Amplification of Enantioselectivity on Solid Surfaces Using Nonchiral

Jun 4, 2015 - Changes in the Enantiomeric Composition of Chiral Mixtures Upon Adsorption on a Non-Chiral Surface. Stavros Karakalos , Junghyun Hong , ...
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Amplification of Enantioselectivity on Solid Surfaces Using Nonchiral Adsorbates Stavros Karakalos‡ and Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: The amplification of enantioselectivity during the adsorption of chiral molecules on solid surfaces by using nonchiral agents was demonstrated for the case of Pt(111) seeded with a small amount of enantiopure propylene oxide and then dosed with propylene as the amplifier. Chiral chemical titration and isothermal kinetic adsorption experiments using collimated effusive molecular beams indicated the possibility of reaching enantioselectivity excesses of over 60% this way. Monte Carlo simulations provided a kinetic explanation for this effect in terms of both an adsorbate-assisted adsorption process and a bias in the chiral configuration the propylene molecules are driven to upon adsorption on the surface.

1. INTRODUCTION Chirality, the manifestation of molecular asymmetry as two nonsuperimposable mirror-image versions of a given compound, R and S (typically containing carbon atoms with four different substituents), is quite prevalent in nature. For reasons still not fully understood, life in nature developed to depend on biochemistry involving only one of those two so-called enantiomers.1,2 Because the chemical interactions between pairs of chiral molecules depend on their relative chirality (the same versus the opposite handedness, between the R and S configurations), it is imperative that pharmaceuticals and other compounds used to affect living organisms are prepared in enantiopure form; otherwise, the wrong enantiomer may lead to unintended, and often undesirable, chemistry.3 This has been an ongoing challenge for the chemical industry in general and for the heterogeneous catalysis community in particular. In a few cases, crystallization experiments have shown a spontaneous symmetry breaking leading to the exclusive precipitation of only one crystal enantiomorph.4,5 Without chiral bias the probability of precipitation is split equally between both enantiomorphs, but if the process is seeded with a small amount of a chiral crystal, it is possible to always obtain the same enantiomorph.6 This type of enantioselectivity amplification provides a promising route for the production of enantiopure compounds. Perhaps more interesting is the fact that enantioselectivity may be amplified by adsorbing or condensing achiral molecules around a chiral center. This is possible because many molecules are prochiral; that is, they can become chiral upon adsorption (thanks to the symmetry breaking provided by the surface).7 Again, in the absence of chiral bias these adsorbates produce equal amounts of the two possible enantiomeric adsorbates (even if they may form homochiral domains), but in the presence of a small chiral perturbation, they may all adopt one single chiral configuration instead.8 This provides a way to create a larger homochiral system out of a small amount of a chiral compound. Such an approach can greatly reduce cost © 2015 American Chemical Society

when dealing with the production of chiral systems or create unique chirally modified catalysts. Enantioselectivity amplification in two dimensions, on singlecrystal surfaces, was first reported for the case of succinic acid monolayers on Cu(110) dosed with small amounts of chiral tartaric acid9 and dubbed a “sergeants-and-soldiers” effect in analogy with similar chiral control previously reported with macromolecules.10 Several examples of this phenomenon have been reported since, mainly focusing on phase transitions within ordered 2D layers (and often on inert substrates such as copper, gold, or graphite). A combination of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) experiments have been used to identify multiple complex surface structures and domains in either chirally seeded systems or layers with small chiral imbalances.9,11−17 There has also been a recent report on the enantioselective effect that aspartic acid adsorbed on Ni surfaces exerts on the keto−enol equilibrium in methylacetoacetate adsorbates.18 Although not directly related to enantioselectivity amplification, that work provides interesting information on chiral seeding.18,19 In our article, we report on the identification of surface enantioselectivity amplification with a prochiral adsorbate by using a chemical method that does not rely on surface ordering and that directly reflects the chemical consequences of this amplified chiral templating in terms of adsorption enantioselectivity. Specifically, enhanced enantioselectivity was achieved on Pt(111) surfaces seeded with enantiopure propylene oxide (PO) via the subsequent adsorption of propylene (Py), a prochiral molecule used here as the chiral amplifier. Our approach represents an extension of a method previously used by us and others to evaluate enantioselectivity in adsorption on surfaces templated by chiral agents.20−22 Enantioselective excesses have been measured, by temperature-programmed Received: May 8, 2015 Published: June 4, 2015 13785

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up of 10 μm glass microcapillary tubes, backed by a gas manifold with a capacitance manometer that is used in conjunction with a leak valve to set the beam flux. A movable stainless steel flag was placed between the doser and the sample in order to be able to intercept the beam at will. The sample, a 0.9 cm diameter platinum single crystal cut in the (111) direction and polished using standard procedures, was mounted by spot-welding two tantalum wires to the back of the crystal. The sample could be cooled to liquid nitrogen temperatures and heated resistively to any desired temperature up to 1300 K by using a chromel−alumel thermocouple spotwelded to the back of the crystal and a homemade temperature controller. The platinum crystal was cleaned in situ by a combination of oxygen treatments at 1100 K and Ar+ ion sputtering−annealing cycles before each experiment. R-, S-, and racemic propylene oxide (PO, 99% purity) were purchased from Aldrich, and propylene (>99% purity) was purchased from Matheson; they were all used as supplied after a series of freeze−pump−thaw vacuum distillations. The pressures in the vacuum chamber were measured by using a Bayard−Alpert nude ion gauge and are reported without any correction for their sensitivity factors. Exposures are reported in Langmuirs (1 L = 10−6 Torr·s). The enantioselectivity amplification induced by the addition of propylene to the chirally seeded Pt surface was quantified systematically as a function of the respective exposures and the order of dosing. The data were processed taking into account several important considerations: (1) the exposures were calibrated into coverages, which are reported here in relative terms, referenced to a saturation layer of S-PO on Pt(111) (θS‑PO,Sat = 1.00 ML) (the uptake curves, measured using both the molecular beam assembly and the TPD experiments, are provided in Figure 2);29 (2) any possible displacement of one type of adsorbate by the other was discarded based on separate

desorption (TPD) and/or infrared absorption spectroscopy, in the uptake of chiral probe molecules (typically PO) after adsorbing a chiral templating agent (2-butanol,20 2-methylbutanoic acid,23,24 2-aminobutanoic acid,23 naphthylethylamine,25,26 different amino acids27,28). Using a similar approach, we have recently proven that the extent of adsorption of PO depends on the enantiocomposition of the adsorbed layer.29,30

2. EXPERIMENTAL DETAILS A three-step experimental procedure was used in the present study, as illustrated in Figure 1. First, a small dose of a chiral

Figure 1. Schematic depiction of the enantioselectivity amplification and chemical titration procedure used in this work. Three dosing steps are performed sequentially, with (1) enantiopure propylene oxide (SPO), used as the seed molecule; (2) propylene (Py), the prochiral amplifier; and (3) either one of the two enantiomers of PO (S- or RPO). The enantiomeric excess, % ee, is calculated using the data from both titration experiments, with S-PO and R-PO, and the formula provided in the upper right corner (eq 1).

seed, enantiopure propylene oxide in our case (S-PO), is added to the Pt(111) surface. Second, propylene (Py), a prochiral molecule, is added as the chiral amplifier. Finally, the surface is saturated with both S- and R-PO (with one of the two enantiopure molecules each time, in two separate experiments), the chemical chiral probe, and TPD experiments are carried out to quantify that uptake. The enantiomeric excess (ee) is then calculated from the differences in coverages (θ) seen between the titrations with S- versus R-PO by using the well-established eq 1, considering only the effect of the probe to separate it from that of the seed (which can be appreciable but is not the subject of this study) % ee =

θS ‐ PO,Probe − θR ‐ PO,Probe θS ‐ PO,Probe + θR ‐ PO,Probe

Figure 2. Uptake curves, in terms of coverage versus exposure, for S(red short-dash line), R- (blue solid line), and Rac-PO (purple short− long-dash line) and for Py (green long-dash line) on clean Pt(111), measured using our molecular beam apparatus. Also reported are values obtained from TPD experiments for S- (red solid circles) and Rac-PO (purple half-filled circles). All coverages are referenced to a saturation monolayer of S-PO, which is assigned a value of 1.00 ML. The uptakes curves for S- and R-PO match each other, as expected, but the uptake for Rac-PO is slower and reaches a lower saturation coverage, as reported before.29 The uptake of Py falls in between, but is quite close to that of the PO racemic mixture. These data (as well as additional similar measurements with mixed PO + Py layers) were used to convert exposures into coverages in the following figures.

× 100 (1)

All experiments were carried out in a small molecular-beam (MB) apparatus described in detail in previous publications,31 with a Pt(111) single crystal mounted inside an ultrahigh vacuum (UHV) chamber equipped with an effusive collimated beam and a UTI quadrupole mass spectrometer, used for both TPD and MB experiments. The collimated beam doser consists of a 2 mm thick and 1.2 cm diameter multichannel array made 13786

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on the surface by such a chiral modifier: the subsequent uptake of R-PO leads to a total coverage of only 80% of what can be reached if S-PO is used instead (since for 0.50 L S-PO θS‑PO,Seed = 0.60 ML, % ee = {[(1.00−0.60) ML − (0.80−0.60) ML)]/ [(1.00−0.60) ML + (0.80−0.60) ML]} × 100% = 33.3%). This effect has been reported by us before and has been explained by a kinetic effect where an adsorbate-assisted adsorption mechanism shows a preference for homo- versus heteropairing of the chiral species;29 it is provided here as a reference for comparison with our other results. The center panel of Figure 4 illustrates the feasibility of creating a similar enantioselective surface by adding propylene (0.40 L) to a surface initially seeded with a small amount (0.15 L) of S-PO. Indeed, a clear effect is seen in this case as well, even if it is not as acute as with S-PO alone: the titration using R-PO yielded a final PO coverage almost 10% lower than that measured with S-PO (θS‑PO,Seed = 0.18 ML, so % ee = {[(0.62−0.18) ML − (0.57− 0.18) ML]/[(0.62−0.18) ML + (0.57−0.18) ML]} × 100% = 6.0%), a difference well outside of the margin of error of these experiments (which were estimated to be less than 0.02 ML in each individual TPD run). For reference, a third experiment is reported in this figure where the propylene is dosed first, as a potential chiral seed (right panel): no detectable effect was seen in that case, since the adsorption of propylene by itself does not lead to any enantiopreference and the subsequent S-PO dose is too small to show any appreciable chiral effects. The results from the quantitation of our systematic studies of the enantioselectivity amplification induced by the addition of propylene to the chirally seeded Pt surface, performing experiments such as those shown in Figure 4, are summarized in Figure 5 in the form of % ee, calculated by using eq 1, versus

control experiments, shown in Figure 3; and (3) the coverages of the probe were estimated from the TPD data after

Figure 3. TPD measurements of displacements between propylene oxide (PO) and propylene (Py) on Pt(111). A surface with an initial coverage of 0.60 ML of the first adsorbate was exposed to different amounts of the second, as indicated in the x axis, and the final coverage of the original adsorbate measured by post-mortem TPD analysis. Several conclusions are clear from the data in this figure: (1) no displacement occurs in any case until doses above approximately 0.2− 0.3 L of the second adsorbate, at which point the surface becomes saturated; (2) Py adsorbs much more strongly than, and cannot be displaced by, PO; (3) the reverse, namely, displacement of adsorbed PO by Py, is possible but is not significant below monolayer saturation; and (4) each molecule can displace itself, a fact that was measured by using fully deuterated compounds.

subtracting the contributions from the PO previously adsorbed either as seed or as amplifier (θPO,Probe = θPO,TPD − θPO,Seed − θPO,Amplifier). The experiments were also carried out multiple times in order to estimate the error bars in our measurements.

3. RESULTS AND DISCUSSION Typical TPD data obtained from our three-step experiments are provided in Figure 4. The left panel, which shows the results from templating the Pt(111) surface with 0.50 L of enantiopure (S-PO) alone, clearly points to the enantioselectivity bestowed

Figure 5. Summary of the enantiomeric excess (% ee) values calculated using TPD data from surfaces dosed with various sequences of PO alone or in combination with Py. Two panels are provided, for data obtained for two different coverage ranges in the initial chiral seeding and chiral amplifying steps (details provided in the text). It is seen that in all cases the addition of Py to surfaces predosed with S-PO increases the % ee of the surface, to an extent comparable to what is obtained with Rac-PO (albeit not as large as what is possible with enantiopure S-PO alone).

the combined coverage of the seed plus the amplifier (examples of how these calculations were carried out were already provided in the previous paragraph). Two panels are provided, to highlight the information acquired at two different initial coverages of the seed (S-PO in all cases). On the left, the changes in % ee seen on a surface initially dosed with 0.05 L of S-PO (equivalent to θS‑PO,Seed = 0.06 ML) are shown as a

Figure 4. PO temperature-programmed desorption (TPD) traces from typical chiral seeding−amplification−probing experiments. Three cases are shown, for: (left) chiral templating with S-PO alone; (center) chiral seeding with S-PO followed by enantioselectivity amplification with Py; and (right) a control Py seeding and S-PO amplification dosing sequence. 13787

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The Journal of Physical Chemistry C function of the extent of subsequent dosing with the amplifier. As mentioned above, the ee’s were estimated by contrasting the uptake of the S-PO versus R-PO probe in the third step of two separate experiments (see Figure 1). If more S-PO is used in the second, amplification, step (as a reference experiment, to compare with the amplification bestowed by Py, the net effect is that of an increase in seed coverage, and with that in the enantioselectivity effect, as clearly evidenced in Figure 5: the % ee reaches ∼15% at θS‑PO = 0.5 ML ML (left panel, red solid circles) and almost 35% at θS‑PO = 0.6 ML (right panel, red open circles). This, of course, is equivalent to adsorbing the appropriate amount of S-PO all in one dose (as opposed to splitting it in twoseeding and amplificationsteps, which was done here only to directly compare with the S-PO + Py experiments) and then testing the uptake of S-PO versus R-PO requited to reach monolayer saturation, so the data only show the enantioselectivity effect caused by the chiral modifier reported before;29 we provide these results here as a reference to contrast with the other experiments. The interesting new observation is that when Py is added as the amplifier instead the enantioselectivity amplification effect is also clearly evident, even if it is not as large: the % ee measured in the probing stage, that is, the difference between the S-PO and R-PO uptake needed to saturate the surface, reaches 5% at θPy,Amp ∼ 0.4 ML (Figure 5, left panel, green open downward pointing triangles). In fact, this enantioselectivity amplification effect with Py, an achiral molecule, is almost as large as that seen with racemic mixtures of PO in the amplification stage (Figure 5, left panel, blue solid upward pointing triangles). Our previous work has indicated that the enantioselectivity amplification seen with the racemic PO is due to a preferential adsorption of one enantiomer of PO over the other on surface sites adjacent to chiral adsorbates, in this case an enhanced uptake of S-PO over R-PO on surfaces seeded with S-PO.29 This kinetic effect is based on the fact that the sticking coefficient is larger for cases where the adsorbate and the incident molecule are of the same chirality rather than being heterochiral pairs. The similarity in uptake behavior between Py and Rac-PO seen here strongly argues for a kinetic effect in the enantioselectivity amplification observed with propylene as well, in this case because there may be a preference for the adoption of one chirality over the other by Py adsorbed on sites next to adsorbed PO. The right panel of Figure 5 provides additional data from experiments with either higher (0.15 L) or variable (x L) dosings of S-PO in the seeding step. Enantioselectivity amplification is seen in all cases, with trends consistent with those seen in the left panel. Only the control experiment where 0.15 L S-PO is dosed on Pt(111) in the amplification stage, after preparing the surface with various precoverages of Py in the seeding phase, shows no enantioselective effect. It should be indicated that the % ee reported in these experiments is in part magnified with increasing seed + amplifier coverages by a reduction in the area of empty sites on the Pt(111) surface (the denominator in eq 1). Nevertheless, the enantioselectivity in these experiments does also increase in absolute terms, as shown more clearly in Figure 6. The main effect appears to be a reduction in total uptake, that is, the combined PO + Py uptake after all three steps of our experiments (seeding, amplification, and probing), when heterochiral combinations are deposited on the surface: the surface density in those cases is measurably lower than with the equivalent homochiral systems.

Figure 6. Summary of total surface coverage (θTotal, referenced to a saturation S-PO layer) calculated from experiments with various combinations of PO and Py doses. Three types of experiments are reported here, mirroring those reported in Figure 4: (left) chiral templating with x L S-PO alone; (center) a x L S-PO plus 0.15 L Py dosing sequence; and (right) a reverse 0.15 L S-PO plus x L Py combination. In all cases an increase in the difference between the saturation coverage attained with S-PO versus R-PO at the final probing stage (green solid diamonds) is seen with increasing exposures (increasing x), a manifestation of the effectiveness of the chiral amplification exerted by addition of Py to S-PO-predosed Pt(111) surfaces. It is also evident that the main factor for the enantioselectivity amplification is a reduction in the total coverage possible in the heterochiral systems.

Isothermal uptake measurements of sticking coefficients (s) versus coverage, obtained by using a molecular beam setup,32 provided further confirmation of the ability of Py to amplify the enantioselectivity of the PO-seeded Pt(111) surfaces. Figure 7

Figure 7. Py sticking coefficient (s) versus coverage for Py adsorption at 150 K on Pt(111) surfaces predosed with various amounts of PO. Data are provided for seeding experiments with (left) enantiopure SPO and (right) a racemic PO mixture.

contrasts data for the uptake of Py on Pt(111) predosed with various amounts of enantiopure (S-PO) versus racemic (RacPO) PO. Clear differences between the two sets of data are obvious upon visual inspection, including the following: (1) the final Py coverage reached at saturation, the point at which the uptake curve reaches a value of s = 0, is always higher with SPO than with Rac-PO; (2) the absolute values of s for Py are always higher on the enantiopure-seeded surface (for equivalent PO and Py coverages); and (3) the sticking probability for Py is 13788

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Figure 7 could also be explained (Figure 10), by making the following key assumptions: (1) adsorption of either PO or Py is

always constant in the initial stages of the uptake, indicating a precursor-mediated adsorption mechanism. Finally, the experimental results reported here were simulated by using a Monte Carlo kinetic algorithm. The general details of these calculations are provided elsewhere.29,33 The trends reported in Figure 5 could be reproduced quantitatively (Figures 8 and 9), and the uptakes shown in

Figure 10. Py sticking coefficient (s) versus PO coverage for Py adsorption on a surface predosed with various amounts of PO, as estimated by Monte Carlo kinetic simulations. Data are provided for uptakes on surfaces chirally seeded with (left) enantiopure S-PO and (right) a racemic PO mixture. These data are to be directly compared with the experimental data reported in Figure 7. Some of the general features could be reproduced with these kinetic modeling calculations, including the constant value of s in the initial stages of the uptake and the lower s values obtained with Rac-PO compared to S-PO.

Figure 8. Summary of enantiomeric excess (% ee) data calculated from Monte Carlo simulations with various combinations of PO and Py doses: (left) 0.05 L S-PO + x L Py and (right) 0.05 L S-PO + x L RacPO. It was possible to reproduce most quantitative aspects of these uptakes with our Monte Carlo simulations using only one adjustable parameter, namely, the ratio of the probability of propylene to adopt the same versus the opposite enantioconfiguration as the adsorbate next to its landing site (2:1). Also worth highlighting is the fact that a similar chiral-amplifying behavior is observed in both cases, with Py and Rac-PO. Two factors contribute to the chiral amplification observed: (1) an enantioselective adsorbate-assisted adsorption mechanism, previously seen with PO alone, and (2) a preferential adsorption with homochiral (vs heterochiral) adsorbate-incoming molecule pairs.

assisted by the presence of adsorbates already on the surface; that is, the sticking coefficient is higher on sites adjacent to adsorbates than on the clean surface (as was already reported for PO alone);29 (2) the sticking coefficients are also higher for homochiral adsorbate-incoming molecule pairs, compared to those for heterochiral pairs; and (3) there is a preference (by a ratio of approximately 2:1) for the conversion of the achiral incoming Py molecules to the same chirality as the adjacent adsorbate, compared to the opposite chirality. Overall, those three assumptions provide a kinetic explanation for the enantioselectivity enhancement seen experimentally upon the addition of prochiral molecules to chirally seeded surfaces.

4. CONCLUDING REMARKS In summary, it was shown here that a clear enantioselectivity amplification can be attained by adding propylene to Pt(111) surfaces seeded with small quantities of propylene oxide. Monte Carlo modeling was used to explain this effect as the result of a kinetic effect on the adsorption probabilities of the molecules incoming from the gas phase, which appears to exhibit particular enantioselectivities. It would have been desirable to image the adsorbates in these systems directly by using scanning tunneling microscopy (STM) in order to determine the spatial distribution of the propylene molecules used as amplifiers with respect to the initial S-PO seeds and, perhaps more interestingly, to establish the chirality adopted by such molecules, but that has so far not been possible. We have provided STM images for the PO system alone in the past to illustrate the random nature of the adsorption and to estimate monolayer densities.29 However, establishing the chirality of the adsorbates is a more difficult task, and we have so far not been able to image that unequivocally. As far as we know, only Ernst and co-workers have had any success, albeit limited, with these systems, and the experiments in that case were done on a highly corrugated Cu(211) surface,34 which provided both high

Figure 9. Summary of enantiomeric excess (% ee) data calculated from Monte Carlo simulations with various combinations of PO and Py doses: (left) 0.15 L S-PO followed by x L Py and (right) x L S-PO followed by 0.15 L Py. In this case the agreement between experiments and simulations is good at low coverages but deviates at higher coverages. Still, this simple Monte Carlo algorithm is capable of explaining the general qualitative trends seen in all cases, specifically, the chiral amplification induced by adsorbing propylene on S-POseeded Pt(111) surfaces.

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contrast, thanks to the nature of the metal, and a reference orientation in the direction of the steps of the underlying surface. Imaging propylene on flat platinum surfaces with sufficient resolution to determine their chirality has turned out to be much more difficult. We offer this challenge to the STM community.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ‡

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DEFG02-12ER16330.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b04452 J. Phys. Chem. C 2015, 119, 13785−13790