Scanning Tunneling Microscopy Measurements of the Full Cycle of a

Letter pubs.acs.org/JPCL

Scanning Tunneling Microscopy Measurements of the Full Cycle of a Heterogeneous Asymmetric Hydrogenation Reaction on Chirally Modified Pt(111) Vincent Demers-Carpentier,† Guillaume Goubert,† Federico Masini,† Yi Dong,† Anton M. H. Rasmussen,‡ Bjørk Hammer,*,‡ and Peter H. McBreen*,† †

CERPIC et Département de chimie, Université Laval, Québec, Qc, Canada G1V 0A6 iNANO and Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark

‡

S Supporting Information *

ABSTRACT: The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements. The experiments were carried out both on a clean surface and on a chirally modified surface, using chemisorbed (R)-(+)-1-(1naphthyl)ethylamine, ((R)-NEA), as the modifier. On the nonmodified surface, introduction of H2 at a background pressure of ∼1 × 10−6 mbar leads to the rapid break-up of TFAP dimer structures followed by the gradual removal of all TFAPrelated images. During the latter step, some monomers display an extra protrusion compared to TFAP in dimer structures. They are attributed to a half-hydrogenated intermediate. The introduction of H2 to a mixture of (R)-NEA and TFAP on Pt(111) leads to the removal of TFAP without any change in the population of the modifier, as required for an efficient chirally modified catalyst. SECTION: Surfaces, Interfaces, Catalysis

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the full cycle of chemisorption, preorganization, and hydrogenation of TFAP on (R)-NEA-modified Pt(111) at room temperature. The absolute configuration of TFPE produced in the surface reaction is determined by the enantiotopic adsorption of TFAP, assuming that the chiral center is formed through hydrogen addition at the face in contact with the metal surface (Scheme 1). The hydrogen source arises through the dissociative chemisorption of H2 to form a supply of atomically adsorbed hydrogen, Hads.15 As reported previously, STM images of TFAP on Pt(111) at room temperature show the formation of homochiral dimer structures.16 TFAP monomers are rarely imaged, although the disassembly and reassembly of TFAP dimers seen in STM videos demonstrates that monomers are also present on the surface. TFAP dimers and TFAP+(R)-NEA complexes are observed when both (R)-NEA and TFAP are present on the surface at room temperature (Figure 1).12 The modifier is observed in two rotameric forms (Figure 1F,G), as for NEA on Pd(111),11 and in both cases TFAP binds at a number of sites around the ethylamine function.12 Hydrogenation of a coadsorbed system of TFAP dimers and TFAP +(R)-NEA diastereomeric complexes can be described as two competing reactions.16 Hydrogenation of TFAP in dimers rather than in (R)-NEA+TFAP complexes is a hindrance to achieving high enantioselectivity.

here is an increasing effort to study heterogeneous catalysis reactions under reaction conditions.1−3 In particular, spatially and molecularly resolved observation of surface reactions can be used to address challenges arising from the heterogeneity of solid catalyst surfaces.4 These systems typically present a range of sites of different activity, and this often translates into nonideal selectivity.5,6 A good example is shown by asymmetric reactions on chirally modified metal surfaces prepared by chemisorbing optically active molecules.7−11 In these reactions combined molecule-metal and modifier-substrate interactions result in the preferred attack of a specific enantiotopic face of the substrate. Recently, Demers-Carpentier et al.12 reported the observation of chirality transfer preorganization of 2,2,2-trifluoroacetophenone (TFAP) on (R)-(+)-1-(1-naphthyl)ethylamine, ((R)-NEA)-modified Pt(111). The same study also showed that the hydrogenation of TFAP on (R)-NEA-modified Pt/ Al2O3 in acetic acid at 10 bar H2 yields an enantiomeric excess (ee) of approximately 34% in favor of the (R)-2,2,2trifluorophenylethanol, (R)-TFPE, enantiomer.12 No evidence for a modifier-substrate condensation reaction was found, consistent with the reported challenges in carrying out this reaction between TFAP and primary amines.13,14 The surface science study using scanning tunneling microscopy (STM), density functional theory (DFT) and reflection absorption infrared spectroscopy (RAIRS) provided direct information on the 1:1 modifier-substrate complexes believed to be responsible for asymmetric induction on the surface.12 Building on those observations, we present a study that reveals several aspects of © 2011 American Chemical Society

Received: October 14, 2011 Accepted: December 6, 2011 Published: December 6, 2011 92

dx.doi.org/10.1021/jz2013853 | J. Phys. Chem. Lett. 2012, 3, 92−96

The Journal of Physical Chemistry Letters

Letter

be assigned to one of 22 calculated stable complexes.12 Over 96% of the observed modifier-substrate complexes involve NH···OC bonding and all of the dimers involve aryl-CH···O bonding. As a result, the directionality of the carbonyl group and hence the prochirality of TFAP in the imaged structures can be determined by visual inspection. For example, 71% of all the observed modifier substrate complexes are formed by pro-R TFAP, and 43% of the dimers are homochiral pro-R structures. Two representative complexes, one for each prochirality, are shown in Figure 1. The observed excess of pro-S dimers will be discussed in a separate publication. The room-temperature interaction of hydrogen with a mixture of (R)-NEA and TFAP at a background pressure of 10−6 mbar H2 was monitored using STM (Figure 2). Data by Ertl and co-workers show that the equilibrium coverage of Hads on clean Pt(111) under these conditions is ∼0.3 ML.15 Introduction of hydrogen in the STM chamber while scanning the (R)-NEA and TFAP exposed surface leads to the gradual decrease of the imaged TFAP population. Figure 2 shows the evolution of the TFAP and (R)-NEA populations in a 30 × 30 nm2 area as a function of time. A smaller area of the analyzed STM video is shown at three different times during the experiment, and the full movie can be seen in the Supporting Information. Fifteen minutes after the initial introduction of hydrogen, ∼75% of TFAP is no longer imaged, whereas the chiral modifier coverage remains constant at its initial value, as expected for an efficient catalytic system. There is no change in the (R)-NEA images under these conditions, and this indicates that hydrogenation of the modifier does not occur on the time scale of our measurements. This is consistent with reported relative rates of modifier and substrate hydrogenation reactions on cinchona modified Pt catalysts.18,19 A closer inspection of the STM data shows that TFAP is removed from the surface in two steps. This behavior is clearly seen in STM images of the hydrogenation of TFAP on the nonmodified surface (Figure 3) and also occurs for the (R)NEA+TFAP system (Figure 2). Upon exposure to 10−6 mbar H2, the TFAP dimers (Figure 3A) quickly break up and a mixture of single molecule species (Figure 3B) and a limited number of nonregular assemblies (shown in the Supporting Information) are observed on the surface. Then, over 30−40 min (Figure 3B−D), almost all TFAP-related species disappear from the nonmodified surface. We assume that TFAP is removed from the surface as the chiral alcohol product from both dimers and diastereomeric complexes. This assessment is based on the following observations. TFAP-related features do not reappear over several hours after cutting the H2 supply. In particular, the subsequent formation of TFAP dimers is not observed even on other areas of the surface. Moreover, previous work17 showed that TFPE undergoes dehydrogenation at room temperature on clean Pt(111) to form TFAP. No evidence for such a dehydrogenation reaction was found after the removal of the TFAP-related features. Furthermore, chemisorbed (R)NEA remaining after H2 treatment (Figure 2) was not seen to form new diastereomeric complexes. These observations effectively rule out the possibility that the almost complete disappearance of TFAP-related features seen in the STM measurements under H2 is the result of an inability to observe fast-moving TFAP or TFPE monomers. The shape and corrugation of the images change every few frames throughout the acquisition of a video, possibly as a result of the tip picking up hydrogen atoms. A variation in tunneling current in the 0.3 to 0.6 nA range was necessary to maintain

Scheme 1. Schematic Illustration of the Asymmetric Hydrogenation of TFAP to TFPE by Adsorbed Atomic Hydrogen on the Pt(111) Surfacea

a

The adsorption configuration of TFAP determines which enantiotopic face of the prochiral molecule is directed toward the surface. ProR and pro-S adsorbed TFAP are defined by reference to the alcohol that is formed following hydrogen addition at the enantiotopic face in contact with the surface.

Figure 1. STM images of coadsorbed (R)-NEA and TFAP on Pt(111) at room temperature. (A) Both TFAP dimers and TFAP + (R)-NEA diastereomeric complexes are imaged. Dimer formation involves arylCH···OC bonding and modifier-substrate complex formation involves NH···OC bonding.12 (B,C) Images of two selected pro-R and pro-S modifier-substrate complexes. The indicated percentages represent the total fraction of pro-R or pro-S diastereomeric complexes in a large sample of complexes of different geometries.12 (D,E) Images of individual pro-R and pro-S TFAP dimers. The indicated percentages show the relative populations of pro-R and pro-S TFAP dimers on the (R)-NEA modified surface. (F,G,H) DFT calculated structures of two different diastereomeric complexes (F,G) and a pro-S TFAP dimer (H). STM images were recorded at room temperature, 1.2 V sample bias, and 0.3 nA tunnel current.

The surface science and catalytic measurements were both carried out at room temperature. Time-lapsed imaging of coadsorbed TFAP and (R)-NEA shows a dynamic situation in which there is interchange between dimers and modifiersubstrate complexes. In a previous paper we addressed the internal structure of TFAP dimers and TFAP+(R)-NEA complexes and identified 11 different complexes that could all 93



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Figure 2. Images from an STM movie of the interaction of H2, at a background pressure of 10−6 mbar, with coadsorbed (R)-NEA and TFAP on Pt(111) at room temperature. A count of the number of molecules in every frame in a sequence of measurements shows a gradual decrease in the TFAP-related features, while the (R)-NEA coverage remains constant. H2 was introduced into the experimental system at time t = 0. STM images were recorded at room temperature, 0.8 V sample bias, and 0.5 nA tunnel current.

Figure 3. Sequential STM images (A−E) of TFAP on Pt(111) at room temperature taken following the introduction of a background pressure of 10−6 mbar H2 into the STM chamber. The initial rapid break-up of the TFAP dimers is followed by the relatively slow removal of TFAP-related features. H2 was introduced into the experimental system at time t = 0. STM images were recorded at room temperature, 1.1 V sample bias, and 0.3− 0.6 nA tunnel current.

rationalized by a mechanism where partial hydrogenation of Hbond activated TFAP in both dimers and diastereomeric complexes occurs followed by a second, slower hydrogenation of the resultant monomeric species, followed by desorption of the alcohol. The transformation of the CO group to C−O− H would break both the TFAP dimers and (R)-NEA+TFAP complexes as observed by STM. In turn, the formation of a relatively strong C−Pt bond could explain the lack of OH···OH bonded assemblies and the relative immobility of the intermediate. Recent studies by Attard et al.20,21 isolated a half-hydrogenated intermediate in the hydrogenation of methyl pyruvate on cinchonidine modified Pt. By fitting SERS spectra to DFTcalculated vibrational data, they identified the intermediate as the hydroxyl species resulting from adding a proton to the oxygen atom of the prochiral carbonyl group.20 DFT calculations (Figure 5) on intermediates formed by TFAP on Pt(111) also suggest that given a two-step hydrogenation

acceptable resolution (Figure 3) during video-type experiments. However, the continually changing imaging conditions do not impede the observation of the surface layer. While the singlemolecule species (Figure 3B,C,D) appear as bowling pin protrusions in most of the images acquired, under some specific tip conditions it is possible to distinguish two different shapes. Figure 4 shows one such set of data where in addition to two typical TFAP features, three adsorbates appear to have a second, dimmer protrusion. The shape of the latter images is reminiscent of STM data for the enol tautomer of acetophenone observed in our previous work.17 In the case of acetophenone, the enol tautomers readily formed a variety of OH···OH bonded dimer assemblies. The structures giving the two-headed images are, in contrast, relatively immobile, indicating that they cannot be attributed to adsorbed TFPE. The two-step process observed in the STM experiments involves rapid dimer break-up followed by a longer period over which all TFAP-related images disappear. This behavior can be 94



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In conclusion, the combination of STM, DFT, and RAIRS allows us to observe key phenomena in the full cycle of chemisoption, chiral preorganization, and hydrogenation responsible for the enantiomeric excess of (R)-TFPE obtained using (R)-NEA modified platinum. The data cannot be used to rule out the participation of a pairwise hydrogenation mechanism such as found by Busygin et al. for the asymmetric hydrogenation of (R)-1-hydroxy-1-phenyl-2-propanone over cinchonidine-modified Pt/Al2O3.22 However, they are consistent with the operation of a stepwise reaction pathway as proposed by Taylor et al.20 and by Rauls and Hammer23 for the hydrogenation of alkyl pyruvates. The results are also consistent with (R)-NEA-directed preorganization prior to asymmetric hydrogenation at modifier sites.12,20

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EXPERIMENTAL METHODS The Pt(111) surface was cleaned by repeated Ar+ sputtering, and treatment in 2 × 10−7 mbar O2 at 900 K, followed by annealing at 1000 K. TFAP (99% purity) and (R)-NEA (99% purity) were purchased from Sigma-Aldrich and further purified by pumping and freeze−thaw cycles. The STM images were acquired using an Aarhus 150 STM instrument. All images were acquired at a tunneling current in the 0.3−0.6 nA range and a bias voltage in the 0.8−1.2 V range. All RAIRS spectra were recorded with the sample at 300 K and were collected using a Bruker Vertex 80v spectrometer equipped with a mercury cadmium telluride (MCT) detector. The DFT calculations were performed using GPAW24,25 a real-space grid implementation of the projector augmented wave (PAW) method. The metal surface is modeled using a c(8×4) super cell and four layers. The system is periodic in two dimensions, and we use a 2 × 2 k-point mesh for integrating over the Brillouin zone. Exchange and correlation effects are treated using the generalized gradient approximation in the Perdew−Burke− Ernzerhof (PBE) form.26 Geometries are optimized until no atom is subject to a force larger than 0.05 eV/Å. During optimization, the bottom metal layer is kept fixed.

Figure 4. Small area of an STM image recorded following exposure of TFAP on Pt(111) at room temperature to a background pressure of 10−6 mbar H2. Under some imaging conditions, a fraction of the observed monomer species are imaged with an additional protrusion (highlighted by the white arrows), whereas the others retain the bowling pin shape of TFAP. STM images were recorded at room temperature, 1.1 V sample bias, and 0.43 nA tunnel current.

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Figure 5. DFT calculated structures of TFAP chemisorbed on Pt(111) (left panel) and two partial hydrogenation intermediates (center and right panels). The calculated energies are expressed relative to the coadsorbed TFAP and 2 Hads system shown in the leftmost panel.

ASSOCIATED CONTENT

S Supporting Information *

STM video recorded during exposure of TFAP + (R)-NEA on Pt(111) to a background pressure of 10−6 mbar H2. STM images of odd-shaped dimers formed by exposure of TFAP to H2. RAIRS data for TFAP/Pt(111) exposure to H2. This material is available free of charge via the Internet at http:// pubs.acs.org

process, the intermediate observed in our STM experiments would result from proton addition to the carbonyl oxygen. Using the most stable TFAP monomer and two chemisorbed hydrogen atoms as the reference, the DFT study finds that the hydroxyl intermediate is the most stable. On the other hand, partial hydrogenation of TFAP at the chiral carbon center is found to be endothermic by 0.35 eV. RAIRS measurements (Supporting Information) support the STM data insofar as they confirm that TFAP/Pt(111) undergoes a transformation as a result of exposure to H2. This is manifested as changes in the relative intensities of CF3 asymmetric and symmetric stretching bands in the 1270−1130 cm−1 region. These data may be rationalized in terms of the H2induced break-up of TFAP dimers to form monomers and/or partially hydrogenated TFAP. The similarity of the calculated monomer and hydroxyl intermediate structures (Figure 5) might make it difficult to use RAIRS to identify the intermediate. Complete removal of TFAP is not seen in the RAIRS measurements. This discrepancy with the STM data is not understood at present.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.H.M.); hammer@ phys.au.dk (B.H.).

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ACKNOWLEDGMENTS This work was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, Canadian Foundation for Innovation (CFI) grants, and by the FQRNT Centre in Green Chemistry and Catalysis (CCVC). This work was supported in part by The Lundbeck Foundation, The Danish Research Councils, and the Danish Center for Scientific Computing. V.D.-C. acknowledges an NSERC graduate student scholarship. 95



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Scanning Tunneling Microscopy Measurements of the Full Cycle of a

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