Stabilization of Thiolate-Protected Gold Clusters Against Thermal

Article pubs.acs.org/JPCC

Stabilization of Thiolate-Protected Gold Clusters Against Thermal Inversion: Diastereomeric Au38(SCH2CH2Ph)24−2x(R‑BINAS)x Stefan Knoppe,† Sophie Michalet,‡ and Thomas Bürgi*,† †

Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland Sciences Mass Spectrometry Platform, University of Geneva, 20 Boulevard d’Ivoy, 1211 Geneva 4, Switzerland

‡

S Supporting Information *

ABSTRACT: Intrinsically chiral thiolate-protected gold clusters were recently separated into their enantiomers, and their circular dichroism (CD) spectra were measured. Introduction of the chiral R1,1′-binaphthyl-2,2′-dithiol (BINAS) into the ligand layer of rac-Au38(2-PET)24 clusters (2-PET: 2phenylethylthiolate, SCH2CH2Ph) was shown to be diastereoselective. In this contribution, we isolated and characterized the diastereomeric reaction products of the first exchange step, A-Au38(2PET)22(R-BINAS)1 and C-Au38(2-PET)22(R-BINAS)1 (A/C, anticlockwise/clockwise) and the second exchange product, A-Au38(2-PET)20(R-BINAS)2. The absorption spectra show minor, but significant influence of the BINAS ligand. Overall, the spectra are less defined as compared to Au38(2-PET)24, which is ascribed to symmetry breaking. The CD spectra are similar to those of the parent Au38(2-PET)24 enantiomers, readily allowing the assignment of handedness of the ligand layer. Nevertheless, some characteristic differences are found between the diastereomers. The anisotropy factors are slightly lower after ligand exchange. The second exchange step seems to confirm the trend. Inversion experiments were performed and compared to the racemization of Au38(2-PET)24. It was found that the introduction of the BINAS ligand effectively stabilizes the cluster against inversion, which involves a rearrangement of the thiolates on the cluster surface. It therefore seems that introduction of the dithiol reduces the flexibility of the gold−sulfur interface.

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INTRODUCTION The last two decades have seen tremendous advances in both the synthesis and understanding of thiolate-protected gold clusters with up to 200 gold atoms.1−21 The clusters can be understood as ligand-protected superatoms (superatom complexes), and molecular behavior is found.2,3 A particularly fascinating observation is the intrinsic chirality that was found in the structures of Au102(p-MBA)44 (p-MBA: para-mercaptobenzoic acid) and Au38(2-PET)24.22,23 While single atoms are achiral, the extension of the periodic system into the third dimension allows for asymmetric superatoms or superatom complexes, a completely new class of materials. Ligand protection enables large-scale synthesis of the clusters by the use of classic wet chemistry.1,6,24 Particular attention has been paid to the intrinsically chiral Au38(SR)24 cluster.9,10,23,25−27 The cluster consists of a Au23 core (face-fused bi-icosahedron) and is protected by three monomeric protecting units AuSR2 and six dimeric units Au2SR3 (Scheme 1).23,25,28 The dimeric units split into two subgroups of three units each. These subgroups form a propeller-like arrangement and protect the polar sites of the prolate core. The unit cell of the crystal contains both enantiomers of the cluster.23 We recently demonstrated the separation of the enantiomers and recorded their CD spectra.26 Assignment of handedness is possible by comparison with simulated spectra.25 The pure enantiomers of the clusters can be handled at room temperature without racemization. However, the barrier for the interconversion of the enantiomers was determined to be quite low; above 40 °C, noticeable © 2013 American Chemical Society

decrease of optical activity is observed within minutes to hours.27 We conclude that the cluster is not suitable for asymmetric catalytic or chiral sensing applications, which often involve heating, release of heat, or rather harsh conditions such as high/low pH or the use of electrolytes. These conditions may induce racemization of the cluster, hence leading to loss of selectivity. Ligand exchange reactions in which the clusters Aum(SR)n are treated with an excess of incoming ligands HSR′ are a useful technique to manipulate the properties of the clusters.29−37 Most obviously, the solubility can be tuned, for example, in the phase-transfer-based synthesis of 2-PET-protected clusters from glutathionate-protected clusters.10,33,38 Introduction of chiral ligands into gold clusters gives significant responses in the CD spectra.39,40 While the cluster can be either achiral (such as Au 2 5 (SR) 1 8 4 ) or intrinsically chiral (Au 3 8 (SR) 2 4 , Au40(SR)2423,41), but racemic, ligand exchange reactions introducing chiral thiols give rise to evolving optical activity. This evolution can be studied with time32 or as a function of the chiral ligands (or both).33,34 It was found that Au25(SR)18 clusters decompose when exposed to the bidentate BINAS ligand,33,37 while Au38(SR)24 and Au40(SR)24 remain stable.33 Other reports seem to confirm a rather peculiar behavior in ligand exchange or direct synthesis of clusters involving dithiols.17,42,43 Received: April 25, 2013 Revised: June 27, 2013 Published: July 1, 2013 15354

dx.doi.org/10.1021/jp4040908 | J. Phys. Chem. C 2013, 117, 15354−15361

The Journal of Physical Chemistry C

Article

methanol. The clusters were redissolved in 10 mL of water. Ten milliliters of acetone and 15 mL of 2-phenylethylthiol were added. The system was stirred at 80 °C for 3 h. After the mixture was cooled to room temperature, water and dichloromethane were added. The aqueous phase was discarded, and the organic phase was washed with brine. After rotovapory concentration, a concentrated solution of the clusters in excess thiol remained. The clusters were precipitated with 50 mL of methanol, and the precipitate was filtered and washed with large amounts of methanol. The clusters were redissolved in dichloromethane and concentrated to dryness. Again, the clusters were washed with methanol. Overall, five washing/ redissolving cycles were applied. Eventually, the clusters (in dichloromethane) were passed over a PTFE syringe filter (0.2 μm) to remove insoluble material. For size-selection, the clusters were repeatedly passed over a size-exclusion column (1 m in length, 2.5 cm in diameter, BioRad BioBeads S-X1, swollen in tetrahydrofuran, eluent: tetrahydrofuran). The Au38(2-PET)24 fraction was monitored with UV−vis spectroscopy and finally subjected to MALDI-TOF mass spectrometry. HPLC. Separation of the enantiomers, collection of the reaction products, and control measurements were performed on a JASCO 20XX series HPLC system equipped with a Phenomenex Lux Cellulose-1 column (5 μm, 250 mm × 4.6 mm). The eluting analytes were detected with a JASCO2070plus UV−vis detector, operating at 370 (enantioseparation) and 630 nm (reaction control and product collection). The column temperature (28 °C) was controlled with a Thermasphere TS-430 column chiller/heater. The samples were injected in toluene (10 μL) and eluted with n-hexane/2propanol 80:20 at a flow rate of 2 and 1.8 mL/min (for enantioseparation and reaction product collection, respectively). For ligand exchange reactions, the pure enantiomers (ca. 3 mg) of Au38(2-PET)24 (at 90% and 95% enantiomeric excess of the two enantiomers) were dissolved in toluene (2 mL), and 5 mg of R-BINAS was added. Note that the exact amount of the clusters was not determined. The reaction was allowed to proceed at room temperature, and HPL chromatograms were recorded to control the conversion.44 At sufficient progress of the reactions, the solvent was removed in a gentle air stream. The clusters were dissolved in tetrahydrofuran and passed over a short size-exclusion column (15 cm in length, 1 cm in diameter) to remove excess ligands. The clusters were then concentrated to dryness, redissolved in toluene, and repeatedly injected to the HPLC (using the same method as for reaction control). The collected reaction products of several injections were combined and concentrated to dryness (rotary evaporation at 27 °C). Total yield of the isolated ligand exchange products was about 1 mg each (less for A-Au38(2-PET)20(RBINAS)2). UV−Vis−NIR Spectroscopy. UV−vis spectra were recorded on a Varian Cary50 or JASCO V670 spectrometer using cuvettes of 10 and 1 mm path length, respectively. UV−vis spectra were recorded in dichloromethane or toluene. Vis−NIR spectra were recorded on a JASCO V670 spectrometer using 1 mm cuvettes and chloroform as solvent. Spectra were normalized at 300 (UV−vis) and 500 (vis−NIR) nm, respectively. CD Spectroscopy. CD spectra were recorded on a JASCO J-815 spectrometer, using 5 mm cuvettes and dichloromethane as solvent. Time course measurements (kinetics) were measured in m-xylene. The temperature was controlled with a

Scheme 1. (Top) Structures of 2-Phenylethylthiol (2-PET) and R-1,1′-Binaphthyl-2,2′-dithiol (R-BINAS); (Bottom) Au38(2-PET)24 in Side View (Left) and Along Its Principal Axis (Right)a

The left-handed A-enantiomer is shown. −CH2CH2Ph groups are omitted for clarity. Colors: green, AuCore; yellow, AuAdatom; blue, sulfur. a

It was demonstrated earlier that the exchange between BINAS and racemic Au38(2-PET)24 is diastereoselective with a preference of R-BINAS for the left-handed A-enantiomer.44 In this contribution, we focus on the isolation of the diastereomeric reaction products A-Au38(2-PET)22(R-BINAS)1 and C-Au38(2-PET)22(R-BINAS)1. The clusters were prepared from the pure enantiomers of Au38(2-PET)24 and isolated with HPLC. This approach is similar to that used by Negishi and coworkers in the separation of the reaction products Au24Pd(SR)18−x(SR′)x by gradient elution45 except that in this work chiral chromatography is used. To the best of our knowledge, this is the first time that diastereomeric clusters were purposely isolated and characterized. The A/C-Au38(2-PET)24−2x(R-BINAS)x clusters were characterized with UV−vis and CD spectroscopy, showing minor, but characteristic, influence of the chiral ligand on the properties of the cluster. We also performed inversion experiments to compare the activation barrier of the inversion to the barrier of racemization of Au38(2-PET)24.27 It was found that the BINAS dithiol significantly reduces the flexibility of the ligand shell.

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EXPERIMENTAL SECTION All materials were purchased from commercial sources and used without further purification. The synthesis of R-BINAS was carried out as reported earlier.44,46 Synthesis and Size-Selection of rac-Au38(2-PET)24. One gram of HAuCl4·3H2O was dissolved in 200 mL of methanol. LGlutathione (3.1 g) was dissolved in 100 mL of water. The two solutions were combined at room temperature and stirred for 15 min at 0 °C, forming a white suspension. A freshly prepared solution of NaBH4 (1.1 g) in water (60 mL) was added, and the white slurry turned dark brown/black immediately. The reaction mixture was allowed to stir for 1 h. The precipitated clusters were collected by decanting and centrifuged in 15355



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Figure 1. Left: HPL chromatograms of the reaction between A-Au38(2-PET)24 and R-BINAS. The bottom trace (black) is the pure enantiomer prior to the reaction. The enantiomeric excess is ca. 90% (the minor signal is C-Au38(2-PET)24). After quenching of the reaction, two new signals arise at 23 and 60 min, which are ascribed to A-Au38(2-PET)22(R-BINAS)1 and A-Au38(2-PET)20(R-BINAS)2. After collection (red and green traces), the pure compounds are found in the chromatograms. Right: HPLC chromatograms for the same experiment using C-Au38(2-PET)24. The reaction was quenched (magenta), and only one new signal at 35 min is observed. The blue trace represents C-Au38(2-PET)22(R-BINAS)1 after collection. Because of the low concentration of the sample, drifts in the baseline are amplified. The curve was cut after 50 min. The full chromatogram is shown in the Supporting Information (Figure S-2).

Figure 2. Left: Vis−NIR spectrum of rac-Au38(2-PET)24 in comparison with A-Au38(2-PET)22(R-BINAS)1 (red) and C-Au38(2-PET)22(R-BINAS)1 (blue). Right: Vis−NIR spectrum of rac-Au38(2-PET)24 in comparison with A-Au38(2-PET)22(R-BINAS)1 (red) and A-Au38(2-PET)20(R-BINAS)2 (green). The basic absorption features of the Au38 cluster are maintained but less defined after substitution with BINAS. The peak at 626.6 nm is shifted toward higher and lower energies for A-Au38(2-PET)22(R-BINAS)1 and C-Au38(2-PET)22(R-BINAS)1, respectively. The transition at 1035 nm is strongly affected by BINAS substitution, leading to a red-shift in C-Au38(2-PET)22(R-BINAS)1 and a blue-shift in A-Au38(2-PET)22(RBINAS)1. Further substitution with BINAS leads to even less defined spectra.

and 60 min. These were ascribed to substitution of the cluster with one and two BINAS ligands, respectively.44 The reaction starting from the C-enantiomer of Au38 proceeds slower. While the reaction with A-Au38(2-PET)24 was quenched after 20 h, the reaction with C-Au38(2-PET)24 was allowed to proceed for 60 h before the reaction product was collected. HPL chromatograms were recorded using the above-described method (Figure 1). All samples were considered pure because only the expected peaks were observed (detection at 630 nm). Optical Properties. UV−vis and NIR spectra of the isolated species show the characteristic absorption features of the Au38(SR)24 cluster. However, in comparison with the parent Au38(2-PET)24 cluster, some deviations become apparent (Figure 2 and Figure S-3). The peak maxima are shifted to shorter wavelengths in A-Au38(2-PET)22(R-BINAS)1 as compared to rac-Au38(2-PET)24, while in C-Au38(2-PET)22(RBINAS)1, the bands are shifted to longer wavelengths (see Table 1). The spectra suggest that C-Au38(2-PET)22(RBINAS)1 has a slightly smaller optical gap than A-Au38(2PET)22(R-BINAS)1 and that the energies of the frontier orbitals are somewhat closer (also, as compared to Au38(2-PET)24). The spectra clearly show the influence of the ligands on the optical properties of the clusters. An entirely electronic effect due to the introduction of an electron-withdrawing (aromatic)

JASCO Peltier element. Time course measurements above 90 °C were performed on a JASCO J-710 spectrometer equipped with a Specac temperature controller. MALDI-TOF-MS. Mass spectra were recorded on a Shimadzu Biotech Axima using DCTB ([3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile) as matrix. The spectra were recorded in positive linear mode. No internal calibration was applied.

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RESULTS AND DISCUSSION Racemic Au38(2-PET)24 clusters were synthesized and sizeselected as reported earlier.13 Chiral HPLC showed the existence of two peaks of the same area, in full agreement with the previously reported method.26,27,44 The two enantiomers were collected by repeated HPLC runs (given the poor solubility of the clusters in the mobile phase, only a very small amount can be injected each time), and the respective fractions were combined. HPLC control of the enantiomers shows enantiomeric excesses of 90% and 95% ee for A- and C-Au38(2-PET)24, respectively (Figure S-1). The pure enantiomers of Au38(2-PET)24 were allowed to react with R-BINAS until the areas of the arising new signals (in situ-HPLC, see below) were judged large enough. In the case of A-Au38(2-PET)24, two new peaks were observed centering at 23 15356



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show strong similarity to their parent enantiomers. The sign of the transitions is preserved; the peak maxima are slightly shifted. First, the CD spectra of A-Au38(2-PET)24, A-Au38(2PET)22(R-BINAS)1, and A-Au38(2-PET)20(R-BINAS)2 are discussed (Figure 3, top). While there are significant differences between the spectra at high energies (below ca. 330 nm), the spectra are very similar in the visible spectral range. Two bands between 500 and 600 nm are more pronounced after BINAS substitution. In the UV, the BINAS ligand is optically active and overlays with the CD spectrum of the Au38 cluster. Anisotropy factors of A-Au 38 (2-PET) 22 (R-BINAS) 1 and A-Au 38 (2PET)20(R-BINAS)2 are consistently smaller than those of AAu 38 (2-PET) 24 (ca. 50−80% of the intensity of the corresponding bands in Au38(2-PET)24). The shape of the CD spectra and anisotropy factors of CAu38(2-PET)22(R-BINAS)1 are similar to those of C-Au38(2PET)24 (Figure 3, bottom left). Again, differences are seen in comparison. The anisotropy factors are slightly smaller, most dominantly in the visible. The sum of anisotropy factors of AAu38(2-PET)22(R-BINAS)1 and C-Au38(2-PET)22(R-BINAS)1 is nonzero for most of the spectral range (Figure 3, bottom right and Figure S-4). If the samples were enantiomers, the anisotropy factors should cancel to zero over the full spectral range (as in Au38(2-PET)24). Deviations from zero highlight the differences between the spectra. The maxima of the most pronounced peaks are slightly red-shifted in C-Au38(2PET)22(R-BINAS)1, in agreement with the UV−vis−NIR spectra.

Table 1. Maxima of Absorption Peaks for the Bands at 630 and 1035 nm for Au38(2-PET)24 and the BINAS-Substituted Diastereomers rac-Au38(2-PET)24 A-Au38(2-PET)22(R-BINAS)1 C-Au38(2-PET)22(R-BINAS)1

630 nm

1035 nm

626.6 625.0 636.2

1035 997 1040

ligand is not thought to occur, because the absorption peaks shift into different directions in the diastereomers. Instead, we assume the differences in the spectra to be based on steric or stereoelectronic effects, at least in part. Also, the spectra are less defined after exchange with BINAS. We ascribe this to symmetry reasons: While the Au38(2-PET)24 cluster has an idealized 3-fold symmetry (along its principal axis), this is broken when one BINAS ligand is introduced into the ligand shell. The actual position of the substitution is unknown, but all of the possible binding sites (inter- and intrastaple binding) would lead to the same point group, C1. Furthermore, the BINAS ligand may affect the surrounding units and induce different absolute configurations at neighboring sulfur atoms. The symmetry lowering could lead to splitting of degenerate energy levels, consequently giving broader absorption features in the experimental spectra. A computational study could shed more light on such effects. Optical Activity. Circular dichroism spectra of the three samples were recorded and compared to those of Au38(2PET)24. All samples show strong Cotton effects with anisotropy factors in the range of 10−3. At a first glance, the CD spectra

Figure 3. Top: CD spectra (left) and anisotropy factors (right) of A-Au38(2-PET)24 (abbreviated as (A-38, 24, 0), black), A-Au38(2-PET)22(RBINAS)1 ((A-38, 22, R-1), red), and A-Au38(2-PET)20(R-BINAS)2 ((A-38, 20, R-2), green). For comparison, the CD spectrum of R-BINAS (magenta) is shown in the top plot. The spectra are offset for clarity. Bottom, left: Anisotropy factors of A-Au38(2-PET)24 (black), A-Au38(2PET)22(R-BINAS)1 (red), C-Au38(2-PET)22(R-BINAS)1 ((C-38, 22, R-1), blue), and C-Au38(2-PET)24 ((C-38, 24, 0), magenta). While the top and the bottom traces are of enantiomers, the middle traces are of diastereomers. Small deviations can be seen. Bottom, right: Anisotropy factors of AAu38(2-PET)22(R-BINAS)1 (red) and C-Au38(2-PET)22(R-BINAS)1 (blue) and their sum (orange). Deviations from zero of the sum of the traces highlight the differences in the spectra of the diastereomers, because a sum of zero over the total spectral range is expected for enantiomers only. 15357



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Figure 4. Top: Normalized CD responses at 360 nm of A-Au38(2-PET)22(R-BINAS)1 during heating between 70 and 130 °C (left). The data for heating at 90 °C are not shown due to inconsistencies stemming from the spectrometer. Normalized CD responses at 360 nm of C-Au38(2PET)22(R-BINAS)1 during heating between 100 and 130 °C (right). Bottom, left: CD response of Au38(2-PET)24 (at 346 nm, black, data from ref 27) and A-Au38(2-PET)22(R-BINAS)1 (at 360 nm, red) during heating at 80 °C. While a clear change is observed for Au38(2-PET)24, exchange with BINAS stabilizes the clusters, hence leading to less change in optical activity. Bottom, right: Eyring plot showing the temperature dependency of the rate constants of the inversion for Au38(2-PET)24 (black, data from ref 27), A-Au38(2-PET)22(R-BINAS)1 (red), and C-Au38(2-PET)22(R-BINAS)1 (blue).

because initial experiments in toluene (boiling point 111 °C) did not show significant change of optical activity at 90 °C. During time course experiments, the CD response at 360 nm was monitored. (At 360 nm, the two diastereomers have opposite sign in their CD spectra but the same anisotropy factor; see Figure S-4. The optical activity at this wavelength should therefore correspond to the diastereomeric excess of the sample.) As compared to Au38(2-PET)24, the decrease of optical activity at 80 °C is much slower, giving rise to the assumption that the inversion of the cluster is effectively hindered (Figure 4, bottom left). A similar experiment was conducted with C-Au38(2-PET)22(R-BINAS)1 at 100, 115, and 130 °C (Figure 4, top right). Only three temperatures were monitored due to lack of compound. HPL chromatograms were recorded for both diastereomers after heating to 130 °C. Because the concentration of the samples was very low, the detection wavelength was set to 370 nm, where the clusters have a higher absorption coefficient than at 630 nm. While the pure diastereomers yield chromatograms showing only one peak at the designated retention time (at 23 and 35 min, respectively), the signal for the other diastereomer is found after thermal treatment (Figure S-5). No other compounds were detected over 85 min of elution time. This indicates that (a) the inversion of the cluster into its diastereomer is successful and (b) no decomposition occurs (at least it cannot be detected under the applied conditions). Quantitative analysis of the chromatograms is avoided because the absorption coefficients of the diastereomers are unknown. UV−vis spectra of the samples were recorded after heating and compared to the pure diastereomers. All UV−vis spectra resemble the Au38

Overall, the CD spectra of the diastereomers (and of AAu38(2-PET)20(R-BINAS)2) show that the influence of the BINAS ligand on the optical activity is subtle. Assignment of handedness of the protecting units on the cluster surface is readily possible by comparison with the spectra of Au38(2PET)24. Nevertheless, characteristic differences are found: most notably, the spectra are less defined than those of the enantiomers. The anisotropy factors are slightly weaker (over most of the spectral range) after BINAS substitution. Stability Against Thermal Inversion. Earlier work demonstrated the flexibility of the gold−thiolate interface when pure enantiomers of Au38(2-PET)24 were heated in the temperature range of 40−80 °C.27 Racemization of the cluster was established by HPLC control experiments prior to and after heating. An activation barrier of 28.1 kcal/mol (with weighted linear regression, see below) was determined using the timedependent CD responses of the cluster. The value is astonishingly low as compared to the typical Au−S binding energies of ca. 50 kcal/mol.47−51 Two possible mechanisms for the inversion of the ligand layer on the cluster surface were proposed. Both are assumed to have the achiral A,C-Au38(SR)24 structure28 as key intermediate. In this structure, the two triblade fans of dimeric units have opposite handedness. The introduction of the rather bulky and rigid binaphthyl system is expected to hinder this rearrangement, because BINAS binds to the cluster via two binding sites. A stock solution of A-Au38(2-PET)22(R-BINAS)1 was prepared and split into aliquots, which were treated at temperatures in the range of 70−130 °C for 20 min each (Figure 4, top left). We used m-xylene (boiling point 139 °C) 15358



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inversion is unknown, we can only speculate on reasons for this behavior. One possible explanation is that the inversion follows a pathway different from that in Au38(2-PET)24. Mass Spectra. MALDI-TOF mass spectra (DCTB matrix) were recorded prior to enantioseparation of Au38(2-PET)24 and after thermal treatment of A-Au38(2-PET)22(R-BINAS)1 and CAu38(2-PET)22(R-BINAS)1 (Figure 5). The mass spectrum of rac-Au38(2-PET)24 shows the expected signal at m/z 10 772 (calculated: m/z = 10 778.08). For A-Au38(2-PET)22(RBINAS)1, a sharp signal at m/z 10 820 is observed, which is in good agreement with the calculated mass of m/z = 10 820.08. The mass spectrum of A-Au38(2-PET)20(R-BINAS)2 shows a single peak at m/z 10 862 (calculated: m/z = 10 862.07). The spectra confirm our assignment based on HPLC. An interesting finding is made for C-Au38(2-PET)22(RBINAS)1: the MALDI-TOF spectra show three peaks, which all correlate with the calculated masses of BINAS-substituted Au38 clusters. The dominant signal at m/z 10 827 is the expected one (x = 1, calculated: m/z = 10 820.08). The signal at m/z 10 871 corresponds to x = 2, and the signal at m/z 10 913 agrees well with Au38(2-PET)18(BINAS)3 (calculated: m/z = 10 904.07). It should be noted that the samples were subjected to MALDI-TOF after heating to 130 °C. While these signals (for x = 2, 3) are not observed in A-Au38(2-PET)22(RBINAS)1, heating may induce conversion of the C-Au38(2PET)22(R-BINAS)1 cluster into species with more BINAS ligands. The C-Au38(2-PET)22(R-BINAS)1 diastereomer might be less stable than A-Au38(2-PET)22(R-BINAS)1 and undergo this transition faster. Higher energy of the ground state is also indicated by the consistent red-shifts in the absorption spectra. Nevertheless, we assume that collection of the pure diastereomer is in principle possible, but heating of the sample over 100 °C should be avoided. The reaction was found to proceed slowly enough that it seems highly unlikely that the products of higher exchange were collected together with the x = 1 product. Furthermore, one would expect different retention times for the different substitution patterns, as in the lefthanded clusters.

cluster (Figure S-6). A closer look to the 630 nm transition reveals a consistent shift of the absorption maximum as the ratio of the diastereomers changes (from 625 to 636 nm starting from A-Au38(2-PET)22(R-BINAS)1 and vice versa). The data displayed in Figure 4 (top) can be used to determine the activation barrier of the inversion reaction, using Eyring analysis (Figure 4, bottom right; see Supporting Information; Eyring and Arrhenius analyses give virtually the same values, Tables S-1 and S-2). Much to our surprise, the activation energy for the inversion of A-Au38(2-PET)22(RBINAS)1 into its diastereomer was determined as 25.2 kcal/mol (for the inversion of C-Au38(2-PET)22(R-BINAS)1, a barrier of 23.0 kcal/mol was found). This value is lower than the activation energy for the racemization of the enantiomers of Au38(2-PET)24, which is 29.5 kcal/mol (the experimental data were again analyzed without weighted linear regression).27 It seems that the ligand exchange with one dithiol (BINAS) leads to slightly lower activation energies. However, we observe experimentally that higher temperatures are needed to induce a significant change of the CD response in the BINAS-substituted clusters (Figure 4, bottom left). The significant vertical offset of the linear fits in the Eyring plot indicates that the different reaction rates are due to different pre-exponential factors of the Arrhenius-equation and therefore due to entropic reasons. Moreover, a change in the sign of the activation entropy is observed (Table 2). This supports the assumption that a Table 2. Activation Parameters for the Thermal Inversion of Au38(2-PET)24, A-Au38(2-PET)22(R-BINAS)1, and C-Au38(2PET)22(R-BINAS)1a

(38, 24, 0) (A-38, 22, R-1) (C-38, 22, R-1)

ΔH⧧ (kcal mol−1)

ΔS⧧ (cal K−1 mol−1)

ΔG⧧ (kcal mol−1)

Ea (kcal mol−1)

28.8 24.4 22.3

9.7 −10.1 −17.6

25.6 28.2 29.1

29.5 25.2 23.0

a

The values were obtained from Eyring analysis of the time course experiments assuming first-order kinetics.

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different pathway is followed. While the Au38(2-PET)24 cluster has a positive entropy of activation (9.7 cal K−1 mol−1), the activation entropy is negative for the BINAS-substituted diastereomers (−10.1 and −17.6 cal K−1 mol−1 for A- and CAu38(2-PET)22(R-BINAS)1, respectively). Negative values indicate associative mechanisms.52 Because the mechanism of

CONCLUSIONS

In summary, we present the first examples of chiral thiolateprotected gold clusters that are protected with a defined ligand shell with respect to its composition (one or two BINAS ligands) and handedness (R-BINAS on A- and C-Au38 clusters).

Figure 5. Left: MALDI-TOF mass spectra of Au38(2-PET)24 prior to ligand exchange (black) and of A-Au38(2-PET)22(R-BINAS)1 (red), C-Au38(2PET)22(R-BINAS)1 (blue), and A-Au38(2-PET)20(R-BINAS)2 (green). Note that the mass spectra of A-Au38(2-PET)22(R-BINAS)1 and C-Au38(2PET)22(R-BINAS)1 were recorded after heating to 130 °C. This may have changed the composition of C-Au38(2-PET)22(R-BINAS)1, leading to formation of species with more than one BINAS ligand. Right: Zoom into the peaks. Fragments are marked with asterisks. 15359



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These diastereomers can be separated with HPLC. The CD spectra readily allow the assignment of handedness in the ligand layer (by comparison with the CD spectra of the enantiomers) and reveal distinct, but overall minor influence of the chiral BINAS ligand on the CD and ordinary absorption spectra. Thermal inversion experiments clearly show that the BINAS ligand hinders the inversion of the cluster, which takes place at higher temperatures as compared to the parent cluster even though the activation enthalpy of the BINAS-functionalized clusters is slightly lower. The significantly lower inversion rates for the exchanged cluster are due to different activation entropies, which may indicate different pathways. The ability to make the Au−thiolate interface more rigid by introducing a dithiol could be important for applications of such particles, for example, in asymmetric catalysis. Future studies should be devoted to the influence of monothiol ligands on the cluster properties, the mechanism of the inversion, and the fate of the diastereomers during extended heating.

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ASSOCIATED CONTENT

S Supporting Information *

HPL chromatograms of the separation of enantiomers of Au38(2-PET)24, and additional CD and UV−vis spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the University of Geneva and the Swiss National Science foundation is acknowledged. We thank Raymond Azoulay (UniGe) for the synthesis of R-BINAS, Julien Meyer and Harry Théraulaz (both SMS, UniGe) for MS measurements, and Stefan Matile and Jerome Lacour (both UniGe) for providing their CD spectrometers.

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REFERENCES

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Stabilization of Thiolate-Protected Gold Clusters Against Thermal

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