Absolute Configuration of Monodentate Phosphine Ligand

The mirror symmetry between two adlayers is demonstrated. On the basis of STM results, structural models are proposed to interpret the chiral adsorpti...
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Anal. Chem. 2004, 76, 627-631

Absolute Configuration of Monodentate Phosphine Ligand Enantiomers on Cu(111) Mei-Juan Han,† Dong Wang, Ju-Min Hao,† Li-Jun Wan,* Qing-Dao Zeng, Qing-Hua Fan, and Chun-Li Bai*

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

Scanning tunneling microscopy (STM) has been employed to investigate the chirality of monophosphine compounds that are highly efficient chiral ligands in transition-metalcatalyzed organic transformations. The absolute configuration of 1-(2-diphenyphosphino-1-naphthyl)isoquinoline enantiomers with axial chirality was discriminated directly by the “marker” group, PPh2 substitutes. Although the two enantiomer molecules adsorb on a Cu(111) surface and form well-defined (4 × 4) structures, the positions of PPh2 substitutes in the molecular adlayers are different. The mirror symmetry between two adlayers is demonstrated. On the basis of STM results, structural models are proposed to interpret the chiral adsorption. The results presented here supply a straightforward method for axial chirality analysis in adsorbed adlayers by STM. Phenomena associated with chirality have held a deep fascination for scientists since Pasteur’s initial discovery of enantiomeric crystals in 1850.1,2 With the increasing demand for pure enantiomer compounds for various applications in the pharmaceutical and catalytic industries, the interest in developing new technology for chirality discrimination has received intensive attention. Chiral discrimination should be the key step to realize the separation of chiral conformers and to understand the stereoselective synthesis and catalysis process. Although traditional analytic techniques such as electrochemistry and spectroscopy are powerful for a microscopic understanding of the structure and dynamics of reactants on electrode surface, they usually give average information.3 On the other hand, with the application of scanning tunneling microscopy (STM), direct insight into chiral investigation at the surface has been addressed including individual adsorbed molecules or those in molecular assemblies and extended enantiomerically pure adlayers.4-12 For example, Lopinski et al. deter* To whom correspondence should be addressed. Fax and Tel: 86-1062558934. E-mail: [email protected]. † Also in Graduate School of Chinese Academy of Sciences, Beijing, China. (1) Bo ¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R. Angew. Chem., Int. Ed. Engl. 1999, 38, 821. (2) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W J. Phys. Chem. B 2000, 104, 7627. (3) Kolb, D. M. Angew. Chem., Int. Ed. 2001, 40, 1162. (4) Lopinski, G. P.; Moffat, D. J.; Wayner, D. D. M.; Wolkow, R. A. Nature 1998, 392, 909. (5) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (6) Ginacarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491. (7) Weckesser, J.; Vita, A. D.; Barth, J. V.; Cai, C.; Kern, K. Phys. Rev. Lett. 2001, 87, 096101-1. 10.1021/ac0347720 CCC: $27.50 Published on Web 12/18/2003

© 2004 American Chemical Society

mined the absolute configuration of isolated chiral centers of alkene molecules chemisorbed on Si(100).4 Lorenzo et al. investigated the adsorption of tartaric acid molecules on a Cu(110) surface, which results in the formation of a chiral surface with chiral channels.5 Ginacarlo and Flynn used a chemical marker group to identify the chirality of the molecular ordering and conformation of individual molecules physisorbed on HOPG.6 Xu et al. reported the discrimination of center chiral molecules in aqueous solution.8 However, investigation of axial chirality is rarely addressed, although molecules with axial chirality are important in asymmetric chemistry. An example is the covalent binding of axial chiral 1,1′-binaphthalene-2,2′-dithiol molecules.12 The chirality of the enantiomeric adlayers was determined based on the chiral holes and step lines of Au(111). However, to our best knowledge, the direct discrimination of the absolute configuration of axial chirality molecules has not been reported up to now. In the present study, we have demonstrated that axial chirality molecules can be directly discriminated with STM. The discrimination was carried out in the adlayers of chiral monophosphines, 1-(2-diphenyphosphino-1-naphthyl)isoquinoline ((R)- and (S)QUINAP) on Cu(111) in 0.1 M HClO4. Chiral monophosphines are highly efficient ligands for transition-metal-catalyzed organic transformations such as in Rh-catalyzed hydroboration and Pdcatalyzed allylic alkylation.13-15 Detailed data on the QUINAP adlayer structure and absolute configuration are therefore of central importance to clarify the underlying microscopic mechanisms of these reactions. Chart 1 shows the chemical structures of the two molecules. The chirality of the two molecules results from the axial chiral 1,1′-binaphthyl skeleton. 1,1′-Binaphthyl derivatives represent an important class of chiral auxiliaries.16 The nonplanar arrangement of two naphthalene moieties in the molecules ensures an asymmetric environment suitable for obtain(8) 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. (9) Feyter, S. D.; Grim, P. C. M.; Ru ¨ cker, M.; Vanoppen, P.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu ¨ llen, K.; Schryver, F. C. D. Angew. Chem., Int. Ed. Engl. 1998, 37, 1223. (10) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021. (11) Ku ¨ hnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891. (12) Ohthni, B.; Shintani, A.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 6515. (13) Alcock, N. W.; Brown, J. M.; Hulmes, D. I. Tetrahedron Asymmetry 1993, 4, 743. (14) Brown, J. M.; Hulmes, D. I.; Layzell, T. P. J. Chem. Soc., Chem. Commun. 1993, 1673. (15) Brown, J. M.; Hulmes, D. I.; Guiry, P. J. Tetrahedron 1994, 50, 4493. (16) Rosini, C.; Franzini, L.; Raffaelli, A.; Salvadori, P. Synthesis 1992, 503.

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Chart 1. Molecular Structures of (a) (R)-QUINAP (b) (S)-QUINAP

ing a high degree of stereoselectivity.17 With STM in the present study, it is found that (R)-QUINAP and (S)-QUINAP molecules form ordered adlayers on Cu(111) in a (4 × 4) structure and show a mirror symmetry. The chirality of individual adsorbed enantiomers is disclosed in high-resolution STM images. EXPERIMENTAL SECTION A commercial Cu(111) single-crystal disk with a diameter of 10 mm (from MaTeck) was used as a working electrode for both electrochemical measurement and in situ STM observation. A welldefined single-crystal surface was prepared as described in the literature.18,19 Briefly, after having been mechanically polished with diamond paste successively down to 0.05 µm in particle diameter, the sample was further treated by electropolishing in a phosphoric acid solution (50 mL of 85% H3PO4 and 50 mL of water) at 0.81.0 A cm-2 for 3-5 s. The Cu crystal was then rinsed repeatedly with ultrapure water (Millipore-Q), and a droplet of water was left on the electrode surface to protect it from contamination during transfer to the electrochemical cell. The in situ STM apparatus used was a Nanoscope E (Digital Instrument Inc.). Tunneling tips were prepared by electrochemically etching a tungsten wire (0.25 mm in diameter) in 0.6 M KOH. The W tips were then coated with a clear nail polish to minimize the faradaic current. The STM images shown here were acquired in the constant-current mode to evaluate corrugation heights of adsorbed molecules. A solution of 0.1 M HClO4 was prepared by diluting ultrapure HClO4 (Cica-Merck, Kanto Chemicals) with ultrapure Millipore water. Saturated solutions of (R)-QUINAP, (S)-QUINAP, and (RS)-QUINAP (from Strem Chemicals, used as received) were prepared with Millipore water. Electrochemical measurements were carried out using a three-compartment glass cell containing 0.1 M HClO4 with or without organic molecules. Solutions were deoxygenated by bubbling purified N2. The homemade electrochemical cell contained a reversible hydrogen electrode (RHE) in 0.1 M HClO4 and a Pt counter electrode. All electrode potentials were reported with respect to the RHE. RESULTS AND DISCUSSION Cyclic Voltammetry. Because the preparation of the monolayer was carried out in solution, it is necessary to understand the electrochemical behavior. Cyclic voltammograms (CVs) of Cu(111) were measured in the absence and presence of organic (17) Whitesell, J. K. Chem. Rev. 1989, 89, 1581. (18) Wan, L.-J.; Itaya, K. Langmuir 1997, 13, 7173. (19) Wang, D.; Wan, L. J.; Xu, Q. M.; Wang, C.; Bai, C. L. Surf. Sci. 2001, 478, L320.

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Figure 1. Cyclic voltammograms of Cu(111) in (a) 0.1 M HClO4, (b) 0.1 M HClO4 + (R)-QUINAP molecule saturated solution, (c) 0.1 M HClO4 + (S)-QUINAP molecule saturated solution, and (d) 0.1 M HClO4 + (RS)-QUINAP molecules saturated solution. Potential scan rate was 50 mV s-1.

molecules in 0.1 M HClO4. The bare Cu(111) electrode was initially examined for comparison with pervious results.18,19 After the examination shown in Figure 1a, the electrode was transferred into a 0.1 M HClO4 solution containing (R)-QUINAP, (S)-QUINAP, and (RS)-QUINAP saturated solutions. In the presence of the chiral molecules, the overall shapes of the CVs in Figure 1b and c are almost the same as that of Cu(111) in pure 0.1 M HClO4 as shown in Figure 1a,18,19 indicating that no electrochemical reaction takes place at least in the double layer potential region, although the electric charge involved in the double layer potential region becomes smaller due to the molecular adsorption. According to electrochemical studies by Attard, the low Miller index singlecrystal surface has no enantioselectivity toward chiral molecules.20 Thus, similar CVs are obtained on the adlayers of the enantiomers. A similar CV is also obtained upon (RS)-QUINAP adsorption shown in Figure 1d. No special feature can be seen in the CV. The result indicates that a similar adlayer structure may exist in the enantiomer and racemate adlayers. In Situ STM. The structural details of the adlayers are revealed by STM. After the examination of Cu(111)-(1 × 1) to determine the substrate lattice direction in 0.1 M HClO4 solution, a small amount of (R)-QUINAP solution was directly injected into the electrochemical cell to form a (R)-QUINAP adlayer. A uniform (R)-QUINAP adlayer was formed on a wide terrace of Cu(111) surface several minutes later. Figure 2a shows a STM image of (20) Attard, G. A. J. Phys. Chem. B 2001, 105, 3158.

Figure 2. (a) STM top view of (R)-QUINAP adlayer on Cu(111) in 0.1 M HClO4 at -0.25 V. Tunneling current was 5 nA. Scanning rate was 15.26 Hz. (b) High-resolution STM image of a Cu(111)-(4 × 4)-(R)-QUINAP adlattice acquired at -0.25 V. Tunneling current was 5 nA. Scanning rate was 17.35 Hz.

(R)-QUINAP on Cu(111) in 0.1 M HClO4 solution in a relatively large area. This image was recorded at -0.25 V. It can be seen that the molecules form a highly ordered adlayer and yield a pattern completely different from Cu(111)-(1 × 1). Although several molecular defects are disclosed in the image, a wellordered molecular array is seen to extend over a wide atomically flat terrace, indicating the stability of the adlayer. The molecular rows cross each other at an angle of either 60° or 120° within an experimental error of (2°. From a comparison with the Cu(111)(1 × 1) atomic image, it is found that all molecular rows are parallel to the 〈110〉 direction of the underlying Cu(111) lattice. The intermolecular distance along the 〈110〉 direction is measured to be ∼1.0 nm, corresponding to 4 times the lattice distance of

Cu(111). On the basis of the orientation of molecular rows and the intermolecular distance, we conclude that the observed structure of the (R)-QUINAP adlayer is (4 × 4) as shown by the superimposed drawn unit cell in Figure 2a. To reveal the intramolecular structure of (R)-QUINAP molecules, high-resolution STM images were acquired under carefully adjusted experimental conditions. Figure 2b is one of the highresolution images. It is clear that the each spot in Figure 2a now appears as two elongated spots with a tail at the end of one of the elongated spots. This feature is outlined by three oval rings. A defect indicated by an arrow corresponds to the position where the molecule is missing. It is clear that the set of spots with three oval rings in Figure 2b represents an (R)-QUINAP molecule. From Chart 1, we know the molecule contains one naphthalene moiety and one quinoline moiety. The most common feature of the naphthalene in STM images is an elongated spot, when the molecules adsorb on Cu(111) in a flat-lying orientation.18 Thus, it is reasonable to attribute spots a and b to the naphthalene ring or the quinoline ring, while the spot marked c corresponds to the diphenylphosphino (PPh2) moiety. From Chart 1, we can see the PPh2 substituent is linked with the naphthalene ring. So we consider that spot b in the STM image corresponds to the naphthalene ring moiety. Then spot a is attributed to the quinoline ring moiety. Detailed investigation showed that the three moieties of this molecule possess a counterclockwise arrangement from the quinoline to naphthalene to the PPh2 as shown in Figure 2b using a visualized symbol, the bent arrow. This feature is consistent with the molecular structure in Chart 1a as shown by an arrow. According to the configuration and the position of the substituent, the absolute configuration of the molecule is determined. Using a similar procedure, the (S)-QUINAP adlayer was prepared on Cu(111). A well-defined defect-free adlayer similar to that of (R)-QUINAP is shown in Figure 3a. This image was recorded at -0.25 V. The same (4 × 4) adlayer structure was determined by analyzing the registry of adlayer to substrate. A unit cell with (4 × 4) symmetry was superimposed in Figure 3a for comparison with the (R)-QUINAP adlayer. Much effort has been made to get the details of the molecular chirality, and the higher resolution STM image was acquired under carefully adjusted experimental conditions. The structural details of the molecule are almost the same as those shown in Figure 2b. It is clear that every three bright spots in Figure 3b represent an (S)QUINAP molecule. The three spots are also two elongated spots with a tail at the end of one of the elongated spots. This feature is marked by three ovals. Analogously, the three bright spots marked a, b, and c on the STM image in Figure 3b are attributed to the corresponding moieties of the quinoline, the naphthalene, and the PPh2. Carefully analyzing the experimental results, a very interesting phenomenon is that the three spots possess a clockwise arrangement from the quinoline ring to naphthalene to the PPh2 as shown in Figure 3b using a visualized symbol, the bent arrow. From the results mentioned above, both the (R)- and (S)-QUINAP have three bright spots in the STM image. Despite this similarity, the two images cannot be superimposed no matter what surface symmetry manipulation was employed. The two adlayers can be related only by mirror symmetry manipulation. That is to say, two different types of chiral adlayer are obtained. Analytical Chemistry, Vol. 76, No. 3, February 1, 2004

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Figure 3. (a) STM top view of (S)-QUINAP adlayer on Cu(111) in 0.1 M HClO4 at -0.25 V. Tunneling current was 5 nA. Scanning rate was 15.26 Hz. (b) High-resolution STM image of a Cu(111)-(4 × 4)-(S)-QUINAP adlattice acquired at -0.25 V. Tunneling current was 5 nA. Scanning rate was 17.35 Hz.

Axial chirality discrimination is difficult using STM. For example, in our previous study, the two naphthalene moieties bearing the same substituent connected to the axis show similar features in the STM image, which makes the determination of molecular chirality very difficult.21 In the present study, the PPh2 substituent serves as the “marker group” to distinguish the two similar naphthalene and quinoline moieties. By virtue of the position of the PPh2 substituent, the spots corresponding to naphthalene and quinoline moieties can be distinguished. Consequently, the aim of discrimination of absolute configuration of enantiomers is achieved. Recognition of the molecular chirality becomes possible by virtue of the position of the asymmetric (21) Han, M.-J.; Zeng, Q.-D.; Wan, L.-J.; Bai, C.-L. Chem. Lett. 2003, 32, 702.

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substituent. The principle used here is similar to that of Flynn’s for the discrimination of central chiral molecules.6 To confirm the rule, after the observation of chiral adlayers of (R)- and (S)QUINAP, respectively, on the Cu(111) surface, we explored the adsorption of the racemic mixture of QUINAP. A few minutes after injecting a racemic mixture of QUINAP solution into the STM cell, a uniform adlayer of racemic QUINAP was formed on the Cu(111) surface. The high-resolution STM images as shown in Figure 4 reveal that the adlayer consists of (R)- and (S)-QUINAP domains. However, no coexistence of (R)- and (S)-QUINAP in the same domain was observed. The molecular chirality in different domains can be identified with the help of the PPh2 substituent. As discussed above, the three spots possessing a counterclockwise arrangement from the quinoline ring to naphthalene to the PPh2 is a feature of the R-isomer while the three spots with a clockwise arrangement is a feature of the S-isomer. According to these features, the chirality of the molecule is discriminated directly. The universal feature of the adsorption geometries proposed for the superstructures of naphthalene on Cu(111), Rh(111), and Pt(111) studied so far is that all naphthalene molecules align their C2 axes along the close-packing direction of the substrate.18,22-24 Based on the above consideration and our STM images, the tentatively proposed models for the adlayers of (R)-QUINAP and (S)-QUINAP on the Cu(111) are depicted in Figure 5a. The projections of the naphthalene ring and quinoline ring by ∼45° with each other are arranged along with the 〈110〉 direction of the Cu(111) substrate. Because of the tilt configuration of naphthalene and quinoline, high-resolution STM images could not reveal the ring features of the molecules. To explain clearly the relationship of our models and STM images, the enlarged STM images of QUINAP were laid on one side of the models. We can see clearly from the STM images that each molecule possesses three spots just like the three ovals drawn on the models. The chiral character of each molecule is indicated by the curved arrows in the models. The mirror symmetry between the adsorption configurations of two molecules can be seen from the models. To provide clearer models commensurate with STM images, a side view of the model was shown in Figure 5b. After recording the images shown in Figures 2b and 3b at -0.25 V, the electrode potential was stepped from -0.3 to 0 V in increments of 10 mV. A set of STM images of the two molecular adlayers was acquired at different potentials during the potential steps. Similar images with the same molecular features were consistently observed. The similar electrochemical adsorption behavior of (R)-QUINAP and (S)-QUINAP adlayers indicates the same adsorption energy of enantiomers on the achiral surface. The present results demonstrate the absolute configuration of axially chiral molecules can be discriminated by STM in solution. However, the molecular structure and the substrate materials play an important role in molecular adsorption. Different structures could be obtained on different substrate materials and crystallographic surfaces. Therefore, to generalize the results and search for some rules, a systematic study of different axially chiral molecules with different substituents on different substrates such (22) Bardi, U.; Magnanelli, S.; Rovida, G. Langmuir 1987, 3, 159. (23) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1997, 101, 3547. (24) Hallmark, V. M.; Chiang, S.; Brown, J. K.; Wo¨ll, Ch. Phys. Rev. Lett. 1991, 66, 48.

Figure 4. High-resolution STM images of (R)-QUINAP and (S)-QUINAP adlayers in the different domains of the racemic mixture adlayer.

Figure 5. (a)Top view of molecular models of (R)-QUINAP and (S)-QUINAP monolayers on the Cu(111) with a structure of (4 × 4) symmetry. (b) Side view of the models.

as Au(111) and Pt(111) is necessary. A more detailed study is now in progress.

results presented here demonstrate that axial chirality could be analyzed by STM.

CONCLUSIONS The adsorption of enantiomeric QUINAP molecules has been studied on the Cu(111) electrode in solution by using ECSTM and CV. With the high-resolution STM image and the position of the PPh2 substituent, the chirality of (R)-QUINAP and (S)QUINAP is discriminated directly on the Cu(111) electrode in solution. The two molecules adsorb on Cu(111) in the same (4 × 4) symmetry, but the molecular configurations differ from each other due to the chirality of the molecules. Although the fcc(111) surface is not a chiral surface, the chiral feature of the molecules and the asymmetry substituent make discrimination possible. The

ACKNOWLEDGMENT Financial support from National Natural Science Foundation of China (20025308, 20177025, and 20103008), National Key Project on Basic Research (Grants G2000077501 and 2002CCA03100), and the Chinese Academy of Sciences are gratefully acknowledged.

Received for review July 9, 2003. Accepted November 11, 2003. AC0347720 Analytical Chemistry, Vol. 76, No. 3, February 1, 2004

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