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Fractal Patterns in Nucleation and Growth of Icosahedral Core of [AunAg44−n(SC6H3F2)30]4− (n = 0−12) via ab Initio Synthesis: Continuously Tunable Composition Control Haifeng Su, Yu Wang, Liting Ren, Peng Yuan, Boon K. Teo,* Shuichao Lin,* Lansun Zheng, and Nanfeng Zheng*

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State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National & Local Joint Engineering Research Center of Preparation Technology of Nanomaterials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: An ab initio one-pot synthesis of the bimetallic clusters [AunAg44−n(SC6H3F2)30]4− (abbreviated (AuAg)44; n ≤ 12) is reported. The mixed-metal (AuAg)44 clusters, synthesized with different reactant Au/Ag ratios, exhibit a fractal-like distribution, suggesting that nucleation of the icosahedral core is a fractal growth process. X-ray crystallographic studies provided unambiguous evidence that the doped Au atoms occupy the icosahedral sites and the maximal doping is 12. The number of Au atoms (n) in [AunAg44−n(SR)30]4− (SR = SC6H3F2) can be continuously tuned from 0 to 12. A three-way correspondence between single-crystal structure, MS, and UV−vis is established, thereby facilitating future identification (finger-printing) of the alloy [AunAg44−n(SR)30]4− clusters. The temperature, solvent, and temporal effects in the synthesis were also investigated.

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INTRODUCTION

There is an upsurge in research activities on atomically precise metal alloy clusters over the past decade, owing to their potential applications in catalysis, magnetism, chirality, fluorescence, bioconjugate labeling and sensing, drug delivery, and medical therapy.8−16 In particular, intense efforts have been focused on doping foreign atoms into metal clusters in order to tune their electronic structure,17−20 to enhance their photoluminescence,21 and to improve their stability, to name just a few.22−24 It is well-known that properties of bimetallic nanoparticles are closely related to their composition and structure.25 In this context, the advantage of a one-pot ab initio synthesis over other multistep strategies is readily apparent. More importantly, it allows continuous tuning of the composition of the resulting clusters. The present study focuses on Au doping of the [Ag44(SR)30]4− cluster (hereafter abbreviated as Ag44), where SR = SPhF, SPhF2, and SPhCF3, recently synthesized and structurally characterized by Zheng and co-workers.26 Singlecrystal X-ray crystallography showed a multishell structure of Ag12@Ag20@Ag12(SR)30. The central icosahedral Ag12 core is capped by a pentagonal-dodecahedral Ag20 shell which is in turn protected by six dimeric Ag2(SR)5 moieties. Upon Au doping, bimetallic analogs [(AuAg)44(SR)30]4− (hereafter abbreviated as (AuAg)44) were formed. Early structure analysis

Clustering phenomena are spontaneous self-assembly processes and the observed structural similarity across scale is a manifestation of the self-organization and self-similarity (SOSS) principle governing many growth processes in nature.1 For molecular clusters, self-organization means spontaneous assemblage of atoms to form energetically stable clusters with relatively efficient packing and reasonably high symmetry. In the process, it often gives rise to the so-called “fractal” patterns.2,3 Studies of fractal pattern formation have allowed a better understanding of the clustering process of atoms and molecules, the aggregation of particles, crystal growth, and so on. However, direct evidence of fractal growth of atomically precise metal clusters, especially in the nucleation stage, is often difficult to obtain. One early example of fractal growth of bimetallic clusters in 3D is the series of AuAg vertex-sharing multi-icosahedral nanoclusters4,5 whose formation pathway was characterized as 3D fractal growth.6 Recent observations of Sierpinsky triangles on surfaces also attest to the importance of fractal growth in 2D.7 Reported herein is the first experimental evidence of a fractal-like pattern in the growth of the icosahedral core of a multishelled bimetallic cluster as exemplified by the title clusters. We believe the characterization of the observed growth pattern as fractal-like may have general implications in the nucleation and growth of bimetallic metal clusters and nanoparticles at the embryonic stage. © XXXX American Chemical Society

Received: August 20, 2018

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DOI: 10.1021/acs.inorgchem.8b02249 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

difluorobenzenethiol (10 μL, 0.091 mmol) and tetraphenyl phosphonium bromide (12 mg, 0.029 mmol) were added and stirred vigorously for 20 min. Then, 1 mL of aqueous NaBH4 solution (45 mg/mL) and 50 μL of triethylamine were added quickly under vigorous stirring. The reaction was aged for 12 h at 0 °C. The aqueous phase was then removed, and the mixture in the organic phase was washed several times with distilled water. Dark red block crystals were crystallized from CH2Cl2/hexane at 4 °C after several days.

suggested that Au substitution occurred in the icosahedral core which may be represented as M12@Ag20@Ag12(SR)30 where M = Au or Ag. However, the exact nature of the doping process was unclear at the time.26 To our surprise, in a recent publication, Xie and co-workers reported that through an exchange mechanism monitored by ESI-MS and DFT calculations up to 12 Au2(SR)5 staples formed from the starting complexes [Au2(SR)2Cl]− anions could replace the 12 surface Ag 2(SR)5 motifs of the [Ag44(SR)30]4− cluster.27 More recently, Pradeep and coworkers carried out exchange reactions between [Ag44(FTP)30][PPh4]4 and [Au25(FTP)18][TOA] (where FTP = 4-fluorothiophenol and TOA = tetraoctylammonium) in different ratios. They concluded from ESI-MS and UV−vis evidence that intercluster Au−Ag exchange reactions could occur in the icosahedral core, the middle dodecahedral shell, or the outermost surface Ag2(SR)5 staple positions.28,29 Neither of these two elegant studies provided experimental structure information with regard to the substitution site(s) of the Au atoms, be it in the icosahedral core, the middle dodecahedral shell, or the surface staples. To resolve the issue of the substitution or doping site(s) and to gain insight into the nucleation and growth of these clusters, in addition to providing conclusive, precise structural information on the resulting clusters, we report herein the investigation of the doping of Au atoms into Ag44 to form (AuAg)44 clusters via one-pot synthesis and characterization of the products by electrospray ionization mass spectroscopy (ESI-MS), UV−vis, and X-ray crystallography. It is demonstrated that the number of Au atoms (n) in [AunAg44−n(SR)30]4− (SR = SC6H3F2) can be continuously tuned from 0 to 12 by employing different molar ratios of reactants AuPPh3Cl and AgNO3. However, distinct from the above mentioned works by other groups,27−29 we found the following: (1) The maximum number of n is 12, achievable only with reactant Au/Ag molar ratio of 1:1. Beyond that, different clusters were formed. Further X-ray crystallographic studies provided unambiguous evidence that Au atoms are doped into the icosahedral core of the cluster. (2) Most interestingly, the mixed-metal (AuAg)44 clusters at various reactant Au/Ag ratios exhibit a fractal-like distribution, suggesting that nucleation of the icosahedral core is a fractal growth process. Also reported herein are the UV−vis and XPS spectra of [AunAg44−n(SR)30]4− which exhibit an Au-dependent absorption behavior. It should also be emphasized that the present work, in addition to providing full structure information on the resulting clusters, involves co-reduction of Au(I) and Ag(I) complexes in one-pot to form the (AuAg)44 clusters. This simple synthetic protocol may be described as ab initio synthesis. The above-mentioned literature works29−31 instead require reactions of preformed Ag44 cluster with other clusters or complexes, thus unavoidably involving complex multistep processes.

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RESULTS AND DISCUSSION Mass Spectra of [AunAg44−n(SR)30]4− with Different Numbers (n) of Au Atoms. Figure 1 depicts 13 MS of

Figure 1. ESI-MS (negative mode) of [AunAg44−n(SR)30]4− (SR = SC6H3F2) synthesized in CH2Cl2 at 0 °C, mass spectra a−m correspond to Au/Ag ratios of 0:1, 1:49, 1:29, 1:19, 1:14, 1:9, 1:5, 1:4, 1:3, 1:2, 2:3, 3:4, and 1:1, designated as 1−13, respectively. The analogous ESI-MS data in CH2ClCH2Cl, designated as 14−19, can be found in Figure S1.

[AunAg44−n(SR)30]4− (SR = SC6H3F2) clusters with different molar numbers (n) of Au atoms. They were synthesized in CH2Cl2 at 0 °C with different ratios of AuPPh3Cl to AgNO3 as reactants. The curves a-m correspond to reactant Au/Ag ratios of 0:1, 1:49, 1:29, 1:19, 1:14, 1:9, 1:5, 1:4, 1:3, 1:2, 2:3, 3:4, and 1:1, hereafter designated as 1−13, respectively. It is apparent from Figure 1 that each MS curve is a distribution of clusters with different Au (n) contents, the center of which increases monotonically from n = 0 for 1(Ag44) to n = 12 for 13. As expected, the curves are truncated at n = 0 (see MS a in Figure 1), corresponding to pure silver icosahedral core in [Ag44(SR)30]4− (1), and as least expected, also at n = 12 (see MS m in Figure 1), corresponding to pure gold icosahedral core in [Au12Ag32(SR)30]4− (13). Fractal-Like Pattern of Mass Spectra. A careful examination revealed that the observed MS a−m exhibit an approximately fractal-like distribution. A typical fractal-like pattern is shown in Figure 2a for Au/Ag = 1:29 (i.e., an

EXPERIMENTAL SECTION

Synthesis of 1−19. AuPPh3Cl and AgNO3 (total metal molarity was kept at 0.091 mmol) were added to an equal mixture of CH2Cl2 (for 1−13) or dichloroethane (for 14−19) and methanol. The molar Au/Ag ratios of the reactants were 1:49 1:29, 1:19, 1:14, 1:9, 1:5, 1:4, 1:3, 1:2, 2:3, 3:4, or 1:1 for 2−13, respectively. The molar Au/Ag ratios of the reactants for 14−19 can be found in Figure S1. The mixture was cooled to 0 °C in an ice bath, and 3,4B

DOI: 10.1021/acs.inorgchem.8b02249 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Nonlinear Relationships between Mole Fractions of Au in the Reactants and Number of Au Atoms in the Clusters [AunAg44−n(SR)30]4−. The average compositions for as-prepared cluster products 1−13 can be determined from the experimental MS shown in Figure 1. Such MS-weighted average compositions30,31 for 1−13, calculated from the mass distributions listed in Table S1 (based on Figure 1), are tabulated in Table S2. Using 9 (Au/Ag = 1:3) as an example, the relative percentage of [Au 6 Ag 38 (SC 6 H 3 F 2 ) 30 ] 4− , [Au 7 Ag 3 7 (SC 6 H 3 F 2 ) 3 0 ] 4 − , [Au 8 Ag 3 6 (SC 6 H 3 F 2 ) 3 0 ] 4 − , [Au9Ag35(SC6H3F2)30]4−, and [Au10Ag34(SC6H3F2)30]4− were measured from MS curve i of Figure 1 as 0.9, 11.0, 43.5, 37.5, and 7.1%, respectively. Therefore, the relative numbers of gold atoms contributed by each species to the total molecular composition are 0.05, 0.77, 3.48, 3.38, and 0.71, respectively. The MS-averaged composition of 9 can then be calculated as Au8.4Ag35.6(SR)304−, as listed in Table S2. Other entries in Table S2 (in CH2Cl2), as well as those in Figure S1 and Tables S4 and S5 (in CH2ClCH2Cl), can likewise be computed. Inductively coupled plasma mass spectrometry (ICP-MS), which transfers all analytes into singly charged gaseous phase ions, can also provide useful information on the ratios of various elements in the study of heteronuclear clusters.32,33 The ratios of Ag/Au acquired by ICP-MS are 13.6, 9.1, 7.3, 4.6 4.3 3.7, and 2.8 for 4, 6−10, and 13, respectively, which are in good agreement with the results calculated from the ESI-MS data (cf. Table S3). It is apparent from the foregoing discussion that the experimentally determined compositions, [AunAg44−n(SR)30]4−, are not directly related to the Au/Ag ratios of the reactants. In order to investigate the relationship between the two, the experimentally determined n values were plotted as a function of mole fraction of Au in the reactants in Figure 3 at different temperatures. These curves can be fitted with the following nonlinear two-parameter model:

Figure 2. Comparison of experimental mass spectra (top) and fractal patterns (bottom) for [AunAg44−n(SR)30]4− synthesized with Au/Ag ratios of 1:29 (MS c,left figures) and 1:1 (MS m,right figures) in CH2Cl2 at 0 °C The simulated spectra (bottom) are based on the Fibonacci series of (1, 1, 2, 3, 5, 8, 13, ...). The experimental MS are truncated at n = 0 (Au12 icosahedral core in [Au2Ag42(SR)30]4− (3), top left) and n = 12 (pure Au 12 icosahedral core in [Au12Ag32(SR)30]4− (13), top right).

expanded version of MS curve c in Figure 1).26 The mass spectrum displays multiple peaks that can be assigned to [AunAg44−n(SR)30]4− clusters where n = 0−6 with intensities resembling a fractal-like distribution profile (Fibonacci series of 1, 1, 2, 3, 5, 8, 13, ...) depicted in Figure 2b. The profile is centered at n = 2, and the experimental MS is, as stipulated above, truncated at n = 0. By the same token, Figure 2c shows an expanded version of MS curve m in Figure 1.26 It corresponds to the mass spectrum for Au/Ag = 1:1, exhibiting n = 9−12 with intensities resembling a fractal-like distribution profile (Figure 2d). The profile is centered at n = 12, and the experimental MS is instead truncated at n = 12. At first glance, some of the observed mass spectra (MS) depicted in Figure 1 do not have a center peak and thus seem not to follow the fractal patterns. We believe this is due to the fact that not all MS match perfectly with an integral n value; the latter being the number of molar Au atoms in the icosahedral core of the [AunAg44−n(SR)30]4− cluster synthesized with an Au/Ag ratio of p:q (n ≠ p, q). As a result, the observed mass spectrum may be a superposition of two or three adjacent growth sequences which, individually, follow the fractal growth pattern (or Fibonacci series). To illustrate this possibility, a comparison of fractal patterns and experimental MS (curve e in Figure 1) of [AunAg44−n(SR)30]4− synthesized with Au/Ag ratios of 1:14 is provided in Figure S2, and as detailed therein: (a) simulated MS based on 1/3 of the Fibonacci series (1, 1, 2, 3, 5, 8, 5, 3, 2, 1, 1) centered at n = 3; (b) simulated MS based on 2/3 of the Fibonacci series centered at n = 4; (c) the superposition of the two simulated MS (a) and (b); (d) the experimental mass spectrum, curve e, of Figure 1. A similar comparison can be found in Figure S3 for [AunAg44−n(SR)30]4− synthesized with Au/Ag ratio of 3:4. It can be seen from these comparisons that the noncompliances of the Fibonacci series may be due to the superposition of two or three adjacent growth sequences. It should be emphasized that each individual peak in the MS curves (Figures 1 and 2) represents a bonafide group of [AunAg44−n(SR)30]4− clusters each of which agrees perfectly with simulation that takes into account isotope distributions of all constituent elements (see comparisons of experimental and simulated mass spectra in Figure S4).

y = a[1 − exp( −x /b)]

(1)

Figure 3. Nonlinear relationships between the mole fractions of Au in the reactants and the number of Au atoms in the resulting clusters [AunAg44−n(SR)30]4− in the CH2Cl2. The curve fitting results are as follows: at −15 °C, y = 11.348[1 − exp(−x/0.1776)]; at −10 °C, y = 11.092[1 − exp(−x/0.1670)]; at 0 °C, y = 10.872[1 − exp(−x/ 0.1656)]; at 10 °C, y = 10.369[1 − exp(−x/0.1357)]; at 25 °C, y = 8.123[1 − exp(−x/0.0743)]. C

DOI: 10.1021/acs.inorgchem.8b02249 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Although a mixture of [AunAg44−n(SR)30]4− was detected by the MS, it was unclear whether or not the crystallization process would alter the composition mix. To evaluate this possibility, one batch of the single crystals was redissolved in CH2Cl2 and its ESI-MS compared to that of the as-prepared product. As shown in Figure S10, the mass spectra of the two were practically identical. This one-to-one correspondence lays the foundation for structure investigations to be described next. Single-Crystal X-ray Crystallography: Au versus Ag Occupancies. Thus far, we have been operating on the assumption that a maximum of 12 Au atoms could be “doped” into the icosahedral core of the [AunAg44−n(SR)30]4− cluster. However, there is no a priori reason to believe that it is the case. There are many other scenarios for the fate (locale) of the “doped” Au atoms, for example, (1) concentrated on the surface shell as in the case of Xie et al.’s work;27 (2) concentrated on the pentagonal-dodecahedral shell, as suggested by Pradeep et al., for n > 12;28,29 (3) random distribution throughout the cluster, and combinations thereof. Worst of all, even if the Au location(s) are known, the occupancies of Au atoms in the [AunAg44−n(SR)30]4− clusters still cannot be determined from MS alone. To remedy this problem, we resort to single-crystal X-ray crystallography which is the only technique that can provide unequivocal structure information with respect to the Au substitution site(s). Needless to say, only painstaking crystallographic studies can provide full structural information to relate to the MS and UV−vis data. Ultimately, this three-way correspondence is important for future identification (signatures) of the alloy [AunAg44−n(SR)30]4− clusters where n = 0−12. Thus, eight crystals were grown from the CH2Cl2 solution, their cell parameters are listed in Table S7. Full crystal data of four mixed crystals, namely, 7−10, were collected and their structures analyzed (Figure 4). In particular, the occupancies of all metal atoms were refined with a disordered Au1−r−Agr

Here y is the MS-averaged number of Au atoms (cf. Table S2) for a given Au molar fraction x (calculated from the Au/Ag ratio of the reactants). The parameter a is the maximum n value attainable at a given temperature and b is a measure of the curvature of the nonlinear curve. The fitted a and b values are compiled in Table S6 for different temperatures and in different solvents (CH2Cl2, CH2ClCH2Cl, and DMF). It should be mentioned that despite our exhaustive attempts to find suitable solvents with different properties and temperature ranges for these experiments, the first two solvents were by far the best and with only limited temperature ranges. DMF is met with only limited success as indicated in Figure S6. Temperature, Solvent, and Temporal Effects. As can be seen from Figure 3, at a given temperature, the y value rises very fast with an initial slope of a/b (for small x values) before entering into the nonlinear region (inflection point) which eventually levels off to reach a plateau at y ≈ b. For example, at 0 °C, the initial slope is a/b = 10.872/0.1656 = 65.65, which bears no relationship to the mole fraction (x) of Au in the reactants. In other words, Au atoms were being incorporated preferentially into the icosahedral core at low x values. This behavior is understandable since Au−Au bonds are stronger than Au−Ag bonds which are in turn stronger than Ag−Ag bonds. Thus, in the initial high-slope (low x) region, the strong Au−Au bonds are formed preferentially. In the nonlinear region, the benefit of extra energy gain owing to the difference in the electronegativities of Au versus Ag resulted instead in maximizing the formation of heteronuclear Au−Ag bonds. Finally, the curve levels off and eventually approaches the maximal value of y = a. These two effects were previously coined “Strong-Bond” and “Hetero-Bond” effects, respectively, in governing the site preference of heteronuclear clusters by Teo and co-workers.34 It is clear from Figure 3 that temperature plays many critical roles in the reaction. It affects not only the reaction rate but also the composition of the products. A higher temperature gives rise to a faster reaction rate, often coupled with a greater initial slope a/b and a lower a value (maximal attainable n value). The analogous curves in CH2ClCH2Cl and DMF can be found in Figures S5 and S6, respectively. The influence of temperature on parameters a and b in CH 2 Cl2 and CH2ClCH2Cl can be found in Figure S7. Both parameters decrease with increasing temperatures; though at or below 0 °C, they seem not to vary significantly under our experimental conditions. Solvent also plays a critical role in the nucleation and growth of metal clusters. Solvent properties such as polarity (or dielectric constant) and viscosity affect not only the transport rate of the reactants but also the formation and relative stabilities of the activation complexes as well as the resulting clusters, in a word, the energy landscape of the reduction, nucleation, and growth reactions. The effects of solvent polarity, dielectric constant, and viscosity on parameters a and b are shown in Figure S7. It is apparent that for a given temperature, parameter a is most affected by the viscosity (a greater viscosity gives rise to a larger a value) and parameter b by the polarity or dielectric constant (a greater polarity leads to a smaller b value) of the solvent. Temporal and/or aging effect were also investigated. As shown in Figure S9, the center (n) of the fractal pattern may shift by ±1, as the reaction proceeds, probably caused by stirring rate and so on. Nevertheless, the fractal pattern “grows” proportionally, as expected.

Figure 4. (a) Icosahedral core (gold), the dodecahedral shell-1 (green), and the surface shell-2 (purple) of [AunAg44−n(SC6H3F2)30]4−. (b) Crystallographically refined occupancies of Au (gold) versus Ag (green) for clusters 7−10. D

DOI: 10.1021/acs.inorgchem.8b02249 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

XPS Spectroscopy. Ag and Au XPS spectra of 9 ([AunAg44−n(SR)30]4− where n = 8) are shown in Figure S15. The binding energies of Ag 3d5/2 were observed at 368.1 and 367.3 eV with a simulated intensity ratio of 1:2 correspond to the Ag(I) (expected at 368.0 eV) and Ag(0) (expected at 367.5 eV), consistent with the two different kinds of Ag atoms in the ratio of 12:24 [(surface shell)/(4 in the icosahedral core + 20 in the dodecahedral shell)]. The binding energy of Au 4f5/2 at 87.2 eV is also consistent with to zero-valent Au, expected at Au(0) (expected at 87.4 eV). The XPS spectra corroborate nicely the single-crystal X-ray crystallographic results in that the eight Au(0) atoms are located in the icosahedral core, whereas the Ag atoms can be found in the icosahedral core (four Ag(0)) as well as in the pentagonal dodecahedron shell (20 Ag(0)) and the outermost surface layer (12 Ag(I)).

model. As shown in Figure S12, the Au atoms are concentrated in the icosahedral core in all cases and evenly distributed among the 12 icosahedral sites. The Au contents of all other locations are less than 5%; they can thus be considered as pure Ag atoms. The overall compositions of 7−10, calculated from the occupancies determined by X-ray crystallography, are listed in Table 1 and compared with the corresponding MS-averaged Table 1. Comparison of the Total Number of Au Atoms in the Icosahedral Core As Obtained by ESI-MS versus X-ray Structure Analysis for Different Au/Ag Molar Ratios of Reactants (in CH2Cl2) cluster

reactant Au/Ag ratio

Au atoms (ESI-MS)

Au atoms (crystallography)

4 6 7 8 9 10 13

1:19 1:9 1:5 1:4 1:3 1:2 1:1

2.8 4.8 6.3 7.6 8.4 9.3 10.8

N/A N/A 6.3 7.0 7.4 9.1 N/A

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CONCLUSION An ab initio one-pot synthesis of the bimetallic clusters [AunAg44−n(SC6H3F2)30]4− ((AuAg)44) is reported. The bimetallic (AuAg)44 clusters, synthesized with different reactant Au/Ag ratios, exhibit a fractal-like pattern, suggesting that, for the first time, nucleation of the bimetallic icosahedral core is a fractal growth process. X-ray crystallographic studies provided unambiguous evidence that the doped Au atoms occupy the icosahedral sites and the maximal doping is 12, beyond which a different cluster is formed. It is shown that the number of Au atoms (n) in [AunAg44−n(SR)30]4− (SR = SC6H3F2) can be continuously tuned from 0 to 12. More importantly, a three-way correspondence between singlecrystal structure, MS, and UV−vis is established, thereby facilitating unambiguous identification (finger-printing) of the alloy [AunAg44−n(SR)30]4− clusters. Finally, the temperature, solvent, and temporal effects were also investigated.

compositions. The excellent agreements are rather gratifying, given the fact that both the samples (single crystals versus solution/gas phases, prepared at different times), and the measurement techniques were drastically different. UV/vis Spectra of 1−13. The optical properties of [AunAg44−n(SC6H3F2)30]4− (n = 1−12) exhibit high sensitivity to Au doping. Figure 5 depicts the ultraviolet−visible (UV/vis)

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02249. Mass spectra, comparison of fractal patterns, nonlinear curve fittings, least-square refined occupancies of Ag, UV/vis spectra, number averages of Au atoms, cluster compositions, ratios of Ag/Au, parameters of the nonlinear fit, summary of crystallographic data (PDF) Accession Codes

CCDC 1849028 and 1849030−1849032 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.

Figure 5. UV/vis spectra of 1, 4, 7, and 13.

spectra of 1, 4, 7, and 13. UV−vis of oth er [AunAg44−n(SC6H3F2)30]4− (n = 1−12) and the molar absorbances of 9 can be found in Figures S13 and S14, respectively, for future finger-printing. In general, the UV−vis displays distinct absorption bands centered at 374, 411, 483, 535, and 641 nm.26 The absorption bands exhibit progressive shifts upon Au substitution of Ag atoms in the icosahedral core. First, the 411 nm band blue-shifted and weakened. Second, the 641 nm band blue-shifted, weakened, and eventually disappeared. Third, the 535 nm band diminished and quickly disappeared.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.K.T.). *E-mail: [email protected] (S.L.). *E-mail: [email protected] (N.Z.). ORCID

Boon K. Teo: 0000-0003-3477-1471 Nanfeng Zheng: 0000-0001-9879-4790 E

DOI: 10.1021/acs.inorgchem.8b02249 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2017YFA0207302), the NSFC of China (21731005, 21420102001, 21721001, and 21701133), the President Research Funds from Xiamen University (20720170100).

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DOI: 10.1021/acs.inorgchem.8b02249 Inorg. Chem. XXXX, XXX, XXX−XXX