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Direct STM Investigation of Cinchona Alkaloid Adsorption on Cu(111) Qing-Min Xu, Dong Wang, Mei-Juan Han,† Li-Jun Wan,* and Chun-Li Bai* Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received June 2, 2003. In Final Form: February 10, 2004 Scanning tunneling microscopy (STM) combined with cyclic voltammetry has been employed to investigate the adsorption of cinchonine on Cu(111). Similar to cinchonidine, cinchonine forms a long-range ordered adlayer with (4 × 4) symmetry on the substrate. The structural details on molecular adsorption were obtained by high-resolution STM images. On the basis of the previous results and obtained STM images, the quinoline ring is proposed to lie parallel to Cu(111) and serve as an anchoring group. The chiral quinuclidine moiety extends out of the surface to facilitate the interaction with the prochiral reactants.
Introduction Chirality synthesis has been one of the most important subjects in organic chemistry and catalytic chemistry since the establishment of stereochemistry. At present, the effective way to synthesize enantiopure chemicals is via homogeneous catalysis. However, much attention has been paid to obtaining chiral products by heterogeneous catalysis because of the special advantage of heterogeneous catalysts in handling and separation.1-8 It has been demonstrated that natural chiral metal surfaces show enantioselectivity toward chiral chemicals.9-11 However, a natural chiral surface with a high Miller index is always expensive and hard to handle, which limits its application in catalysis. Another promising way to construct chiral surfaces is via chiral modification of achiral catalysts with chiral molecules.12-15 One of the few successful examples of chiral-modified heterogeneous catalysis is the hydrogenation of R-ketoesters on supported platinum catalysts modified with cinchona alkaloids.1-8,14,15 These catalysts have been shown to be effective for performing enantioselective hydrogenation on a range of CdO bond containing molecules with high enantiomeric excess of up to 9095%.1 Scheme 1 shows the chemical structure of two widely used cinchona alkaloid modifiers, cinchonidine (CD) and cinchonine (CN), and a protypical reaction involving hydrogenation of methyl pyruvate by CD- or CN-modified * To whom correspondence should be addressed. Tel & Fax: +8610-62558934. E-mail:
[email protected]. † Also in Graduate School of Chinese Academy of Sciences, Beijing, China. (1) Baiker, A. J. Mol. Catal. A: Chem. 1997, 115, 473-493. (2) Blaser, H.-U. Tetrahedron: Asymmetry 1991, 2, 843-866. (3) Baiker, A. J. Mol. Catal. A: Chem. 2000, 163, 205. (4) Studer, M.; Blaser, H.-U.; Exner, C. Adv. Synth. Catal. 2003, 345, 1-21. (5) Osawa, T.; Harada, T.; Takayasu, O. Top. Catal. 2000, 13, 155168. (6) Blaser, H.-U.; Jalett, H. P.; Mu¨ller, M.; Studer, M. Catal. Today 1997, 37, 441-463. (7) Von Arx, M.; Mallat, T.; Baiker, A. Top. Catal. 2002, 19, 75-87. (8) Pfaltz, A.; Heinz, T. Top. Catal. 1997, 4, 229-239. (9) Attard, G. A. J. Phys. Chem. B 2001, 105, 3158-3167. (10) Sholl, D. S.; Asthagiri, A.; Power, T. D. J. Phys. Chem. B 2001, 105, 4771-4782. (11) Horvath, J. D.; Gellman, A. J. J. Am. Chem. Soc. 2002, 124, 2384-2392. (12) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376-379. (13) Humblot, V.; Haq, S.; Muryn, C.; Hofer, W. A.; Raval, R. J. Am. Chem. Soc. 2002, 124, 503-510. (14) Blaser, H.-U.; Jalett, H. P.; Lottenbach, W.; Studer, M. J. Am. Chem. Soc. 2000, 122, 12675-12682. (15) Kacprzak, K.; Gawron´ski, J. Synthesis 2001, 961-998.
Scheme 1. Molecular Structure of Cinchonidine (A) and Cinchonine (B); (C) Catalytic Reaction Scheme for the Enantioselective Hydrogenation of Methyl Pyruvate over the Cinchona-Modified Pt Catalyst
Pt catalysts. Although CD and CN are diastereomeric pairs, they are always called “pseudo- or near-enantiomers” because of their opposite stereochemistry at the crucial carbons C8 and C9. In fact, the Pt catalysts modified with CD or CN can catalyze the same prochiral reactant to generate nearly identical values of enantiomeric excess but with a reversal of the sign. To obtain catalysts with high activity and enantioselectivity, great effort has been made by preparing different cinchona derivatives and optimizing reaction conditions such as solvents, temperature, modifier concentration, and catalyst supports. At the same time, an investigation on the catalysis mechanism, including the structure of active intermediates and dynamics of their transformation to products, was also intensively carried out. As a result, it is believed that the conformation of cinchona modifier on the Pt surface is closely related to the efficiency of the catalysts.16-19 So far, various modern surface analysis techniques and theoretical simulations have been applied to study the adsorption mode of cinchona, particularly CD and CN, on the surface under simulated or real catalysis environ-
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ments. For example, a near-edge X-ray adsorption fine structure (NEXAFS) investigation revealed that 10,11dihydrocinchonidine (DHCD) molecules take a parallel orientation by the quinoline ring at 25 °C and a tilted orientation at 50 °C on a Pt(111) surface under ultrahigh vacuum (UHV) conditions.16 Carley et al. found that DHCD molecules form a disordered stable monolayer on Pt(111) in UHV conditions by X-ray photoelectron spectroscopy (XPS) and low-energy electronic diffraction (LEED). 17 Baiker et al. demonstrated that CD molecules have three stable conformations in solution and the population of the conformers depends on the dielectric constant of solvents by combining density functional theory (DFT) calculations and NMR.18 Recently, other in situ methods such as reflection-absorption infrared spectroscopy (RAIRS), attenuated total reflection IR spectroscopy (ATRIRS), and surface-enhanced Raman spectroscopy (SERS) were also employed to study the adsorption of CD in solution.19-24 A parallel orientation via the quinoline ring was observed, while at a higher concentration CD with a tilted orientation was also observed. The effect of solvent and coverage on the adsorption mode was discussed. Scanning tunneling microscopy (STM) is a powerful tool for the investigation of the adsorption mode of molecules on surfaces. STM has been successfully used to investigate the adsorption of tartaric acid on Cu and Ni surfaces. The “chiral channel” formed on Cu(110) or “chiral footprint” on Ni(110) formed upon tartaric adsorption provides insight into the enantioselective mechanism of the tartaric acid/Ni catalyst.12,13 Recently, the direct chiral discrimination of chiral molecules at the solid/liquid interface and the investigation of chiral adlayer orderliness of enantiomers and racemic mixtures were carried out by using electrochemical scanning tunneling microscopy (ECSTM) working in solution.25,26 Here, we report a recent STM result on the adsorption mode of two widely used cinchona alkaloids: CD and CN on a Cu(111) surface in solution. In the present study, the Cu(111) substrate is used because the mobility of adsorbates on the surface is helpful to form an ordered adlayer.27-29 It was found that the adsorption of CN and CD molecules results in the same ordered structures with a (4 × 4) symmetry. The internal structure and coordination of the molecules on Cu(111) were revealed in high-resolution STM images. The result presented here could serve as a model to understand the adsorption mode of cinchona alkaloid on the solid surfaces and further the understanding of the catalysis process of cinchona alkaloid based catalysis systems. (16) Evans, T.; Woodhead, A. P.; Gutie´rrez-Sosa, A.; Thornton, G.; Hall, T. J.; Davis, A. A.; Young, N. A.; Wells, P. B.; Oldman, R. J.; Plashkevych, O.; Vahtras, O.; Ågren, H.; Carravetta, V. Surf. Sci. 1999, 436, L691-L696. (17) Carley, A. F.; Rajumon, M. K.; Roberts, M. W.; Wells, P. B. J. Chem. Soc., Faraday Trans. 1995, 91, 2167-2172. (18) Bu¨rgi, T.; Baiker, A. J. Am. Chem. Soc. 1998, 120, 12920-12926. (19) Ferri, D.; Bu¨rgi, T.; Baiker, A. Chem. Commun. 2001, 11721173. (20) Ferri, D.; Bu¨rgi, T. J. Am. Chem. Soc. 2001, 123, 12074-12084. (21) Kubota, J.; Zaera, F. J. Am. Chem. Soc. 2001, 123, 11115-11116. (22) Kubota, J.; Ma, Z.; Zaera, F. Langmuir 2003, 19, 3371-3376. (23) Chu, W.; LeBlanc, R. J.; Williams, C. T. Catal. Commun. 2002, 3, 547-552. (24) Chu, W.; LeBlanc, R. J.; Williams, C. T.; Kubota, J.; Zaera, F. J. Phys. Chem. B 2003, 107, 14365-14373. (25) Xu, Q.-M.; Wang, D.; Wan, L.-J.; Wang, C.; Bai, C.-L.; Feng, G.-Q.; Wang, M.-X. Angew. Chem., Int. Ed. 2002, 41, 3408-3411. (26) Wang, D.; Xu, Q.-M.; Wan, L.-J.; Bai, C.-L.; Jin, G. Langmuir 2003, 19, 1958-1962. (27) Wan, L.-J.; Itaya, K. Langmuir 1997, 13, 7173-7179. (28) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1997, 101, 3547-3553. (29) Wang, D.; Xu, Q.-M.; Wan, L.-J.; Bai, C.-L. Langmuir 2002, 18, 5133-5138.
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Figure 1. Cyclic voltammograms of the Cu(111) electrode in (a) 0.1 M HClO4, (b) 0.1 M HClO4 + 0.1 mM CD, and (c) 0.1 M HClO4 + 0.1 mM CN. The scan rate was 50 mV/s.
Experimental Section A commercial Cu(111) single-crystal disk with a diameter of 10 mm (from Mateck Co., Germany) was used as a working electrode for both electrochemical measurement and STM observation. A well-defined single-crystal surface was prepared by electrochemical polishing as described in the literature.27 A solution of 0.1 M HClO4 was prepared by diluting ultrapure HClO4 (Cica-Merck, Kanto Chemicals) with ultrapure Millipore water. CN (99%) and CD (98%) were obtained from Fluka and used directly as received. A homemade electrochemical cell with a reversible hydrogen electrode (RHE) in 0.1 M HClO4 and a Pt counter electrode was employed for electrochemical measurement. All electrode potentials are reported with respect to the RHE. The STM apparatus used was a Nanoscope E (Digital Instruments Inc.). W tips were electrochemically etched in 0.6 M KOH. The details of the experiment were the same as those described in a previous paper.27
Results and Discussion Cyclic Voltammetry. The cyclic voltammogram (CV) of Cu(111) in 0.1 M HClO4 is shown in Figure 1a to compare with the published results.27 The cathodic current at -0.35 V corresponds to the hydrogen evolution, while the anodic current at 0.2 V corresponds to the anodic oxidation of the Cu electrode. The CV of Cu(111) in 0.1 M HClO4 + 0.1 mM CD solution is shown in Figure 1b. The featureless CV indicates no electrochemical reaction involving the double layer potential region, although the electric charge involved in the double layer becomes smaller due to the molecular adsorption. A similar CV is observed in the electrolyte containing 0.1 mM CN (Figure 1c). Recently, an electrochemical study on the adsorption of CD on achiral and chiral single-crystal Pt surfaces was reported by Attard.9 Two pairs of redox processes were identified independent of the chirality of the surface, indicating no selectivity of the adsorption of CD on the Pt surface.
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Figure 2. Atomic structure of the Cu(111)-(1 × 1) surface in 0.1 M HClO4 acquired at -0.2 V. The scanning rate was 20 Hz. The tunneling current was 10 nA.
Detailed investigation suggests that the peaks correspond to the adsorbate-blocked H2 adsorption/desorption. In addition, according to the electrochemical results by Attard, the single-crystal surface with a low Miller index with high symmetry has no enantioselectivity toward chiral molecules.9 This is in accordance with the result that similar CVs were obtained on CD- and CN-modified Cu(111) electrodes. In Situ STM. The adsorption of CD on Cu(111) has been reported in a previous communication.30 To compare it with the CN adlayer, the results are briefly described here. As disclosed by STM, CD forms a uniform adlayer on a wide terrace of the Cu(111) surface in 0.1 M HClO4 + 0.1 mM CD solution. A (4 × 4) structure of the molecular adlayer was concluded after comparing the registry of the adlayer with the Cu(111) lattice. In contrast, no ordered structure of CD was reported by LEED in UHV on Pt(111).17 This can be attributed to the different substrate surface.28,29 Higher-resolution STM images clearly show that each CD molecule consists of three spots. The details of the results can be found in our previous paper.30 Before the investigation of the CN adlayer on Cu(111) by STM, the Cu(111)-(1 × 1) structure was first acquired in pure 0.1 M HClO4 for the ease of determining the orientation between the Cu(111) lattice and the CN adlayer. Figure 2 shows an atomic image of the Cu(111) surface acquired at -0.2 V. The Cu(111) surface shows a hexagonal lattice with the atomic distance of 0.26 nm, indicating that the well-defined Cu(111) surface has a (1 × 1) structure. Then, a small amount of CN solution was directly added to the STM cell to form a molecular adlayer. The concentration of CN in the solution is ca. 0.1 mM. A well-defined adlayer similar to that of CD is observed as shown in Figure 3a. The molecular rows are seen to extend along the 〈110〉 directions of the Cu(111) (30) Xu, Q.-M.; Wang, D.; Wan, L.-J.; Bai, C.-L.; Wang, Y. J. Am. Chem. Soc. 2002, 124, 14300-14301.
Figure 3. (a) STM image of the cinchonine adlayer on Cu(111) in 0.1 M HClO4 + 0.1 mM CN at -0.2 V with a setpoint of 5 nA. (b) High-resolution STM image of CN molecules on Cu(111) with a setpoint of 5 nA. (c) The cross-section profile of the CN adlayer shows the corrugation height between spots a and c.
substrate as indicated by the arrows in Figure 3a. The intermolecular distance along the close-packed molecular rows is ca. 1.00 nm, nearly 4 times the lattice distance of Cu(111). Thus, CN, similar to CD, forms a (4 × 4) adlayer structure on Cu(111). A unit cell is outlined in Figure 3b. The results indicate that the adlayer symmetry is not affected by the chirality of molecules. Figure 3b shows a high-resolution STM image of CN on Cu(111) in 0.1 M HClO4 at -0.2 V. Each CN molecule consists of three bright spots labeled by a, b, and c. The distance between spots a and b is measured to be 0.49 ( 0.02 nm, and that between spots a and c is 0.85 ( 0.02 nm. The corrugation height of spots a and c is disclosed from the cross-section profile shown in Figure 3c.
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The STM images are found to be very stable. To investigate the potential-dependent adsorption behavior of molecules, the electrode potential was scanned in the double layer potential range during the experiment. No potential-dependent structural change was found in the experiment. This result is in accordance with the cyclic voltammetry measurement. Adsorption Mode of CN. Cinchona is composed of a quinuclidine moiety and a planar quinoline moiety interconnected by two carbon-carbon single bonds. The difference between CD and CN and other cinchona derivatives is the chirality of the interconnecter C8 and C9. Blaser and other groups have carried out detailed investigations to disclose the dependence of the enantioselectivity of cinchona alkaloid based catalysts on the structures of the derivatives.14 It is found that the aromatic moiety is important for the high enantioselectivity of catalysts. This is consistent with the other spectral results, where the quinoline group is attributed to the anchoring group for the cinchona modifier. In our previous studies, the adsorption of pyridine and naphthalene, molecules with structural features similar to the quinoline moiety, on Cu(111) was investigated by in situ STM.27,29 A flat-on conformation of pyridine at the negative potential region and naphthalene was proposed. The naphthalene was proposed to locate at the 2-fold bridge sites and produce two protrusions in STM images on the basis of STM results and theoretical simulation.31 Recently, the flat orientation of quinoline on a Pd(111) surface was determined by XPS and NEXAFS.32 On the basis of the above results and the present STM images, it is reasonable to propose that spots a and b in Figure 3b correspond to two aromatic rings of the quinoline moiety with a flat-on orientation on the Cu(111) surface. Figure 4a is another higher resolution STM image showing the molecular structure in the CN adlayer. To understand the interaction of the cinchona modifier with the substrate in the catalysis process, it is critical to investigate the conformation of cinchona. So far, various simulation methods, such as molecular mechanism and ab initio,18,33-35 have been applied to model the conformation of cinchona. Three preferable conformers, close (1), close (2), and open (3), in polar solvents were identified, while the open (3) conformer is thought to be with high population in apolar solvents. Particularly, the conformation analysis by the theoretical simulation is in agreement with the NMR investigation of CD in different solvents.18,33 However, the intermediate of the complex seems to be complicated. Although several models based on the open (3) conformer, whose lone pair of quinuclidine N points toward the quinoline ring and thus facilitates the interaction with the reactant, have been proposed, there is no direct evidence available. On the other hand, Margitfalvi’s calculations yielded an active complex between the ethyl pyruvate and the CD in its closed form, which can also explain the enantioselectivity and was denominated as the “shielding model”.36 In our previous work, the close (1) structure was proposed and thought to agree with the STM image of CD on Cu(111).30 Similarly, a possible model for CN molecules on Cu(111) is temporarily proposed and (31) Chiang, S. Chem. Rev. 1997, 97, 1083-1096. (32) Bonello, J. M.; Lindsay, R.; Santra, A. K.; Lamber, R. M. J. Phys. Chem. B 2002, 106, 2672-2679. (33) Dijkstra, G. D. H.; Kellogg, R. M.; Wynberg, H.; Svendsen, J. S.; Marko, I.; Sharpless, K. B. J. Am. Chem. Soc. 1989, 111, 8069-8076. (34) Dijkstra, G. D. H.; Kellogg, R. M.; Wynberg, H. J. Org. Chem. 1990, 55, 6121-6131. (35) Margitfalvi, J. L.; Tfirst, E. J. Mol. Catal. A 1999, 139, 81-95. (36) Margitfalvi, J. L.; Hegedu¨s, M.; Tfirst, E. Tetrahedron: Asymmetry 1996, 7, 571-580.
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Figure 4. (a) Magnified STM images of the CN adlayer with the overlaid structural model. (b) Structural model proposed for the CN adlayer on Cu(111).
shown in Figure 4b. After a comparison of the STM image with the chemical structure of CN, spot c in Figures 3b and 4a is thought to be produced by the quinuclidine group. According to the generally accepted STM image mechanism, the apparent protrusion in the STM image is produced by either high electronic density or actual physical height.37 Because of the special steric structure of CN(CD), the quinuclidine moiety should extend out of the surface, which accounts for the exceptional contrast of spot c. In the proposed model, the nitrogen atoms in the quinoline moiety are located on atop sites.30 The rings of quinoline are slightly shifted from the 2-fold bridge sites of the Cu(111) lattice, similar to the adsorption geometry of naphthalene.27 Hints for Heterogeneous Catalysis. For the heterogeneous catalysts, three important factors, the adsorption sites, the adlayer structure, and the adsorption conformation of the modifier on the solid surface are all of special importance for the catalysis process and the enantioselectivity. The adlayer structure of the modifier on the surface is important for the enantioselectivity of (37) Sautet, P. Chem. Rev. 1997, 97, 1097-1116.
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chiral heterogeneous catalysts. For example, chiral “channels” can be obtained on tartaric acid modified Cu(110) and are thought to serve as a template to induce the enantioselective reaction of prochiral reactants.12 Similarly, the chiral “screws” created on Au(111) upon adsorption of dithiol-binaphthalene show enantioselectivity toward amino acid adsorption.38 However, the mechanism of the chiral template does not seem to be applicable for the cinchona/Pt catalysts. For example, previous LEED results indicate CD adsorbed on Pt(111) randomly.17 In the present study, the different adsorption behavior of cinchona, that is, a high-ordered array, was found on Cu(111). However, there are no chiral sites created by the adsorption of the modifier. As demonstrated in the present and previous studies, the adsorption of CD and CN by the quinoline moiety determines the quinuclidine group extending into the solution. Thus, the interaction of ketoesters with the cinchona alkaloid modifier was facilitated by the amine group in the quinuclidine moiety and the hydroxy group connected to the chiral C8. Recently, direct evidence of 1:1 interaction between a cinchona modifier and a prochiral reactant has been obtained by the combination of various UHV experimental techniques.39 The present work is only a small step toward the understanding of the catalytic mechanism of cinchona/Pt catalysts. The most urgent requirement is to conduct in situ investigation on the adsorption of cinchona on the Pt surface to get insight into the catalysis process. The modifiers always have different adsorption behavior on different metal surfaces. A robust example may be the entirely different adlayer structure of tartaric acid on
Cu(110) and Ni(110).12,13 In addition, the effect of the environment, for example, UHV versus solvent, including the polarity of the solvent, should also be paid great attention. For instance, p-xylene can only form an ordered adlayer on Pt(111) at the solid-liquid interface.40 Barto´k et al. found the acetic acid solvent plays an important role in the catalytic process.41 Finally, the investigation of the interaction between the modifiers and reactants is obviously important, although it is very difficult to study and requires the combination of different analysis techniques.
(38) Nakanishi, T.; Yamakawa, N.; Asahi, T.; Osaka, T.; Ohtani, B.; Uosaki, K. J. Am. Chem. Soc. 2002, 124, 740-741. (39) Bonello, J. M.; Williams, F. J.; Lambert, R. M. J. Am. Chem. Soc. 2003, 125, 2723-2729.
(40) Suto, K.; Wakisaka, M.; Yamagishi, M.; Wan, L.-J.; Inukai, J.; Itaya, K. Langmuir 2000, 16, 9368-9373. (41) Barto´k, M.; Bala´zsik, K.; Szo¨llo¨si, G.; Barto´k, T. J. Catal. 2002, 205, 168-176.
Conclusion In summary, the adsorption of CD and CN on the Cu(111) surface has been investigated by electrochemical STM. It is found that the molecules adsorb on Cu(111) and form a long-range ordered adlayer. The high-resolution STM images reveal the internal structure of the molecules. The structural models for the adlayers are tentatively proposed. The quinoline ring is proposed to lie parallel to Cu(111), and the chiral quinuclidine moiety extends out from the substrate surface. The present study is a preliminary step toward the understanding of the adsorption of cinchona on various substrates and may supply structural information for the enantioselective mechanism of real cinchona alkaloid-Pt catalysts. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20025308, 20177025, and 20121301), the National Key Project on Basic Research (Grants G2000077501 and 2002CCA03100), and the Chinese Academy of Sciences. LA034964Q
