Ligand Exchange Mechanism on Thiolate Monolayer Protected Au25

Article pubs.acs.org/JPCC

Ligand Exchange Mechanism on Thiolate Monolayer Protected Au25(SR)18 Nanoclusters Amendra Fernando and Christine M. Aikens* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States

J. Phys. Chem. C 2015.119:20179-20187. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/30/15. For personal use only.

S Supporting Information *

ABSTRACT: Ligand exchange reactions are important to functionalize and modify the optical and electronic properties of thiolate-protected gold nanoparticles. A theoretical investigation of the kinetics of the ligand exchange process was performed for the Au25(SH)18− nanocluster with CH3SH as the incoming thiol ligand. Three possible ligand exchange sites were investigated: between the core gold atom and the terminal −SH unit, between the staple gold atom and the terminal −SH unit, and between a staple gold atom and the central −SH unit. We found that the most favorable ligand exchange takes place between terminal −SH units and staple gold atoms.

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INTRODUCTION Thiolate-protected gold nanoparticles gained wide popularity over the past few years because of their remarkable properties and applications.1−9 Investigation of surface modification of these gold nanoparticles by means of ligand exchange reactions is of interest because the ligands affect optical and electronic properties.8,10−20 It has been suggested that ligand exchange on these nanoparticles proceeds by an associative mechanism.21−23 Knowledge of the structure of these nanoscale clusters is essential to understand their properties and reactivity. The thiolate-protected gold nanoclusters are typically surrounded by oligomeric thiolate ligands (often called “staples”) such as SR− Au−SR monomer units and SR−Au−SR−Au−SR dimer units.24 The Au25(SR)18 cluster is a widely studied nanoparticle, especially for the experimental study of ligand exchange reactions. The cluster consists of an icosahedral Au13 core surrounded by six SR−Au−SR−Au−SR dimeric units. The central SR groups in these staples are attached to gold atoms in the staple units, whereas the terminal SR groups are also attached to the core gold atoms. The cluster can be prepared in three different charge states: −1,25,26 0,27,28 and +1.29 In one of the earliest studies of ligand exchange on the Au25(SR)18− nanoparticle, Guo et al. examined the kinetics for exchange of phenylethanethiolate ligands (SCH2CH2Ph) with para-substituted arylthiols (p-X-PhSH, where X = NO2, Br, CH3, OCH3, and OH).21 (Note: The Au38 nanoparticle discussed in that study was later reassigned as Au25.) They found that the rate constants varied linearly with the incoming arylthiol concentration and that the ligand exchange was a second-order process overall. They also ascertained that the forward and reverse reactions had identical substituent effects, which suggested an associative mechanism. Interestingly, they © 2015 American Chemical Society

determined that the rates of exchange differed modestly for early and late stages of the reaction, which implied that more than one type of SR site was present in the nanoparticle. In 2008, Dass et al. reported mass spectrometric studies of ligand exchange reactions on Au25(SCH2CH2Ph)18− with hexanethiol (HSC6) and thiophenol (HSPh) and found mixed or partially exchanged ligands for both of these cases.30 They determined that the exchange is relatively slow and depends on the nanoparticle concentration and the ratio between the incoming and exchanging ligands. In 2009, Si and co-workers investigated exchange on Au25(SCH2CH2Ph)18− in tetrahydrofuran with two chiral ligands: R/S-BINAS (R/S-1,1′-binaphthyl-2,2′-dithiol) and NILC/NDIC (NILC = N-isobutyryl-L-cysteine, NIDC = Nisobutyryl-D-cysteine).31 They also reported partial ligand exchange and observed significant induced optical activity even with a mixed ligand shell environment. In this early study they noticed that ligand exchange resulted in a change of particle size, suggesting decomposition of the Au25 nanocluster. Parker et al. revisited exchange on Au25(SCH2CH2Ph)18− with arylthiols with electron withdrawing substituents (p-XPhSH; X = Br, NO2).32 These authors reported that strong electron withdrawing groups on the incoming −SPhX ligands shift the redox waves toward more positive potentials with a nearly linear relationship. Density functional theory calculations showed that the HOMO−LUMO gap remains unchanged during the exchange reaction. A charge analysis showed no significant changes in the Au13 core even after complete exchange. Received: July 15, 2015 Revised: August 7, 2015 Published: August 10, 2015 20179

DOI: 10.1021/acs.jpcc.5b06833 J. Phys. Chem. C 2015, 119, 20179−20187

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The Journal of Physical Chemistry C

the reaction energies predicted by BP86 with a triple-ζ basis set were in good agreement with the larger quadruple-ζ basis set. Barrier heights were found to be about 40−50 kJ/mol at the BP86/TZP level of theory. The transition states predicted were consistent with an associative type mechanism that is first order in both the gold cluster and the incoming ligand. For larger systems, Heinecke and co-workers reported a study of the ligand exchange mechanism on the Au102(p-MBA)44 nanocluster with p-BBT as the incoming ligand (p-MBA = paramercaptobenzoic acid, p-BBT = para-bromobenzene thiol).23 They used a constrained bond optimization method in predicting the transition states and suggested that the ligand exchange mechanism essentially follows an associative pathway. They also found experimentally that the reaction can occur at only two sites initially, which they attributed to the solvent accessibility of these sites. In this study, we employ DFT to examine ligand exchange on a model Au25(SH)18− nanocluster with an incoming methanethiol. Intermediates and transition states are calculated, and the barrier heights and reaction energies are predicted for this ligand exchange process. This study represents the first theoretical investigation of the ligand exchange mechanism on the ubiquitous Au25 nanocluster.

Fields-Zinna and co-workers reported ligand exchange chelation of Au25(SCH2CH2Ph)18− by CH3C6H3(SH)2.33 Using mass spectrometry, they found that at most six dithiolates were incorporated in the nanoparticle. The voltammetric and UV−vis spectral evidence suggested that the bidentate binding of the dithiolate ligand alters the standard electrochemical and optical features of the Au25 nanocluster. Two different binding modes were proposed: interstaple binding and intrastaple binding. In 2011, Jupally et al.34 investigated exchange on the Au25(SR)18− cluster with varying chain lengths of the incoming ligand, HS−(CH2)n−SH where n = 2−6. It was found that C3 and C4 have optimum lengths for interstaple coupling with more than six dithiolates binding, whereas C5 and C6 prefer interstaple binding to a lesser extent than C3 and C4. The C2 ligand does not have a long enough chain length for bidentate binding. Recently, Knoppe and Burgi35 reinvestigated the ligand exchange experiment between Au25(SCH2CH2Ph)18− and BINAS that was previously done by Si et al.31 The MALDITOF spectral analysis suggested successful ligand exchange with bidentate binding of the BINAS ligand and an intact Au25 cluster. Decomposition of the cluster was not observed; however, optical properties changed as expected with the ligand exchange. The binding mode between the dithiolate and the cluster was proposed to be very similar to the previous suggestion by Jupally et al.34 and Fields-Zinna et al.33 where two neighboring staples are cross-linked by bidentate thiol ligands. Ni et al. 36 reported a single crystal structure of Au25(SCH2CH2Ph)16(p-BBT)2 (p-BBT = para-bromobenzenethiol) that resulted from ligand exchange of Au25(SCH2CH2Ph)18 with the p-BBT incoming ligand. The incoming ligands were seen to exchange with terminal SR units that are attached to core Au atoms. In 2015, Carducci et al.37 investigated the charge transfer effects of Au25(SCH2CH2Ph)18 nanomolecule in various charge states for ligand exchange with PhSH and PhSeH ligands. A higher rate constant was observed for the PhSeH ligand exchange suggesting strong electronic coupling of selenolate ligands compared to thiolates, affecting the assembly and behavior of the cluster. They found behavior consistent with an associative exchange mechanism. Selenolate-stabilized nanoparticles were also studied by Zhong et al.,38 who used density functional theory (DFT) calculations to investigate the electronic structure and ligand exchange of 11 selenolateprotected Au nanoclusters including the Au25 nanocluster. They found that the selenolate-protected nanoclusters are essentially identical to the thiolated ones. Although they examined the overall energetics of the ligand exchange process, the mechanism and barrier heights were not calculated. In a recent experimental paper, Niihori and co-workers39 investigated ligand exchange reactions on a Au 24 Pd(SC2H4Ph)18 nanocluster, which is a Pd-doped Au25(SR)18 system. The isomer distribution was investigated by reversedphase high-performance liquid chromatography. The authors suggested that the ligand exchange reaction starts to occur preferentially at the thiolates attached to core gold atoms in all the reactions.39 Very few theoretical studies have investigated ligand exchange mechanisms on gold nanoparticles. Using DFT, Hadley and Aikens investigated ligand exchange on Au1 and Au11 clusters with several incoming ligands including cysteine and aliphatic ligands with various lengths.22 It was found that

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COMPUTATIONAL METHODOLOGY All the geometry optimizations were calculated using the Amsterdam Density Functional program (ADF).40 A double-ζ basis set is used with a [1s2-4f14] frozen core for gold atoms, a polarized double-ζ basis set is utilized for sulfur atoms and carbon atoms with [1s 2 -2p 6 ] and [1s 2 ] frozen cores, respectively, and a polarized double-ζ basis set is employed for hydrogen atoms. The generalized gradient approximation (GGA) Becke−Perdew (BP86) functional is used as the exchange-correlation functional. Scalar relativistic effects are accounted by adopting the zero-order regular approximation (ZORA). It should be noted that BP86, like other generalized gradient approximation (GGA) functionals, is expected to underestimate the barrier heights,41 and thus, the experimental barrier heights may be somewhat larger than those presented in this study. The starting structure for Au25(SH)18− is taken from ref 26 and then optimized at our level of theory. A constrained structural optimization technique is used in calculating the transition states similar to the procedure used in ref 23. For site A (shown in Figure 1), in the first transition state (TS1) search we found that the appropriate bond to use is

Figure 1. (a) Au25(SH)18− nanocluster with three possible ligand substitution sites marked as A, B, and C. Bond distances (in Å) are shown. The orange, yellow, black, and white color spheres represent gold, sulfur, carbon, and hydrogen atoms, respectively. This color code is consistent in all the figures presented in this article. 20180


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Figure 2. Ligand exchange mechanism at site A.

motif. Site B represents insertion of the incoming thiol between a terminal −SH group and a staple gold atom. Site C depicts ligand substitution between the central −SH group and a staple gold atom. The bond lengths between the core gold atom and terminal −SH ligands are 0.1 Å longer than the bond lengths between terminal −SH units and staple gold atoms as well as 0.1 Å longer than the bond lengths between staple gold atoms and central −SH ligands. This suggests a weaker bond between the core gold atom and terminal −SH, which could thus be susceptible for more reactive ligand exchange than sites B and C. However, since site A is located between the core and the staple motif, one might expect that the more accessible B and C sites would be preferred. These questions motivated us to study the kinetics of ligand exchange in detail for this model system. In the future, the dependence of the kinetics on the stabilizing ligand of the Au25 nanocluster as well as on the incoming ligand can be examined. Ligand Exchange at Site A. As the incoming thiol approaches the Au25(SH)18− cluster, the bond between the core gold atom and the terminal −SH breaks to form the first transition state with a barrier height of 0.56 eV (Figure 2). The atom labels are shown in Figure 3 and will be referred to in this manner throughout the entire ligand exchange process (i.e., Sterminal is replaced by Sincoming during the reaction such that the final products include H2Sterminal and a nanoparticle with a terminal −SincomingH group). The incoming thiol is preferentially oriented with its hydrogen pointing toward the sulfur atom of the terminal −SH unit. Going from reactants to transition state TS1A, the distance between the core gold atom and the sulfur of the terminal −SH increases to 3.82 Å from 2.50 Å (Table 1). The bond length between the core gold atom and the sulfur atom of the incoming thiol is 3.20 Å in TS1A, which is much longer than the original Aucore−Sterminal distance

the distance from the sulfur atom of the incoming thiol to the core gold atom. For the second transition state (TS2) search, we examined the distance between the leaving sulfur atom of the H2S and the staple gold atom. For the TS1 transition state search at sites B and C, the distance between the sulfur of the incoming thiol and the staple gold atom is considered. For the TS2 search at site B the distance from the sulfur of the leaving H2S to the core gold atom is used, and for the TS2 search at site C the distance between the sulfur of the leaving H2S and the staple gold atom is utilized. Constrained optimizations are not guaranteed to reach a true transition state, so caution must always be used with this approach. All of the transition states in this study possess one imaginary frequency; 100% offsets along the corresponding normal mode led to the expected reactants/ intermediates and products/intermediates. Throughout the article, subscripts will be used to denote which site is considered for each step in the mechanism.

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RESULTS CH3SH (methanethiol) was examined as the incoming thiol for the ligand exchange reaction on the Au25(SH)18− nanocluster. The methanethiol can substitute at three different sites on the Au25 cluster marked A, B, and C in Figure 1. It should be noted that the sulfur atoms are stereocenters with two S−Au bonds, one S−R (R = H, CH3) bond, and one lone pair. Each SR group has two possible orientations relative to the Au−S−Au plane, so many different conformers are possible for each structure discussed in this work. As discussed below, generally the conformers lie close in energy for small R groups such as H and CH3, so only one set of A, B, and C sites is examined in this work. At site A, the incoming thiol can insert between a core gold atom and the terminal −SH ligand of the V-shaped staple 20181


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between the core gold atom and the sulfur atom of the incoming thiol is fully formed at this intermediate state, with a bond distance of 2.50 Å. To complete the ligand exchange process from the intermediate state, the reaction proceeds through a second transition state, TS2A, with a 0.76 eV barrier height (0.57 eV above the intermediate state). Going from TS1A to IntA and from IntA to TS2A, the bond length between the hydrogen of the incoming thiol and the terminal sulfur atom shortens by 0.20 and 0.14 Å in the two processes, respectively. In TS2A, the terminal −SH unit starts to detach from the staple gold atom (with a bond distance of 2.88 Å) as a H2S molecule. This H2S molecule has a partial bond formation between the terminal sulfur with the hydrogen from the incoming thiol in which the partial S−H bond is 0.6 Å longer than the other free S−H bond of H2S. In the final ligand exchange product, the two hydrogen atoms and the CH3 group are oriented toward the same side of the staple motif. The final products are 0.01 eV lower in energy than the reactants. It should be mentioned that the product (and reactant) molecules calculated at a finite distance apart are typically lower in energy than when they are calculated separately. This energy difference ranges from −0.13 eV up to a maximum of 0.02 eV. Ligand Exchange at Site B. Ligand exchange at site B (Figure 4) involving ligand exchange occurring between the terminal −SH and the staple gold atom is also an associative type mechanism similar to the above ligand exchange mechanism at site A. The approach of the thiol breaks the bond between the staple gold atom and the terminal −SH unit. In TS1B, the bond distance between the sulfur of the terminal −SH unit and the staple gold atom reaches 3.72 Å and the distance between the sulfur of the incoming thiol and the staple


Figure 3. Structure of IntA. Atom labels are shown with arrows.

Table 1. Relevant Bond Lengths (in Å) for Ligand Exchange at Site A bond

reactants

TS1A

IntA

TS2A

products

Aucore−Sterminal Aucore−Sincoming Austaple−Sterminal Sterminal−Hincoming Austaple−Sincoming

2.50 N/A 2.40 N/A N/A

3.82 3.20 2.37 2.30 4.00

4.20 2.50 2.38 2.10 3.95

4.22 2.49 2.88 1.96 3.93

N/A 2.48 N/A 1.35 2.40

of 2.50 Å. The coordinates of the structures discussed in this work are provided in the SI. TS1A corresponds to a transition state representing an associative type mechanism for the ligand exchange process. Next, the metastable intermediate state IntA is formed with a relative energy of 0.19 eV compared to the reactants. The bond

Figure 4. Ligand exchange mechanism at site B. 20182


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gold atom is 2.79 Å (Figure 5 and corresponding Table 2). The first energy barrier to yield TS1B is 0.62 eV, which is 0.06 eV higher in energy than the first barrier height for ligand exchange at site A.

IntB at 0.29 eV above the reactants is 0.10 eV higher in energy than IntA. In IntB, the incoming thiol forms a bond with staple gold atom, and the Austaple−Sincoming bond distance is 2.39 Å. The hydrogen atom attached to the sulfur on the incoming thiol ligand is oriented in a similar way to the intermediate for site A, facilitating the removal of a H2S molecule. This intermediate state leads to the final product via TS2B with a 0.65 eV energy barrier compared to reactants (0.36 eV above the intermediate state). Compared to ligand exchange at site A, ligand exchange at site B is more favorable with lower barrier heights. (The highest 0.76 eV second barrier height for ligand exchange at site A is higher compared to the highest 0.65 eV second barrier height for site B). For TS2B, the bond between the terminal −SH and the core gold atom dissociates (distance of 3.18 Å) and the sulfur of the terminal −SH unit partially forms a bond (bond distance of 1.42 Å) with the hydrogen attached to the sulfur atom of the incoming thiol ligand. This new S−H bond is 0.06 Å longer than the other free S−H bond on H2S. The final ligand exchanged products (ProdB and H2S) at site B are 0.05 eV higher in energy than reactants. The final products from ligand exchange on the Au25 nanocluster should be essentially the same for both sites A and B (ProdA and ProdB). However, we observed a slightly higher energy isomer for the final product ProdB arising from ligand exchange at site B. The difference is that both hydrogen atoms and the CH3 in ProdB do not point in the same direction unlike ProdA. Instead, the CH3 group and the hydrogen of the terminal −SH group on the other side of the staple unit are oriented opposite to each other. Even so, the energy differences between these two ProdA and ProdB isomers are very small (0.06 eV).

Figure 5. Structure of IntB. Atom labels are shown with arrows.

Table 2. Relevant Bond Lengths (in Å) for Ligand Exchange at Site B bond

reactants

TS1B

IntB

TS2B

products

Aucore−Sterminal Austaple−Sterminal Austaple−Sincoming Sterminal−Hincoming Aucore−Sincoming

2.50 2.40 N/A N/A N/A

2.46 3.72 2.79 2.07 4.01

2.48 3.87 2.39 1.78 4.01

3.18 3.82 2.36 1.42 3.80

N/A N/A 2.40 1.35 2.49

Figure 6. Transition state for the conformational change of the terminal −SH group. Arrows are included to draw the eye to the SH group whose conformation is changing. 20183


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Figure 7. Transition state for the conformational change of the terminal −SCH3 group. Arrows are included to draw the eye to the −SCH3 group whose conformation is changing.

Figure 8. Ligand exchange mechanism at site C.

Austaple−Scentral−Austaple plane are 0.55 and 0.60 eV respectively. The transition states corresponding to these movements have

The calculated barrier heights for conformational change of the −SH (Figure 6) and −SCH3 (Figure 7) groups through the 20184


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The Journal of Physical Chemistry C hydrogen and methyl respectively positioned in the Austaple− Scentral−Austaple plane. The transition state for the methyl conformational change has elongated Aucore−Austaple distances (up to 0.21 Å). The transition state corresponding to the hydrogen conformational change has a 0.43 Å elongated bond length between the core gold atom and the terminal −SH unit (2.97 Å). It should also be noted that multiple isomers of these structures with different methyl and hydrogen orientations are possible based on their position and direction. These isomer energies vary in the range of 0.01 to 0.07 eV. Ligand exchange at site B is similar to the mechanism examined for Au102(SH)44,23 although that work examined ligand exchange on a short staple (SR−Au−SR). Using the PBE functional, which is another GGA functional, the previous study found a first barrier height of 0.65 eV, which is similar to our first barrier height of 0.62 eV for mechanism B. Their intermediate was found to lie 0.19 eV higher in energy than reactants, which is slightly lower in energy than the 0.29 eV intermediate found for site B in our Au25(SH)18− system. This difference in energy may be due to the relative stability of the intermediate as the nanoparticle size changes or to the difference between single and V-shaped staple motifs. Their second barrier height for H2S removal was found to be 0.81 eV, which is higher than the 0.65 eV barrier height we find for site B, although it is relatively close to the barrier height for site A (0.76 eV). Their overall reaction energy for replacement of a SH moiety with SCH3 was 0.05 eV, which is identical to the reaction energy found in our current work. Although the barrier heights differ by up to 0.16 eV (which as stated above for the intermediate may be due to differences in the staple motifs and the size of nanoparticle), they are quite close considering the differences in the system, which suggests that other thiolatestabilized gold nanoparticles may have similar barrier heights and reaction energies. Ligand Exchange at Site C. Similar to sites A and B, ligand exchange at site C with substitution between the central −SH and a staple gold atom also proceeds through an associative type mechanism (Figure 8). The bond between the central −SH moiety and the staple gold atom breaks with increasing proximity of the incoming thiol ligand. In this mechanism, the hydrogen atom bonded to the sulfur of the incoming thiol approaches in an orientation such that a bond forms with the central −SH unit. In TS1C, this bond is partially formed with a H−S bond distance of 2.20 Å (Figure 9 and corresponding Table 3). At the metastable intermediate state IntC this bond distance decreases to 1.92 Å. IntC, at 0.35 eV above the reactants, is less stable than the intermediates arising from ligand exchange at the other two sites. In TS1C, the S−Au distance between the central −SH unit and the nearest staple gold atom is 3.57 Å (Austaple‑2− Scentral), while the bond length between the incoming thiol and this staple gold atom is calculated to be 2.89 Å (Austaple‑2− Sincoming). This TS1C is reached by overcoming a 0.78 eV energy barrier from the reactants to generate IntC. This is higher than the first barrier heights of the ligand exchange reactions at both sites A and B. In the second step of the reaction, the Au−S bond between the central −SH and the other staple gold atom in the intermediate state must be broken (Austaple‑1−Scentral distance of 3.39 Å in TS2C) in order to allow the formation and removal of H2S (Scentral−Hincoming distance of 1.42 Å in TS2C), which will permit the sulfur atom of the incoming thiol to bind to the staple gold atom. The newly formed S−H bond in H2S is found

Figure 9. Structure of IntC. Atom labels are shown with arrows.

Table 3. Relevant Bond Lengths (in Å) for Ligand Exchange at Site C bond

reactants

TS1C

IntC

TS2C

products

Austaple‑1−Scentral Austaple‑2−Scentral Austaple‑1−Sincoming Austaple‑2−Sincoming Scentral−Hincoming

2.40 2.40 N/A N/A N/A

2.38 3.57 4.60 2.89 2.20

2.39 4.00 4.52 2.39 1.92

3.39 4.36 4.26 2.36 1.42

N/A N/A 2.39 2.39 1.35

to be 0.06 Å longer than the other S−H bond. The second transition state has a 1.15 eV barrier height, which is the highest barrier height of all the ligand exchange sites and is nearly 0.40 eV higher than the first barrier height for this ligand exchange reaction at site C. The resulting final products are 0.04 eV lower in energy than the initial reactants.

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DISCUSSION All ligand exchange processes at sites A, B, and C have barrier heights varying between 0.56 to 1.15 eV. For site A the second barrier height is 0.2 eV higher than the first one, and for site B, the two barrier heights are nearly similar. For site C, the second barrier height is 0.40 eV higher than the first barrier height. The ligand exchange process at site A has the lowest energy first barrier height, which is 0.06 eV lower than the comparable transition state for site B. However, the second barrier height that leads to the final products is 0.11 eV lower for site B than site A. This suggests that the most favorable ligand exchange mechanism occurs at site B where the incoming thiol ligand substitutes between the staple gold atoms and the terminal −SH unit. It is surprising why this site is favored over the other two sites considering the weaker bond between the core gold atom and the terminal −SH group for site A and the accessibility for ligand exchange at site C. It should be noted that a recent crystal structure of Au25(SC2H4Ph)16(p-BBT)2 reported by Ni et al.36 with p-BBT ligands substituted at terminal positions also provides support for ligand exchange via either mechanism A or B. Given the bulky nature of the SC2H4Ph and p-BBT ligands, a ligand exchange mechanism through site B may be more likely. Previous work by Murray and co-workers21 showed that Au25(SR)18− undergoes two different rates of ligand exchange. We suggest that the initial faster ligand exchange is due to exchange via mechanism A or B for substitution of the terminal ligands; the slower second phase of exchange is expected due to higher barrier heights for exchange at site C. 20185


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(3) Wang, M.; Wu, Z.; Yang, J.; Wang, G.; Wang, H.; Cai, W. Au25(SG)18 as a Fluorescent Iodide Sensor. Nanoscale 2012, 4, 4087− 4090. (4) Whetten, R. L.; Price, R. C. Nano-Golden Order. Science 2007, 318, 407−408. (5) Zhu, Y.; Qian, H.; Drake, B. A.; Jin, R. Atomically Precise Au25(SR)18 Nanoparticles as Catalysts for the Selective Hydrogenation of α,β-Unsaturated Ketones and Aldehydes. Angew. Chem., Int. Ed. 2010, 49, 1295−1298. (6) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. A DNA-based Method for Rationally Assembling Nanoparticles into Macroscopic Materials. Nature 1996, 382, 607−609. (7) Mulvaney, P. Surface Plasmon Spectroscopy of Nanosized Metal Particles. Langmuir 1996, 12, 788−800. (8) Ackerson, C. J.; Powell, R. D.; Hainfeld, J. F. Site-Specific Biomolecule Labeling with Gold Clusters. In Methods in Enzymology; Grant, J. J., Ed.; Academic Press: New York, 2010; Vol. 481, pp 195− 230. (9) Fernando, A.; Weerawardene, K. L. D. M.; Karimova, N. V.; Aikens, C. M. Quantum Mechanical Studies of Large Metal, Metal Oxide, and Metal Chalcogenide Nanoparticles and Clusters. Chem. Rev. 2015, 115, 6112−6216. (10) Guo, R.; Murray, R. W. Substituent Effects on Redox Potentials and Optical Gap Energies of Molecule-like Au38(SPhX)24 Nanoparticles. J. Am. Chem. Soc. 2005, 127, 12140−12143. (11) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Dynamics of Place-Exchange Reactions on Monolayer-Protected Gold Cluster Molecules. Langmuir 1999, 15, 3782−3789. (12) Song, Y.; Murray, R. W. Dynamics and Extent of Ligand Exchange Depend on Electronic Charge of Metal Nanoparticles. J. Am. Chem. Soc. 2002, 124, 7096−7102. (13) Sousa, A. A.; Morgan, J. T.; Brown, P. H.; Adams, A.; Jayasekara, M. P. S.; Zhang, G.; Ackerson, C. J.; Kruhlak, M. J.; Leapman, R. D. Synthesis, Characterization, and Direct Intracellular Imaging of Ultrasmall and Uniform Glutathione-Coated Gold Nanoparticles. Small 2012, 8, 2277−2286. (14) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. Redox and Fluorophore Functionalization of Water-Soluble, Tiopronin-Protected Gold Clusters. J. Am. Chem. Soc. 1999, 121, 7081−7089. (15) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. The Monolayer Thickness Dependence of Quantized Double-Layer Capacitances of Monolayer-Protected Gold Clusters. Anal. Chem. 1999, 71, 3703−3711. (16) Brown, L. O.; Hutchison, J. E. Convenient Preparation of Stable, Narrow-Dispersity, Gold Nanocrystals by Ligand Exchange Reactions. J. Am. Chem. Soc. 1997, 119, 12384−12385. (17) Brown, L. O.; Hutchison, J. E. Controlled Growth of Gold Nanoparticles during Ligand Exchange. J. Am. Chem. Soc. 1999, 121, 882. (18) Woehrle, G. H.; Brown, L. O.; Hutchison, J. E. Thiolfunctionalized, 1.5-nm Gold Nanoparticles through Ligand Exchange Reactions: Scope and Mechanism of Ligand Exchange. J. Am. Chem. Soc. 2005, 127, 2172−2183. (19) Woehrle, G. H.; Hutchison, J. E. Thiol-Functionalized Undecagold Clusters by Ligand Exchange: Synthesis, Mechanism, and Properties. Inorg. Chem. 2005, 44, 6149−6158. (20) Woehrle, G. H.; Warner, M. G.; Hutchison, J. E. Ligand Exchange Reactions Yield Subnanometer, Thiol-Stabilized Gold Particles with Defined Optical Transitions. J. Phys. Chem. B 2002, 106, 9979−9981. (21) Guo, R.; Song, Y.; Wang, G.; Murray, R. W. Does Core Size Matter in the Kinetics of Ligand Exchanges of Monolayer-Protected Au Clusters? J. Am. Chem. Soc. 2005, 127, 2752−2757. (22) Hadley, A.; Aikens, C. M. Thiolate Ligand Exchange Mechanisms of Au1 and Subnanometer Gold Particle Au11. J. Phys. Chem. C 2010, 114, 18134−18138. (23) Heinecke, C. L.; Ni, T. W.; Malola, S.; Mäkinen, V.; Wong, O. A.; Häkkinen, H.; Ackerson, C. J. Structural and Theoretical Basis for

In general we have seen that a lone pair on the sulfur of the incoming thiol points toward the core or staple gold atoms, while the proton of the incoming thiol interacts with one of the lone pairs of the sulfur atoms in the nanoparticle staple motifs. This is similar to the mechanism examined for the Au102(SH)44 nanoparticle.23 This suggests that each of these reactions starts with an initial nucleophilic attack of the sulfur of incoming thiol ligands. Overall, our kinetic studies using SH and SCH3 suggest ligand substitution preferentially occurs between the staple gold atoms and terminal −SH groups, which is in excellent agreement with available experimental results. Future work should be performed on bulkier ligands similar to those used experimentally to ensure that the mechanism does not change with the size of the ligand and to ascertain the effects of sterics and solvent accessibility.

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CONCLUSIONS In summary, we employed density functional theory to investigate the potential energy surfaces of the ligand exchange mechanism on the Au25(SH)18− nanocluster. The nanocluster possesses three possible sites for the ligand exchange process: substitution of incoming ligands between a core gold atom and a terminal −SH (denoted site A), between a terminal −SH and a staple gold atom (site B), and between a central −SH and a staple gold atom (site C). All these ligand exchange processes follow though an associative type mechanism where the incoming thiol induces breaking of the existing Au−S bonds between the −SH units and relevant gold atoms. All these ligand exchange processes have barrier heights ranging from 0.6 to 1.2 eV. Our kinetic results indicate that the most favorable ligand exchange process is at site B, which is in good agreement with available experimental data.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06833. XYZ coordinates of all the structures mentioned in this study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1-785-532-0954. Fax: 1785-532-6666. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation under Grant CHE-1213771. C.M.A. is grateful to the Camille and Henry Dreyfus Foundation for a Camille Dreyfus Teacher-Scholar Award (2011−2016).

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REFERENCES

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