Deciphering the Ligand Exchange Process on Thiolate Monolayer

Article pubs.acs.org/JPCC

Deciphering the Ligand Exchange Process on Thiolate Monolayer Protected Au38(SR)24 Nanoclusters Amendra Fernando and Christine M. Aikens* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States S Supporting Information *

ABSTRACT: Ligand exchange reactions are used widely to modify and tune the properties of thiolate monolayer protected nanoclusters. We employed density functional theory calculations to investigate the ligand exchange process on the Au38(SH)24 nanocluster. The Au38 cluster consists of eight possible unique sites for ligand exchange, including six sites from the dimeric staple motifs and two sites from monomeric units. The most favorable site for ligand exchange is found to be between a core gold atom and the −SH moiety on the monomeric unit. Ligand substitution between a core gold atom and a terminal −SH group or between a terminal −SH group and a staple gold atom at both sides of the dimer units is a more favorable site for the ligand exchange process than substitution of the central −SH moiety. However, substitution of the central −SH moiety on Au38 is more favorable than for the corresponding position on Au25.

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INTRODUCTION

Over the past few years there have been several experimental studies related to ligand exchange on the Au38 nanocluster. Knoppe and co-workers39 investigated ligand exchange reactions on Au38(2-PET)24 and Au40(2-PET)24 (2-PET = 2phenylethanethiolate) clusters with BINAS (BINAS = 1,1′binaphthyl-2,2′-dithiol) as the incoming ligand. The ligand exchange reaction on both these clusters was found to be incomplete, with an average of 1.5 bidentate BINAS ligands replacing 3 monodentate 2-PET ligands for Au38 and a maximum of 3 BINAS ligands incorporated in any cluster. The rate of the reaction slowed down after a few hours. They suggested that the reason behind this is that the number of short staples (monomeric SR-Au-SR units) is a limiting factor, indicating selective binding of BINAS to these monomeric units. However, ligand exchange with monodentate thiophenol led to up to 9 ligands exchanged, which exceeds the number of SR groups present on the short staples. These authors later investigated ligand exchange reactions on racemic mixtures of the same Au38 and Au40 clusters with monodentate and bidentate chiral thiols.40 A nonlinear behavior between optical activity and the number of chiral ligands was found for the BINAS ligand, which suggested different exchange rates for BINAS with the two enantiomers of Au38. For the monodentate 1S,4R-camphor-10-thiol ligand, fast exchange of up to 10 2-PET ligands was observed; as for thiophenol, this exceeds the number of SR groups present on the monomeric units. In 2012, Knoppe and co-workers41 reported that the reaction between racemic Au38(PET)24 and R-BINAS is diastereoselective and that the exchange reaction is found to be 4 times faster for the preferred A-enantiomer. The ligand exchange

In general, monolayer protected gold nanoparticles have a wide range of applications in catalysis,1−5 sensors,6,7 nanomedicine,6,8 etc.6,9−11 Ligand exchange reactions are a popular technique to functionalize and tune the optical and electrochemical properties of monolayer protected gold nanoparticles.12−21 Such modified nanoparticles can then be used for many different applications based on their properties. Controlling the kinetics of ligand exchange reactions leads to the possibility of selective functionalization of the ligand shell of these systems. Several stable cluster sizes of gold nanoparticles have been successfully synthesized with structures determined by X-ray crystallography techniques.22−35 Many of these gold nanoclusters have a general structural feature in that they have a central gold core protected by −SR(Au-SR)x− oligomer units. For example, the Au38(SR)24 cluster (henceforth simply abbreviated as Au38) consists of a biicosahedral Au23 central core that is protected by six SR-Au-SR-Au-SR dimeric units and three linear monomeric SR-Au-SR units.33,36 This chiral cluster has an elongated structure where the two poles are each covered by three dimeric units and the linear monomer units can be found at the equator of the cluster. Following our study of ligand exchange kinetics for the Au25 nanocluster,37 we implemented the same approach to investigate the Au38 nanocluster to determine trends in the preferred exchange sites. Our results with the Au25 nanocluster were in agreement with recent reversed-phased high-performance liquid chromatography experiments by Niihori et al.38 These authors observed a ligand exchange process that initiates at sulfur atoms attached to core sites of the Au25 nanoparticles. Compared to the Au25 cluster, Au38 is a more complex structure with an increased number of unique sites exposed to ligand exchange processes. © 2016 American Chemical Society

Received: May 4, 2016 Revised: June 6, 2016 Published: June 6, 2016 14948

DOI: 10.1021/acs.jpcc.6b04516 J. Phys. Chem. C 2016, 120, 14948−14961

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The Journal of Physical Chemistry C rate for exchange of the second ligand was observed to be much slower than for the first one indicating a kinetic effect. Molina and co-workers42 investigated R-BINAS monosubstituted Au38(SCH3)24 clusters with density functional theory. The theoretical calculations suggested that the most thermodynamically stable binding mode bridges two dimer units in an interstaple binding fashion rather than bidentate binding within the same monomer unit, in contrast to the previous suggestions of intrastaple binding. The interstaple bidentate binding mode of the BINAS ligand has been previously established for the Au25 nanocluster as well.43,44 Beqa et al.45 investigated the ligand exchange reaction between Au38(2-PET)24 and [2.2] paracyclophane-4-thiol (PCP-4-SH) using HPLC and mass spectrometry. These authors found that three out of the four symmetry unique sites are exchanged, leading to different regioisomers. Of the three, two sites appear to be more populated than the third. These four unique sites will be discussed in our Results section. A few researchers have looked at other types of processes occurring during ligand exchange. Zeng et al.46 demonstrated a ligand-induced disproportionation mechanism that converts the rod-like biicosahedral Au38(2-PET)24 clusters into tetrahedral Au36(TBBT)24 (TBBT = 4-tert-butylbenzenethiol) and Au40 nanoclusters. Formation of Au36 nanoclusters was also observed during ligand exchange with phenylthiol ligands.39,40 Our goal in this study is to investigate the kinetics of the ligand exchange process on a model Au38 cluster. We believe this understanding will be helpful in the future for experimentalists to selectively modify these gold nanoparticles and thus to optimize them for potential applications. In this study, we focus entirely on an associative mechanism between the nanoparticle and the incoming ligand. It should be noted that Niihori et al.38 suggested that interparticle ligand exchange may also occur after an initial associative ligand exchange between the nanocluster and thiol; however, interparticle exchange is not addressed in this study.

Figure 1. (a) Au38(SR)24 nanocluster with eight possible ligand exchange sites marked as A−H. Bond distances are given in Å. (b) Wire frame view of the Au38(SR)24 nanocluster with ligand exchange sites marked A−H. 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.

For ligand exchange at the B, C, D, and H sites, the bond distance between the sulfur of incoming thiol ligand and the staple gold atom is used for TS1. For TS1 of site E, we found that the distance between the staple gold atom and the sulfur of the terminal −SH generates the correct transition state rather than the bond distance between the sulfur of the incoming thiol ligand and the staple gold atom. For TS2 of sites B, E, and H, the bond distance between the sulfur of the leaving H2S molecule and the core gold atom is considered. For both sites C and D, TS2 was found using the distance between the sulfur of the leaving H2S and the staple gold atom. Throughout the article, subscripts will be used to denote which site is considered for each step of the ligand exchange process.

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COMPUTATIONAL METHODOLOGY Density functional theory with the Becke−Perdew (BP86)47,48 exchange-correlation functional implemented in the Amsterdam Density Functional (ADF)49 program is employed in all theoretical calculations. It should be mentioned that generalized gradient approximation (GGA) functionals such as BP86 tend to underestimate barrier heights. Therefore, larger barrier heights may be expected experimentally. A polarized double-ζ basis set is used with a [1s2−4f14] frozen core for gold atoms, a [1s2−2p6] frozen core for sulfur atoms, and a [1s2] frozen core for carbon atoms. Scalar relativistic effects are incorporated by employing the zero-order regular approximation (ZORA). All the transition states in this study are calculated with a constrained structural optimization technique used in refs 21 and 37. It should be noted that constrained optimizations do not necessarily reach a true transition state, so caution must always be used with this approach. All transition states considered in this work possess one imaginary frequency. Offsets of the geometries along their normal modes reached the expected reactants/ intermediates and products/intermediates. For ligand exchange at sites A, F, and G (shown in Figure 1a,b) we found that the suitable bond to use in calculating the first transition state (TS1) is the distance between the sulfur of the incoming thiol ligand to the core gold atom. For the second transition state (TS2), the distance between the leaving sulfur atom of the H2S molecule and the staple gold atom is considered.

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RESULTS AND DISCUSSION Similar to our previous study with the Au25 nanocluster,37 we investigated the Au38(SH)24 ligand exchange mechanism using CH3SH (methanethiol) as the incoming ligand. The Au38 nanocluster has 8 possible sites for ligand substitution: six sites on dimeric staples and two sites on monomeric motifs. These 8 sites are marked as A−H in Figure 1. Sites A and F, B and E, and C and D represent ligand substitution on a dimer unit between a core gold atom and a terminal −SH unit, a terminal −SH unit and a staple gold atom, and a staple gold atom and a central −SH unit, respectively. Sites G and H represent possible substitutions on a monomer unit between a core gold atom and terminal −SH unit and between a terminal −SH unit and a staple gold atom. Although there are 8 unique sites for the incoming ligand, some of the products are essentially indistinguishable: substitution at A/B, C/D, E/F, and G/H would lead to four unique products. 14949


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Figure 2. Ligand exchange mechanism at site A.

In dimer units, sites A, B, and C are located at the poles (high curvature end) of the Au38 cluster whereas sites D, E, and F are at the equator (low curvature side) of the elongated Au 38 nanocluster (the bond distances in these two environments are very similar). Note that at the equator the core gold atoms are more coordinated (coordination number of 6) than the gold atoms at the poles (coordination number of 5) of the cluster. This different ligand environment can change the preferentiality toward ligand exchange; thus, we investigated all these sites to check the effect of this ligand environment. Sites G and H of the monomer units are located at the equator of the Au 38 nanocluster. Sites A, F, and G exhibit the longest bond distances between gold and sulfur for the Au38 cluster, suggesting a weaker bond in these sites that therefore could be the most vulnerable for ligand exchange. However, this was not found to be the case for Au25 where we observed that a stronger bond undergoes bond breaking for ligand substitution in the lowest energy pathway.37 One can also argue that sites A, F, and G between core gold atom and terminal −SH units are less accessible to ligand exchange; thus, sites B, C, D, E, and H would be preferred. These questions and the complexity of ligand substitution sites led us to investigate the Au38 nanocluster and determine patterns for the ligand exchange process on thiolate-protected nanoparticles. Ligand Exchange at Site A. The ligand exchange process at site A is given in Figure 2. The atom labels are shown in Figure 3 and will be referred this way throughout the ligand exchange process (i.e., Sterminal is replaced by Sincoming during the reaction, and the final products include H2Sterminal and a nanoparticle with a −SincomingCH3 group). We found many similarities with the ligand exchange process at site A in the Au25 nanocluster. With the approach of the incoming thiol ligand, we see that the bond between the core gold atom and the terminal −SH unit breaks to form the first

Figure 3. (a) and (b) represent alternative viewing angles of the structure of IntA. Atom labels are shown with arrows.

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staple gold atom with a bond length of 2.87 Å. The hydrogen of the incoming thiol ligand is oriented preferentially to facilitate the removal of the H2S molecule. The Sterminal−Hincoming bond distance changes from 2.23 Å in TS1A to 2.01 Å in IntA and 1.52 Å in TS2A. In TS2A, the H2S molecule that is forming shows a partial bond formation between the terminal sulfur atom and the H from the incoming thiol with a bond distance that is 0.16 Å longer than that of the other free S−H bond. In the final ligand exchange product ProdA, the Austaple− Sincoming bond distance is now equal to the original bond distance of 2.39 Å between Austaple−Sterminal. The bond distance between the core gold atom and the sulfur of the −SCH3 unit in ProdA is now 0.02 Å shorter than that of the bond length between the core gold atom and the sulfur of terminal −SH unit in the reactants. The final products are 0.06 eV lower in energy than the reactants. The terminal −SCH3 group of ProdA is oriented in the same direction as that of the terminal −SH unit of the reactant Au38 nanocluster. This was not the case with ProdA of the Au25 nanocluster where we observed an opposite orientation with respect to the reactant.37 It should be noted that the reaction energies and barrier heights of the Au25 nanocluster are nearly comparable to the values of Au38. In Au25, we calculated that ligand exchange at site A led to a TS1A with a 0.56 eV barrier height followed by an intermediate state that is 0.19 eV above the reactants.37 The second transition state of Au25 nanocluster TS2A has a 0.76 eV barrier height.37 These values are comparable to 0.68, 0.22, and 0.80 eV values for site A of the Au38 nanocluster as discussed above. Ligand Exchange at Site B. At site B, ligand exchange occurs between the terminal −SH group and the staple gold atom (Figure 4). The atom labels are shown in Figure 5. The first transition state TS1B has a barrier height of 0.61 eV, which is 0.07 eV lower than the first barrier height for site A. The incoming thiol ligand breaks the bond between the terminal −SH unit and

transition state with a barrier height of 0.68 eV. Similar to the Au25 nanocluster,37 the incoming thiol is preferentially oriented with its hydrogen pointing toward the sulfur atom of the terminal −SH unit. The distance between the core gold atom and the sulfur of the terminal −SH unit in the reactant increases from 2.48 to 4.16 Å in TS1A (Table 1). The sulfur of incoming thiol 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

2.48 N/A 2.39 N/A N/A

4.16 3.18 2.36 2.23 4.13

4.24 2.48 2.37 2.01 3.99

4.20 2.44 2.87 1.52 4.01

N/A 2.46 N/A 1.35 2.39

binds to the Au38 nanocluster in TS1A with a bond distance of 3.18 Å, which is longer than the Aucore−Sterminal bond distance of 2.48 Å in the reactants. With the detachment of the terminal −SH unit from the core gold atom, the Austaple−Sterminal bond distance in TS1A decreases from 2.39 Å in the starting Au38 nanocluster to 2.36 Å. The formation of TS1A corresponds to an associative type mechanism similar to the ligand exchange process in Au25 nanoparticles.37 The metastable intermediate state IntA is formed with a relative energy of 0.22 eV in comparison to the reactants. The bond between the core gold atom and the sulfur of the incoming thiol is completely formed in IntA, with a bond distance of 2.48 Å. The Aucore−Sterminal bond has increased to 4.24 Å, and the Austaple−Stermnial bond distance is 0.01 Å longer than in TS1A. The ligand exchange process then continues via a second transition state with a barrier height of 0.80 eV (0.58 eV above the intermediate state). In the second transition state TS2A, a terminal −SH unit starts to detach as a H2S molecule from the

Figure 4. Ligand exchange mechanism at site B. 14951


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In TS1B a new bond is formed with a bond length of 2.79 Å between the staple gold atom and the sulfur of the incoming thiol ligand. This bond breaking and new bond formation suggests that the ligand exchange process at site B is also an associativetype mechanism; in fact, all reaction processes studied in this article demonstrates a similar pattern suggesting that they follow an associative-type mechanism for the ligand exchange process. With the detachment of the staple gold atom from the terminal −SH unit, the Aucore−Sterminal bond distance is seen to be shortened by 0.02 Å in TS1B. The intermediate state IntB is 0.29 eV above the reactants and 0.07 eV higher in energy than IntA. In IntB, a new bond between the staple gold atom and the sulfur of incoming thiol is fully formed with a bond distance of 2.39 Å. The Austaple−Sterminal bond is further lengthened to 4.00 Å. Similar to ligand exchange at site A, the hydrogen of the incoming thiol ligand is oriented in the precise direction to facilitate removal of the H2S molecule. The ligand exchange proceeds through a second transition state with a 0.76 eV barrier height (0.47 eV above the intermediate state). This second barrier height is 0.04 eV lower in energy than the second barrier height for ligand exchange at site A. The bond between the core gold atom and the sulfur of terminal −SH is starting to dissociate with a 3.15 Å bond length. The Austaple− Sincoming bond distance is further shortened by 0.03 Å. The bond between the sulfur of the terminal −SH unit and H of the incoming thiol ligand is partially formed with a bond distance of 1.43 Å. This is 0.07 Å longer than that of the other free S−H bond of H2S. The final product is 0.07 eV lower in energy than the reactants. ProdB has a different terminal −SR orientation with respect to the reactants. The final products of ligand exchange in sites A and

Figure 5. Structure of IntB. Atom labels are shown with arrows (Hincoming is marked with a green circle).

the staple gold atom in TS1B. This bond distance reaches 3.70 Å in TS1B from 2.39 Å (Table 2) in the reactant Au38 nanocluster. 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

2.48 2.39 N/A N/A N/A

2.44 3.70 2.79 2.13 4.13

2.45 4.00 2.39 1.92 4.02

3.15 4.03 2.36 1.43 3.60

N/A N/A 2.38 1.35 2.46

Figure 6. Ligand exchange mechanism at site C. 14952


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The Journal of Physical Chemistry C B are nearly the same except for the terminal −SCH3 orientation. Experimentally, the final ligand exchange products at sites A and B would be indistinguishable. However, ProdB with terminal −SH and −SCH3 groups pointed in the same direction is slightly (0.01 eV) lower in energy than ProdA. We observed the same scenario for the Au25 nanocluster where the terminal −SR orientation in the same direction is slightly lower in energy than the orientation to opposite directions.37 It should be noted that in the Au25 nanocluster37 we observed ProdA with terminal −SR units oriented in the same direction to be lower in energy whereas in Au38 the lower energy product is ProdB. The barrier height to reorient the −SR group in Au25 was found to be 0.55− 0.60 eV.37 Similar to the Au25 nanocluster we can expect that −SR group reorientation in the Au38 nanocluster may have barrier heights accessible under experimental reaction conditions. The ligand exchange processes at sites A and B have similar barrier heights. Although site B is slightly more favorable than site A, the difference between the highest barrier heights of site A and B is 0.04 eV. Similar to the comparable energies in site A of Au25 and Au38 nanoclusters, site B of both nanoclusters also has comparable energies. TS1B, IntB , and TS2B of the Au25 nanocluster have energies of 0.62, 0.29, and 0.65 eV relative to the reactants, respectively.37 These values are comparable to 0.61, 0.29, and 0.76 eV, respectively, for site B of the Au38 nanocluster. Ligand Exchange at Site C. Ligand exchange at site C takes place between the staple gold atom and the sulfur of the central −SH group. The reaction process is given in Figure 6, and the atom labels are shown in Figure 7. In transition state TS1C, the

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

2.37 4.51 4.18 3.10 2.24

2.38 4.37 3.99 2.40 1.97

2.98 4.40 3.67 2.38 1.46

N/A N/A 2.39 2.38 1.35

Austaple‑2−Sincoming bond distance is now 2.38 Å in TS2C, and the central −SH unit is seen to detach (2.98 Å) from Austaple‑1 to leave as a H2S molecule after abstraction of Hincoming from the incoming thiol. The Scentral−Hincoming bond distance is reduced to 1.46 Å in TS2C from 2.24 Å in TS1C and 1.97 Å in IntC. This bond is 0.1 Å longer than that of the other free S−H bond of H2S molecule. ProdC is 0.05 eV lower in energy than the initial reactants. This product has a central −SCH3 unit. The bond distances of Austaple‑2−Sincoming and Austaple‑1−Sincoming when fully formed in ProdC are 2.38 and 2.39 Å, respectively, which are 0.02 and 0.01 Å shorter than the initial bond of the reactant Au38 nanocluster with a central −SH unit. Note that the central −SCH3 is oriented opposite to the −SH orientation of the reactant Au38 nanocluster. The ligand exchange process at site C in the Au25 nanocluster is different from the Au38 nanocluster in that we observed a higher barrier height for the second transition state TS2C of Au25.37 In contrast, as discussed above, we noticed two similar barrier heights of 0.85 eV for TS1C and TS2C of the Au38 nanocluster. Even so, comparing the ligand exchange processes at sites A, B, and C of the Au38 nanocluster shows that the ligand exchange process at site C has a higher barrier height than that of sites A and B. This behavior is similar to site C of the Au25 nanocluster, although the barrier height in this work is much smaller than the 1.15 eV barrier height predicted for Au25.37 We also observed slightly different transition states and intermediate state for ligand exchange at site C depending upon the approaching side of the incoming thiol ligand. For this exchange, the first transition state, intermediate state, and second transition state have energies of 0.93, 0.33, and 0.73 eV, respectively. Both approaches produced the same products. Because the first barrier height is higher for this approach, we will not further discuss this pathway. It should be noted that we have investigated other possible approaches for the incoming thiol ligand for other sites of the Au38 nanoparticle as well and some of these pathways are discussed herein. Ligand Exchange at Site D. Ligand exchange at site D (Figure 8) is conceptually similar to that of the ligand exchange process on site C where the ligand exchange takes place between a staple gold atom and the central −SH unit. As discussed at the beginning of the Results and Discussion section, due to the differences in the gold and sulfur environment we also decided to investigate the ligand exchange mechanism between the staple gold atom and the sulfur on the other side (site D) of the central −SH unit. In this case, it occurs between the Austaple‑1 atom (Figure 9a) and the sulfur of the central −SH unit. There are similar patterns in the bond breaking and formation processes at site D as in site C. The first transition state TS1D has a barrier height of 0.59 eV, which is 0.34 eV lower in energy compared to TS1C. Similar to the ligand exchange process at site C, with the approach of the incoming thiol ligand the bond between Austaple‑1 and the sulfur of the central −SH unit breaks (Table 4). With this detachment, the Austaple‑2−Scentral bond shortens. Similar to TS2C, we observed

Figure 7. Structure of IntC. Atom labels are shown with arrows.

approach of the incoming thiol ligand breaks the bond between the sulfur of the central −SH unit and the gold atom denoted Austaple‑2. The bond distance between these atoms reaches 4.51 Å in TS1C from 2.40 Å in the reactants (Table 3). The bond between Austaple‑2 and the sulfur of the incoming thiol ligand is partially formed at 3.10 Å. The first transition state has a barrier height of 0.85 eV. The metastable intermediate state IntC has a relative energy of 0.21 eV. In IntC, the Austaple‑2−Sincoming bond distance decreases to 2.40 Å while the Austaple‑2−Scentral bond reduces to 4.37 Å. In the second step of the reaction, the second transition state TS2C is formed with a 0.85 eV barrier height (0.64 eV above the intermediate state). This is same as the first barrier height. The 14953


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Figure 8. Ligand exchange mechanism at site D.

Table 4. Relevant Bond Lengths (in Å) for Ligand Exchange at Site D bond

reactants

TS1D

IntD

TS2D

products

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

2.40 2.40 N/A N/A N/A

3.86 2.37 2.88 4.71 2.20

3.95 2.38 2.38 4.44 1.91

4.16 2.98 2.37 4.24 1.44

N/A N/A 2.39 2.39 1.35

The metastable intermediate state IntD is similar in energy to that of IntC at 0.34 eV compared to the initial reactants (IntC lies 0.33 eV above the reactants). The reaction proceeds via TS2D through a second barrier height of 0.89 eV (0.55 eV above the intermediate state), which is 0.30 eV higher in energy than the first barrier and 0.16 eV higher than that of TS2C. This barrier height is nearly similar to the barrier heights of the ligand exchange process at site C, suggesting that the ligand exchange process at site D is also less likely to take place compared to sites A and B. In comparison to the Au25 nanocluster, we noticed a similar behavior where ligand exchange in dimer units between the sulfur of the central −SH unit and the staple gold atom is less likely to occur. ProdD is 0.11 eV lower in energy than the initial reactants. ProdD is 0.06 eV lower in energy than ProdC. Both ligand exchange processes at sites C and D should produce essentially the same products (which are expected to be experimentally indistinguishable). However, there is a difference in energy between ProdC and ProdD, which is due to the orientation of the −SCH3 group. ProdD has −SCH3 oriented in the same direction

Figure 9. (a) Structure of IntD. Atom labels are shown with arrows. (b) Structure of TS1D showing the newly formed bond between staple gold atoms (marked in green circles).

that the same Austaple‑1 atom of the dimeric unit forms a bond with the staple gold atom of the monomeric unit (Figure 9b). 14954


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Figure 10. Ligand exchange mechanism at site E.

as the reactant −SH unit whereas in ProdC the −SCH3 group is oriented in the opposite direction of the −SH direction. Ligand Exchange at Site E. Ligand exchange at site E (Figure 10) is similar to site B in that it takes place between a staple gold atom and a terminal −SH unit (Figure 11); in this

Ligand exchange via TS1E has a first barrier height of 0.59 eV; in comparison to TS1B this is only 0.02 eV lower in energy. The incoming thiol ligand breaks the bond between the sulfur of the terminal −SH unit and Austaple‑1 in TS1E, and similar to the other sites, with the detachment of this bond the bond between the core gold atom and the terminal −SH unit shortens (Table 5). Table 5. Relevant Bond Lengths (in Å) for Ligand Exchange at Site E bond

reactants

TS1E

IntE

TS2E

products

Aucore−Sterminal Austaple‑1−Sterminal Austaple‑1−Sincoming Sterminal−Hincoming Aucore−Sincoming

2.49 2.41 N/A N/A N/A

2.48 2.99 2.43 1.56 4.23

2.49 3.96 2.38 1.55 4.21

3.57 4.31 2.36 1.41 4.25

N/A N/A 2.40 1.35 2.46

The reaction then follows through the metastable intermediate IntE, which is 0.21 eV above the reactants. Compared to IntB, IntE is 0.08 eV lower in energy. In the next step, the ligand exchange process proceeds through TS2E with a barrier height of 0.80 eV (0.59 eV above the intermediate). Compared to the second barrier height of the ligand exchange at site B, this is 0.04 eV higher in energy. The terminal −SH unit starts to detach and leave as a H2S molecule with the abstraction of the hydrogen from the incoming thiol ligand in TS2E. The final product from ligand exchange at site E is 0.04 eV lower in energy than the reactants. The −SCH3 orientation is similar the −SH orientation of the Au38 reactant. It should be noted that ProdB also retained the −SR group orientation. Ligand exchange processes at sites B

Figure 11. Structure of IntE. Atom labels are shown with arrows.

case it takes place between Austaple‑1 and the neighboring terminal −SH unit. The difference in site E compared to site B is primarily the environment of the gold and sulfur atoms, and the bond formation and breaking processes are similar to site B. Site E lies near the equator whereas site B is at the pole of the Au38 nanocluster. 14955


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Figure 12. Ligand exchange mechanism at site F.

The first transition state TS1F has a barrier height of 0.75 eV, which is 0.07 eV higher than that of site A. Similar to the previous exchange sites, the approach of the incoming thiol ligand breaks the bond between the core gold atom and the sulfur of terminal −SH unit. Note that bond breaking and formation processes are similar to those of site A; these bond distances are given in Table 6.

and E of the Au38 nanocluster have nearly similar barrier heights. This suggests that the difference in the environment between sites B and E does not drastically affect the ligand exchange at least for these sites. However, there is a slight preference toward ligand exchange at site B (0.04 eV lower for the highest barrier height). Ligand Exchange at Site F. Ligand exchange at site F (Figure 12) takes place between a core gold atom and a sulfur of a terminal −SH group. This site is similar to site A other than the difference in environment. The atom labels are shown in Figure 13.

Table 6. Relevant Bond Lengths (in Å) for Ligand Exchange at Site F bond

reactants

TS1F

IntF

TS2F

products

Aucore−Sterminal Aucore−Sincoming Austaple‑1−Sterminal Sterminal−Hincoming Austaple‑1−Sincoming

2.48 N/A 2.39 N/A N/A

4.03 3.78 2.37 2.26 4.26

4.28 2.48 2.37 2.12 3.98

4.29 2.48 2.82 2.01 3.92

N/A 2.46 N/A 1.35 2.40

The metastable intermediate state IntF is 0.06 eV in energy above reactants; this is significantly (0.16 eV) lower in energy than IntA. In the next step, the ligand exchange process proceeds through the second transition state TS2F with a 0.56 eV barrier height (0.50 eV above the intermediate state). This is 0.19 eV lower in energy than TS1F. In comparison to TS2A this second barrier height is significantly (0.24 eV) lower in energy. The final product ProdF is 0.04 eV lower in energy than the reactants and 0.02 eV lower in energy than ProdA. Unlike in ProdA, ProdF retains its −SCH3 orientation similar to that of the initial −SH orientation. There is only a 0.05 eV energy difference between TS2A and TS1F, which are the highest barrier heights for ligand exchange at sites A and F, respectively. This suggests that ligand exchange at sites A and F may have nearly similar rates, similar to

Figure 13. Structure of IntF. Atom labels are shown with arrows. 14956


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Figure 14. Ligand exchange mechanism at site G.

those of sites B and E. The final ligand exchange products for sites E and F are the same; it is not possible to distinguish between ligand exchange mechanisms occurring through these two pathways based on the final products. Ligand Exchange at Site G. The ligand exchange process for sites A−F takes place within the dimeric SR-Au-SR-Au-SR staple units whereas sites G and H correspond to ligand exchange at monomeric units. Similar to sites A and F, ligand exchange at site G (Figure 14) also occurs between a core gold atom and a sulfur atom of a terminal −SH unit (Figure 15), and the bond breaking and formation processes behave similar to sites A and F. The first transition state TS1G has a barrier height of 0.54 eV. The incoming thiol ligand breaks the Aucore−Sterminal bond and

partially forms a bond with core gold atom (Table 7). In the next step, the reaction follows through a second transition state, Table 7. Relevant Bond Lengths (in Å) for Ligand Exchange at Site G bond

reactants

TS1G

IntG

TS2G

products

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

2.50 N/A 2.39 N/A N/A

3.58 3.11 2.36 2.35 4.49

3.80 2.51 2.37 2.11 4.27

4.20 2.42 2.87 1.45 4.20

N/A 2.48 N/A 1.35 2.38

TS2G, with a barrier height of 0.65 eV (0.44 eV above the intermediate state). With respect to the first barrier height this is 0.11 eV higher in energy. When the terminal −SH detaches from the staple gold atom, it abstracts the hydrogen of the incoming thiol ligand. The bonds between these atoms are partially formed. The bond between the sulfur of −SCH3 and the staple gold atom is similar to that of staple gold atom and the −SH unit. The final products are 0.05 eV lower in energy than the initial reactants. The orientation of the −SCH3 group is in the same direction to that of the original −SH group. We also found a higher energy potential energy pathway with 0.69, 0.06, and 1.09 eV for the first transition state, intermediate state, and the second transition state, respectively. The final ligand exchange product was found to be 0.03 eV higher in energy than the one described above. Ligand Exchange at Site H. Ligand exchange at site H (Figure 16) also takes place at a monomeric staple unit in the Au38 nanocluster. This site has a similar bonding arrangement to

Figure 15. Structure of IntG. Atom labels are shown with arrows. 14957


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Figure 16. Ligand exchange mechanism at site H.

sites B and E of the dimeric units. The first transition state TS1H has a high barrier height of 0.91 eV, suggesting that the ligand exchange processes at both sites on these monomeric units are less favorable than sites A, B, E, and F on the dimeric units. The bond between the staple gold atom and the terminal sulfur of the terminal −SH unit (Figure 17a) reaches 3.75 Å in TS1H with the partial bond formation between the sulfur of the incoming thiol ligand and the staple gold atom with a bond distance of 3.29 Å (Table 8). As seen with TS1D, the first transition state TS1H also demonstrates the formation of a new type of a staple unit where the Austaple‑1 atom of the dimeric unit binds with the staple gold atom of the monomeric unit (Figure 17b). Even though the structures with these new bonds do not represent the structures with the highest barrier height in the D and H ligand exchange processes, in general these processes do have higher overall barrier heights than other sites. The metastable intermediate state IntH is 0.15 eV in energy with respect to reactants. The ligand exchange process has a second barrier height of 0.77 eV (0.62 eV above the intermediate state) in TS2H. Compared to the first barrier height, this is 0.14 eV lower in energy. At TS2H the bond between the core gold atom and the sulfur of the terminal −SH unit breaks, and the H2S molecule starts to dissociate. The Austaple−Sincoming bond is fully formed, and the Sterminal−Hinoming bond shortens in TS2H. The final ligand exchange products for sites G and H are also the same and experimentally indistinguishable. The final products at site H are 0.05 eV lower in energy than the initial reactants. The orientation of the −SCH3 unit in ProdH is retained with respect to the initial −SH orientation of the Au38 nanocluster. However, we noticed that the other −SH unit is rotated to the opposite direction in ProdH. The first barrier height of the ligand exchange at site H is high in energy; thus, the ligand exchange at site H is less favorable. Compared to the

Figure 17. (a) Structure of IntH. Atom labels are shown with arrows. (b) Structure of TS1H showing the newly formed bond between staple gold atoms (marked in green circles).

highest barrier height of ligand exchange at site G that is 1.09 eV, the highest barrier height of site H (0.91 eV) is 0.18 eV lower in energy. Ligand exchange at both sites G and H of monomeric units have higher energy barrier heights. However, comparing G 14958


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and the central −SH units (sites C and D) have comparatively higher barrier heights than other sites in the dimeric units. However, the difference between the highest barrier heights for sites C and D compared with A, B, E, and F is only 0.05−0.14 eV, which is much less than the 0.4 eV difference for Au25 for related sites. For the monomeric staple unit, site G has a 0.37 eV lower barrier height than site H. In fact, site G exhibits the lowest barriers of Au38 ligand exchange process. Recalling that the experimental HPLC and mass spectrometry studies reported by Beqa et al.45 suggest three out of the four symmetry unique sites are exchanged leading to different regioisomers, we predict that sites G, A/B, and E/F are the three most thermodynamically favorable sites for the ligand exchange process. It should also be noted that the formation of a new type of gold−gold bond occurs in the TS1D and TS1H transition states. This new motif is formed between a staple gold atom of the dimeric unit and the staple gold atom of the monomeric unit, which results in reducing the number of monomeric units in the cluster as well as forming a new type of Au−Au bond. Although a direct correlation between these structures and the barrier heights in the corresponding transition states is not established, it is noted that the sites for which this occurs (D and H) have higher barrier heights than the other ligand exchange sites. It should also be noted that these barriers may vary depending on the initial and incoming ligands, which should be studied in future work.

Table 8. Relevant Bond Lengths (in Å) for Ligand Exchange at Site H bond

reactants

TS1H

IntH

TS2H

products

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

2.50 2.39 N/A N/A N/A

2.42 3.75 3.29 2.33 5.03

2.47 3.98 2.39 1.71 4.36

4.07 4.34 2.37 1.38 3.98

N/A N/A 2.38 1.35 2.48

and H, site H is more favorable than site G for the ligand exchange process.

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DISCUSSION Understanding the ligand exchange process on Au38 with eight possible exchange sites is more complex than the Au 25 nanocluster. The bond breaking and formation during ligand exchange at sites A−H on Au38 indicates an associative-type reaction mechanism, which is similar to Au25. Similar to the previously studied Au2537 and Au10221 nanoclusters, we observed that the 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 the lone pairs of the sulfur atoms in the nanoparticle staples. This suggests that each of the ligand exchange processes starts with a nucleophilic attack of the sulfur of incoming thiol. All these ligand exchange processes in Au38 have barrier heights varying from 0.54 to 0.91 eV (Table 9). In

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CONCLUSIONS In summary, we performed density functional theory calculations to investigate the ligand exchange process on the Au38 nanocluster. The Au38 nanocluster has eight unique possible sites for ligand exchange that includes six sites (A−F) from dimeric staple motifs and two sites (G and H) from monomeric units. All ligand exchange processes proceed through an associative-type reaction mechanism where the incoming thiol ligand induces the breaking of existing Au−S bonds between the −SH units and respective gold atoms. The barrier heights are similar to those of the Au25 nanocluster and range from 0.54 to 0.91 eV. Our studies suggest that ligand exchange between the staple gold atoms and the sulfur atom of central −SH units will proceed at a slower rate. We observed a similar phenomenon for the Au25 nanocluster as well.37 Ligand exchange processes at A, B, E, F, and G sites are the most favorable for ligand exchange. Ligand exchange at site H on the monomeric unit has the highest barrier and is the least favorable.

Table 9. Summary of Energies (in eV) for Ligand Exchange Au38 A Au38 B Au38 C Au38 D Au38 E Au38 F Au38 G Au38 H Au25 A (ref 37) Au25 B (ref 37) Au25 C (ref 37) Au102 (ref 21)

TS1

Int

TS2

products

0.68 0.61 0.85 0.59 0.59 0.75 0.54 0.91 0.56 0.62 0.78 0.65

0.22 0.29 0.21 0.34 0.21 0.06 0.21 0.15 0.19 0.29 0.35 0.17

0.80 0.76 0.85 0.89 0.80 0.56 0.65 0.77 0.76 0.65 1.15 1.0

−0.06 −0.07 −0.05 −0.11 −0.04 −0.04 −0.05 −0.05 −0.01 0.05 −0.04 0.04

Au25 this range varies between 0.56 and 1.15 eV.37 We have investigated eight possible ligand exchange sites. Six sites A−F are located in the dimeric units of the cluster and the remaining two sites G and H are on the monomeric units of the Au38 nanocluster. In the dimer units, sites A, B, and C are related to sites F, E, and D, respectively, because of the bonds broken and formed by the incoming thiol. Even though sites A, B, C and F, E, D are different due to the environment (A, B, C sites are at the pole of the Au38 nanocluster whereas F, E, D are at the equator of the cluster), no drastic differences in maximum barrier heights were observed. However, whether TS1 or TS2 is lower in energy does depend on the site. For the dimer units, sites A, B and F, E have the lowest barriers and are more favorable for ligand exchange. If we consider only the highest barriers of all these sites, we find that site F has a 0.05 eV lower barrier height than sites A and B has a 0.04 eV lower barrier height than site E. Site C has a 0.04 eV lower barrier height than site D. Similar to the Au25 results, we noticed that the ligand exchange between the staple gold atom

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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.6b04516. XYZ coordinates of all the structures mentioned in this study (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel 1-785-532-0954 (C.M.A.). Present Address

A.F.: Department of Chemistry, Yale University, New Haven, CT 06520. Notes

The authors declare no competing financial interest. 14959


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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). The computing for this project was performed on the Beocat Research Cluster at Kansas State University, which is funded in part by NSF Grants CNS1006860, EPS-1006860, and EPS-0919443.

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