Tailoring the Electronic and Catalytic Properties of Au25 Nanoclusters

Jul 21, 2016 - To explore the electronic and catalytic properties of nanoclusters, here we report an aromatic-thiolate-protected gold nanocluster, [Au...
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Tailoring the Electronic and Catalytic Properties of Au25 Nanoclusters via Ligand Engineering Gao Li,*,† Hadi Abroshan,‡ Chong Liu,§ Shuo Zhuo,‡ Zhimin Li,† Yan Xie,† Hyung J. Kim,‡,∥ Nathaniel L. Rosi,§ and Rongchao Jin*,‡ †

Gold Catalysis Research Centre, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡ Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States § Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, United States ∥ School of Computational Sciences, Korea Institute for Advanced Study, Seoul 02455, Korea S Supporting Information *

ABSTRACT: To explore the electronic and catalytic properties of nanoclusters, here we report an aromatic-thiolate-protected gold nanocluster, [Au25(SNap)18]− [TOA]+, where SNap = 1-naphthalenethiolate and TOA = tetraoctylammonium. It exhibits distinct differences in electronic and catalytic properties in comparison with the previously reported [Au25(SCH2CH2Ph)18]−, albeit their skeletons (i.e., Au25S18 framework) are similar. A red shift by ∼10 nm in the HOMO−LUMO electronic absorption peak wavelength is observed for the aromatic-thiolate-protected nanocluster, which is attributed to its dilated Au 13 kernel. The unsupported [Au25(SNap)18]− nanoclusters show high thermal and antioxidation stabilities (e.g., at 80 °C in the present of O2, excess H2O2, or TBHP) due to the effects of aromatic ligands on stabilization of the nanocluster’s frontier orbitals (HOMO and LUMO). Furthermore, the catalytic activity of the supported Au25(SR)18/CeO2 (R = Nap, Ph, CH2CH2Ph, and n-C6H13) is examined in the Ullmann heterocoupling reaction between 4-methyl-iodobenzene and 4-nitro-iodobenzene. Results show that the activity and selectivity of the catalysts are largely influenced by the chemical nature of the protecting thiolate ligands. This study highlights that the aromatic ligands not only lead to a higher conversion in catalytic reaction but also markedly increase the yield of the heterocoupling product (4-methyl-4′-nitro-1,1′-biphenyl). Through a combined approach of experiment and theory, this study sheds light on the structure−activity relationships of the Au25 nanoclusters and also offers guidelines for tailoring nanocluster properties by ligand engineering for specific applications. KEYWORDS: gold, nanoclusters, Au25, ligand effects, Ullmann coupling

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strong influence on the physical and chemical properties. Although the fundamental properties of different size Aun(SR)m nanoclusters are still under exploration, the catalytic potential of Au25(SR)18 nanoclusters has been studied in several reactions including carbon−carbon coupling, hydrogenation, oxidation, and so forth.10,11 The properties of a nanocluster can be influenced by the chemical nature of its protecting ligands. For example, Murray and co-workers reported the ligand effects on the redox potentials of the gold clusters protected by −SPhX (where X =

ecently, well-defined Au nanoclusters with atomic precision and molecular purity have emerged as a promising class of nanomaterials with various applications in areas such as nanocatalysis, sensing, photovoltaics, biology, and so forth.1−4 In this regard, a series of size-discrete gold nanoclusters capped by organic ligands (e.g., thiolate, phosphine, alkyne, etc.) have been synthesized and characterized.5−8 In particular, the thiolate-protected nanoclusters, Aun(SR)m (where −SR represents thiolate, n and m denote the gold atom and ligand numbers, respectively) have gained significant interest owing to the availability of a wide range of sizes for systematic studies of size dependency5 and catalytic application.9 These nanoclusters (typically smaller than 2 nm) exhibit nonmetallic behavior due to the electron energy quantization arising from quantum size effect, which exerts a © 2016 American Chemical Society

Received: June 15, 2016 Accepted: July 21, 2016 Published: July 21, 2016 7998

DOI: 10.1021/acsnano.6b03964 ACS Nano 2016, 10, 7998−8005

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ACS Nano NO2, Br, H, CH3, and OCH3).12 It is shown that the clusters with electron-withdrawing ligands energetically favor reduction and disfavor oxidation.12 DFT calculations carried out by Aikens showed that an electron donating group such as −OCH3 at the para-position of Au25(SPhX)18 leads to a decrease in HOMO−LUMO gap, whereas nanoclusters with X = H, F, Cl, and Br have a similar gap.13 Recent work also demonstrated that the optical properties of the Au36(SR)24 nanoclusters are influenced by surface-protecting ligands (i.e., aliphatic thiolate, cyclopentanethiolate (−SC5H9) vs aromatic thiolate, 4-tert-butylbenzenethiolate (−SPhtBu)).14 Although the general features of UV−vis spectra of the Au36(SC5H9)24 and Au36(SPhtBu)24 nanoclusters are found to be similar, the spectrum of the Au36(SC5H9)24 is overall blue-shifted in comparison to that of the Au36(SPhtBu)24 nanocluster.14 In order to precisely correlate the structure of gold nanoclusters with their electronic properties, one should achieve the crystal structure of the nanoclusters with different protecting thiolate ligands. The crystal structures of the [Au25(SCH2CH2Ph)18]− and the [Au25(SCH2CH3)18]− show that the skeleton of these particles is composed of an Au13 icosahedral kernel with six staplelike surface motifs in the form of Au2(SR)3 (that is, −SR−Au−SR−Au−SR−).15−17 The two types of Au25 nanoclusters are capped by sulfur atoms which are bonded to methylene groups (−CH2−), thereby resulting in nearly identical optical spectra. However, aromatic thiolate ligands can possibly impart the [Au25(SR)18]− nanocluster with new properties. Thus, it would be highly desirable and valuable to study the nanoclusters ligated by the aromatic thiolates. In turn, this may also offer an opportunity to tune the catalytic properties of Au25 nanoclusters for specific reactions via ligandsubstrate interactions. We herein report the synthesis, crystallization, electronic, and catalytic properties of an aromatic-thiolate-protected [Au25(SNap)18]−[TOA]+ nanocluster (where −SNap = 1naphthalenethiolate). The results show that the antioxidation capability of the [Au25(SNap)18]−[TOA]+ nanoclusters is largely improved compared to the previous [Au25(SCH2CH2Ph)18]− [TOA]+.18 The [Au25(SNap)18]− nanocluster is also found to be very robust under thermal conditions (e.g., at 80 °C for hours) and in the presence of excess oxidants (e.g., air, O2, H2O2, and TBHP). The nanocluster is further explored for catalytic application in the Ullmann heterocoupling reaction between 4-methyl-iodobenzene and 4-nitro-iodobenzene, and our results indicate that aromatic ligands notably increase the conversion of substrate and the selectivity for the heterocoupling product.

Because the nanocluster is expected to be anionic, the negative mode spectrum shows an expected intense peak at m/z = 7789.70 (Figure S2), indicating the high purity of the assynthesized product. The spacing of the isotope patterns is 1 (Figure S2, inset), confirming that the nanocluster is singly charged. The experimental isotope pattern matches well with the simulated one (Figure S2, inset). Based upon the singly charged peak, the molecular mass of the gold nanocluster is determined to be 7789.70 and accordingly its chemical formula to be [Au25(SNap)18]− (theoretical m/z = 7790.31, deviation = −0.61). Further, thermogravimetric analysis (TGA) of unsupported Au25(SNap)18 nanoclusters shows a weight loss of 40.9 wt % (Figure S3), consistent with the theoretical value of 40.4 wt % calculated based on the complete loss of the surface thiolate ligands and the cation TOA+. To gain insights into the electronic properties of the nanoclusters capped by aromatic ligands vs aliphatic ones, we prepared dichloromethane solutions of Au25(SR)18 with R = −Nap, −Ph, and −CH2CH2Ph to compare their optical absorption spectra. The syntheses of Au25(SPh)18 and Au25(CH2CH2Ph)18 followed the literature reports.9,15 The general features of the UV−vis profiles are similar for all the three Au25 nanoclusters (Figure 1a), which indicates that gold

RESULTS AND DISCUSSION The [Au25(SNap)18]−[TOA]+ nanocluster (denoted as the Au25(SNap)18 hereafter) was synthesized via two steps. First, polydispersed Au nanoclusters protected by hexanethiolate (i.e., −SC6H13) were produced in tetrahydrofuran with hexanethiolto-Au precursor ratio at 5.0 (molar). Second, the as-obtained polydispersed nanoclusters were dissolved in 1 mL of toluene and then reacted with neat 1-naphthalenethiol (1 mL) at 80 °C for overnight. The UV−vis spectrum of the polydispersed gold nanoclusters showed a decay-like profile (Figure S1, black line). The final product exhibits three peaks at 520, 679, and 796 nm (Figure S1, red line), which are similar to the characteristic optical absorption features of [Au25(SR)18]− (denoted as Au25(SR)18 hereafter). The final product was further analyzed by electrospray ionization mass spectrometry (ESI-MS).

Figure 1. (a) Comparison of the optical spectra of the Au25(SCH2CH2Ph)18, Au25(SPh)18, and Au25(SNap)18 nanoclusters. The spectra are up-shifted for the ease of comparison. Inset: the calculated lowest-energy peaks of Au25(SCH3)18 and Au25(SPh)18. (b) Positive-mode MADLI spectrum of Au25(SNap)18 nanoclusters. The asterisks (*) in (b) indicate the fragments (i.e., Au21(SNap)14 and Au25(SNap)16) caused by MALDI method.

and sulfur atoms possess the same structural framework (Figure S4, confirmed by X-ray analysis, vide inf ra). However, the spectra of the aromatic thiolate-capped Au25(SPh)18 and Au25(SNap)18 nanoclusters are red-shifted compared to that of the aliphatic thiolate-capped Au25(SCH2CH2Ph)18. As shown in Figure 1a, the peak of the Au25(SCH2CH2Ph)18 nanocluster centered at 670 nm is red-shifted to 679 and 697 nm for the 7999

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possible reason for losing two −SNap is the higher absorption coefficient of naphthalene in comparison to that of phenyl group at 337 nm where the laser of MALDI-MS operates. Of note, extending the pi-conjugated systems usually leads to larger coefficients of absorption with a red shift in the peak wavelengths. At high temperatures and in the presence of oxidants such as H2O2, the anionic Au25(SCH2CH2Ph)18 nanoclusters undergo an oxidation process, resulting in the charge neutral cluster.18 The thermal antioxidation stability of the Au nanoclusters can be influenced by the protecting ligands. For example, Yuan et al. recently reported that the stability of gold nanoclusters can be enhanced by using two different thiol-terminated ligands with oppositely charged functional groups.19 To investigate the ligand effects on the stability of the Au25(SR)18 nanoclusters, we monitored UV−vis spectra of toluene solutions of [Au25(SCH2CH2Ph)18]−, [Au25(SPh)18]−, and [Au25(SNap)18]− nanoclusters at 80 °C under atmospheric condition (air, 1 atm.) for 1 h. As shown in Figure 2a, the

Au25(SNap)18 and the Au25(SPh)18, respectively. To rationalize the red-shifts in the UV−vis of the Au25 nanoclusters, we performed density functional theory (DFT) calculations of Au25(SCH3)18 and Au25(SPh)18, which act as models of the aliphatic and aromatic ligands-protected clusters. In agreement with the results of a previous study,12 the replacement of −SCH3 ligands with −SPh leads to a slight decrease (0.03 eV) of the HOMO−LUMO gap of the gold nanoclusters. This induces a red-shift of the optical spectrum of the clusters protected by aromatic ligands (−SPh). Further, the timedependent density functional theory (TD-DFT) was applied to calculate the theoretical optical absorption spectra of the clusters. We considered only the first (lowest energy) absorption peak. The maximum-intensity lines of the absorption peak of the Au25(SCH3)18 and the Au25(SPh)18 are located at 844 and 868 nm, respectively (Figure 1a, insets). The most salient aspect of our results is that the first theoretical peak of the Au25(SPh)18 is red-shifted by ∼24 nm compared to that of the Au25(SCH3)18, which is in good agreement with the experimental result (27 nm, Figure 1a). Of note, the aliphatic chain length (e.g., −CH3 vs −CH2CH2Ph) does not cause any appreciable shift in the peak wavelength. The Au25(SCH2CH2Ph)18 and Au25(SNap)18 nanoclusters were also characterized by matrix-assisted laser desorption/ ionization mass spectrometry (MALDI-MS) (Figure 1b and Figure S5). It is worth noting that MALDI-MS is somewhat destructive in ionization compared to the soft ionization in ESIMS and thus may cause somewhat fragmentation of the nanoclusters. While fragmentation is generally undesirable, it can sometimes offer insights into the internal structure of the nanoclusters. The Au25(SCH2CH2Ph)18 nanocluster shows two peaks (Figure S5), which are assigned to the intact nanocluster (m/z = 7394.3, theoretical m/z = 7394.2) and its fragment Au21(SCH2CH2Ph)14 at m/z = 6057.2 (i.e., after loss of Au4(SCH2CH2Ph)4). In the case of Au25(SNap)18, the main peak at m/z = 7789.30 is assigned to the intact nanocluster (Figure 1b). Interestingly, two fragment ions were formed for the Au25(SNap)18 nanocluster, with one fragment at m/z = 6365.0 (assigned to Au21(SNap)14, that is, losing a unit of Au4(SNap)4) and the other fragment at m/z = 7471.4 (assigned to Au25(SNap)16, which indicates the loss of two protecting thiolate ligands). We attempted to explain the fragmentation results using DFT calculations. A −SCH3 group bridging an exterior Au atom and Au13 kernel of the Au25(SCH3)18 model was replaced with −SNap, resulting in Au25(SCH3)17(SNap)1 (Figure S6). DFT shows the replacement of −SCH3 with −SNap leads to elongation of Au(kernel)−S bond by 0.032 Å. Results also indicate that the formation of Au25(SCH3)17 species via removal of −SNap group in forms of anion or radical is energetically favored by 27.6 and 12.3 kcal/mol over that of SCH3, as expected from the change in the Au(kernel)−S bond length. These findings are in line with the fact that naphthalene group of −SNap is more capable of stabilizing the electrons of sulfur atom through π-resonance than the alkyl group of −SCH3. Therefore, losing −SNap groups during MALDI-MS is more probable than −SCH3 as confirmed by our experimental results (comparing Figure 1b and Figure S5). Also notable is that naphthalene groups of −SNap ligands can mutually interact through π−π stacking on the surface of the nanocluster (Figure S7 and see discussion below on X-ray crystallography data). This opens up a possibility that a leaving −SNap group can pull out another ligand, that is, losing two −SNap ligands (Figure S8) to form Au25(SNap)16. Another

Figure 2. Thermal stability of (a) [Au25(SCH2CH2Ph)18]− and (b) [Au25(SNap)18]− nanoclusters (dissolved in toluene, 80 °C, in air, 1 h).

anionic [Au25(SCH2CH2Ph)18]− nanoclusters gradually become oxidized (i.e., one-electron loss) in the presence of O2 (air) and are converted to neutral [Au25(SCH2CH2Ph)18]0, then to cationic [Au25(SCH2CH2Ph)18]q+ nanoclusters (q = 1, 2, etc.), consistent with previous reports.18,20,21 Of note, the nanoclusters may decompose finally, as the characteristic optical peak at ∼670 nm almost disappears. It is worth mentioning that some white solids were collected after centrifugation of the solution (at 8000 rpm for 5 min), suggesting that the Au25(SCH2CH2Ph)18 nanoclusters were ultimately decomposed to polymeric Au(I):SR complexes (Scheme S1a). In comparison with the [Au25(SCH2CH2Ph)18]− cluster, the [Au25(SNap)18]− and [Au25(SPh)18]− nanoclusters are much more stable under atmospheric conditions at 80 °C, evidenced by no changes 8000

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ACS Nano observed in the time-dependent UV−vis spectra (Figure 2b and Figure S9). We further investigated the antioxidation properties of the two Au25(SR)18 nanoclusters (R = CH2CH2Ph and Nap) in the presence of H2O2 oxidant (H2O2:Au25 = 75:1, mol/mol). Figure S10a shows that, similar to the thermal stability test in air, the [Au 25 (SCH 2 CH 2 Ph) 18] − nanoclusters gradually changed to neutral and cationic nanoclusters during the first hour of the experiment. Of note, the new peak appeared at ∼960 nm belongs to the overtone absorption of water, which is produced in the presence of H2O2. The UV−vis spectra of [Au25(SNap)18]− nanocluster treated under the identical conditions showed no changes even after 2 h (Figure S10B). For completeness, the Au25 nanoclusters are studied in the presence of tert-butyl hydroperoxide (TBHP), which is a toluene-soluble oxidant. It is found that the [Au25(SCH2CH2Ph)18]− becomes oxidized to form neutral and cationic nanoclusters when the molar ratios of Au25 to TBHP are 1:5 and 1:10 (mol/mol), as the peak at 800 nm disappears (Figure S11A). At higher concentration of TBHP (Au25:TBHP = 1:15), the Au25(SCH2CH2Ph)18 nanoclusters even decompose. In contrast, the [Au25(SNap)18]− nanoclusters remain intact at all the three concentrations of TBHP (Figure S11B), which indicates the [Au25(SNap)18]− nanoclusters possess an excellent antioxidation stability (Figures S10 and S11, and Scheme S1). The [Au25(SCH2CH2Ph)18]− nanoclusters tend to be oxidized more easily than [Au25(SNap)18]−. DFT calculations also show that HOMO and LUMO levels of [Au25(SCH3)18]− are higher by 0.28 and 0.31 eV, respectively, than those of [Au25(SPh)18]−. Therefore, from the electronic point of view, the [Au25(SCH3)18]− is more accessible to react with oxidizing agents like H2O2 and TBHP or loses electrons at the surface of the anode compared to the aromatic-thiolate-protected nanoclusters. To correlate the structure of the Au25(SNap)18 nanocluster with its properties, we grew single crystals of [Au25(SNap)18]−[TOA]+ in a solution of CH2Cl2/ethanol (1:1, v/v). The structure was solved by X-ray crystallography (see details in the Supporting Information). Results show that the [Au25(SNap)18]−[TOA]+ crystallizes in the triclinic group P-1.22 The full structure of the [Au25(SNap)18]−[TOA]+ nanocluster is shown in Figure 3a. Similar to the structure of [Au25(SCH2CH2Ph)18]−, the [Au25(SNap)18]− nanocluster also comprises an icosahedral Au13 kernel (Figure 3b) with an exterior shell composed of six dimeric Au2(SNap)3 staples (Figure 3c). The counterion, tetraoctylammonium (TOA+), was also found (Figure 3a). The Au−Au, Au−S, and S−C bond lengths of the [Au25(SNap)18]− are given in Table 1 and compared with those of the [Au25(SCH2CH2Ph)18]−. The average bond length of Au(staple)−Au(central) in Au25(SNap)18 is shorter than that in Au25(SCH2CH2Ph)18 (4.873 vs 4.892 Å, Table 1, entry 1). However, the average of Au−Au bonds within the Au13 kernel of the [Au25(SNap)18]− cluster is longer than that of the [Au25(SCH2CH2Ph)18]− (2.784 vs 2.775 Å, Table 1, entry 2). These results indicate that, compared to the [Au25(SCH2 CH2 Ph) 18]− cluster, the Au25 core size of [Au25(SNap)18]− is slightly smaller (0.38%), but the Au13 kernel is slightly looser (0.3%). The Au13 kernel is mainly responsible for the HOMO−LUMO electronic transition.15,18,21,23 Therefore, the slight expansion of the Au13 kernel accounts for the red-shift in the UV−vis spectrum of

Figure 3. (a) Total structure of the [Au25(SNap)18]−[TOA]+ nanocluster (Au25S18 is shown in ball-and-stick mode, thiolate ligands and TOA+ in wire-and-stick mode). (b) Icosahedral Au13 kernel. (c) Six dimeric Au2(SR)3 staple motifs. Gold, green or orange; sulfur, yellow; carbon, gray; hydrogen, white; TOA, magenta.

Table 1. List of the Average Au(Staple)−S and Au(kernel)−S Bond Lengths and the Distances, Au(staple) − Aucenter, between the Au Atoms on the Staple Motif and the Cluster Center in the Au25(SCH2CH2Ph)1815,16 and Au25(SNap)18 Nanoclusters Au25(SR)18 entry

lengths (average, Å)

R = Nap

R = CH2CH2Ph

1 2 3 4 5

Au(staple)−Au(central) Au−Au (within kernel) Au(staple)−S Au(kernel)−S S−C

4.873 2.784 2.306 2.400 1.772

4.892 2.775 2.311 2.371 1.855

the [Au25(SNap)18]− nanocluster. The average Au(staple)−S bond in the Au25(SNap)18 is also slightly shorter than that in the Au25(SCH2CH2Ph)18 (2.306 vs 2.311 Å, Table 1, entry 3). In good agreement with our DFT results, the average bond length of Au(kernel)−S is longer for the case of Au25(SNap)18 (2.400 vs 2.371 Å, Table 1, entry 4). This indicates that the thiolate ligands on the Au13 kernel of the Au25(SNap)18 nanocluster is relatively weakly bounded and would be lost easier under an external force, for example, under laser irradiation in the MADLI mass spectrometry analysis. We rationalize that the Au25(SNap)18 and Au25(SCH2CH2Ph)18 nanoclusters would exhibit different catalytic activities given their differences in electronic properties caused by the ligands; thus, we choose a carbon−carbon coupling reaction for a test. The carbon−carbon coupling reactions catalyzed by transition metals have profoundly improved the protocols to synthesize natural products, building blocks for supramolecular chemistry, organic materials, polymers, and pharmaceuticals, and so forth. 24,25 The Ullmann-type coupling reactions play a critical role in shaping domains of organic synthesis by the formation of C−C bond between two aryl halides. Recently, Li et al. studied the catalytic activity of oxide-supported Au25(SCH2CH2Ph)18 nanoclusters for the homocoupling reaction of aryl iodides to yield biaryls and attained excellent activity and recyclability of the catalyst.26 Heterobiphenyl products are usually synthesized via the Suzuki 8001

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activity of Au(I)−SNap polymer (supported on CeO2) as catalyst for the control reaction. Such a polymer may form due to possible decomposition of the gold clusters under harsh reaction conditions. Results show the Au(I)−SNap catalyst only leads to 21% conversion of nitroiodobenzene with selectivity of 14% (Table 2, entry 6), which is significantly lower than those using the gold nanoclusters. Therefore, both the blank and control experiments confirm that catalytic activity arises from the nanoclusters. Furthermore, the recyclability of the gold nanocluster catalysts is investigated. The catalyst after the reaction (entry 4 of Table 2) is collected and reused in fresh reactions under identical conditions as indicated in Table 2 (footnotes). The conversion of nitroiodobenzene is found to drop to 79% (2nd cycle) and 50% (3rd cycle). Also notable is the selectivity decreases to 47% and 15% for the second and third cycles, respectively (Table 2, entries 7 and 8). A possible reason is gradual removal of the protecting −SNap ligands and thus decomposition of the gold nanoclusters. Nevertheless, the results indicate a reasonably good recyclability of the catalysts. In agreement with the control experiments discussed above, the recyclability test implies that the catalytic performance of the Au25(SNap)18/CeO2 is from the gold nanocluster and is largely affected by the surface ligands. To explain the catalytic results, we further carried out DFT calculations. All surface gold atoms of [Au25(SR)18]− are protected by thiolate groups; therefore, the nanoclusters are shown to be inert toward adsorption of nonradical molecules.28−34 In agreement with previous studies, our calculations indeed show that there are no strong interactions of [Au25(SCH3)18]− with reactants of the Ullmann heterocoupling reaction. Previous studies showed that nanoclusters can catalyze some reactions;10 however, the exposure of lowcoordinated gold atoms of the nanocluster on oxide support at elevated temperatures is more efficient in catalyzing the reactions.29,33,34 Therefore, we hypothesize that a thiolate is lost and two gold atoms of the staple motif are thus exposed to reactants (Figure S14). To account for the effects of protecting ligands on the catalytic activity of the nanoclusters, two remaining thiolate ligands on the same staple motif are set to be either −SNap or −SCH3. The rest of thiolate ligands are −SH in order to reduce the computational costs (Figure S14). DFT calculations show that the interaction energy of 4-nitroiodobenzene with the exposed gold atoms of Au25(SCH3)2(SH)15 and Au25(SNap)2(SH)15 is −4.3 and −7.2 kcal/mol with Au−I bond length of 2.87 and 2.82 Å (Figure S15). These results indicate that replacement of aliphatic thiolate ligands with aromatic ones leads to a better activation of the C−I bond in the substrate, thereby increasing the reaction conversion. These finding are in good agreement with experimental results. To shed light on the catalytic selectivity results, the nudged elastic band (NEB) approach was used to calculate transition states and activation energy of homo- and heterocoupling reactions.35 For efficient calculations, only the staple motif and four gold atoms of the Au13 kernel that are closely bound to the motif are taken into account (Figure S16−18). Of note, the later four gold atoms were kept fixed during the NEB calculations. Results indicate the activation energy for the homo- and heterocoupling reactions with −SCH3 ligands are comparable (Figure 4a). Interestingly, the activation energy of heterocoupling reaction is 2.7 kcal/mol less than that of homocoupling if −SNap ligands are used (Figure 4b). These

cross-coupling reaction between boric acids and phenyl halides. As halides are much cheaper than boric acids, it is worthwhile and desirable to develop catalysts for the synthesis of heterobiphenyl products by the Ullmann heterocoupling reaction where only halides are used. Herein, we report the catalytic properties of the Au25 nanoclusters protected by different ligands (−SC6H13, −SCH2CH2Ph, −SPh, and −SNap) in the Ullmann heterocoupling reaction. The Au25(SR)18/CeO2 catalysts were prepared by impregnation of CeO2 powders in a dichloromethane solution of the nanoclusters, followed by centrifugation and air drying for 12 h. Next, the catalysts were annealed for 1 h at 150 °C (note: this temperature is well below the thiolate desorption temperature ∼200 °C). Further, Z-contrast scanning transmission electron microscopy (STEM) analysis indicates that the particle size of Au25(SNap)18 nanoclusters remains ca. 1.4 nm (Figure S12), consistent with the original size of monodisperse nanoclusters. The catalytic reaction was carried out under the conditions as indicated in Table 2 and the conversion and product selectivity Table 2. Catalytic Performance of Au25(SR)18/CeO2 Catalysts for Ullmann Heterocoupling between 4-Methyliodobenzene and 4-Nitro-iodobenzene.a

entry

catalyst

conversion [%]b

selectivity [%]b

1 2 3 4 5 6 7c 8d

Au25(SC6H13)18/CeO2 Au25(SCH2CH2Ph)18/CeO2 Au25(SPh)18/CeO2 Au25(SNap)18/CeO2 CeO2 “Au(I)−SNap”/CeO2 Au25(SNap)18/CeO2 Au25(SNap)18/CeO2

69 72 80 91 n.r. 21 79 50

16 19 50 82 14 47 15

a

Reaction conditions: 0.06 mmol 4-methyl iodobenzene, 0.05 mmol 4nitroiodobenzene, 0.3 mmol K2CO3, 100 mg Au25(SR)18/CeO2 (∼1 wt % Au25(SR)18 loading), 1 mL DMF, 130 °C, 24 h. bThe conversion based on 4-nitroiodobenzene and selectivity for the heterocoupling product 4-methyl-4′-nitro-1,1′-biphenyl were determined by 1H NMR. c 2nd reuse of the catalyst recovered from entry 4. n.r. represents no reaction. d3rd reuse of the catalyst recovered from entry 4. n.r. represents no reaction.

were determined by NMR (Figure S13). The Au25(SC6H13)18/ CeO2 catalyst resulted in 69% conversion of 4-nitroiodobenzene with 16% selectivity toward the heterocoupling product, 4-methyl-4′-nitro-1,1′-biphenyl (Table 2, entry 1). The performance of Au25(SCH2CH2Ph)18 is comparable, with a conversion of 72% and a selectivity of 19% (entry 2). Interestingly, the oxide-supported Au 25 (SPh) 18 and Au25(SNap)18 nanoclusters lead to higher conversion (80% and 91%, respectively, see entries 3 and 4). The most salient aspect of our results is that the use of Au25 nanoclusters protected by the aromatic ligands dramatically increases the selectivity toward the heterocoupling product, that is 50% and 82% selectivities over the Au25(SPh)18 and Au25(SNap)18 nanoclusters, respectively, indicating that the chemical nature of the protecting ligands exerts a major influence on the catalytic properties of the nanoclusters. Of note, no activity is found in blank experiment in which the clusters are absent and only CeO2 is present (Table 2, entry 5). To make sure that the catalytic activity is indeed due to the gold clusters, we tested the 8002

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ACS Nano

[Au25(SNap)18]− TOA+ nanoclusters. The organic phase was thoroughly washed with methanol to remove excess thiol. Pure [Au25(SNap)18]− TOA+ nanoclusters were simply extracted with dichloromethane. Preparation of Au25(SR)18/CeO2 Catalysts, Where RS = n-C6H13S, PhS, PhCH2CH2S, and 1-NapS, ∼ 1 Wt % Loading. The CeO2 is chosen as the support for Au25(SR)18 nanoclusters as previous studies indicate CeO2-supported clusters usually show better catalytic performance in comparison to use of other oxide supports.25,27 Typically, 2 mg of Au25(SR)18 nanoclusters were dissolved in 8 mL of DCM, and 200 mg of CeO2 were added. After stirred overnight at room temperature, the supernatant became light yellow or colorless. The Au25(SR)18/CeO2 catalysts were collected by centrifugation and then were dried and annealed in a vacuum (150 °C for 1 h). Typical Procedure for Heterocoupling Reaction. In a typical heterocoupling reaction, p-nitro iodobenzene (0.05 mmol) and p-methyl iodobenzene (0.06 mmol), K2CO3 (0.33 mmol), Au25(SR)18/CeO2 (100 mg), and 1 mL of DMF were added to a 6 mL vial. The mixture was stirred under N2 atmosphere at 130 °C for 48 h as indicated in Table 1. After the catalytic reaction, 5 mL of water was added to the vial, followed by extraction with EtOAc. The product was obtained after removal of EtOAc. The conversion of iodobenzene was determined by 1H NMR (300 MHz) spectroscopic analysis. X-ray Crystallographic Analysis. Single-crystal X-ray diffraction data of [Au25(SNap)18]−TOA+ was collected on a Bruker X8 Prospector Ultra system equipped with an Apex II CCD detector and an IμS microfocus Cu Kα X-ray source (λ = 1.54178 Å). A piece of brown crystal with dimensions 0.40 × 0.06 × 0.02 mm was mounted onto a MiTeGen micromount with fluorolube. The data was collected under cold N2 flow at 150 K. The structure was solved by direct methods using Bruker program SHELXTL, which located all Au and S atoms. Remaining non-hydrogen atoms were generated via subsequent difference Fourier syntheses. However, it was found that several naphthalene-rings were disordered, so a well-defined naphthalene fragment adapted from Cambridge Crystallographic Data Center WebCSD (entry identifier: VEZDUB36) was incorporated, using previously resolved aromatic C coordinates. Idealized atom positions were calculated for all aromatic hydrogen atoms (with d-(Cphenyl−H) = 0.95 Å). More details are provided in the Supporting Information. Characterization. The UV−vis absorption spectra were recorded on a Hewlett-Packard (HP) 8543 diode array spectrophotometer. MALDI-TOF mass spectrometry analyses were performed with a PerSeptive Biosystems Voyager DE super-STR time-of-flight (TOF) mass spectrometer using DCTB (trans-2-[3-(4-tert-butylphenyl)-2-methyl-2propenylidene]malononitrile) as the matrix. For comparison, the excitation wavelength was fixed at 500 nm (from a Xe arc source) for all the cluster species in emission measurements. STEM images of the Au25(SR)18/CeO2 catalysts were obtained on a Hitachi 7000 transmission electron microscope operated at 75 kV. Thermal gravimetric analysis (using ca. 2 mg sample) was conducted in an air atmosphere (flow rate ca. 40 mL/min) on a TG/DTA 6300 analyzer (Seiko Instruments, Inc.) with a 10 °C/min heating rate. Computational Details. DFT optimization and TD-DFT calculations of optical absorption spectra of nanoclusters were carried out using Gaussian 09 package and TPSS functional.37−40 The basis sets 6-31G** were employed for H, C,

Figure 4. Energy vs reaction coordinate of the hetero- and homocoupling reactions with (a) −SCH3 and (b) −SNap ligands.

results altogether show that replacement of aliphatic thiolate ligands with aromatic ones not only increases the conversion rate of the reaction but also favors formation of a heterocoupling product over the homocoupling one, consistent with our experimental results. Though our DFT calculations proposed that removal of some protecting ligands is required to provide catalytic active sites, the −SR ligands adjacent to the open metal sites play a key role in catalytic efficiency and selectivity. In other words, DFT calculations suggest that Au25(SR)18−x (where x = 1 or few) should be the active catalyst.

CONCLUSIONS In summary, an aromatic-thiolate-protected [Au25(SNap)18]− TOA+ nanocluster is obtained and the distinct effects of the aromatic ligands in electronic and catalytic properties are observed. The Au25(SNap)18 nanocluster possesses the same Au 25 S 18 framework as the Au 25 (SCH 2 CH 2 Ph) 18 and Au25(SCH2CH3)18 nanoclusters, that is, an icosahedral Au13 kernel and six dimeric Au2(SR)3 staple motifs. On the other hand, a red shift in the UV−vis spectrum is observed in Au25(SNap)18 compared to the Au25(SCH2CH2Ph)18, which is attributed to the slight expansion of Au13 kernel induced by the aromatic ligands. The Au25(SNap)18 nanoclusters exhibit significantly improved thermal stability (e.g., at 80 °C for several hours) and antioxidation properties (e.g., in the presence of excess H2O2 and TBHP oxidants) in comparison with the Au25(SCH2CH2Ph)18. More importantly, the catalytic activity and selectivity of the Au25(SR)18 nanoclusters in the Ullmann heterocoupling reactions are also largely enhanced by the aromatic thiolate ligands. Overall, this work demonstrates the effectiveness and promise of ligand engineering for tailoring the electronic and catalytic properties of nanoclusters. METHODS Synthesis of the [Au25(SNap)18]− TOA+ Nanoclusters. HAuCl4·3H2O (0.1 mmol) and TOABr (0.12 mmol) were dissolved in 10 mL THF in a trineck round-bottom flask. The solution was stirred at ∼600 rpm for 10 min. n-Hexanethiol (6.0 equiv per mole of gold) was added. The solution color slowly changed from deep red to colorless. After half an hour, 3 mL of aqueous solution of NaBH4 (12 equiv per mole of gold) was rapidly added to the solution. The solution color quickly changed to dark. After 2 h, the organic phase was removed and washed with methanol. The black solid was extracted with 1 mL toluene and transferred to a flask. 1-Naphthalenethiol (400 μL) was added and the solution was heated to 80 °C and maintained at this temperature. The thermal process was allowed to continue overnight at 80 °C. Over the long time etching process, the initial polydisperse Aun(SC6H13)m nanoclusters were finally converted to monodisperse 8003

DOI: 10.1021/acsnano.6b03964 ACS Nano 2016, 10, 7998−8005

Article

ACS Nano and S.41−45 For the gold atom, the LANL2DZ basis set was used.46 To investigate the proposed reaction mechanisms, Quantum Espresso package was used.46 The projector augmented wave (PAW) method was applied to describe the interaction between the electrons and nuclei.47 The Perdew− Burke−Ernzerhof (PBE) form of the generalized gradient approximation was employed for electron exchange and correlation.48 The gold cluster was placed at the center of a cubic box of 25.0 Å × 25.0 Å × 25.0 Å. The kinetic energy cutoff was chosen to be 450 eV and integration in the reciprocal space was carried out at the Γ k-point of the Brillouin zone.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03964. CIF of [Au25(SNap)18]− TOA+. (CIF) 1 H NMR spectrum of the catalytic reaction, and details of the X-ray crystallographic analysis, structural parameters, DFT calculation. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (R. Jin): [email protected]. *E-mail (G. Li): [email protected]. Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.6b03964 ACS Nano 2016, 10, 7998−8005