Ligand-protected gold thiolate nanoclusters have shown several unique physical and chemical properties which can be applied to diverse areas such as photocatalysis, 1 biomedicine, 2 and chemical or biomedical sensors. 3 Most gold-thiolate nanoclusters are found to be "atomically precise nanoclusters", 4 such as Au20(SR)16, 5,6 Au25(SR)18 -, [7][8][9] Au38(SR)24, 10,11 and Au104(SR)46. 12 The general structure of a gold-thiolate nanocluster consists of a gold kernel and gold-thiolate protecting motifs (of which the basic unit is SR-Au-SR). 13 The R group on the thiolate (SR) ligand can be SPh, SCH2CH2Ph (aka PET, phenylethanethiolate), p-MBA (para-mercaptobenzoic acid), etc.
The Brust-Schiffrin (B-S) method was one of the earliest methods to synthesize goldthiolate nanoclusters. The B-S method starts with a Au(III) complex (such as HAuCl4) and follows with a two solvent phase reduction to synthesize gold-thiolate nanoclusters. 14 Several modifications to the Brust-Schiffrin methods (i.e. changing the solvent, changing from two-phase to one-phase reactions) were then studied to refine the reaction process. 15,16 Recently, a mixture of diglyme (dg) and tetrahydrofuran (THF) solvents was applied to the gold-thiolate nanocluster selfassembly process, and diglyme can be observed as a ligand in the final generated product. [17][18][19][20] Diglyme was found to yield a gold-thiolate nanocluster dimer as well, which indicates that diglyme can mediate the gold nanoparticle assembly process. 18 Moreover, a Au20(SR)15-diglyme system was also found experimentally in which diglyme mediates dimerization of two nanoclusters to yield a photoluminescent cluster with a very high quantum yield. 21 Therefore, it is very important to investigate the interaction between diglyme and gold clusters which will help unravel the photoluminescent mechanism in these systems in the future.
With the growth of experimental research on ligand-protected gold nanoparticle synthesis, [14][15][16][22][23][24][25] the mechanisms involved in this process have also been investigated using quantum chemistry calculations. [26][27][28][29][30] A plausible reaction pathway from Au(III) chloride to Au(I) thiolate via the addition of thiols was first proposed by Barngrover and Aikens using the density functional theory (DFT) method in 2011, and the involved reaction pathways were demonstrated to be thermodynamically favorable. 26 Later, solvent effects and R group effects on the Au(III) to Au(I) reaction were examined theoretically. 27 The DFT method was also applied to study the Au nanoparticle growth pathway starting from gold halide complexes. 28 However, the previous discussions of the reaction mechanism were mainly focused on the original Brust-Schiffrin method. For the modified Brust-Schiffrin method that has diglyme as a solvent, the question arises: how does diglyme play a role in the gold nanocluster synthesis process?
In this work, we computationally model reasonable reaction pathways to generate Au(I)containing species from the starting Au(III) complex. 1 H NMR spectra are calculated to examine the proton environment of the gold-diglyme system. The interactions between diglyme and gold-containing species are discussed using chemical bonding theory. In this way, a plausible mechanism for the diglyme-assisted Brust-Schiffrin synthesis can be proposed.
All calculations were performed with the Amsterdam Modeling Suite (AMS) 2021.102 software. 31 Geometry optimizations were performed at the BP86/TZP level of theory with a frozen core approximation (1s for C, 1s for O, 2p for S, 2p for Cl, 4f for Au), where BP86 32,33 is a generalized gradient approximation (GGA) exchange-correlation functional and TZP stands for the triple-zeta polarized basis set. Scalar relativistic effects were included by using the Zero Order Regular Approximation (ZORA). 34,35 Grimme3 parameters were added to correct for the dispersion interaction. 36,37 Solvent effects were taken into consideration using the Conductor-like Screening Model (COSMO). [38][39][40] All reaction pathways were modeled in diethyl ether solvent, which is similar in dielectric constant and functionality to the diglyme and tetrahydrofuran (THF) solvents used experimentally.
Because 1 H NMR spectra are sensitive to the atomic environment, NMR is a powerful experimental tool to characterize geometric structure. In our current work, 1 H NMR spectra for three featured molecular systems were calculated with respect to TMS (tetramethylsilane), because it is the standard reference for NMR spectra, both experimentally and theoretically. To examine the chemical shifts, single-point energy calculations were performed without the frozen core approximation in the basis set. Then, 1 H NMR calculations were performed on all protons in the diglyme molecule. Finally, ETS-NOCV theory [41][42][43][44] (see details in SI) was applied to analyze the bonding between the gold moiety and the diglyme molecule and to compare the differences between the gold-thiolate system and the gold-chloride system.
Previous theoretical work has suggested two different pathways that lead to the reduction of Au(III) to Au(I). 26 Our proposed reaction mechanisms are initially based on this previous work; however, our current proposed reaction pathway includes the diglyme molecule throughout the entire reaction pathway. The reaction mechanism consists of two main parts: 1. Au(III)-Au(III) linking in the presence of diglyme; 2. thiol addition reaction (including reduction of gold atoms).
For the proposed reaction mechanism, the first step is the binding between the goldchloride complex and diglyme (dg). The reaction starts with a Au(III) complex (here we use HAuCl4), which binds to the diglyme molecule, and the proton and one chloride from HAuCl4 generate a HCl molecule and a AuCl3 species that possesses a dative bond with diglyme (Reaction 1A, Figure 1A). Then, a second HAuCl4 is added, which binds with the (AuCl3)dg product to generate (AuCl3)2dg (Figure 1B) and release another HCl as well (Reaction 2A). The optimized structure of (AuCl3)2dg (Figure 1B) shows that two AuCl3 moieties are bound to the diglyme, with two of the chlorides bridging the two gold atoms. The Au2Cl6 moiety is known to be the preferred structure for the AuCl3 dimer. 45 The generated (AuCl3)2dg species can subsequently react with HAuCl4 and generate a HCl and a (AuCl3)3dg species (Figure 1C, Reaction 3A). The reaction energies for 1A, 2A, and 3A are -0.90 eV, -0.46 eV, and -0.80 eV, respectively. These reaction energies indicate that the Au-Au linking reactions are thermodynamically favorable. Previous work has shown that transition states for similar reactions have barriers near 0.3 eV, 26 so we expect these reactions to have reasonable kinetics at room temperature, which is a common temperature for nanoparticle synthesis. 22,24 Due to the increased size of the present systems and the additional degrees of freedom with the addition of diglyme, transition state searches are not practical; however, the Au(III) and Au(I) species considered in this work are very close to those in Ref. 26.
Due to the flexibility of the diglyme and PET ligands, it should be noted that the systems present experimentally may vary dynamically from the lowest energy isomers found in this work, but these calculations demonstrate that the reactions are thermodynamically feasible.
For the thiol addition reactions, the (AuCl3)2dg or (AuCl3)3dg product can subsequently react with thiols to form a mixed Au(III)-Au(I) species. In this work, we focus on the species with two gold atoms as the simplest multi-atom species. 2-Phenylethanethiol (HPET, HSCH2CH2Ph) is used as the reactive thiol because it is the ligand used experimentally 18,19,21 in the formation of the known diglyme-containing nanoclusters. The added HPET reacts between the two Au atoms to bridge the two Au(III) species, and the proton on HPET reacts with one chloride to generate a (AuCl2)PET(AuCl3)dg (Figure 2A) species and HCl (Reaction 1B), where PET is 2phenylethanethiolate. The reaction energy for the first thiol addition is -0.97 eV, which is thermodynamically favorable. Then, the (AuCl2)PET(AuCl3)dg species reacts with another thiol; the proton on the thiol reacts with a chloride to generate another HCl and product (AuCl2PET)2dg
(Figure 2B) with a reaction energy of -0.60 eV (Reaction 2B). Comparing between (AuCl3)2dg and (AuCl2PET)2dg, we found that the gold chloride moieties tend to form a dimer and weakly bind to the diglyme molecule, whereas the mixed gold chloride-thiolate moieties tend to bind to the diglyme through coordinate covalent/dative bonding. The generated product (AuCl2PET)2dg can continue to react with HPET to yield a disulfide (PET)2 and the product (AuCl2)PET(AuCl)dg (Figure 2C) with a reaction energy of -0.79 eV (Reaction 3B). The resulting product, (AuCl2)PET(AuCl)dg, is a mixed Au(III)-Au(I) species, in which one gold is in the +3 charge state and the other gold is in the +1 charge state. Formation of a disulfide is common in thiol-based reactions that reduce the oxidation state of gold. 15,26 Reaction Scheme 2. Thiol addition reactions in the presence of diglyme
To compare the differences between the reaction energy with diglyme participants and without diglyme participants, the thiol addition reactions were modeled without the presence of diglyme molecule as well (Reactions 1B', 2B', and 3B'). The products for these reactions are shown in Figure 3A-C. Compared between the (AuCl2PET)2dg (Figure 2B) and (AuCl2PET)2 (Figure 3B), we found the two AuCl2PET moieties will generate a dimer without the presence of the diglyme. The reaction energy results show that the reaction steps will be somewhat less energetically favorable when no diglyme is present in the molecular system (Table 1). These results indicate that diglyme can increase the exothermicity of the reaction for the thiol addition process. the HPET generate a HCl, and then the PET binds to the gold to form (AuClPET)2dg (Figure 4A).
This reaction energy is -0.01 eV (Reaction 1C). Then, (AuClPET)2dg can subsequently react with an additional HPET, and that HPET substitutes for a second chloride on the Au(III) atom to yield (AuClPET)Au(PET)2dg (Figure 4B) with a reaction energy of -0.62 eV (Reaction 2C).
(AuClPET)Au(PET)2dg then reacts with a third HPET and generates a PET2 molecule and (AuCl)PET(AuHPET)dg (Figure 4C, Reaction 3C). The reaction energy for this last step is -0.76 eV. The final product is a species that includes both Au(I)-thiolate and Au(I)-chloride motifs. Of note, the SR-Au-SR-Au-Cl moiety forms a V-shaped motif similar to the V-shaped staple motifs commonly found on the surface of gold nanoclusters. 4,46 Reaction Scheme 5. Pathway 2 (AuCl2)PET(AuCl)dg + HPET → (AuCl)2dg + PET2 + HCl (1D) Pathway 2. Another pathway is that the Au(III)-Au(I)-mixed motif subsequently reacts with HPET to generate a disulfide (PET2), HCl, and (AuCl)2dg (Figure 4D, Reaction 1D). Through this pathway, the mixed Au(III)-Au(I) motif can be transformed into a species with two Au(I)chloride motifs. However, the calculated reaction energy is 1.13 eV, which indicates that the endothermicity for the reaction is relatively high. Disulfides (and the corresponding gold reduction) are unlikely to be formed in this manner.
The reaction energies for the mechanisms discussed in this work are summarized in Table 2. (Reaction free energies with zero-point energy and entropic corrections are shown in the SI.)
Comparing these two distinct pathways, pathway 1 generates both Au(I)-thiolate and Au(I)chloride motifs, while pathway 2 generates a gold-chloride motif. Pathway 1 exhibits negative reaction energies for each step and exhibits an overall exothermic process, which indicates that it is a more thermodynamically favorable reaction pathway than pathway 2. Compared with previous calculations for systems without diglyme, 26 the synthetic pathway in the presence of the diglyme molecule shows more negative reaction energies for each step, which consequently indicates that the Au(III)-chloride motif can be reduced to a Au(I) motif with the assistance of diglyme in an energetically favorable pathway. We note that in the experiment, diglyme is present in great excess as a solvent; Le Chatelier's principle will likely aid in the formation of the diglyme-bound AuCl3 species. In this work, we only considered the binding of a single diglyme to the gold systems, in part because only 1 or at most 2 diglyme species have ligated experimental nanoclusters, [17][18][19][20][21] although future work could consider the binding of additional diglyme molecules.
1 H NMR calculations of the protons on the diglyme ligand were performed on three different species to examine how the protons of this ligand are affected by environment. Similar calculations on the protons of the HPET moieties are discussed in the SI (Tables S1-S2, Figures S1-S2). The 1 H NMR spectra of the pure diglyme molecule was investigated previously and the theoretical chemical shifts lie in the range from 3.26 ppm to 3.98 ppm. 19 In (AuCl3)dg (Figure 5A), AuCl3 is a gold-chloride moiety with a +3 charge state for gold. The calculated 1 H NMR spectrum has chemical shifts that range from 3.35 ppm to 4.95 ppm (Table 3). For (AuCl2PET)2dg (Figure 5B), two AuCl2PET motifs bind to two oxygen atoms of the diglyme molecule, and the two gold atoms are both in the +3 charge state. The 1 H NMR spectra have chemical shifts that range from 3.44 ppm to 6.32 ppm (Table 3). Atom 15 is located near two chlorides, and it has the highest chemical shift of 6.32 ppm. Atom 10 (δ = 3.45 ppm) and atom 21 (δ = 3.44 ppm) have the two lowest chemical shifts; these atoms are oriented away from the AuCl2PET moiety.
For the (AuCl)PET(AuHPET)dg system (Figure 5C), the two gold atoms are in the +1 charge state. The 1 H NMR spectra have chemical shifts ranging from 2.97 ppm to 5.35 ppm (Table 3). Atom 20, which is oriented towards the gold atom, has the highest chemical shift. Atom 22 has a very similar orientation to atom 20 and its calculated chemical shift is 4.97 ppm, which is the second-highest chemical shift. Atom 11 (δ = 3.13 ppm) and atom 14 (δ = 2.97 ppm) move upfield;
both atoms are pointed towards the PET group.
Overall, by analyzing chemical shifts, the proton environment is observed to be strongly affected by the environment around diglyme. Protons that are oriented near gold and chloride will shift downfield, whereas protons that are pointed towards the PET group will move upfield. It was found experimentally in the diglyme-protected Au20(PET)15dg2 system that some of the protons on diglyme move downfield significantly, and theoretical results suggest that these protons are located near the gold atoms in the nanocluster. 19 For (AuCl2PET)2dg, the binding energy between the two AuCl2PET moieties and diglyme is -209.1 kJ/mol (Table 4), which is higher compared with AuCl3dg. Two dominant orbital interactions lead to the charge transfer, and the orbital interaction energies for these interactions are -77.9 kJ/mol and -77.3 kJ/mol, respectively (Figure 7). The first interaction is between the exterior oxygen and the exterior gold moiety. The second interaction is between the center oxygen and the center gold moiety. As a result, a total of 0.39 e was transferred from diglyme to the gold (Table 4). For (AuCl)PET(AuHPET)dg, the binding energy between the two fragments (diglyme and (AuCl)PET(AuHPET)) is -106.5 kJ/mol (Table 4), and only 0.06 e was transferred from diglyme to gold (Table 4). The dominant orbital interaction energy is -9.4 kJ/mol, which is relatively weak compared with the other two molecular systems (Figure 8). In this system, both Au atoms are in the +1 oxidation state and have two chloride or thiolate ligands, which causes the Au atoms to be less likely to accept electrons from the diglyme ligand. Since the diglyme has been found to have weak bonding with a gold nanocluster in experimental work, 18,19 the interaction between diglyme and Au20(SCH3)15 + was examined in the research as well. We started with the proposed Au20(SCH3)15dg + computational model 19 and reoptimized the structure in diethyl ether solvent. The ETS-NOCV analysis shows a total of 0.24 electron (e) was transferred from diglyme to the gold nanocluster (Table 4). The dominant orbital interaction energy is -59.1 kJ/mol (Figure 9), and that orbital interaction energy is relatively weak compared with the interaction between the Au(III) species and diglyme. For (AuCl3)dg and (AuCl2PET)2dg, the gold atoms are all in the +3 charge state, while in (AuCl)PET(AuHPET)dg and gold motifs in Au20(SCH3)15dg + , the gold atoms are in the +1 charge state. Both the binding energy results and the charge transfer analysis indicate that Au(III) species have relatively strong orbital interaction with diglyme compared with Au(I), while Au(I) has a more intense dispersion interaction with diglyme (Table 5).
Energy decomposition results (Table 5) for the gold motifs show that (AuCl2PET)2dg has the highest total interaction energy while (AuCl)PET(AuHPET)dg has the lowest total interaction energy. (AuCl2PET)2dg has the highest orbital interaction energy, which accounts for the highest charge transfer value. (AuCl)PET(AuHPET)dg system has the highest dispersion correction interaction energy, which is related to the dispersion-based bonding between two fragments.
For the Au20(SCH3)15dg + system, the dispersion interaction energy (-182.6 kJ/mol) is higher than the orbital interaction energy (-150.8 kJ/mol). The total interaction energy is -178.8 kJ/mol, which is lower than the interaction energy (-209.5 kJ/mol) between diglyme and two AuCl2PET moieties. Since the gold atoms on the ligand-protected motifs are all in +1 charge states, the energy decomposition results demonstrate that the bonding between diglyme and Au(I) is weaker than the bonding between diglyme and Au(III).
A series of experimental studies have previously demonstrated that diglyme can mediate the synthesis of gold nanoclusters and assist with the self-assembly of gold nanoparticles. This work represents the first investigation of the intrinsic mechanism of diglyme-assisted synthesis. In the current research, by exploring plausible reaction pathways and calculating the reaction energy for each step, we propose a reasonable reaction mechanism for synthesizing gold-thiolate motifs in the presence of diglyme. The negative reaction energy in each step theoretically demonstrates that diglyme can assist the synthesis process of the gold nanocluster in an energetically favorable pathway.
By analyzing the NMR spectrum, we found that the diglyme ligand interaction on the gold motif can be characterized by examining the chemical shifts. Protons that are pointed towards chloride have chemical shifts that are shifted downfield, and protons that are pointed to the PET group will have low chemical shifts that may even lie upfield from the bare diglyme molecule. The NMR results in this work can be a good reference for the experimental research.
Finally, we applied the ETS-NOCV method to examine the chemical bonding of the featured system. We found that the Au(III)-chloride system can bind strongly with diglyme through dative bonding, while the Au(I)-thiolate moiety tends to form dispersion-based bonding with diglyme. The highest dispersion interaction energy was found in the Au20(SCH3)15 dg + system, and that demonstrates the weak bonding between diglyme and nanocluster. Comparing between the three featured systems, we can determine that the interaction between diglyme and gold motif becomes weaker when the oxidation state of gold is reduced from Au(III) to Au(I).