitate is collected and washed several times with ethanol to remove the redundant AdmSH. Target Metal Exchange of [Au16.1Ag1.9(Dppm)6Br4](BPh4)2 Nanoclusters. The [Au16.1Ag1.9(Dppm)6Br4](BPh4)2 is synthesized by AdmSAg target metal exchange with [Au18(Dppm)6Br4](BPh4)2. [Au18(Dppm)6Br4](BPh4)2 (10 mg) is dissolved in 10 mL of mixed solution (containing 7 mL of methylene chloride and 3 mL of methanol), and 2 mg of AdmSAg (powder) is added to the solution. The reaction lasts for 1 h at room temperature. After that, the reaction mixtures are centrifuged at 8000 rpm. The organic layer is separated from the precipitate and evaporated to dryness. The [Au16.1Ag1.9(Dppm)6Br4](BPh4)2 is obtained. Greenish-yellow block crystals are crystallized from CH2Cl2/hexane at room temperature after 7 d. X-ray Crystallographic Determination of Four Nanoclusters. The data collection of [Au18(Dppm)6Br4](BPh4)2 is performed on a Bruker Smart APEX II CCD diffractometer at 173 K, using graphitemonochromatized Cu Kα radiation (λ = 1.541 78 Å). The data collection of [Au13(Dppm)6](BPh4)3, [Au16.1Ag1.9(Dppm)6Br4](BPh4)2, and [Au20(Dppm)6(CN)6] is performed on a Bruker Smart APEX II CCD diffractometer at 173 K, using graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å). Data reductions and absorption corrections are performed using the SAINT and SADABS programs, respectively. The structure is solved by direct methods and refined with full-matrix least-squares on F2 using the SHELXTL software package. Note: There are two A alerts for the crystal data of Au13 nanocluster. Both alerts are due to the low quality of the crystal; however, the structure is without doubt. At first, all the metal atoms in the [Au16.1Ag1.9(Dppm)6Br4](BPh4)2 cluster are solved as Au (R1 = 7.89% and GOF = 1.253). X-ray diffraction data refinement involving partial occupancy is used to locate the Ag atom. After refinement and convergence, the free variables correspond to all positions. Results show that all gold atoms without connecting the halide ligands are larger than 1.0 (from 1.02 to 1.04), which indicates the atoms in these positions are 100% Au. The free variables corresponding to gold atoms that connected with halide show less than 1 (0.96 for Au04 and 0.75 for Au09) indicate residency of Ag. In the converged result, these two positions have Ag occupancy of 0.15 and 0.8, respectively (see CIF file

EXPERIMENTAL SECTION

Chemicals. Tetrachloroauric(III) acid (HAuCl4·3H2O, 99.99%), silver nitrate (AgNO3, 98%), tetraoctylammonium bromide (TOAB, 98%), sodium borohydride (NaBH4, 99.99%), sodium cyanoborohydride (NaBH3CN, 99.99%), bis(diphenylphosphino)methane (Dppm, 98%), 1-adamantane mercaptan (C10H16S, 99%), tetraphenylboron sodium (NaBPh4,98%), toluene (Tol, HPLC grade, Aldrich), methanol (CH3OH, HPLC, Aldrich), n-hexane (Hex, HPLC grade, Aldrich), dichloromethane (CH2Cl2, HPLC grade, Aldrich), and pure water were purchased from Wahaha Co. Ltd. All reagents were used as received without further purification. Synthesis of [Au13(Dppm)6](BPh4)3 Nanocluster. Typically, gold salt (HAuCl4·3H2O, 120 mg, 0.30 mmol) is added to the 20 mL methanol solution under vigorous stirring. Then bis(diphenylphosphino)methane (Dppm, 100 mg, 0.26 mmol) and adamantane mercaptan (AdmSH, 100 mg, 0.60 mmol) are added to form a colorless solution. After 10 min, a freshly prepared solution of NaBH3CN (200 mg, 3.18 mmol in 2 mL of H2O) is added. The color of the solution gradually changes from colorless to dark over 1 min. The reaction lasts for 5 h at room temperature. Precipitate is collected by centrifugation (5 min at ∼7000 rpm). The orange precipitate is washed with excess methanol and collected by centrifugation again. Later, the obtained product is dissolved in 2 mL of CH2Cl2, and NaBPh4 (20 mg, 0.06 mmol) in 2 mL of CH3OH is added into the solution to replace the anions for crystallization. Orange block crystals are crystallized from CH2Cl2/hexane at room temperature after 4 d. The yield of [Au13(Dppm)6](BPh4)3 is ∼40% (Au atom basis). Synthesis of [Au18(Dppm)6Br4](BPh4)2 Nanocluster. Typically, gold salt (HAuCl4·3H2O, 80 mg, 0.20 mmol) is added to a 30 mL toluene solution containing TOAB (200 mg, 0.366 mmol) under vigorous stirring. The solution color changes from light yellow to red gradually after 15 min. Then, bis(diphenylphosphino)methane (Dppm, 50 mg, 0.13 mmol) and adamantane mercaptan (AdmSH, 50 mg, 0.30 mmol) are added to form a colorless solution. After 30 min, a freshly prepared solution of NaBH4 (38 mg, 1 mmol in 2 mL H2O) is added. The color of the solution immediately changes from B

DOI: 10.1021/acs.inorgchem.7b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Reasonable prediction of reaction diagram. Color labels: golden = Au; red = S; light green = P; gray = C.

Figure 2. (A) UV−vis absorption spectrum of the purified three nanoclusters in dichloromethane. (B) TGA and (C) XPS results of the purified three nanoclusters. (D) Positive-mode ESI-MS in dichloromethane. for detailed information specifying atoms and positions). This shows the total Ag number is 1.9 [(0.15 + 0.8) × 2]. All non-hydrogen atoms are refined anisotropically, and all the hydrogen atoms are set in geometrically calculated positions and refined isotropically using a riding model. Free solvent molecules are highly disordered, and location and refinement of the solvent peaks are unsuccessful. The diffuse electron densities from these residual solvent molecules are removed from the data set using the SQUEEZE routine of PLATON and refined further using the data generated. All the refinement parameters are summarized in Tables S1−S4. Characterization. All UV/vis absorption spectra of [Au13(Dppm)6](BPh4)3, [Au18(Dppm)6Br4](BPh4)2, [Au16.1Ag1.9(Dppm)6Br4](BPh4)2, and [Au20(Dppm)6(CN)6] nanoclusters dissolved in CH2Cl2 are recorded using an Agilent 8453. Thermogravimetric analysis (TGA) is performed on a thermo

gravimetric analyzer (DTG-60H, Shimadzu Instruments, Inc.) with 5 mg of the [Au13(Dppm)6](BPh4)3, [Au18(Dppm)6Br4](BPh4)2, and [Au20(Dppm)6(CN)6] nanocluster in a SiO2 pan at a heating rate of 10 K min−1 from 323 to 1073 K. Prior to measurement, all samples were drying for at least 12 h in vacuum oven under 50 °C. X-ray photoelectron spectroscopy (XPS) measurements are performed on a Thermo ESCALAB 250 configured with a monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, a flood gun to counter charging effects, and the analysis chamber base pressure lower than 1 × 10−9 mbar; data are collected with FAT = 20 eV. Electrospray ionization time-of-flight mass spectrometry (ESI-TOFMS) measurement is performed by MicroTOF-QIII high-resolution mass spectrometer. The sample is directly infused into the chamber at 5 μL/min. Nuclear magnetic resonance (NMR) analysis is performed on a Bruker Avance spectrometer operating at 400 MHz for 1H NMR. C

DOI: 10.1021/acs.inorgchem.7b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Total structures of the [Au13(Dppm)6](BPh4)3, [Au18(Dppm)6Br4](BPh4)2, and [Au20(Dppm)6(CN)6] nanoclusters: (A) Total structure of the [Au13(Dppm)6](BPh4)3. (B) The metallic core of Au13. (C) Total structure of the [Au18(Dppm)6Br4](BPh4)2. (D) Twisted Au11 structure. (E) The Au18 core by sharing a face consisting of four gold atoms. (F) Total structure of the [Au20(Dppm)6(CN)6]. (G) Twisted Au11 structure. (H) The Au20 core by sharing two gold atoms. Color labels: golden/spring green = Au; purple = P; gray = C; brown = Br, blue = N. Note: the counterions are not shown for clarity. CD2Cl2 and CH3OD is used as the solvent to dissolve ∼5 mg clusters; the residual solvent is used as reference.

three nanoclusters. The results of the TGA show weight losses of 37.90%, 47.50%, and 58.10%, respectively, which are in agreement with the theoretical value of [Au20(Dppm)6(CN)6] (37.95 wt %), [Au18(Dppm)6Br4](BPh4)2 (47.90 wt %), and [Au13(Dppm)6](BPh4)3 (58.20 wt %) (Figure 2B). Moreover, the XPS analysis shows clear signal of N only in [Au20(Dppm)6(CN)6] (the green circle, Figure 2C) and Br signal only in [Au18(Dppm)6Br4](BPh4)2 (the pink circle, Figure 2C and Figure S1). Atomic Structure. The total structure of [Au13(Dppm)6](BPh4)3 is shown in Figure 3A. According to previous reports, most of the structures of Au13 are arranged in an icosahedron core cocapped by phosphorus and thiol ligands (or phosphorus and halogen ligands).35 In 1981, J. W. A. Vandervelden synthesized phosphine-protected [Au13(dppm)6](NO3)4 nanocluster, of which the valence state is +4.35b Meanwhile, MO calculations predicted Au13(Dppm)6 nanocluster with different valence state, which regrettably has not been proved yet. In this work, we successfully obtained [Au13(Dppm)6](BPh4)3 (Figure 3A) with 10 electrons (ns = 13 − 3); the 3+ valence state is further confirmed by ESI-MS (vide infra). For the reported Au13 with eight electrons, the icosahedron core is complete without distortion. Compared with [Au13(dppm)6](NO3)4, which has nine electrons and possesses relatively complete icosahedron, the [Au13(Dppm)6](BPh4)3 with 10 electrons has a more distorted icosahedral configuration (Figure 3B). Electronic numbers may have certain influence on Au13 icosahedron distortion. The Au−Au bond length varies from 2.544 to3.201 Å. Each two gold atoms (except for the central gold atom) are protected by one Dppm ligand (Figure 3A). The total crystal structure of [Au18(Dppm)6Br4](BPh4)2 is shown in Figure 3C. Different from the reported structural core of [Au18(P)2(PPh)4(PHPh)(dppm)6]Cl3, which consits of Au3P trigonal pyramids and Au4P rectangular pyramids

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RESULTS AND DISCUSSION Synthesis and Characterization. The [Au13(Dppm)6](BPh4)3, [Au18(Dppm)6Br4](BPh4)2, and [Au20(Dppm)6(CN)6] nanoclusters are all synthesized by the thiol-induced size-focusing strategy. Figure 1 shows the reasonable prediction of reaction diagram for these nanoclusters. The UV−vis absorption spectra of these three nanoclusters in dichloromethane are shown in Figure 2A. The [Au18(Dppm)6Br4](BPh4)2 exhibits a series of peaks centered at 655, 492, 440, 335 nm and a weak shoulder peak at 780 nm. The [Au13(Dppm)6](BPh4)3 shows two main peaks at 450 and 800 nm. Meanwhile, three main peaks are observed at 420, 510, and 760 nm in [Au20(Dppm)6(CN)6]. Then, ESI-MS without the addition of cesium acetate (CsOAc) is used to confirm the exact formula. As shown in Figure 2D, the predominant peak at m/z 3085.89 Da corresponding to the [Au18(Dppm)6Br4]2+ is observed. A characteristic peak separation of m/z 0.50 illustrates the charge-state of Au18 is 2+ (i.e., +1/0.5). Similarly, the predominant peak at m/z 1622.30 Da is attributed to [Au13(Dppm)6]3+, and the charge-state 3+ is confirmed from a characteristic peak separation of m/z 0.33. Specially, on the basis of the well-determined [Au20(Dppm)6(CN)6], we first perform the ESI-MS with the addition of CsOAc; unfortunately, no signals of [cluster + xCs]x+ adducts are observed in positive mode, which is different from the other neutral clusters. Then, we perform the ESI-MS without the addition of CsOAc. After careful analysis, the signal at m/z 3174.50 Da belongs to the [Au20(Dppm)6(CN)4]2+ derived from the removal of two −CN from complete [Au20(Dppm)6(CN)6].32a Further, the TGA is performed to confirm the purity of these D

DOI: 10.1021/acs.inorgchem.7b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry polyhedra as well as phosphine ligands,33 the Au18 core of [Au18(Dppm)6Br4](BPh4)2 can be viewed as being assembled from two twisted Au11 (Figure 3D,E) units sharing an Au4 face. Compared with the individually icosahedral Au11, the Au11 section in Au18 core has been completely distorted. The gold− gold bond on the edge is protected by Dppm ligands to form a bidentate “Au−P−CH2−P−Au” pattern. Additionally, four bromine atoms are coordinated to the shared Au4 plane with Au−Br bond lengths in the range of 2.458−2.470 Å. Furthermore, the other four Dppm ligands also protect the twisted Au 11 in bidentate Au−P−CH2 −P−Au fashion. A c c o r d i n g l y , t h e va l u e o f v a l e n c e elec t r o n s o f [Au18(Dppm)6Br4]2+ is calculated to be 12 (18 − 4 − 2). The structure of the [Au20(Dppm)6(CN)6] is exhibited in Figure 3F. Similar with t he structural core of [Au20(PPhpy2)10Cl4]Cl2,32a which is built from two Au11 fused via two gold atoms, the Au20 core of [Au20(Dppm)6(CN)6] can also be viewed as the fusion of the two Au11 sharing two gold atoms (Figure 3G,H) despite the difference in the ligands. The value of valence electrons of [Au20(Dppm)6(CN)6] is calculated as 14 (20 − 6). On the basis of the structure of these three nanoclusters, we can reasonably predict that the bidentate Au−P−CH2−P−Au pattern contributes to the stabilization of the nanocluster. Reaction Process Tracking of [Au18(Dppm)6Br4](BPh4)2. Particularly taking the synthesis of [Au18(Dppm)6Br4](BPh4)2 as an example, HS-Adm ligands are essential in the synthesis of these three nanoclusters (Figures S2 and S3).To prove whether this is a special case or a common phenomenon, the synthesis of Au18 nanocluster is used for the case study of this process. As shown in Table 1, a variety of thiol ligands are used to replace HS-Adm. The results illustrate that most thiol ligands can work well in this process.

beginning of reaction. The signals from Cl are very strong in the beginning, indicating the nanoclusters are capped by Cl at first. When prolonging the reaction time, the signals from both O and Cl become weak, accompanied by the enhancing of Br signal, which represent that the nanocluster has been capped by unoxidized Dppm and Br. For more deeply understanding the process, ESI-TOF-MS is used to monitor the precipitate (Figure 4), and the [Au18(Dppm)6Br4]2+ (m/z 3085.89) peak

Figure 4. Time-dependent ESI mass spectra of the samples precipitating in toluene solution.

at the beginning is not obvious. With the reaction proceeding, the ratio of [Au18(Dppm)6Br4]2+ increases through size focusing, as unstable nanoclusters decompose. Time-dependent MALDI mass spectroscopy of the product in toluene solution and the precipitate are performed when synthesizing the [Au18(Dppm)6Br4]2+ with and without AdmSH: (1) It is easy to observe the initial size distribution in toluene solution ranging from ∼5000 to ∼7000 with AdmSH and Dppm (Figure S3A), while there is no obvious size distribution in toluene solution when only Dppm is used (Figure S3B). So, the participation of thiols can induce a small size distribution of the initial product in toluene solution. (2) Subsequently, the initial mixture with the small size range has been converted into phosphine clusters that contain [Au18(Dppm)6Br4]2+ (Figure 4 and Figure S3C), accompanied by the formation of the complex in solution. Further, [Au18(Dppm)6Br4]2+ increases, as other less-stable phosphine gold nanoclusters decrease when size focusing. By contrast, the initial mixture without obvious size range (Figure S3B) can not produce [Au18(Dppm)6Br4]2+ in conversion and enrich [Au18(Dppm)6Br4]2+ in the size focusing step, leading to polydisperse phosphine products (Figure S3D). Meanwhile, the UV−vis spectrum of the precipitation of 5 h shows the obvious peaks of [Au18(Dppm)6Br4]2+ (Figure S3E inset) because of the high quantity of [Au18(Dppm)6Br4]2+ (Figure S3E) during synthesis with Dppm and AdmSH. The UV−vis spectrum of the precipitation of 5 h only shows a peak at 515 nm when synthesizing only with Dppm (Figure S3F inset) due to polydisperse products (Figure S3F). Taking these into consideration, we can reasonably predict the thiols play an

Table 1. Experimental Results under Different Thiol Ligands Atmosphere ligands Dppm 0.130 0.130 0.130 0.130 0.130 0.130 0.130 0.130 0.130

mmol mmol mmol mmol mmol mmol mmol mmol mmol

thiol ligand (0.300 mmol)

product

adamantane mercaptan tert-butyl mercaptan tert-butyl thiophenol 4-tert-butyl benzyl mercaptan 2,4-dimethyl thiophenol 3,4-difluorothiophenol cyclohexyl mercaptan phenylethanethiol none

Au18 Au18 Au18 Au18 Au18 Au18 Au18 Au18 polydisperse

To obtain clearer understanding of this process, we first monitor the precipitate of reaction intermediate period by NMR measurement for qualitative detection. As shown in Figure S4, no signal is found in the range of 0−2 ppm, which suggests the thiol ligand does not exist in the obtained precipitate. XPS survey spectra of precipitates are detected (Figure S5) on synthetic process of [Au18(Dppm)6Br4]2+ with increase of reaction time to 5 h for details. No S signal is found during the whole process, which further indicates the precipitates are phosphine-protected gold nanoclusters. More specifically, when reacted for 1 h, the signals from O (O1s, ∼532 ev) and Cl (Cl2p, ∼197 ev; Cl2s, ∼270 ev) are obvious, while signals from Br (Br3d, ∼68 ev) are relatively weak. The signals from O show that some Dppm ligands are oxidized at the E

DOI: 10.1021/acs.inorgchem.7b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Structure transformation of these nanoclusters under different condition.

Figure 6. (A) The [Au16.1Ag1.9(Dppm)6Br4]2+ obtained by target metal exchange of [Au18(Dppm)6Br4]2+. (B) The UV−vis spectra of [Au 16.1Ag1.9(Dppm) 6Br4 ] 2+(black line) and [Au 18 (Dppm) 6Br 4] 2+ (red line). The ESI-MS of (C) [Au18(Dppm) 6Br 4] 2+ and (D) [Au16.1Ag1.9(Dppm)6Br4]2+. Color labels: golden = Au; silver = Ag; purple = P; brown = Br; the C, H atoms are not shown for clarity.

absorption peaks of [Au18(Dppm)6Br4](BPh4)2 disappeared, and a new absorption peak at 530 nm appeared. With the extension of reaction time, a new UV−vis absorption at 450 nm appeares and becomes obvious, which indicates the increasing ratio of [Au13(Dppm)6](BPh4)3. After reaction for 150 min, the characteristic absorption peaks of [Au13(Dppm)6](BPh4)3 nanocluster at 450, 600, and 800 nm could be observed, demonstrating that the Au18 were successfully converted to Au13 with the existence of PPh3 ligands (Figures 5 and S6A). The role of PPh3 is necessary in the conversion of Au18 to Au13. Previously, Jin reported the Au24 nanoclusters can be obtained by adding excess PPh3 to Au25 nanocluster, in detail, two PPh3 ligands taking one gold atom from Au25 nanocluster, forming (PPh3)2AuX (X = Br/Cl).50 Konishi K. had outlined the examples of phosphine-capped gold clusters syntheses via

important role in inducing initial size distribution and decomposing unstable nanoclusters in precipitation, which results in the enrichment of [Au18(Dppm)6Br4]2+ when size focusing. The Vivid Conversion from [Au18(Dppm)6Br4](BPh4)2 to [Au13(Dppm)6](BPh4)3 and [Au20(Dppm)6(CN)6] Nanocluster. Comparing the structures of Au18, Au13, and Au20 nanoclusters, we notice that they have the same number of phosphorus ligands but with different anionic groups. In this context, we are motivated to intertransfer these three nanoclusters under different conditions. Specifically, the obtained [Au18(Dppm)6Br4](BPh4)2 nanocluster (20 mg) is dissolved in a mixture of 2 mL of methanol and 6 mL of dichloromethane. Then, PPh3 (200 mg) is added. After they reacted for a few minutes with stirring, the characteristic F

DOI: 10.1021/acs.inorgchem.7b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry etching reactions, such as the conversion of [Au9(PPh3)8]3+ to [Au8(PPh3)8]2+ under PPh3.51 Indeed, we also find the (PPh3)2Au+ signal in the ESI-MS analysis. We propose phosphine ligands are taking the gold atoms out of the Au18 nanocluster and, meanwhile, the outside Br ligands are leaving as an anti-ion at the same time. This is somewhat different from ligand exchange-induced structural transformation of thiolated Au18(SR)14 metal core into Au21(S-Adm)15.52 Meanwhile, [Au18(Dppm)6Br4](BPh4)2 can be converted to [Au20(Dppm)6(CN)6] under the presence of NaBH3CN (Figures 5 and S6B). According to the synthesis of [Au20(Dppm)6(CN)6], the NaBH3CN not only acts as a reductant but also provides CN− group to protect the nanocluster. For this purpose, NaBH3CN (60 mg dissolved in 2 mL of methanol) is added to the [Au18(Dppm)6Br4](BPh4)2 solution (20 mg dissolved in a mixture of 2 mL of methanol and 6 mL of dichloromethane). After 30 min, accompanied by the decay of the characteristic absorption peak of [Au18(Dppm)6Br4](BPh4)2 nanoclusters at 650 nm, the peak at 760 nm for [Au20(Dppm)6(CN)6] nanoclusters gradually emerges. This indicates that Au18 has been converted to Au20 under NaBH3CN environment. With the extension of reaction time to 1 h, the peaks at 420 and 510 nm can also be observed, illustrating that the Au20 becomes the predominant product. Accordingly, the color of solution alters from greenish-yellow to orange. After the completion of the two conversions, the UV− vis spectra of purified products are in agreement with the spectra of directly synthesized products, and further the purified products are confirmed to be Au13 and Au20 by ESI-MS (Figure S7). The conversion from [Au18(Dppm)6Br4](BPh4)2 to [Au13(Dppm)6](BPh4)3 and [Au20(Dppm)6(CN)6] nanocluster is very interesting, and the reaction mechanism remains to be unraveled in future work. Targeted Metal Exchange of [Au18(Dppm)6Br4](BPh4)2, Forming [Au16.1Ag1.9(Dppm)6Br4](BPh4)2. Further, for the [Au18(Dppm)6Br4](BPh4)2, there is a shared face consisting of four gold atoms capped by four bromine atoms. According to previous reports of (AuAg)25,46,47 (AuAg)37,48 and (AuAg)38,49 halogen atoms are more prone to connect with silver and further stabilize the nanoclusters. Inspired by these reports, we apply the metal exchange method to find whether the Br-linked gold atoms can be exchanged by silver atoms or not. Because the Au18 nanoclusters are highly stable in the presence of thiol ligands, the Ag-SAdm is used to react with Au18 nanoclusters. X-ray structural analysis illustrates silver atoms in [Au16.1Ag1.9(Dppm)6Br4](BPh4)2 locate in the predicted position (Figure 6A). The [Au18(Dppm)6Br4](BPh4)2 shows four peaks at 650, 490, 445, and 335 nm (Figure 6B, red line). After doping Ag atoms, the UV−vis spectra are not obvious, showing weak peaks at 450 and 550 nm. The [Au18(Dppm)6Br4](BPh 4) 2 and [Au 18−xAgx(Dppm)6Br4 ](BPh4)2 (x = 1, 2) are further determined by ESI-MS (Figure 6C, D). Meanwhile, the XPS analyses reveal that the Au/Ag atomic ratio in [Au18−xAgx(Dppm)6Br4](BPh4)2 is 15.9/1.9 (Figure S8), which is similar to the results obtained from singlecrystal X-ray measurement (cal. 16.1/1.9). The structural details of [Au16.1Ag1.9(Dppm)6Br4]2+ are shown in Figure S9. For example, location 1 in alloy core (Figure S9A) can be occupied by either Ag or Au with Ag being present at 15% occupancy and Au at 85%, and the location 2 can be occupied with Ag being present at 80% occupancy and Au at 20%. Additionally, location 1′ is occupied by Ag with 15%, and location 2′ is occupied with 80% Ag occupation.

Therefore, the overall composition of the 18-atom core is determined as [email protected] (Figure S9B). Structurally, the four gold atoms located in the sharing face were capped by four Br groups. On the one hand, the distance of Br−Au (location 1) was 2.470 Å, and Br−Au (location 2) was 2.458 Å obtained from the structure of [Au18(Dppm)6Br4](BPh4)2. On the other hand, because of the occupancy of Ag or Au, the distances of Br−metal are enlarged to be 2.477 and 2.499 Å at locations 1 and 2, respectively. According to these results, the target metal exchange by using the different combination ability of ligand with metal is possible to control the synthesis of alloy nanoclusters. The Kinetic Stability in Solution. We further investigate the kinetic stability of these phosphine-protected nanoclusters dissolved in a mixture of CH2Cl2 and CH3OH at room temperature in air. As to these homogold clusters, the UV−vis spectra are essentially unchanged over time, which indicates their kinetic stability (Figure S10). After doping of silver into the gold nanocluster, the stability of AgxAu18−x is slightly decreased. On the basis of the well-determined structures, we assumed that the structure and the influence of the bidentate Au−P−CH2−P−Au pattern may contribute to the kinetic stability.

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CONCLUSION In summary, this work demonstrates a new strategy for synthesizing phosphine-protected gold nanoclusters. Thiol is used to destroy the unstable nanoclusters and enrich the ultrastable one. By using this strategy, [Au13(Dppm)6](BPh4)3, [Au18(Dppm)6Br4](BPh4)2, and [Au20(Dppm)6(CN)6] nanoclusters are synthesized, and their structures are determined by single-crystal X-ray diffraction. The core structures of [Au18(Dppm)6Br4](BPh4)2 and [Au20(Dppm)6(CN)6] suggest that incomplete icosahedral Au11 can form a series of atomically precise gold nanoclusters by distorting and sharing gold atoms. Meanwhile, the transformation between [Au18(Dppm)6Br4](BPh4)2, [Au13(Dppm)6](BPh4)3, and [Au20(Dppm)6(CN)6] nanoclusters contribute to more intensive understanding on the conversion of phosphine-protected nanoclusters. Moreover, according to the different binding ability of silver with halogen, we successfully achieve target metal exchange on [Au18(Dppm)6Br4](BPh4)2 with Ag-SAdm (where HS-Adm stands for 1-admantanethiol) complex and obtain [AgxAu18−x(Dppm)6Br4](BPh4)2 (x = 1, 2) alloy nanoclusters. These phosphine-protected clusters remain very stable. Our work will contribute to more intensive understanding on synthesizing phosphine-protected nanoclusters and structure transformation by ligand engineering.

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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.7b01458. Details of structure analysis, XPS, UV−vis. Crystallographic information file for [Au13(Dppm)6](BPh4)3. CCDC: 1552975 Crystallographic information file for [Au18(Dppm)6Br4](BPh4)2. CCDC: 1552978 Crystallographic information file for Au 20 (Dppm) 6 (CN) 6 . CCDC:1552976 Crystallographic information file for [Ag1.9Au 16.1 (Dppm)6 Br4 ](BPh4 ) 2 . CCDC: 1552977 (PDF) G

DOI: 10.1021/acs.inorgchem.7b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

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CCDC 1552975−1552978 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 emailing [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 W. D contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support by NSFC (21372006, U1532141, 21631001, 21602002), the Ministry of Education, the Education Department of Anhui Province, 211 Project of Anhui Univ.

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