Transformation Chemistry of Gold Nanoclusters: From One Stable Size

Jul 9, 2015 - The FCC structure was once thought to be unstable (e.g., less stable than the multiple-twined structure in the ultrasmall Aun(SR)m nanoc...
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Transformation Chemistry of Gold Nanoclusters: From One Stable Size to Another Chenjie Zeng, Yuxiang Chen, Anindita Das, and Rongchao Jin* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. This dream has first been realized in the case of gold nanoparticles. We previously discussed a size-focusing methodology for the syntheses of atomically precise gold nanoclusters protected by thiolate ligands (referred to as Aun(SR)m, where n and m represent the exact numbers of gold atoms and surface ligands). This methodology led to molecularly pure nanoclusters such as Au25(SR)18, Au38(SR)24, Au144(SR)60, and many others in recent work. In this Perspective article, we shall further discuss a new methodology for controlling the size and structure of nanoclusters through ligand-exchange-induced transformation of Aun(SR)m nanoclusters. Notable examples include the transformations of Au25(SR)18 to Au28(SR′)20, Au38(SR)24 to Au36(SR′)24, and Au144(SR)60 to Au133(SR′)52. Total structures of the new nanoclusters have also been attained. The transformation processes are remarkable and resemble the organic transformation chemistry. We have also achieved mechanistic understanding on the transformation process, and a disproportionation mechanism has been for the first time identified. This new methodology (i.e., ligand-exchange-induced size/structure transformation, LEIST for short) has not only demonstrated the important role of thiolate ligand in the transformation chemistry of clusters but also paved the way for creating an expanded “library” of Aun(SR)m nanoclusters for exploration of their magic sizes, structures, properties, and applications. synthesis, the first step produces a number of sizes upon NaBH4 reduction of the Au(I):SR polymeric intermediate, but these different sizes of Aux(SR)y nanoclusters are vastly different in stability. The second step, ligand-induced size conversion or decomposition of relatively unstable sizes, gives rise to the most stable species under the size-focusing conditions, and ultimately, only the most robust size survives the size-focusing process. This methodology has led to a rapid expanding of the discrete sizes of nanoclusters3,25−27 and tremendous progress in crystallization and structure determination, deep understanding of the fundamental properties of Aun(SR)m nanoclusters and the development of their applications in a range of fields.1,5−8

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ecent years have witnessed significant advances in controlling gold nanoparticles with atomic precision.1−3 Such unique nanoparticles are often called nanoclusters in order to distinguish them from the regular nanoparticles, which are more or less polydispersed and can only be represented with an average diameter of the metal cores typically measured by transmission electron microscopy.4 In contrast, atomically precise nanoclusters possess definite formulas like organic molecules,3 and for the first time, molecularly pure nanoparticles have now become available, which open up many new, exciting opportunities in both fundamental studies and promising applications.5−16 Although achieving atomic precision and molecular purity is routine in organic chemistry, it is by no means so in nanochemistry, because nanoparticle growth is extremely complicated and remains the least understood,17 and also because nanochemistry is still very young (about two decades), whereas organic chemistry has been developed for more than a century. Nevertheless, intense research in the past years has established a systematic methodology based on earlier work,18,19 called size focusing, for achieving atomically precise nanoparticles with molecular purity.20−24 This methodology consists of two primary steps, (i) kinetically controlled synthesis of an Aux(SR)y mixture with a properly controlled size range (note: this is critical for achieving singe-sized nanoclusters in the second step) and (ii) thermodynamically dictated size-focusing of the mixture to single-sized nanoclusters. The basis of the methodology is the stability property of different sized Aun(SR)m nanoclusters. In the size focusing © XXXX American Chemical Society

The prerequisite for the discovery of precise transformation chemistry between nanoclusters is the accessibility of atomically precise nanoclusters as “precursors”. A major question is what stable sizes are existent in Aun(SR)m nanoclusters. This is important not only for understanding the stability issue of magic-size nanoclusters Received: June 1, 2015 Accepted: July 9, 2015

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Au25(SeR)18 was found to be enhanced.39,40 Bakr and coworkers reported a rapid (less than 30 s) thiolate-for-thiolate exchange on the [Ag44(SR)30]4− nanocluster with ligands containing a variety of functional groups, and the as-obtained nanoclusters could be processed into smooth thin films for integration into devices due to the replacement of the hydrophilic thiolate by the hydrophobic thiolates.41 In regards of characterization of mixed-ligand shell nanocluster, Negishi and co-workers successfully achieved the isolation of each component of the mixed-ligand PdAu24(SR)18−x(SR′)x and Au25(SR)18−x(SR′)x series using HPLC.42 The mixed-thiolate ligand-shell has also been observed by X-ray crystallography. Heinecke et al. reported the partial ligand exchange of parabromobenzenethiol (p-BBT) onto the Au102(p-MBA)44 nanocluster (p-MBA = para-mecaptobenzoic acid).43 When the feed ratio of p-BBT to the p-MBA on the cluster was kept at 2:44, partially exchanged Au102(p-MBA)40(p-BBT)4 was resulted at a very fast kinetics (e.g., in 5 min). The X-ray crystal structure shows that the exchanged thiolate positions were on the two poles of the 5-fold symmetric dodecahedral kernel, which are the most accessible sites on the Au102 nanocluster. A similar ligand exchange reaction was also performed on the Au25(PET)18 by the same group, and the partially ligand-exchanged product Au25(PET)16(p-BBT)2 was successfully crystallized.44 It is worth noting that the mixedligand shelled nanoclusters could also be synthesized via direct reduction of gold salts in the presence of multiple types of ligands. For example, Yuan et al. reported a facile synthesis of mono-, bi-, and trithiolate-protected Au25(SR)x(SR″)y(SR‴)18−x−y nanoclusters via a NaOH-mediated NaBH4 reduction method.45 Ligand exchange has also been extensively employed to introduce chirality into the gold nanoclusters by using chiral thiolate ligands. Burgi and co-workers reported the use of a chiral bidentate thiol, (R)-BINAS (note: BINAS = 1,1′binaphthyl-2,2′-dithiol), to perform ligand exchange with the racemic Au38(PET)24 nanoclusters at room temperature with a ratio of BINAS:Au38(PET)24 =120:1 or BINAS:PET = 5:1.46 It was found that two PET ligands on the Au38 were exchanged by one (R)-BINAS, and further exchange with a second (R)-BNAS ligand showed a much slower rate. They also found that the (R)-BINAS had the preference for the lefthanded Au38 cluster, manifested in a ligand exchange rate four times higher compared to that for the right-handed Au38 cluster.47 In terms of ligand-exchange-induced size change of nanoclusters, early research by Hutchison group reported the thiolate ligand exchange on gold nanoclusters protected by phosphine, for example thiolate ligand exchange reaction with the Au55(PPh3)12Cl6 cluster would lead to the 1.7 nm (diameter) thiolate-protected gold clusters.48 Tsukuda group reported a large-scale synthesis of Au25(SG)18 from the phosphine-protected Au11 clusters via ligand exchange with glutathione (HS-G).49 Furthermore, using HS(CH2)nCH3 to exchange with phosphine, they also obtained another 25-atom nanocluster formulated as [Au25(PPh3)10(SR)5Cl2]2+ and successfully solved its biicosahedral structure.50 These are distinct examples of thiol-for-phosphine exchange. Qian et al. developed a ligand exchange approach for obtaining large quantities of Au38(SC12H25)24 nanoclusters by thiol-for-thiol exchange of a mixture of aqueous Aux(SG)y nanoclusters with organic soluble HS-C12H25,51 which solved the issues of low yield and nontrivial postsynthetic isolation of Au38(PET)24 in

but also for the exploration of new properties for real-world applications.3 Although the previously developed size-focusing methodology has led to a series of stable sizes,20−27 those bottom-up syntheses are nevertheless somewhat “trial and error”. We have recently developed and pursued a new approach, which is to utilize ligand exchange to induce size and structure transformation and, hence, to attain new Aun(SR)m nanoclusters. At first glance, ligand exchange would not cause any change in nanoparticle size as reported in early work.28−31 However, it should be pointed out that early work involved polydispersed nanoparticles and atomic level information was smeared out.28−31 Thus, the prerequisite for the discovery of precise transformation chemistry between nanoclusters is the accessibility of atomically precise nanoclusters as “precursors”. In addition, the previous work typically involved only partial exchange of ligands and the process occurred at room temperature,32 but our ligand exchange approach involves thermal conditions and the addition of large excess of thiol. Moreover, only when the incoming thiol has some significant difference than the original thiolate on the nanocluster, would the transformation process occur; ligand exchange such as the −SC6H13 with −SC7H15 would not lead to the size change. The difference in ligands would activate the starting nanoclusters for further chemical reactions, socalled transformation chemistry of nanoclusters. Below, we first give a brief overview of the early ligandexchange chemistry and then focus on recent progress in the new transformation chemistry of nanoclusters induced by thermal ligand exchange. Overview of Ligand Exchange. In nanoscience, ligand exchange is a highly versatile strategy for functionalization and tailoring of the surface properties of nanoparticles. For examples, citrate-protected Au nanoparticles can be readily functionalized with DNA for biomedical applications,33 thiolateprotected Au nanoparticles can be ligand exchanged to introduce drug molecules,34 spin-labeled thiolate can be exchanged onto the Au nanoparticle surface for probing the dynamics,35 and so forth.

In nanoscience, ligand exchange is a highly versatile strategy for functionalization and tailoring of the surface properties of nanoparticles. Here, we utilize it to create magic-sized nanoclusters. With respect to atomically precise nanoclusters, thiolate ligand exchange process has also been applied to introduce functionality to the nanolcusters’ exterior ligand shell. Murray and co-workers performed extensive work of ligand exchange on Au25(PET)18 with various SR (where PET = SCH2CH2Ph, R = Ph−CH3, Ph−F, etc.) and this Au25 nanocluster can accommodate many different types of thiolate ligands, evidenced in the observation of the Au25(SR)18−x(SR′)x series (where x can be up to 12).32,36,37 Pradeep and co-workers38 also performed ligand exchange on Au25(SG)18 with functionalized glutathione, with the purpose of tuning the optical properties of the Au25(SR)18. Furthermore, selenolate for thiolate exchange was reported in the Au25(SR)18 case by Negishi and Zhu groups, and the stability of the resultant 2977

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Figure 1. (A) Scheme of the transformation of Au38(PET)24 to Au36(TBBT)24, (B) mass spectrum (CsOAc was added to form a Cs+-adduct with nanoclusters, (C) thermogravimetric analysis (N2 atmosphere, 10 °C/min), (D) tetrahedral two-shell Au28 kernel with the inner Au4 tetrahedron shown in space-fill and the outer shell in ball-and-stick fashion, (E) the Au28 kernel exhibits bulk-like (111) and (100) faces, and (F) the surface protection of the Au28 tetrahedron by four dimeric staples (containing 12 thiolates) and 12 bridging thiolates. Color coding: yellow = sulfur; gray = carbon; other colors = gold. Adapted from ref 54.

the approach reported by Chaki et al.19 Following this thiolfor-thiol exchange approach, Qian et al. subsequently obtained phenylethanethiolate-protected Au38(PET)24 in high yield and also successfully solved the crystal structure.52 In terms of the mechanism, the ligand exchange (from −SG to −SC2H4Ph) on the Aux(SG)y nanoclusters was rather fast (5−10 min) under the heating conditions, and the subsequent thermal etching process (in excess thiol) caused gold core etching, and eventually the starting polydisperse nanoclusters were converted to monodisperse Au38(PET)24 with high purity.21 In later work, Nimmala et al. reported HS-Ph exchange with a phenylethanethiolate-protected 15−25 kDa mixture and obtained an Au36(SPh)23 nanocluster.53 Taken together, all these thiol-for-thiol exchange examples involved polydisperse Aux(SR)y nanoclusters as the starting material. Size and Structure Transformation via Ligand Exchange. With the development of the size-focusing synthetic methodology, atomically precise Au 25 (PET) 18 , Au 38 (PET) 24 , and Au144(PET)60 nanoclusters became available by 2010 (where PET = SC2H4Ph, and here after),20,21,23 which opened up new opportunities for ligand exchange synthesis of different sized nanoclusters. Because the starting nanocluster is of molecular purity (rather than a mixture in earlier thiol-for-thiol exchange

work), we thought that one might be able to obtain new magic sizes in high yield and molecular purity; this was indeed so. Transformation of Au38(PET)24 to Au36(TBBT)24. In 2012, we first succeeded in thiol-for-thiol thermal exchange of molecularly pure Au38(PET)24 nanoclusters with 4-tertbutylbenzenelthiol (TBBT, the same abbreviation of its thiolate form), Figure 1A.54 Briefly, the Au38(PET)24 nanocluster reacted with TBBT at 80 °C for >12 h. The molar ratio of incoming TBBT to the PET on the Au38 cluster was around 160:1, much higher than that of the previous ligand exchange processes. This process produced a new Au36(TBBT)24 nanocluster in high yield (>90%, Au atom basis) and with molecular purity, evidenced by clean mass spectrometry and thermogravimetry analyses (Figure 1B−C). This direct transformation chemistry from the Au38 to Au36 cluster is remarkable, which resembles the precise organic transformation reaction. The discovery of this elegant yet simple transformation chemistry greatly facilitated the crystallization of the Au36 nanocluster, and its atomic structure was successfully solved by single-crystal X-ray crystallography (Figure 1D−F). Surprisingly, the Au36(TBBT)24 structure possessed a face-centered cubic (FCC) kernel, which is 2978

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significantly different from the starting Au38(PET)24 cluster, although they only differ by two gold atoms. The Au36(TBBT)24 was indeed the first observation of FCC structure in Aun(SR)m nanoclusters. The FCC structure was once thought to be unstable (e.g., less stable than the multiple-twined structure in the ultrasmall Aun(SR)m nanoclusters) and only when the size reaches to a certain threshold, the FCC atomic-packing mode of gold atoms would appear.55,56 It turned out that the ligand played a critical role in stabilizing the FCC kernel in the Au36 cluster. The overall structure of Au36(TBBT)24 consists of a 28-atom (Au28) FCC kernel with a truncated tetrahedral shape (Figure 1E), and the four exposed {111} facets are each protected by a Au2(SR)3 dimer, and the exposed six {100} facets are protected by 12 simple bridging −SR− thiolates (Figure 1F). The simple bridging thiolates (i.e., without incorporating any gold atoms as in Au(SR)2 and Au2(SR)3 staple motifs) was a new feature. The emergence of FCC-structured Au28 kernel at such a small size came as a surprise, as the previously thought general trend was from icosahedral (e.g., Au25(PET)18 and Au38(PET)24)52,57 to decahedral (e.g., Au102(p-MBA)44)58 to FCC structure in plasmonic nanoparticles and bulk gold. We note that the conversion of Au 38 (PET) 24 to Au36(TBBT)24 does not mean that the Au38(SC2H4Ph)24 nanocluster is not stable; instead, Au38(PET)24 exhibits high thermal and chemical stability (e.g., resistant to reduction and oxidation by common reagents).59 Herein, what the ligand exchange does is activation of the nanocluster (see mechanistic discussion). An example from organic transformation chemistry is that benzene can be converted to hexane via catalytic hydrogenation and the latter can also be converted back to benzene via dehydrogenation; apparently, both benzene and hexane are very stable molecules. Transformation of Au25(PET)18 to Au28(TBBT)20. Following the success in the Au38(PET)24 to Au36(TBBT)24 conversion, Zeng et al. further performed ligand exchange on the Au25(PET)18 nanocluster and indeed obtained a new stable size formulated as Au28(TBBT)20, see Figure 2A.60 The ligand-exchange-induced size/structure transformation was fast in the case of Au25(PET)18; after ∼2 h, all the Au25(PET)18 nanoclusters were found to be completely converted to Au28(TBBT)20 in high yield (>90%, Au atom basis). The crystal structure of Au28(TBBT)20 was solved (Figure 2), and it was found that the Au28(TBBT)20 structure was also significantly different from that of the starting Au25(PET)18; the latter has an icosahedral Au13 kernel protected by six Au 2 (SR) 3 dimeric staple motifs (Figure 2A). The Au28(TBBT)20 nanocluster contains a FCC Au20 kernel and four dimeric staples (Figure 2B−D) and eight bridging thiolates (−SR−, Figure 2E−G). Within the kernel, two cuboctahedra interpenetrate each other, forming a rod-like Au20 kernel. The rotative arrangements of the four dimeric staples as well as the arrangements of the bridging thiolates give rise to chiral Au28 structures (i.e., quasi-D2 symmetry, Figure 2H−I), and a pair of enantiomers was found in the unit cell of Au28(TBBT)20 crystals. The enantiomers were separated by chiral-HPLC.60 Transformation of Au25(PET)18 to Au20(TBBT)16. Interestingly, the Au25(PET)18 nanocluster can also be transformed to a Au20(TBBT)16 nanocluster via ligand exchange.61 In this case, by carefully tuning the transformation kinetics, that is, thermal reaction of Au25(PET)18 with excess TBBT thiol at 40 °C (lower than the case of Au28(TBBT)20 synthesis), a new

Figure 2. (A) Scheme of the transformation of [Au25(PET)18]− to Au28(TBBT)20, (B−I) thiolate-binding modes in Au28(TBBT)20, (B− D) dimeric staples (total: four), (E−G) bridging thiolates (total: eight), and (H, I) overall Au28S20 framework. Color coding: magenta = Au atoms in the kernel; blue = Au in dimeric staples; yellow = sulfur; the four dimeric staples are highlighted in green in panels C and I. Adapted from ref 60.

cluster Au20(TBBT)16 was obtained. The large excess of TBBT thiol (i.e., 150:1 molar ratio of TBBT to PET in Au25(PET)18) facilitated the complete replacement of PET to TBBT in the nanoclusters. The crystal structure of Au20(TBBT)16 was successfully solved (Figure 3A). The cluster structure is also chiral, and each enanotiomer comprises a vertex-shared bitetrahedral Au7 kernel. Surprisingly, we for the first time observed an octameric macrocyclic ring, Au8(SR)8, as a surface-protecting motif; other surface motifs include two monomers and one trimer, Figure 3B. It is noteworthy that this Au 20 (TBBT) 16 , together with Au28(TBBT)20, Au36(TBBT)24, and Au44(TBBT)28 constitute a neat “magic series” with a uniform progression of Au8(SR)4.61,62 Transformation of Au23(S-c-C6H11)16 to Au24(SCH2Ph-tBu)20. Another example is the transformation of [Au 23(S-cC6H11)16]− TOA+ (where S-c-C6H11 = 1-cyclohexanethiol, and TOA+ = +N(n-C8H17)4)63 to the Au24(SCH2Ph-tBu)20 nanocluster following a thermal ligand exchange reaction of [Au23(S-c-C6H11)16]− with excess HSCH2Ph-tBu thiol.64 Structural characterization showed that the charge-neutral Au24(SCH2Ph-tBu)20 nanocluster comprises a bitetrahedral Au8 kernel, which is protected by four tetrameric staple motifs (Figure 4). The Au24(SCH2Ph-tBu)20, together with Au23(S-cC6H11)16 and Au25(PET)18 nanoclusters, constitute the first crystallographically characterized “trio”, but their structures are dramatically different in terms of both the kernel and surface protecting motifs, resulting from different protecting thiolate 2979

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Figure 3. (A) Total structure of one of the enantiomers of Au20(TBBT)16 with the Au7 kernel shown in space-fill fashion, the surface motifs in ball-and-stick, and the carbon tails in wire-frame. (B) Au8(SR)8 macrocyclic ring as protecting motif in Au20(TBBT)16, together with two Au(SR)2 monomeric staple motifs and one Au3(SR)4 trimeric staple motif. Adapted from ref 61.

Figure 4. Crystal structure of Au24(SCH2Ph-tBu)20. Color coding: magenta = Au; yellow = S; blue/green = Au in tetrameric staple motifs. Adapted from ref 64.

ligands. Another trio is Au 38(SR) 24 , Au 39(SR)24 , and Au40(SR)24, but the latter two structures have not solved.65,66 Transformation of Au144(PET)60 to Au133(TBBT)52. A major achievement is the transformation of Au144(PET)60 to Au133(TBBT)52 (Figure 5A),67 which demonstrated the effectiveness of the transformation chemistry in larger sized nanoclusters. The Au133(TBBT)52 nanocluster was obtained via thermal reaction of molecularly pure Au144(PET)60 with excess TBBT (TBBT:PET = ∼370:1) at 80 °C for 4 days. Significantly, Zeng et al. succeeded in the crystallization of this nanocluster, which is the hitherto largest structure. Prior to this, Au102(p-MBA)44 was the largest structure.58 The total structure of Au133(TBBT)52 is shown in Figure 5B. The diameter of the metal core is 1.7 nm and the entire particle is ∼3.0 nm (including the ligand shell). The Au133(SR)52 nanocluster is charge-neutral, evidenced by X-ray crystallography and electrospray ionization mass spectrometry analyses. The 133 gold atoms in the particle are distributed in four shells. Starting with a central atom, the successive shells are 12, 42, 52, and 26 gold atoms, respectively. A 55-atom Mackay icosahedron (MI) is for the first time observed in gold nanoclusters, which has long been sought since Schmid et al. reported the phosphine-protected Au55 nanocluster in 1981.68 But the Au55:phoshphine cluster has not been crystallized yet. The third shell in the Au133(TBBT)52 nanocluster caps the 20 exposed triangular {111} facets of Au55-MI and is a transition layer between the Au55-MI and the surface staple layer. Among the 20 triangular {111} facets of Au55-MI, 16 facets are each capped by three gold atoms, and the remaining 4 facets are each capped by only one gold atom, hence, total 16 × 3 + 4 × 1 = 52 gold atoms in this third shell. These 52 gold atoms indeed as the “footholds” for the 26 monomeric Au(SR)2 staples, with each staple requiring two footholds; hence, the overall structure is highly symmetric

Figure 5. (A) Scheme of the transformation of Au144(PET)60 to Au 133 (TBBT) 52 , (B) total structure of chiral Au 133 (TBBT) 52 determined by single crystal X-ray crystallography. Color coding: magenta = gold; yellow = sulfur; gray = carbon; white = hydrogen. Adapted from ref 67.

and renders a high stability of the structure. Of note, Au133(TBBT)52 possesses 81 nominal valence electrons (i.e., 133−52 = 81) and does not fall in the superatom series (e.g., 2, 8, 18, 58, 92, ...). Several asethetic features are manifested in the Au133(TBBT)52 nanocluster.67 First, 24 monomeric staples (−S−Au−S−) form four beautiful “helical stripes” which are evenly distributed on the spherical surface of the Au107 kernel (Figure 6). Within each stripe, the six staples are parallel to each other and form a ladder-like helix (Figure 6A,C). The four helices emanate from one pole of the globular Au107 kernel and are converged at the other pole (Figure 6B, C). Although the Au107 kernel is achiral, the clockwise and anticlockwise rotative distributions of the four helices give rise to the left- and right-handed Au133(TBBT)52 nanoclusters. The arrangement of the carbon tails (Ph-p-tBu) on the 2980

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Scheme 1. Comparison of Structures of Au38(PET)24 and Au36(TBBT)24a

a

Adapted from ref 74.

nanoclusters chirality. By contrast, Au36(TBBT)24 contains a truncated tetrahedral FCC Au28 kernel, and both the kernel and arrangement of surface staples give rise to a planesymmetric structure, hence, achiral. To address the question of how the size and structure transformation occurs along with ligand exchange, we carried out time-dependent mass spectrometry and optical spectroscopy analyses.74 Our results unambiguously mapped out a detailed size-transformation pathway. The whole process can be roughly divided into four stages (Scheme 2, vide infra). An interesting disproportionation mechanism was identified in the transformation of Au38(PET)24 to Au36(TBBT)24.

Figure 6. Self-assembled −S−Au−S− helical stripes on the spherical Au107 kernel. (A, B) Side views and (C, D) top views; the two chiral isomers are shown in (D). Color coding: yellow = sulfur; orange/ red/blue/green = gold in the helices; purple = gold in the independent monomeric staples. Adapted from ref 67.

spherical surface is also surprising. The carbon tails do not form the helical stripe pattern as the underlying −S−Au−S− motifs; instead, the tails are self-organized into multiple “swirls” on the particle surface, and each swirl consists of four rotatively arranged phenyl rings. This arrangement of carbon tails also induces chirality. Overall, the unique structural features in the large Au133 structure are elegant, such as the first observed Au55-MI, highly ordered surface patterns of helical ladders of −S−Au−S− motifs and swirly distribution of carbon tails. Transformation Chemistry in Nanoclusters with Other Ligand Types and of Other Metals. It is worth noting that the ligandinduced size/structure transformation process was also observed in gold nanoclusters with other types of ligands as well as in thiolate-protected silver nanoclusters. For example, Konishi and co-workers reported the transformation of a [Au9(PPh3)8](NO3)3 cluster into a [Au8(dppp)4](NO3)2 cluster by reacting with excess of dppp ligand at room temperature, where dppp = Ph2P(CH2)3PPh2, a bidentate phosphine ligand.69,70 The structure of the cluster changes from a toroidal shape in Au9 to an edge-shared tritetrahedral shape in Au8 during the transformation. This experiment also demonstrates the strategy of using the size/structure transformation to obtain the structure which is hard to be accessed by the direct reduction of metal salts. The size transformation process was also recently demonstrated in the silver nanoclusters. Bakr and co-workers reported the transformation of Ag35(SG)18 into Ag44(4-FTP)30, where SG = glutathione, and 4-FTP = 4-fluorothiophenol.71 Mechanism of Ligand-Exchange-Induced Size/Structure Transformation (LEIST). In the case of simple ligand exchange on the surface of nanoparticles, the mechanism typically adopts a second-order nucleophilic attack (SN2).72,73 Below, we discuss the mechanism of ligand-exchange-induced size/structure transformation (LEIST for short). We chose the Au38 to Au36 transformation as a typical example, as this case has been thoroughly investigated.74 The Au38(PET)24 nanocluster consists of a face-sharing biicosahedral Au23 kernel, which is protected by three Au(SR)2 monomers and six Au2(SR)3 dimers (Scheme 1).52 While the Au23 kernel is achiral, the dual-propeller-like rotative arrangement of the six dimer staples renders the

Scheme 2. Reaction pathway for conversion of Au38(PET)24 to Au36(TBBT)24a

a

Stage (I) ligand exchange; (II) structure distortion; (III) disproportionation; (IV) size focusing. Adapted from ref 74.

Stage I. This very first stage (0−5 min, Figure 7) features ligand exchange without any size or structure transformation (Scheme 2, stage I), evidenced by the optical spectra being identical to the starting point. The product was the mixedligand-shelled Au38(SR)24, that is, Au38(TBBT)m(PET)24‑m with a maximum of 12 TBBT ligands exchanged onto the Au38 cluster. The reaction of this stage can be written as Au38(PET)24 + TBBT → Au38(TBBT)m (PET)24 − m + PET

(m < ca 12)

(1)

Stage II. During this stage (10−15 min, Figure 7), ligand exchange continues, but the changing absorption spectra (Figure 7B, stage II) indicate that the more bulky TBBT ligands (from ∼12 to ∼21) on the Au38 core start to induce 2981

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structure transformation was unexpected, and it is indeed first observed in the reaction of nanoclusters. In later research, a similar disproportionation process was suggested to be responsible for the bottom-up growth of Au25(SR)18−.75 Stage IV. After all the Au38(TBBT)n(PET)24−m clusters are transformed into Au36(TBBT)m(PET)24−m and Au40(TBBT)m+2(PET)24−m, the reaction enters the fourth stage (120−300 min), during which ligand exchange occurs toward completion (i.e., until 24 TBBT ligands), and the Au40(TBBT)m+2(PET)24−m species are gradually converted to Au36 under the harsh environment of high temperature and excess thiol Au40(SR)26 → Au36(SR)24 + 2Au(0) + 2Au(I)SR

and eventually molecularly pure Au36(SR)24 was obtained. The overall release of two free Au(0) atoms is evidenced by the observation of the yellow golden film on the stir bar after the reaction. The above mechanism should give a theoretical yield (94%) of final Au36(SR)24, and our experimental yield is 90%, consistent with the reaction route. Equation 4 in combination with eq 3 gives the following overall equation:

Figure 7. Size/structure transformation process from Au38 to Au36 monitored by ESI-MS and UV−vis. Adapted from ref 74.

structural distortion of the Au38 cluster. The metastable cluster gave a new absorption band at 550 nm in UV−vis spectra (Figure 7B, see the arrow), and its intensity increases as the population of the heavily exchanged product [Au38(TBBT)m(PET)24−m] increases. Compared to the PET ligand, TBBT is a bulky, secondary thiol with its S atom connecting directly to a benzene ring (as opposed to the S atom connected to the CH2 group). When more and more TBBT ligands replace PET, the original Au38 structure becomes distorted in order to accommodate more TBBT ligands (Scheme 2, stage II). This structural distortion is the prelude to the size and structural transformation, entering stage III (vide infra). The reaction for stage II can be written as below, with Au38* denotes the distorted structure

2Au38(SR)24 + 2TBBT → Au40(SR)26 + Au36(SR)24 Overall: Au (SR) → Au (SR) + 2Au(0) 40 26 36 24 + 2Au(I)SR 2Au38(SR)24 → 2Au36(SR)24 + 2Au(0)

(5)

The transformation of the highly robust biicosahedral Au38(PET)24 nanocluster into an extremely stable tetrahedral Au36(TBBT)24 nanocluster raises an intriguing question: is the transformation due to the bulkiness effect of TBBT or the electronic conjugation effect of the aromatic ligand in contrast with the nonaromatic PET (i.e., SCH2CH2Ph)? We pursued the synthesis and crystallization of Au36 protected by cyclopentanethiolate, Au36(SC5H9)24.76 The obtained crystal structure is essentially identical with that of Au36(TBBT)24. This rules out that the FCC structure of Au36(TBBT)24 is dictated by the aromatic property of the ligand. Remarks on the Transformation Chemistry of Nanoclusters LESIT Reaction versus Simple Ligand Exchange. The “size/ structure transformation” methodology has been demonstrated to be quite powerful. As discussed above, the difference between simple ligand exchange reaction and the LEIST reaction lies in (i) the amount of incoming thiols, (ii) the reaction temperature, and (iii) the differences between the incoming thiol and outgoing thiol. The structural transformation of nanoclusters typically requires high concentration of thiol and thermal treatment,54,60,67 whereas the simple ligand exchange process is performed at room temperature and under low ratios of incoming thiol molecules relative to the bound ligands.41,42 LESIT Reaction versus Size Focusing. Together with the size focusing methodology, more sizes of stable Aun (SR) m nanoclusters are expected to be generated in future work. The basis of both methodologies is the stability property of different sized Aun(SR)m nanoclusters.24 Below, we further comment on the advantages and disadvantages of the size/ structure transformation reaction vs the size-focusing-based direct synthesis method.

Au38(TBBT)m (PET)24 − m (m < ca 12) + TBBT → Au38*(TBBT)m (PET)24 − m (m > ca 12) + PET

(4)

(2)

Stage III. This stage (20−60 min) is critical as both the size and structure are converted. A “disproportionation” reaction is identified, that is, when one Au38(SR)24 cluster releases two gold atoms to form Au36(SR)24, another Au38(SR)24 cluster captures the released two gold atoms and also uptake two free TBBT ligands to form Au40(SR)26. Strong evidence lies in the ESI-MS analysis: two new sets of peaks with comparable intensities were observed on the left and right sides of the Au38 peak set, with the left set being Au36(TBBT)m(PET)24−m (m = 19 to 24) and the right-side being Au40(TBBT)m+2(PET)24−m (m = 21 to 26). As the reaction continued, the intensities of Au36 and Au40 peaks increased, while the intensity of Au38 decreased. The reaction in this stage can be written as 2Au38(TBBT)m (PET)24 − m + 2TBBT → Au36(TBBT)m (PET)24 − m + Au40(TBBT)m + 2 (PET)24 − m (3)

The transformation occurs in the original cluster via internal reconstruction (as opposed to complete disintegration followed by reassembly of pieces), and simple releasing and capturing processes lead to the formation of new Au36 and Au40 clusters. Such a disproportionation process during the size and 2982

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recent progress in size-focusing synthesis of Au n(SR)m nanoclusters with different thiol ligands opens a new avenue and reveals the diversity of Aun(SR)m nanoclusters. Three levels of ligand effect have been identified (Scheme 3).

One advantage of the LEIST reaction is that it is a simpler process compared to the size-focusing-based direct synthesis approach. The size-focusing method involves complicated processes such as reduction, nucleation, growth, etching, and so forth, and obtaining a pure size often requires fine-tuning of each reaction step.25 Thus, each size-focusing synthesis requires much effort in optimization before one can obtain the single-sized, pure product. In contrast, the LEIST process simply start with pure nanoclusters of specific sizes (e.g., Au38), the reaction can exclusively lead to the close sizes of high stability (e.g., Au36) that is to a large extent controlled by the reactant thiolates. Such a simple reaction significantly facilitates the obtaining of new magic sizes. One limitation of the LEIST reaction lies in its indirect nature. It relies on the synthesis of pure magic sizes first by the size focusing method. Also, not all types of thiolates are suitable for the size transformation reaction; some thiolates with very bulky carbon groups and also weak affinity to gold would be reluctant to undergo ligand exchange, and the reaction would be quenched in the early stage of ligand exchange (for example, the case of using bulky admantanethiol as the incoming ligand). The LEIST reaction could often result in a mixed-ligand shell on nanoclusters due to the incomplete ligand exchange. Kinetics of the LEIST Reaction. The kinetics of the LEIST reaction is relatively simple to control, compared to the size focusing method. One variable in the LEIST reaction is the temperature, because the cluster structure reorganization requires heat to overcome the activation energy barrier.74 Adjusting the reaction temperature could lead to different stable sizes, as demonstrated in the case of Au25 transforming into Au28 (at 80 °C) and also Au20 (at 40 °C).60,61 We speculate that the transformation should occur from the outside ligand shell into the gold kernel, that is, surfaceinduced transformation. The energy barrier is expected to increase as the size of the cluster increases. For example, transformation of Au144 to Au133 requires a much longer reaction time than that of Au25 to Au28. An interesting question is at what size the size transformation would become ineffective, that is, the stress force generated by the tension of surface ligand exchange can no longer penetrate further inside to cause structural rearrangement. This remains to be elucidated in future work. The reaction time and the thiolate concentration are another two parameters that control the result of size transformation. As shown in the case of Au38 transforming into Au36, an intermediate cluster Au40 coexists at the early stage of the transformation reaction, but it finally disappears due to its less stability compared to Au36 under the TBBT thiolate condition.74 This simpler kinetics in the size transformation reaction renders facile control during the synthesis of nanoclusters. On the Ligand Effects in the LEIST Reaction. As mentioned above, the main force that triggers the LEIST reaction is the difference in the structure between the incoming and outgoing ligands. Then, a question arises: how different between the ligands is sufficient to induce size and structure transformation? This is a much more complicated question than we originally thought, as demonstrated by the recent experiment. In the early research of Aun(SR)m nanoclusters, the dominant thiolates used in the synthesis were the alkanethiols (HSCnH2n+1) or derivatives such as phenylethanethiol (PET) or glutathione (HSG).19,66,77−81 The

Scheme 3. Three levels of thiolate ligand effect on the size and structure of Aun(SR)m nanoclusters: (A) bulkiness in the α-carbon; (B) bulkiness in the isomeric methylbenzenethiol; (C) bulkiness in the para-position of the benzenethiol.

It is probably safe to say that the ligand selects the magic sizes. The most obvious and direct ligand effect pertains to the bulkiness of the α-carbon, that is, the carbon directly connecting to the sulfur (Scheme 3A). The α-carbon is the closest to the Au−S interface, thus the change of the bulkiness in the α-carbon most directly results in the reconstruction of the Au−S interface. Changing from a primary thiol (e.g., −S−CH2−R) to a secondary thiol (e.g., −S−CH−R2) or tertiary thiol (e.g., −S−C−R3), the magic sizes in gold nanocluster could change correspondingly.82−84 This is reflected in the size-focusing synthesis of Au64(SR)32 nanocluster with secondary cyclohexanethiol, Au38S2(SR)20 with tertiary adamantanethiol, and so forth.25,26,84 The αcarbon bulkiness effect is somewhat similar to the early research on the phosphine-protected metal clusters, and the concept of “cone angle” of the ligands can partially explain this ligand effect.85−87 Recent research revealed that the ligand effect is more complicated than initially thought. Chen et al. found that the position of the methyl group in the isomeric methylbenzenethiol (MBT, Scheme 3B) could also significantly alternate the magic size.27 When changing the position of methyl group on the benzenethiol from para- to meta- to ortho-, the size of gold nanoclusters can be tuned from Au130(p-MBT)50 to Au104(m-MBT)41 to Au40(o-MBT)24, respectively. This result demonstrates that the size and structure of gold nanoclusters are highly sensitive to the subtle change in the methylbenzenethiolate ligand’s structure. The rigidity of the methylbenzenethiol plays a partial role in this ligand effect.27 The thiolate ligand effect can further come from the paraposition of benzenethiol (Scheme 3C). In the similar size 2983

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range, we found that 4-methylbenzenethiol selected the Au130(p-MBT)50,27 while the 4-tertbutylbenzenethiol selected the Au133(p-TBBT)52.67 It is intriguing to see the steric hindrance could work at the far end of the thiolate ligand. Overall, these three levels of different ligand structures demonstrate the complexity of ligand effect on the size and structure of gold nanoclusters. Finally, it is worth pointing out the major role of ligands in controlling the size and structure of nanoclusters. The discovery of thiolate ligand’s effects on the stability of magic sizes is a key toward expanding the library of gold nanoclusters. It is probably safe to say that the ligand selects the magic sizes, and that the geometric structure of the ligand (as opposed to its electronic effect) plays a more important role in controlling the structure. Overall, the ligand-exchangeinduced size/structure transformation reaction (LEIST) not only will become a tool for nanochemists to discover new magic sizes but also will provide further insight into the nanoscale transformation chemistry. Such transformation chemistry may become a promising branch of research on atomically precise nanoclusters.

interests include atomically precise nanoparticles, optics of nanoparticles, and catalysis (http://www.chem.cmu.edu/groups/jin/).

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ACKNOWLEDGMENTS We acknowledge financial support of our research by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-15-1-9999 (FA9550-15-1-0154).

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The ligand-exchange-induced size/structure transformation reaction (LEIST) not only will become a tool for nanochemists to discover new magic sizes but also will provide further insight into the nanoscale transformation chemistry.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Chenjie Zeng is a Ph.D. candidate in chemistry at Carnegie Mellon University. She obtained her B.S. in chemistry from Nankai University (Tianjin, China) in 2011. She works under the supervision of Prof. Jin, and her research is focused on the structure and property evolution of gold nanoclusters. Yuxiang Chen is a graduate student in Professor Jin’s group at Carnegie Mellon University. He received his B.S. in chemistry from Nankai University. His research interests focus on the synthesis and catalytic applications of precious metal nanoclusters. Anindita Das is a Ph.D. candidate in the Jin group at Carnegie Mellon University. She obtained her B.Sc. from St. Ann’s College/ Osmania University (Hyderabad) in 2007 and M.Sc. from the University of Pune (Pune) in 2009. Her thesis research focuses on the synthesis of noble metal nanoclusters with atomic precision for applications in catalysis. Rongchao Jin received his Ph.D. in chemistry from Northwestern University in 2003 and then performed postdoctoral research at the University of Chicago. He joined the chemistry faculty of Carnegie Mellon University in 2006 and was promoted to tenured Associate Professor in 2012 and Full Professor in 2015. His current research 2984

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DOI: 10.1021/acs.jpclett.5b01150 J. Phys. Chem. Lett. 2015, 6, 2976−2986