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Total Structure Determination of Au16(S-Adm)12 and Cd1Au14(StBu)12 and Implications for the Structure of Au15(SR)13 Sha Yang, Shuang Chen, Lin Xiong, Chong Liu, Haizhu Yu, Shuxin Wang, Nathaniel L Rosi, Yong Pei, and Manzhou Zhu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04257 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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Journal of the American Chemical Society

Total Structure Determination of Au16(S-Adm)12 and Cd1Au14(StBu)12 and Implications for the Structure of Au15(SR)13 Sha Yang,a,‡ Shuang Chen,a,‡ Lin Xiong,b,‡ Chong Liu,c Haizhu Yu,a Shuxin Wang,*,a Nathaniel L. Rosi,c Yong Pei,*,b Manzhou Zhu*,a a

Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui, 230601, China. b Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of MOE, Xiangtan University, Xiangtan, Hunan, 411105, China. c Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15213. Abstract: Ultra-small nanoclusters (e.g. Au15(SR)13) are crucial in not only real applications such as bio-application but also in understanding the structure transition from gold complexes to gold nanoclusters. However, the determination of these transitionsized gold nanoclusters has long been a major challenge. In this work, two new nanoclusters in the transition regime, including the thus far smallest alloy nanocluster Cd1Au14(StBu)12 and the homogold nanocluster Au16(S-Adm)12, are obtained and their atomic structures are fully determined by single crystal X-ray diffraction. Moreover, based on the structures of Cd1Au14(SR)12 and Au16(SR)12, we perform DFT calculations to predict the structure of the “transformation” nanocluster, Au15 (Au15(SR)12- and Au15(SR)13). Overall, this work bridges the gaps between gold complexes and nanoclusters.

1. Introduction Ultra-small noble metal nanoclusters (NCs) have attracted intensive attention due to their fascinating molecular-like physical and chemical properties (such as photoluminescence and intrinsic magnetism).1-7 In recent years, gold NCs protected by thiolate ligands have been extensively studied, which have shown promising applications in catalysis,8 bio-therapy9 and sensing.10 A grand challenge to scientists in this field is to explore the inherent geometric structure patterns of nanoclusters. To date, many thiolated gold clusters and alloy ones with various atomic structures have been determined by single crystal X-ray diffraction.11-19 Base on the precise atomic structures, the structure-dependent properties have been extensively studied,20,21 and the stabilities of thiolated gold clusters have also been clarified by DFT calculations.22-25 In addition, the structure evolution from non-metallic gold nanoclusters to metallic gold nanoparticles have been explored. Jin’s group reported the structures of Au133(TBBT)52 and Au246(p-MBT)80 nanoclusters and femtosecond transient absorption spectroscopic measurements revealed that these sizes are non-metallic.26,27 Meanwhile, Au279(TBBT)84 nanoclusters exhibit metallic behavior.28 All these works indicate that the transformation from non-metallic to metallic occurs between Au246(p-MBT)80 and Au279(TBBT)84. Despite the great experimental and theoretical research efforts, some fundamental issues about the geometric and electronic structure evolutions of thiolate-stabilized gold nanoclusters remain to be addressed; for example, how the gold complex evolves into gold nanoclusters (Scheme 1)?

Scheme 1. Evolution from gold complexes to gold nanoclusters protected by thiolates. The size-dependent evolution can be divided into three states: complex (yellow), transition-sized nanoclusters (magenta) and nanoclusters (blue).

To map out the structure evolution process, the key issue is to synthesize and determine the structures of nanoclusters in the transition size range. Tsukuda and co-workers earlier reported a separation of GS-protected (GSH = glutathione) gold nanoclusters,29 in which Au15(SR)13 and Au18(SR)14 were found to be the key transition-size gold nanoclusters with 2e and 4e free valence electrons, respectively. Recently, Xie and co-workers studied the reduction of Au(I)-thiolate complexes to the evolution of Au25(SR)18 nanoclusters and the Au15(SR)13 nanocluster was also found to be the smallest gold nanocluster which plays a key role in the evolution.30 Monodisperse Au15(SG)13 and Au18(SG)14 were later reported by Pradeep and co-workers.31 Moreover, Zheng and co-workers discovered that Au15(SG)13 and Au18(SG)14 nanoclusters are more easily

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and physically retained by the glycocalyx of the glomeruli.32 This enhances their passive targeting to cancerous tissues through an enhanced permeability and retention effect.32 However, due to the flexible GS ligand, it is difficult to obtain the crystal structures of these transition-size nanoclusters.29,33 Exchanging the flexible GS ligand with rigid mercaptan ligands can facilitate crystallization of nanoclusters.13,34 Indeed, the crystal structure of Au18(SR)14 has been attained by replacing GS with cyclohexane thiolate ligand and the structure collapse and growth mechanism has thus been found.14a,35 However, the structure of the smallest thiolated gold nanocluster, i.e. Au15, which is of crucial importance for understanding the evolution from gold complexes to gold nanoclusters and biological applications, has not been determined yet. Herein, to reveal the structure of the smallest gold nanocluster (Au15), two strategic routes are adopted. First, rigid thiol ligands are used to replace the flexible GS ligand in Au15(SG)13. Interestingly, a new size of gold nanocluster, that is, Au16(S-Adm)12 (S-Adm = adamantine thiolate) was obtained. On the other hand, a Cd atom was doped into Au15(SG)12- (another Au15) to reveal its structure. It is worth noting that a small amount of Cd doping into nanoclusters does not change the framework of nanoclusters.17,36,37 In this work, the smallest Au-based alloy nanocluster, Cd1Au14(StBu)12 (StBu = Tert-butyl thiolate), is obtained. The crystal structures of both nanoclusters are revealed by X-ray crystallography. We further predict the geometry structure of Au15(SR)12- and Au15(SR)13 via DFT calculations based on the precise atomic structures of Cd1Au14(StBu)12 and Au16(SAdm)12 (as shown in Scheme 2). Scheme 2. Schematic illustration of crystal structure of Cd1Au14(StBu)12, Au16(S-Adm)12 nanocluster and predictive structure of Au15(SR)12-, Au15(SR)13.

2. Experimental Section 2.1. Chemicals. All reagents and solvents were obtained from commercial suppliers and used without further purification, unless otherwise stated. Tetrachloroauric(III) acid (HAuCl4•4H2O, ≥99.99% metals basis) was purchased from the China Nonferrous Metal Mining (Group) Co., Ltd. (Shenyang, China). CdCl2 (≥98%), glutathione (reduced, ≥98%), tButyl mercaptan (≥99%), adamantane thiol (≥99%), tert-

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butylamine complex (≥97%) were purchased from SigmaAldrich (Shanghai, China). Toluene (C7H8, ≥99.9%), dichloromethane (CH2Cl2, ≥99.9%), methanol (CH3OH, ≥99.9%), and hexane (C6H14, ≥99.9%) were purchased from Guangfu (Tianjin, China). CO (≥99.98%) was purchased from chen hong special gas company (Nanjing, China). 2.2. Synthesis and crystallization of Au16(S-Adm)12 nanocluster. Starting with the water soluble Au15(SG)13 nanocluster reported in the previous work,38 20 mg Au15(SG)13 was dissolved in 10 mL aqueous solution, then 100 mg adamantane thiol (HSAdm) which was dissolved in dichloromethane was added to the above aqueous solution. The water-oil two phase transfer was kept for about 12 hours at 40 oC, during which the aqueous solution became colorless. The colorless aqueous solution was discarded, whereas the dichloromethane solution containing the Au16(S-Adm)12 nanocluster was evaporated. The crude product was washed several times with methanol and collected by centrifugation to give Au16(S-Adm)12. The Au16 nanocluster was crystallized in toluene/methanol solution over one week. 2.3. Synthesis and crystallization of Cd1Au14(StBu)12 alloy nanocluster. Starting from a water-soluble mixture containing Au15(SG)12nanocluster prepared by Xie’s method,30 a two-phase ligand exchange method was utilized to gain the target Cd1Au14(StBu)12. For the first step, we synthesized the Au15(SG)12- nanocluster according to the literature. In the second step, 30 mg CdCl2 was dissolved in the above mixture solution, and then 10 mL toluene solution of 500 µL tert-Butyl mercaptan was added at 55 oC. After about 12 hours, the toluene solution changed to a magenta color and the aqueous solution changed to colorless. The toluene solution containing the target product was evaporated with the aid of methanol. The crude product was washed several times with methanol and collected by centrifugation to give pure Cd1Au14(StBu)12. The pure Cd1Au14(StBu)12 nanocluster was crystallized in CH2Cl2/methanol over 2−3 days. 2.4. Synthesis and crystallization of Cd1Au14(S-Adm)12 alloy nanocluster. First, 30 mg CdCl2 was dissolved in 20 mL water and poured to a 50 mL flask. After ~2 min stirring under 55°C, 500 mg GSH (glutathione) was added to the aqueous solution. The stirring was continued for about 15 min, and then 1 mL HAuCl4·4H2O (0.2 g/mL) was added. After ~20 min stirring, a solution of borane tert-butylamine complex (220 mg) and 100 mg adamantane thiol (dissolved in 10 mL toluene) was quickly added to the above solution to initiate the reaction. The twophase mixture was allowed to react for 4 hours (the aqueous solution turned colorless). After the reaction, the colorless aqueous solution was removed and the toluene organic phase (note: nanoclusters were dissolved in the organic phase) was evaporated with the aid of methanol. The crude product was washed several times with CH3OH to remove the excess thiol and the by-products. The pure Cd1Au14(S-Adm)12 nanocluster was crystallized in CH2Cl2/methanol over 2−3 days. 2.5. X-ray crystallographic determination Cd1Au14(StBu)12 and Au16(S-Adm)12 nanoclusters.


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The diffraction data of the single crystals were collected on Bruker APEX-II CCD diffractometer using Mo Kα radiation (λ= 0.71073 Å) and Cu Kα radiation (λ= 1.54178 Å) for Cd1Au14(StBu)12, Cd1Au14(S-Adm)12 and Au16(S-Adm)12 nanoclusters, respectively. The crystal structures were determined by direct methods and refined by using the full-matrix least-squares methods within the ShelXT program (Sheldrick, 2015) for these two nanoclusters. The placement of the heteroatoms and fractional site occupancy in the Cd1Au14(StBu)12 alloy nanoclusters were ascertained by the method of modifying the disorderly free variables. 2.6. Computational method and details. Density functional theory (DFT) calculations were employed to optimize the geometric structures of all the clusters, all within the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional39 and the dpolarization basis set (DND) was used for the elements C, H and S. The DFT semi-core pseudopotential (DSPP) approximation with some degree of relativistic correction into the core was used for Au and Cd implemented in the Dmol3 package.40 The structural optimizations were performed without symmetry restrictions, using a force and displacement tolerance criterion of 0.004Ha/Å and 0.005Å, respectively. Based on the COSMO model with the solvent of benzene, the single point energies of all species were computed using Dmol3 package. Time-dependent DFT (TD-DFT), as implemented in Amsterdam Density Functional (ADF) software package41 was utilized for the study of the ultraviolet-visible (UV-vis) optical properties. The PBE0 density functional combined with the triple-zeta polarized (TZP) basis set with inclusion of the scalar relativistic effect via a zeroth-order regular approximation (ZORA) implemented in the ADF package are adopted. The TD-DFT calculations evaluated the lowest 200 singlet-tosinglet excitation energies. For the convenience of calculation, the R groups of all clusters simplified as methyl groups in geometric optimization and spectral calculations. 2.7. Characterization. Ultraviolet-visible (UV-vis) absorption spectra were recorded on an Agilent 8453 spectrophotometer with the CH2Cl2 as solvent. X-ray photoelectron spectroscopy (XPS) measurements were performed on a thermal ESCALAB 250, equipped with a monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, and a flood gun (to counter charging effects). The analysis chamber base pressure was lower than 1×10−9 mbar, and data was collected with FAT = 20 eV. Electrospray ionization mass spectra (ESI-MS) recorded using a Bruker Q-TOF mass spectrometer. The source temperature maintained at 80 oC. The sample was directly infused into the chamber at 5 µL/min. ESI sample was prepared by dissolving it in dichloromethane (0.1 mg/mL). Thermal gravimetric analysis (TGA) was conducted on samples of about 10 mg, under an atmosphere of anhydrous N2 (flow rate 50 mL/min), using a TG/DTA 6300 analyzer (Seiko Instruments, Inc), with a heating rate of 10 oC/min. 3. Results and Discussion 3.1. Synthesis and characterization of the two new nanoclusters We tried a series of rigid thiol ligands to maintain the structure of water-soluble Au15(SG)13 nanoclusters based on our

reported two-phase ligand exchange method (for details, see supporting information),14a however, all attempts failed (Figure S1) to get the Au15(SR)13 nanoclusters. The adamantane thiol reaction with the water-soluble Au15(SG)13 led to Au16(SAdm)12 (Au16 for short), instead of Au15(S-Adm)13, and other thiol ligands (such as 2,4-dimethylbenzenethiol, cyclohexyl thiol, 4-tert-butylbenzenethiol, 4-tert-butylbenzylmercaptan, phenylethane thiol) led to decomposition of the cluster. The accurate formulation of Au16 was determined by ESI-MS and TGA. As shown in Figure S2 A, the ESI-MS (positive ion mode) analysis revealed a prominent peak at m/ z 5290.8 Da, corresponding to the formula of the intact cluster [Au16(C10H15S)12Cs]+ (calculated formula weight: 5291.4 Da). The observed isotopic pattern of the Au16 cluster is in agreement with the simulation results (Figure S2 A, inset). As shown in Figure S2 B, TGA reveals a weight loss of 38.66%, which is very close to the theoretical value (38.87%) for Au16. In addition, the HPLC separation further confirms our homogeneous preparation (Figure S2 C). The Au16 nanocluster exhibits a major absorption peak centered at 485 nm with two shoulder peaks at 378 and 604 nm (Figure S2 D). Pink crystals of Au16 NC were obtained from a toluene/methanol solution over one week, which was used for single crystal X-ray diffraction. The Au16 nanocluster was crystallized in a monoclinic space group -P 2ybc (Table S1) and the crystal structure of Au16 is shown in Figure S3. As shown in Figure 1a, the framework of Au16(SR)12 is composed of a Au7 kernel (Figure 1b, core), one Au3(SR)4 motif (Figure 1c, motif i), one tetrameric Au4(SR)5 motif (Figure 1d, motif ii), as well as one dimeric Au2(SR)3 motif (Figure 1d, motif iii). The Au7 kernel comprises one octahedron unit with one face-capping Au atom. The average Au-Au bond is 2.741 Å. The distances of Au−S bonds in staple motifs range from 2.198 to 2.349 Å (average: 2.289 Å, Figure 1 c-d).

Figure 1. The crystal analysis of Au16 nanocluster. (a) framework; (b) core; (c)-(d) staple motif (color labels: cyan/gray = Au, red = S; no C or H atoms are shown).



On the other hand, another Au15 nanocluster (Au15(SR)12-) was detected via ESI-MS in a recent study conducted by Xie and co-workers.30 Negatively charged gold nanoclusters are relatively rare compared with neutral gold nanoclusters. To date, Au23(SR)16- and Au25(SR)18- nanoclusters are the only two negatively charged gold nanoclusters. Interestingly, the stability of both nanoclusters can be enhanced by single Cd atom doping with the total structure maintained.17,36,37 We anticipated that using the single-Cd-atom doping strategy could synthesize a single-Cd-doped Cd1Au14(SR)12 nanocluster. With the help of density functional theory (DFT) calculation, we found that the metal exchange reaction between the single Cd atom and Au15(SG)12- nanocluster is unfavorable in the benzene solution (eq. 1). It is noted that the Gibbs free energy (G) is calculated to obtain a more accurate theoretical description for the thermodynamics of eq.1 and eq.2. In the previous report, synthesis of Cd1Au24(SR)18 from Au25(SR)18 nanocluster is also unfavorable (eq. 2), however, the precipitation of Au(SR) from the reaction system as well as electrically neutral Cd1Au24(SR)18 played a key role in the synthesis of Cd1Au24(SR)18.36 Au15(SCH3)12- + Cd(SCH3)2 → CdAu14(SCH3)12 + Au(SCH3) + SCH3- (1) ∆G = 2.83 eV Au25(SCH3)12- + Cd(SCH3)2 → CdAu24(SCH3)12 + Au(SCH3) + SCH3- (2) ∆G = 2.77 eV Therefore, for the synthesis of Cd1Au14(StBu)12 (Cd1Au14 for short) nanocluster, we dissolved the Au15(SG)12- nanocluster in water phase and applied a two-phase (water and toluene) ligand exchange method, since the by-product (Au-SG) dissolves more easily in the aqueous phase which can promote the metal exchange reaction. By using this two-phase exchange method, the Cd1Au14 nanocluster was obtained in toluene. The optical absorption spectrum of the Cd1Au14 (Figure S4) shows only two main absorption bands centered at 402 and 555 nm. The formula of Cd1Au14 NC was determined by electrospray ionization mass spectrometry (ESI-MS) in positive mode and was further confirmed by thermogravimetric analysis (TGA). As shown in Figure S5, the mass spectrum of Cd1Au14 displays a prominent peak at m/z = 4072.7 Da, which is assigned to [Cd1Au14(SC4H9)12Cs+] (calculated formula weight: 4072.9 Da). The observed isotopic pattern of the Cd1Au14 cluster is in agreement with the simulation results (Figure S5, inset). As shown in Figure S6, TGA reveals a weight loss of 27.38%, which is close to the theoretical value (27.16%) for Cd1Au14(SC4H9)12. X-ray photoelectron spectroscopy (XPS) was also applied to confirm the composition revealed by X-ray crystallography (Figure S7). The quantitative measurement indicates the mole ratio of Au and Cd, that is, 14:1. The Au 4f spectrum of this alloy nanocluster is shown in Figure S8. With the width multiple component fitting of the Au 4f 5/2 peak, we found that there have the most of Au+1 (71%) and a small percentage of the Au0 (29%) in the Cd1Au14(StBu)12 nanocluster. The Cd signal (Cd 3d, 405.8 eV) was also observed (Figure S9), but the charge state is uncertain as the binding energies of Cd0 and CdII are quite similar (404.9 vs 405.2 eV). The single Cd signal is CdII due to the lack of the spectral feature of Cd0 (the peak in the 414 nm) in the Cd 3d XPS spectrum.

Figure 2. The crystal analysis of Cd1Au14 nanocluster. (a) framework; (b) core; (c)-(d) staple motifs (color labels: cyan = Au, orange = Cd, red = S; no C or H atoms are shown).

Magenta crystals of Cd1Au14 were obtained via crystallization in CH2Cl2/methanol over 2−3 days. The structure of Cd1Au14 was determined by X-ray crystallography (Figure S10). Of note, we also obtained the Cd1Au14(S-Adm)12 nanocluster with the in situ two phase ligand exchange method (Figure S11). Both of them have the similar crystal structure. The Cd1Au14 nanocluster is crystallized in a monoclinic space group -2C 2yc (Table S2). As revealed by the X-ray crystallography, the structure of Cd1Au14 is centrosymmetric (Figure 2 a). The structure dissection of Cd1Au14 is shown in Figure 2 b-d. First, a Cd1Au5 core is identified and the M-M (M = Au/Cd) bond lengths in this core are from 2.620 to 3.083 Å (average: 2.856 Å). The Cd1Au5 core can be treated as formed by assembling two tetrahedrons via edge sharing. The Cd1Au5 core was capped with a pair of tetrameric Au4(SR)5 staple motifs (Figure 2c, labelled i) and one monomeric Au(SR)2 staple motif (Figure 2d, ii). The bond lengths of Au−S in the tetrameric Au4(SR)5 staple motif range from 2.290 to 2.319 Å (average: 2.300 Å) and are slightly shorter than those of the Au−S bonds in the monomeric Au(SR)2 staple motif on average (2.311 Å in Figure 2 d, motif ii). 3.2 The prediction of the geometric framework of Au15(SR)12In order to find out whether or not the Au15(SR)12- nanocluster has the same metal atom and ligand shell configurations as that of Cd1Au14(SR)12, we applied density functional theory (DFT) calculation. From the DFT calculation of energies of NCs, we found that the Au atom substitution of Cd in Cd1Au14 to form Au15(SCH3)12- is energetically favorable (∆G= -2.83 eV, reverse process of eq. 1) and the Au15(SCH3)12- is a stable minimum structure without imaginary frequency.


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have been totally converted into the Au16(S-Adm)12 nanoclusters. The proposed NC conversion process includes two steps: (i) Cd1Au14 converts to Au15(SR)12- with the Au atom replacing the Cd atom, described by eq. 1; (ii) Au15(SR)12- grows to Au16(SR)12 by capturing an exterior Au atom from the Au-SR species. The latter step is written as eq. 3. The computed reaction energies of eq. 3 with R are –CH3 and –Adm substituent groups are displayed in Table S5. When R = Adm, the reaction energy is 0.34 eV (considering the solvent effect). Figure 3. The computed growth of Au15(SCH3)12- to Au16(SCH3)12 (color labels: cyan/green/magenta = Au red = S; no C or H atoms are shown).

To further confirm the framework of the Au15(SR)12nanocluster (assuming that Au15(SCH3)12- has the same structure as that of Cd1Au14), a possible cluster conversion pathway from Au15(SR)12- to Au16(SR)12 is explored by the DFT calculations on the basis of the idea of “gold-atom insertion, thiolate-group elimination” scheme.35 As shown in Figure 3, the Au15(SR)12- and Au16(SR)12 are different only by one gold atom in the metal core, and their ligand shell structure are slightly different. Following the “gold-atom insertion” scheme, we insert an extra gold atom between the neighboring thiolate in Au15(SCH3)12- to gain a series of the Au16(SCH3)12 nanoclusters. Revealed by the DFT calculations (Table S4), among all possible Au-atom insertion positions, the positions between the thiolate groups labeled by 2 and 9 (2–9), 1 and 4 (1–4) have the lowest Au-atom insertion energy (-0.14 eV and -0.09 eV, respectively). Note that due to the centrosymmetric Au-S framework of Au15(SCH3)12-, two insertion positions (2-9 and 1-4) are same (without considering the orientation of R ligands). When inserting an exterior Au atom at either 2–9 or 1–4 site, the DFT geometric optimization shows that the whole cluster will spontaneously convert to the experimentally determined Au16(SR)12 (the dynamic process of transformation is shown in Supporting Movie). These results suggest that the Au15(SR)12- cluster may easily convert to the Au16(SR)12 when binding an exterior gold atom. 3.3 The growth of Cd1Au14(StBu)12 to Au16(S-Adm)12 nanocluster Since the Au atom substitution of Cd in Cd1Au14 to form Au15(SR)12- is energetically favorable and Au15(SCH3)12- can easily convert to Au16(SCH3)12 nanocluster (Figure 3) upon binding an exterior Au atom from the theoretical calculations, a reaction of Cd1Au14(StBu)12 with Au-S-Adm is designed to gain the Au16(S-Adm)12 nanocluster. The time-dependent UVvis spectra of the Cd1Au14(StBu)12 nanocluster in reaction with Au-S-Adm are displayed in Figure 4a. The black curve is the UV-vis absorption spectrum of the solution before the reaction, which belongs to the Cd1Au14(StBu)12 nanocluster. After addition of Au-S-Adm complex, the intensity of the main peak at 402 nm (Figure 4a, peak α) gradually disappeared and a new feature peak centered at 378 nm (peak α’) appeared. Meanwhile, with the decreasing of the peak at 555 nm (Figure 4a, peak γ), two new peaks appear at 485 nm (peak β) and 605 nm, respectively. After 20 hours, the UV-vis absorption spectrum of the solution only shows feature absorption peaks at 378 (peak α) and 485 (peak β) nm with a shoulder peak at 605 nm, and the original peaks (402 and 555 nm) totally disappeared. This indicates that the Cd1Au14(StBu)12 nanoclusters

Au15(SR)12- + Au(SR) → Au16(SR)12 + [SR]-

(3)

The UV-vis spectra of Cd1Au14, Au16, as well as Au15(SR)12are simulated from calculating the excitation energies via the TD-DFT method (Figure 4b) to correlate the reaction processes. From Figure 4b, the feature peaks in the experimental UVvis absorption spectra of Cd1Au14 and Au16 NCs, as marked by α, β, and γ, are well reproduced by the theoretical simulations. For the Cd1Au14, TD-DFT calculations predicted two feature absorption peaks centered at 391 nm and 530 nm, close to the experimental feature peaks located at 402 nm (peak α) and 555 nm (peak γ), respectively. For the Au16 NC, theoretically predicted feature absorption peaks located at 375 nm and 460 nm, which are close to the experimental peaks located at 378 nm (peak α) and 485 nm (peak β). The weak shoulder peak (located at 605 nm) is also well reproduced by the theoretical spectrum centered at 630 nm.

Figure 4. (a) The time-dependent UV-vis spectra of the Cd1Au14(StBu)12 conversion to Au16(S-Adm)12; (b) Calculated UV-vis spectra of Cd1Au14(SR)12, Au16(SR)12 as well as Au15(SR)12-.



From Figure 4b, it is seen that the spectral features of Au16 are quite different from those of Cd1Au14 and Au15-. In comparison to the Cd1Au14 and Au15-, the first dominant absorption peak of Au16 blue shifted significantly and a weak shoulder peak appeared at 605 nm (experimental curve) and 630 nm (theoretical curve). Such differences are attributed to the bare Au atom in the Au16. Figure S12 displays the Kohn-Sham (KS) molecular orbital levels and the electronic density diagrams of Cd1Au14, Au15- and Au16 NCs. We found that the three NCs have very similar electronic density distribution in the HOMO and the HOMO densities of three NCs all show 1P superatom orbital characters and distributed majorly in the metal core. However, the LUMO of three NCs are different. The Cd1Au14 and Au15- have very similar LUMO density distribution, while the LUMO of the Au16 is major contributed by the unprotected bare Au atom. By further checking the electronic density of LUMO+1 of the Au16, we interestingly found that the LUMO+1 of the Au16 is very similar to the LUMO of both CdAu14 and Au15- NCs. The molecular orbital transitions involved in some major feature peaks are further assigned (Figure S13 and Table S6). For the Cd1Au14 and Au15-, their feature absorption peaks γ both involve majorly the HOMO→ LUMO transitions. For the Au16, the weak shoulder absorption peak located at the 605 nm (experimental curve) and 630 nm (theoretical curve) is resulted from the HOMO→ LUMO transitions, but the LUMO is largely contributed by the bare gold atom. As for the feature peak β of the Au16 NC, it originates from the HOMO → LUMO+1 transition, which corresponds the blue shift of the absorption peak γ of Au15-. Taking the above analyses together, the large differences between the optical spectra of the Au15- and Au16 are attributed to the bare gold atom in the metal core of Au16. In step of cluster conversion from the Au15- to Au16, a new MO energy level is introduced, which leads to a new shoulder feature peak at 605 nm (experimental curve) and 630 nm (theoretical curve) and the red shift of first absorption peak (β). 3.4 The prediction of the geometric framework for Au15(SR)13 from Au15(SR)12The Au15(SR)13 with 2 free electrons was experimentally identified as the smallest cluster and may hold the key to the origin of nucleation of a gold core in the thiolated gold clusters.42 In Xie’s work, the Au15(SR)12- (4 free electrons) could grow from the Au15(SR)13 nanocluster. Therefore, we employ the structure of Au15(SR)12- nanocluster via the growth mechanism of small nanocluster (similar to the growth mechanism of Au15(SR)12- to Au16(SR)12) to reversely rationalize the geometric framework for Au15(SR)13. As shown in Figure 5, adopting the reverse process, we inset a thiolate group at the triangle Au3 site of gold core. When a thiolate group is inserted, the Au3 unit in the metal core of Au15(SR)12- cluster is broken. After the DFT geometric optimizations, as shown in Figure 5, we found that a gold(I) atom collapsed to the bottom triangle Au3 site and form a triangle Au4 core. The resulted Au15(SR)13 NC structure is essentially the same to the previously theoretical prediction by Whetten et al.43 The DFT optimization and energy calculations of the resulted Au15(SR)13 (R = CH3) is also energetically comparable to that predicted by Whetten et al. (energy difference is only 0.01 eV), (Figure S14). A possible conversion process from Au15(SR)12- to Au15(SR)13 is given

in Figure 5 with energies in each step are calculated by DFT (R = CH3). The reverse process indicates that the Au15(SR)13 nanocluster can transform into the Au15(SR)12- nanocluster via breaking the tetrahedron gold core and eliminating a thiolate (SR) group. From Au15(SR)13 to Au15(SR)12-, the elimination of the SR group is found to be the rate-determining step.

Figure 5. Proposed intermediate structures and computed reaction energies for the reversely process of Au15(SR)13 converts to Au15(SR)12- nanocluster. The R group is not displayed for clarity.

4. Conclusion In this work, starting from the water-soluble Au15 nanoclusters (i.e. Au15(SG)12- and Au15(SG)13), we obtained two transitionsized nanoclusters, including the thus far smallest thiolated alloy nanocluster and smallest thiolated homogold nanocluster, that is, Cd1Au14(StBu)12 and Au16(S-Adm)12. The atomic structures of both nanoclusters are fully determined by X-ray crystallography. In addition, Au16(S-Adm)12 could also be formed form the Cd1Au14(StBu)12 nanocluster. With the help of DFT calculations, we have mapped out the geometric structure of the Au15(SG)12- and Au15(SR)13 nanoclusters. This work affords reliable reference and provides scientific basis for understanding the structure evolution from gold complexes to gold nanoclusters.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Details of the crystal data, and supporting figures and tables(PDF) Crystallographic information for Cd1Au14(StBu)12 (CIF) Crystallographic information for Cd1Au14(S-Adm)12 (CIF) Crystallographic information for Au16(S-Adm)12 (CIF) Dynamic process of transformation from Au15(SCH3)12- to Au16(SCH3)12 (Movie)

AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] (S.W.). * E-mail: [email protected] (Y.P.). * E-mail: [email protected] (M.Z.).

Author Contributions ‡S. Y., S. C., and L. X. contributed equally to this work.

Notes The authors declare no competing financial interest..

ACKNOWLEDGMENT M. Z. acknowledge financial support from the National Natural Science Foundation of China (21631001, 21372006, and U1532141), the Ministry of Education, the Education Department of Anhui Province, and the 211 Project of Anhui University. Y. P. acknowledge financial support by NSFC (21773201).

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Total Structure Determination of Au16(S-Adm)12 ... - ACS Publications

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