Catalytic Chiral Metal Surfaces Generated by Adsorption of O-Phenyl

J. Phys. Chem. C 2007, 111, 9349-9358

9349

Catalytic Chiral Metal Surfaces Generated by Adsorption of O-Phenyl Derivatives of Cinchonidine Norberto Bonalumi, Angelo Vargas, Davide Ferri, and Alfons Baiker* Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering, ETH Zurich, Ho¨nggerberg, HCI, CH-8093 Zurich, Switzerland ReceiVed: February 26, 2007; In Final Form: April 5, 2007

Chirally modified platinum surfaces as used in enantioselective hydrogenation have been generated by adsorption of O-phenyl derivatives of cinchonidine on Pt/Al2O3 films. The adsorption behavior as well as the structure of the resulting chiral solid-liquid interfaces has been investigated using attenuated total reflection infrared (ATR-IR) spectroscopy. In particular O-phenyl-cinchonidine, O-(3,5-dimethylphenyl)-cinchonidine, and O-[(3,5-bis(trifluoromethyl)phenyl]-cinchonidine have been analyzed, and their behavior has been compared to that of the parent alkaloid. The ATR-IR spectroscopic investigation provided insight into submolecular structural details of the conformation of the adsorbates under conditions close to those existing during the catalytic enantioselective hydrogenation. As a complement to the spectroscopic observations, electronic structure calculations were performed using density functional theory (DFT). The result of the conformational study of each adsorbed modifier revealed a correlation between the spatial orientation of the substituted phenyl rings and the enantioselectivity of the catalyst observed in the asymmetric hydrogenation of ketopantolactone and other R-activated ketones. Experiments and calculations support an interpretation according to which the orientation in space of the phenyl ring reshapes the surface chiral space formed by the adsorbed modifiers, thus generating surfaces (and catalysts) having different, in some cases even opposite, enantioselective properties without altering the absolute configuration of the modifier. Competitive adsorption experiments between modifiers were also carried out in order to determine their affinity toward the metal and allow assignment of a relative adsorption strength scale.

Introduction Chemical routes to enantio-pure compounds have become a topic of great importance in the past decades, stimulating enormous progress in the field of homogeneous enantioselective catalysis1 and culminating in the Nobel Prize in Chemistry of 2001 being awarded to Sharpless,2 Noyori,3 and Knowles.4 Despite considerable effort, covered in several reviews,5 similar advancement could not be witnessed in heterogeneous enantioselective catalysis. Nevertheless, the intrinsic technical advantages of heterogeneous asymmetric catalysis related to easy separation, reuse, and stability of the catalyst are permanently spurring research in this area. Various strategies are known for generating chiral recognition on catalytic surfaces.6 To date the best studied heterogeneous catalytic systems include the tartaricacid-modified nickel catalyst for the enantioselective hydrogenation of β-keto esters7 and the cinchona-modified platinum and palladium systems for the enantioselective hydrogenation of R-ketoesters8 and activated CdC bonds,8b,9 respectively. It is generally accepted that the role of the modifier is that of generating surface chiral sites. Substrate and adsorbed chiral modifiers generate a 1:1 interaction which is critical for enantiodifferentiation.10-16 In the case of platinum (and palladium) chirally modified by cinchona alkaloids, the structure of the modifier is characterized by an anchoring moiety (quinoline ring), one or more stereogenic atoms that impart the asymmetry to the environment, and a basic nitrogen (quinuclidine moiety), likely responsible for the docking of the substrate * To whom correspondence should be addressed. E-mail: alfons. [email protected]. Tel.: +41 44 63 23 153. Fax.: +41 44 63 21 163.

in the chiral site.14 These modifiers possess a rich conformational chemistry in solution, which is retained to some extent upon adsorption on a metal surface.17 A striking feature of these systems is the possibility to invert the sense of enantiodifferentiation by applying pairs of enantiomeric18 and diastereomeric19 modifiers. Diastereomeric pairs include cinchonidine-cinchonine (CD-CN) and quininequinidine (QN-QD) in which CD and QN bear opposite absolute configuration at C(8) and C(9) with respect to CN and QD (Scheme 1). These so-called pseudoenantiomers (only two of the four stereogenic centers are inverted) lead to opposite enantioselectivity.19 Such pairs are also characterized by a different adsorption potential, which allows the development of enantioswitching devices.20 Although inversion of enantioselectivity within cinchona alkaloids has been known for decades, only recently the investigation of ether derivatives of CD demonstrated that O-phenyl-cinchonidine (PhOCD), although having the same absolute configuration of CD, leads to opposite enantioselectivity compared to the parent alkaloid.21 Another recent study22 uncovered that adsorption of CD and PhOCD is similar and attributed the differences in enantioselection to the position of the phenyl ring within the chiral site, therefore to the reshaping of the asymmetric environment of the site with respect to CD. This shows that the opposite absolute configuration of the alkaloids is not a necessary condition for obtaining inversion of enantioselectivity and opens perspectives for the tuning of enantioselection and catalyst tailoring. In order to further understand the relation between modifier structure and enantioselectivity, O-phenyl derivatives of CD

10.1021/jp071572+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007

9350 J. Phys. Chem. C, Vol. 111, No. 26, 2007 SCHEME 1: Heterogeneous Enantioselective Hydrogenation of Ketopantolactone over Pt/Al2O3 Chirally Modified by Ether Derivatives of Cinchonidine.

bearing substituents at the phenyl ring were prepared and tested.21,23 Indeed, the enantioselectivity induced by the chiral modifiers is affected by the particular substitution applied to the ring. The hydrogenation of ketopantolactone (1) to pantolactone (2) over Pt/Al2O3 (Scheme 1) modified by O-(3,5dimethylphenyl)-cinchonidine (dMePhOCD) afforded 36% ee (S)-2, whereas the same reaction using O-[(3,5-bis(trifluoromethyl)phenyl]-cinchonidine (tFPhOCD) as modifier afforded 16% ee (R)-2.23 Intrigued by this observation, we started to investigate the structural changes induced in the adsorbed modifier upon introduction of different ring substituents, as reported in a recent communication.24 Here we present a detailed structural analysis of the role of the ring substituents in reshaping the surface chiral space. Attenuated total reflection infrared (ATR-IR) spectroscopy24-26 complemented by electronic structure calculations using density functional theory (DFT) is employed to garner a submolecular understanding of the possible roles of subunits within the modifiers. It is a fact that the great conformational flexibility of the modifiers is a reason for the versatility of the catalytic system, which also complicates the achievement of a simple understanding of the surface sites. Only the combination of methods seems to provide a sufficient level of understanding useful for the further development of these catalysts. Affinity of modifiers to the metal is critical for the formation of surface chiral sites. Differences in adsorption energies between modifiers can in fact steer the dynamic change of enantioselective properties of a chirally modified metal.20,27,28 Furthermore knowledge of the features of a modifier that are able to influence the anchoring to the metal provides useful insight for catalyst tailoring. For this reason, to complete our understanding, adsorption competition experiments between the modifiers in study have been carried out in order to evaluate a relative scale of adsorption strength.

Bonalumi et al. Experimental Methods Materials. Cinchonidine (CD, Fluka, 98%), O-phenyl-cinchonidine (PhOCD, Ubichem, > 99.8%), O-(3,5-dimethylphenyl)-cinchonidine (dMePhOCD, Ubichem, > 98.5%), and O-[(3,5bis(trifluoromethyl)-phenyl]-cinchonidine (tFPhOCD, Ubichem, >99.8%) were used as received. Dichloromethane solvent (Baker, technical grade) was stored over 5 Å molecular sieves. N2 (99.995 vol %) and H2 (99.999 vol %) gases were supplied by PANGAS. Film Preparation. The Pt/Al2O3 thin films used for ATRIR spectroscopy were prepared on a trapezoidal Germanium internal reflection element (IRE, 50 × 20 × 1 mm3) by electron beam vapor deposition. Pt and Al2O3 targets were heated with an electron beam as described in detail elsewhere.29 First, 50 nm Al2O3 were deposited followed by 1 nm Pt. ATR-IR Spectroscopy. In situ ATR-IR spectra were recorded on a Bruker IFS-66/S spectrometer equipped with a commercial ATR accessory (Optispec) and a liquid-nitrogencooled MCT detector. Spectra were collected by co-adding 200 scans at 4 cm-1 resolution. After cell mounting and optics alignment, the probe chamber was purged overnight with N2 gas. The assembled stainless steel flow-through cell was maintained at 293 K throughout the experiments. N2-saturated CH2Cl2 was circulated over the thin film at 1.0 mL/min for about 45 min using a peristaltic pump till achievement of steady-state conditions, under which the infrared spectra did not change significantly. Before adsorption, the Pt film was treated with H2-saturated solvent for about 3 min. An H2-saturated solution of the modifier under investigation at the desired concentration was then admitted to the cell for about 1 h followed by rinsing with H2-saturated solvent in order to remove weakly adsorbed and dissolved species. Adsorption competition experiments were performed by alternatively pumping CH2Cl2 solutions of two modifiers from separate bottles followed by rinsing with H2saturated CH2Cl2. All spectra are presented in absorbance units and, where needed, signals from atmospheric water were subtracted in the 1700-1400 cm-1 spectral range. Spectra of CH2Cl2 solutions and neat liquids were obtained in the transmission mode (200 scans, 4 cm-1 resolution). Theoretical Calculations. Normal-mode analyses were performed with the Gaussian 98 set of programs,30 using the density functional theory formalism with a B3LYP Hamiltonian31,32 and a 6-31G(d-p) basis set.33 Adsorption studies have been performed using the Amsterdam density functional (ADF) program package.34 The surface was simulated using a Pt 38 cluster as described in detail elsewhere.17,35 A frozen core approximation was used for the description of the inner core of the atoms. Orbitals up to 1s were kept frozen for the second row elements, whereas orbitals up to 4f were kept frozen for platinum. Decreasing the Pt frozen core to 4d, which implies the explicit calculation of 14 additional electrons per platinum atom, has been shown to increase the adsorption energy by only about 5 kJ/mol for the adsorption of benzene.36 The importance of relativistic effects has been shown for calculations involving platinum;37,38 therefore, the core was modeled using a relativistically corrected core potential created with the DIRAC utility of the ADF program. The DIRAC calculations imply the local density functional in its simple X-R approximation without any gradient correction, but the fully relativistic Hamiltonian is used, including spin-orbit coupling. Furthermore, the relativistic scalar approximation (mass-velocity and Darwin corrections) was used for the Hamiltonian, with the zero-order regular approximation (ZORA) formalism,39

Catalytic Chiral Metal Surfaces

Figure 1. Transmission IR spectra of (a) cinchonidine (CD) and its derivatives: (b) O-phenyl-cinchonidine (PhOCD), (c) O-(3,5-dimethylphenyl)-cinchonidine (dMePhOCD), and (d) O-[3,5-bis(trifluoromethyl)phenyl]-cinchonidine (tFPhOCD), (conc. ) 10 mM, CH2Cl2 solvent). The direction of the dynamic dipole moment associated with signals of the phenyl rings is schematically shown by blue (parallel to the phenyl ring) and red (normal to the ring plane) arrows. (b) Interference due to solvent signals.

where spin-orbit coupling is included already in zero order. It was shown that the scalar relativistic correction could account for up to 70% of the total energy in the adsorption of carbon monoxide on platinum and that also the calculated adsorption site was influenced by the use of relativistic corrections.37 The ZORA formalism requires a special basis set, to include much steeper core-like functions, implemented in the code. Within this basis set the double-ζ (DZ) basis functions were used for platinum. For second row elements and hydrogen, double-ζ polarized (DZP) basis functions were used.40 The local part of the exchange and correlation functional was modeled using a Vosko, Wilk, and Nuisar parametrization of the electron gas.41 The nonlocal part of the functional was modeled using the Becke correction for the exchange42 and the Perdew correction for the correlation.43 All calculations were run unrestricted. Catalytic Tests. Catalytic tests were performed using a parallel pressure reactor system (Endeavor, Argonaut Technologies).23 Conditions: 42 mg of prereduced 5 wt % Pt/Al2O3, 1.84 mmol of substrate (ketopantolactone) and 6.8 µmol of modifier in 5 mL of THF, room temperature at 1 bar H2 pressure for 2 h. Results Transmission IR Spectra. The transmission IR spectra of CH2Cl2 solutions of the O-phenyl ether derivatives of cinchonidine (Figure 1) are the primary reference for the analysis of the spectra of the adsorbed compounds. A complete theoretical study

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9351 of the vibrational modes of the modifiers in the most stable conformation in vacuum (Open 3) has been performed (Table 1). The 1650-1450 cm-1 spectral range contains common diagnostic vibrations to all modifiers. The signal at 1636 cm-1 belongs to the vinyl group ν(CdC) of C10-C11.44 This is detectable only in the transmission spectra since it is typically hydrogenated in the presence of the Pt catalyst under the experimental conditions used.26,45 The signals at 1590, 1570, 1530, and 1510 cm-1 have been assigned26,46 to a combination of deformation and in-plane δ(C-H) modes of the quinoline ring, whereas the quinuclidine moiety shows collective δ(CH) vibrational modes as a broad signal at ca. 1460 cm-1. Additionally, the characteristic features of the infrared spectrum of the phenyl derivatives of CD are the signals associated with the quadrant and semicircle ring stretches (ν(CC) and in-plane δ(C-H)) of the O-phenyl-moiety (Figure 1 and Table 1). These signals exhibit different energies according to the substitution on the phenyl ring and overlap with those of the quinoline and quinuclidine rings. They are observed at 1598 and 1495 cm-1 in PhOCD, at 1613 and 1468 cm-1 in tFPhOCD, and at 1613, 1592, and ca. 1470 cm-1 in dMePhOCD. In the low-frequency region, dMePhOCD exhibits two bands at 1167 and 1154 cm-1 (Figure 1, coupled in-plane ν(C-CH3) and δ(C-H) modes). The signal at 1323 cm-1 is an in-plane deformation of the phenyl ring coupled with ν(C-CH3) and an out-of-plane δ(C-H) of the methyl groups. The calculated spectrum indicates that the dynamic dipole moment of the two former modes is in the plane of the ring (as illustrated in Figure 1), whereas that of the latter mode is almost normal to the plane of the ring. The signal at 1378 cm-1 in the spectrum of tFPhOCD is assigned to a coupled νs(C-CF3) and δ(C-H) mode and displays the dynamic dipole moment in the plane of the ring and parallel to the main C2 axis. The signal at 1137 cm-1 is composed of two signals associated with the CF3 groups (Table 1): the first (prominent) was calculated at 1145 cm-1 and the second (very weak) at 1143 cm-1. Both modes have the dynamic dipole moments perpendicular to the plane of the ring. Finally, the signal at 1181 cm-1 originates also from the overlap of two vibrational modes similar to the previous one but with a contribution from an in-plane δ(C-H) mode. The first mode (calculated at 1174 cm-1) is the strongest and should dominate over the second one (calculated at 1168 cm-1) in the experimental spectrum. For this reason, we assume in first approximation that the signal of tFPhOCD at 1181 cm-1 is due to a vibrational mode characterized by a dynamic dipole moment parallel to the plane of the ring and oriented along the C2 axis (C(1′′)-C(4′′)) of the phenyl ring (Scheme 1). ATR-IR Spectra of Adsorbates. Since the behavior of the modifiers on Pt will be analyzed in comparison with the adsorption of the parent alkaloid CD, it is useful to briefly summarize the main features of the adsorption of the latter.26 CD adsorbed on a polycrystalline platinum surface as that used in this study in the presence of H2-saturated CH2Cl2 displays three different adsorption modes competing for Pt sites: (i) a strongly adsorbed species, with the quinoline moiety predominantly parallel to the surface plane and characterized by a signal at 1570 cm-1; (ii) a weakly adsorbed species, N-lone pair bonded, having the quinoline ring tilted with respect to the plane of the metal, and associated with the signals at 1590 and 1510 cm-1; (iii) another weakly adsorbed species, called R-quinolyl, also characterized by a tilted geometry of the quinoline moiety on Pt and associated with a signal at 1530 cm-1 (absent in the

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TABLE 1: Assignment of Major Vibrational Modes of dMePhOCD and tFPhOCD in Solution, on Pt/Al2O3, and Calculated (Conformers Open 3 in the Vacuum, See Theoretical Calculations for Details)

dMePhOCDa

tFPhOCDa -1

vibrational frequencies (cm ) solb 1636 1613 1613 1592 1590 1570 1510 ∼1470 ∼1460 ∼1460 1324

1167 1167 1154 1154 1154

Ptc n.o. 1613 1613 1592 15903 15701,2,3 15302 15103

calcd

solb

1654 1604 1598 1585 1579 1556

1636 1613 1613 1590 1570

1495 1465

∼1460 ∼1460

1460 1451

1323

1320

1167 1167 1151 1151 1151

vibrational frequencies (cm-1) calcd

description of the vibrational mode

n.o 1613 1613

1654 1604 1593

15903 15701,2,3 15302 15103

1580 1556

∼1468 ∼1468 ∼1450 ∼1450 ∼1450

1441 1447 1464 1452 1437

1378 1181 1181

1372 1181 1181

1343 1174 1168

1137 1137

1137 1137

1145 1143

(C10-C11), ν(CdC) QN, ring stretching + δ(C-H) Ph, δ(CC) + δ(C-H) Ph, δ(CC) + δ(C-H) QN, ring stretching + δ(C-H) QN, ring stretching + δ(C-H) QN, ring stretching + δ(C-H) QN, ring stretching + δ(C-H) Ph, δ(CC) + δ(C-H) Ph, δ(CC) + δ(C-H) Ph, δ(CC) + δ(C-H) QD, δas(C-H) QD, δas(C-H) QD, δas(C-H) Ph, ν(CC) + νs(C-CH3) + δ (C-H) Ph, νs(C-CF3) + δ(C-H) Ph, νas(C-F) + δs(C-CF3) + δas(C-H) Ph, νas(C-F) + δas(C-CF3) Ph, ν(C-CH3) + δs(C-H) + QD, δas(C-H) Ph, ν(C-CH3) + δs(C-H) Ph, ν(C-CH3) + δs(C-H) + QD, δas(C-H) Ph, ν(C-CH3) + δs(C-H) Ph, ν(C-CH3) + δs(C-H) + QD, δas(C-H) Ph, νas(C-F) + δs(C-CF3) Ph, νas(C-F) + δas(C-CF3)

1510 ∼1468 ∼1468 ∼1460 ∼1460 ∼1460

Ptc

1496

1153 1149 1142 1138 1133

a For adsorbed dMePhOCD and tFPhOCD different species are observed: 1 ) flat adsorbed; 2 ) R-quinolyl; and 3 ) N-lone pair bonded see text for details. b IR spectrum of 0.01 M solution in CH2Cl2. c ATR-IR spectra of 0.5 mM solution in CH2Cl2 on Pt/Al2O3. d DFT calculations performed using the B3LYP method with the 6-31G(d,p) basis set (see text for details). A scaling factor of 0.961 has been applied. Vibrational mode description using the following notation: QN ) quinoline group; QD ) quinuclidine group; Ph ) O-phenyl group; n.o. ) not observed; νs ) symmetric stretching; νas ) asymmetric stretching; δs ) symmetric bending; δas ) asymmetric bending.

transmission IR spectrum, Figure 1a-d). The flat species resists desorption, whereas the other two species mostly disappear during rinsing with H2-saturated CH2Cl2. Figure 2 shows that the 1600-1500 cm-1 spectral range is dominated by the signals of the adsorbed quinoline ring, similarly to CD26 and PhOCD.47 Since beside the signals of the phenyl ring the other features appear very similar, the quinoline ring behaves as the anchoring part of all the molecules in this study,26,48 and species evolve on the metal surface that exhibit identical orientation of the quinoline ring to that shown by CD. However, these species may differ for the orientation of the quinuclidine and phenyl rings. As mentioned above, the flat species is the most stable on the metal surface. In the same region of the signals of the quinoline ring also the modes of the phenyl ring of each O-phenyl derivative can be observed. Since these signals are still detected at the adsorption step if followed by rinsing with solvent, the phenyl ring cannot be oriented parallel with respect to the surface in the strongly

adsorbed species. For such orientation of the phenyl moiety, the surface selection rules do not allow detection of signals on this frequency region. The ATR-IR spectra of adsorbed PhOCD showed two characteristic signals of the fundamental vibrations of the phenyl ring (1601 and 1497 cm-1).22 Theoretical calculations49 indicate that the dynamic dipole moment of these modes is nearly parallel to the plane of the phenyl ring and oriented along the main C2 axis; therefore, they become IR active when a component of the dipole moment is perpendicular to the metal surface. This indicates that there exist adsorbed species of PhOCD for which the phenyl ring is oriented nonparallel with respect to the surface plane, for low modifier concentration.22 Interestingly, the quinuclidine moiety, characterized by the signal at ca. 1460 cm-1, is almost silent in the spectra of PhOCD (Figure 2b). In dMePhOCD and tFPhOCD, this signal is hardly separable from the signal of the CH3 vibrations and from that of the fluorinated phenyl ring, respectively. A close observation

Catalytic Chiral Metal Surfaces

Figure 2. ATR-IR spectra of (a) CD, (b) PhOCD, (c) dMePhOCD, and (d) tFPhOCD adsorbed on Pt/Al2O3. Spectrum (e) was recorded after rinsing with H2-saturated CH2Cl2 after adsorption of tFPhOCD. Conditions: Cmodifier ) 0.5 mM, H2-saturated CH2Cl2 solvent, 293 K, ∼1 h on stream (1 mL/min). Signals between 1650 and 1500 cm-1 are representative of three coexisting adsorbed species, but exhibiting a different orientation of the quinoline ring with respect to the surface plane. 1570 cm-1: flat species; 1590 and 1510 cm-1: tilted species (N-lone pair bonded); 1530 cm-1: R-quinolyl species. Negative signals at ca. 1400 cm-1 and at 1338 cm-1 are typical of methylene and ethylidine species, respectively removed from the metal surface upon adsorption of the chiral modifier (see ref 26).

of the spectrum shown in Figure 2d reveals the presence of a shoulder at ca. 1450 cm-1 suggesting a similar orientation of the quinuclidine moiety for CD and tFPhOCD, whereas its absence in the spectrum of dMePhOCD (Figure 2c) suggests a similar situation as for PhOCD (Figure 2b). Beside the signals of the quinoline and phenyl rings, the ATRIR spectrum of dMePhOCD exhibited signals associated with the methyl groups (1167 and 1151 cm-1) that remained unperturbed upon adsorption of the molecule on Pt and were still detectable after rinsing of the surface (Figure 3a-c), suggesting that these groups are unaffected by the metal. On the contrary, the signal at 1324 cm-1 was much weaker than in the transmission spectrum (Figure 2c), information that together with the previous one indicates that the phenyl ring adopts a well defined orientation with respect to the surface. The spectral range below 1200 cm-1 is the most interesting for tFPhOCD in that the behavior of the signals at 1181 and 1137 cm-1 is dissimilar from that found in solution as shown in Figure 3. In the early stages of adsorption, only the signal at 1181 cm-1 is detected (Figure 3d). The signal at 1132 cm-1 appears with delay, and after ca. 8 min (Figure 3e), the ratio of the intensity of the two signals approximately equals that observed in the transmission spectrum. After rinsing (Figure 3f) which affords predominantly the species with the quinoline ring parallel to the surface plane, this ratio changed again in favor of the signal at higher frequency. As mentioned above, these two signals are associated with dynamic dipole moments

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9353

Figure 3. Left panel: ATR-IR spectra of dMePhOCD adsorbed on Pt/Al2O3 at 293 K (CdMePhOCD ) 0.5 mM, traces a and b) spectra were recorded within 40 min. Spectrum c was recorded after rinsing with H2-saturated CH2Cl2 for 50 min. Right panel: ATR-IR spectra of tFPhOCD adsorbed on Pt/Al2O3 at 293 K (CtFPhOCD ) 0.5 mM, traces d and e), spectra were recorded within the first 8 min. Spectrum (f) was recorded while rinsing with H2-saturated CH2Cl2 for 40 min.

having a specific orientation (Figure 1); thus, the adsorbed modifier has stabilized on a particular conformation. It should be noted that the signal at 1378 cm-1 is silent after rinsing with the solvent (Figure 2e), indicating that it does not belong to the most strongly adsorbed species. As mentioned above the dipole moment of this mode is approximately parallel to the C2 axis, whereas in the free anisole derivative it is perpendicular to it.24 This means that the contribution of the rest of the cinchona molecule in tFPhOCD affects this dipole more than it does in the two modes below 1200 cm-1, which bear dipole moments identical to those of the free anisole. We attribute this apparent discrepancy to the fact that adsorbed tFPhOCD exhibits a specific orientation on the surface, which is totally different from that calculated in the vacuum, as it will be clear in the following. Competitive Adsorption between Modifiers. As shown in Table 2, during the hydrogenation of 1 to 2 (Scheme 1), the enantioselectivity observed in catalytic tests where pairs of modifiers were added to the reaction mixture was not proportional to the relative concentration of the modifiers in the reactor, indicating a nonlinear phenomenon.18 This nonlinearity has been attributed to differences in the adsorption strength of the modifiers,27,50 and thus to their ability to modify a surface. Therefore, competition experiments were performed in order to evaluate the relative adsorption strength of the chiral modifiers.20,22,28 The spectroscopic analysis is performed by alternately admitting two solutions of different modifiers with the same concentration to the Pt surface. Characteristic signals of the modifiers like that at 1497 cm-1 in the case of PhOCD are useful to follow the behavior of the adsorbed modifier as described in ref 22. For example, this signal vanished when replacing PhOCD by CD in contrast to the signals of the

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TABLE 2: Catalytic Hydrogenation of Ketopantolactone in THF (1 bar; ee (%), [Conversion]), Using Different Mixtures of Cinchonidine Modifiersa

a

Conditions: 42 mg of catalyst (Pt/Al2O3 5 wt %, Engelhard, type 4759), 1.84 Mmol substrate, 6.8 µmol modifier, 5 mL of THF, T ) 293 K.

quinoline ring; in the reverse experiment (pre-equilibration with CD followed by PhOCD) the same signal was less intense and disappeared upon rinsing with H2-saturated solvent. This behavior indicates the relatively fast displacement of PhOCD from the metal surface by CD. Thus CD generates a more stable layer and is more strongly adsorbed than PhOCD. In the case of the present derivatives of CD, the signals below 1200 cm-1 are more prone than those in 1600-1500 cm-1 region to be used as indicators for the displacement of modifiers by analogous molecules (Figure 4), thus this frequency region appears more suited for discrimination and for orientation studies. The ATR-IR spectra of the competition between dMePhOCD and PhOCD (dMePhOCD-PhOCD) showed that after equilibration of Pt with dMePhOCD, rinsing with H2saturated solvent and admission of a solution of PhOCD the signal of PhOCD at 1497 cm-1 immediately appeared and resisted desorption. This experiment demonstrates that PhOCD and dMePhOCD coexist on the Pt surface. This is best shown in the low frequencies region (Figure 4A) where the characteristic signals of dMePhOCD appear at 1167 and 1151 cm-1 and reveal traces of adsorbed dMePhOCD also after the final rinsing of the surface with solvent. The experiment carried out in the opposite sequence (PhOCD-dMePhOCD) displayed an identical behavior (not shown) with a clear signal at 1497 cm-1 confirming the presence of adsorbed PhOCD. Figure 4B shows the spectra of the dMePhOCD-tFPhOCD competition. The signals of the trifluoromethyl groups of the tFPhOCD at 1181 and 1137 cm-1 clearly appear in the spectrum after surface pre-equilibration with dMePhOCD, and their intensity ratio is different from that in the transmission spectrum suggesting that tFPhOCD interacts with the metal surface on dMePhOCD-covered Pt. Nevertheless, rinsing with solvent and admitting of dMePhOCD causes the displacement of tFPhOCD from the surface (Figure 4Bc) and the disappearance of the signals at 1181 and 1137 cm-1. The experiment demonstrates that although a mixed chiral adsorbed layer is formed on Pt, dMePhOCD is more strongly adsorbed than the fluoro-derivative. This becomes even more obvious from the reverse experiment (tFPhOCD-dMePhOCD, Figure 4B (e-h)) showing that the signals of tFPhOCD were replaced by the signals of dMePhOCD, which persisted after the rinsing. Finally, Figure 4C shows the ATR-IR spectra of the tFPhOCD-CD competition. In this case the 1600-1500 cm-1

Figure 4. ATR-IR spectra of adsorption competition experiments. Conditions: Cmodifier ) 0,5 mM at 293 K adsorbed from H2-saturated CH2Cl2. Panel A: (a) adsorption of dMePhOCD after 10, 15, 25, 35, and 60 min on stream; (b) rinsing with H2-saturated CH2Cl2; (c) flowing PhOCD (spectra taken at 5, 10, 30, 45, min from step b); (d) rinsing with H2-saturated CH2Cl2. Panel B: (a) adsorption of dMePhOCD (40 min); (b) flowing H2-tFPhOCD for 40 min after dMePhOCD; (c) flowing dMePhOCD after tFPhOCD; (d) rinsing with H2-saturated CH2Cl2. (e) Adsorption of tFPhOCD after 45 min on stream; (f) flowing dMePhOCD for 40 min after tFPhOCD; (g) flowing tFPhOCD; (h) rinsing with H2-saturated CH2Cl2. Panel C: (a) adsorption of tFPhOCD (40 min); (b) flowing CD for 40 min after tFPhOCD; (c) flowing tFPhOCD after CD; (d) rinsing with H2-saturated CH2Cl2. (e) Adsorption of CD (30 min); (f) flowing tFPhOCD for 40 min after CD; (g) flowing CD; (h) rinsing with H2-saturated CH2Cl2.

spectral range is also indicative (not shown). Admission of tFPhOCD to Pt generates a signal at 1613 cm-1 that is peculiar only for the fluorinated phenyl ring (Figure 1(d), Table 1; ring deformation + in-plane δ(C-H)). Although already present, signals at 1510 and 1590 cm-1 (tilted species) were considerably enhanced after changing to CD whereas the signal at 1613 cm-1 almost vanished. Both signals below 1200 cm-1 appeared as after rinsing with solvent following tFPhOCD adsorption (Figure 3) and indicated that this modifier was still present on Pt despite CD. However, admission of tFPhOCD again was not able to restore the intensity of the signals of this modifier back to the initial values of Figure 4C(a). This indicates that tFPhOCD is not able to displace CD from the surface but can compete with it for surface sites. On the other hand, the reverse experiment (CD-tFPhOCD), Figure 4C(e-h)) shows that the adsorbed layer of CD is far more stable than that created by tFPhOCD, because surface pre-equilibration with CD did not allow adsorption of tFPhOCD. Note that in Figure 4C(f) the signals of the fluoro-derivative appear close to those observed in the transmission spectrum (Figure 1d) and suggest that the PttFPhOCD interaction was inhibited by CD. DFT Studies of Surface Conformations. Calculating surface conformers of the modifiers helps the detailed and submolecular

Catalytic Chiral Metal Surfaces

Figure 5. Calculated SQB(1) structure of PhOCD indicating the additional degrees of freedom with respect to CD inherent to the presence of the phenyl ring. Beside τ1 and τ2, the torsional angles τ3 and τ4 describe the rotation around the ether connectivity and around the C2 axis of the ring (C1′′-C4′′, Scheme 1), respectively.

determination of the geometries of the chiral sites. The adsorption of CD and that of PhOCD has been investigated in some detail using DFT.14,22,24,35,51 Figure 5 shows a view of one of the minimum energy conformations of PhOCD that helps to visualize the conformational degrees of freedom characteristic of this kind of modifier. This species is equivalent to the one previously reported22 but has been refined and completed by the addition of the quinuclidine moiety. The chiral space in proximity of the adsorbed molecule has a remarkable conformational complexity and can depend on the particular conformation assumed by the subunits of the modifier (quinoline, quinuclidine, phenyl ring). The values of the torsional angles τ1 and τ2 determine the existence of several conformers characterized by a different orientation of the quinuclidine moiety,17 and simultaneously, the torsional angles τ3 and τ4 identify conformers having different positioning of the phenyl ring. Figure 6 shows three different conformers of PhOCD, which lay within an energy difference range of 2 kcal/mol. The arrows on the phenyl ring correspond to a C2 symmetry axis, indicating the rotational degree of freedom of the ring and emphasizing the different position in space assumed by this moiety. Conformations A and B originate from the same adsorption geometry, where the quinuclidine nitrogen is bound to the metal. For CD this conformation has been called surface quinuclidine bound (1) (SQB1)51 (Figure 7E) since it is characterized by the binding of the quinuclidine moiety to the surface. In the case of PhOCD (Figure 6), this same surface species allows two positions of the phenyl ring: (i) that in Figure 6A where the phenyl ring is above the anchoring group, and far from the surface, and (ii) that in Figure 6B where the phenyl ring is on the contrary directly above the metal surface, and perpendicular to it (more correctly, the plane identified by the phenyl ring is perpendicular to the plane of the metal surface). The conformations described above could have different catalytic behavior because the reaction takes place on the surface, in the space confined between the metal atoms and the skeleton of the modifier. In one case (Figure 6A), the phenyl cannot interfere with an incoming substrate, since it is above the quinoline ring. This absence of the phenyl ring close to the metal reactive site virtually reproduces the chiral space displayed by CD. On the contrary, in the other case (Figure 6B,C), the phenyl ring can hinder the access to some metal sites close to the modifier, which are critical for the docking of the incoming substrate. It should be noted that in conformation B the phenyl ring can oscillate around its C2 axis, thus generating an almost spherical steric impediment near the surface.

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9355 The conformation depicted in Figure 6C is quite different from the preceding two, since the quinuclidine moiety in this case is detached from the metal, while the phenyl ring is partially chemisorbed parallel to the metal. It should be noted that in this case the space directly adjacent to the modifier is changed, as in conformation B, and this conformation could thus give rise to a catalytic behavior different from that of CD. Due to their proximity in terms of energy, each of the three conformations A-C could generate surface sites. These calculations help confining the possible behaviors that such complex organic molecules can take in contact to a platinum surface. The same structures shown in Figure 6 were calculated for tFPhOCD. Comparing these two modifiers was chosen since they yield opposite enantiomers in catalytic reactions where they are used as modifiers.21,23 The structures are not shown for economy of space, but the main results are that (i) the analogous of structure C does not exist, due to the low affinity of the 3,5trifluorinated phenyl ring to the surface,52 and (ii) the difference between the energy of conformations A and B increases in favor of A, so that A results more stable than B by ca. 4 kcal/mol. Agreement between Computed Structures and Infrared Spectra. The O-phenyl moieties of structures A and B in Figure 6 are in principle detectable by ATR-IR spectroscopy since the phenyl rings have components of the dynamic dipole moment normal with respect to the metal surface. On the contrary, according to the surface selection rule, the O-phenyl moiety of structure C is not spectroscopically detectable, and other signals must be used for its identification. In particular Figure 6 shows that the quinuclidine unit of conformation C changes its orientation with respect to the other two. The signal at ca. 1460 cm-1 results from combination of the C-H scissor modes of the quinuclidine skeleton which induce a dynamic dipole moment vector along the N-C(4) axis (Scheme 1).26,46 Studying the changes in intensity of this signal makes it possible to follow qualitatively the orientation of this subunit: passing from CD (Figure 2a) to PhOCD (Figure 2b) the ATR-IR spectra show that this signal vanishes almost completely, indicating that the surface conformer of PhOCD is characterized by a different orientation of the quinuclidine moiety with respect to CD. PhOCD and dMePhOCD are lacking the signal at ca. 1460 cm-1; therefore, we can tentatively assume that they both have the quinuclidine ring oriented differently from that of the CD. In tFPhOCD, the signals belonging to the vibrations of the phenyl ring overlap with that of the quinuclidine ring, but a shoulder around 1450 cm-1 indicates an orientation of the quinuclidine ring similar to that of CD. In conclusion, the infrared spectra indicate that conformation C of PhOCD is also populated on the platinum surface and that the position of the phenyl ring in dMePhOCD is very similar to that of PhOCD thus pointing to the structures in Figure 6. On the other hand, tFPhOCD seems to adopt a different conformation: also in this case the phenyl ring is tilted with respect to the surface plane but the ring is oriented with the C2 axis approximately perpendicular and far-off from the metal surface, as shown in Figure 7D, and in contrast to PhOCD and dMePhOCD. Calculations show that for tFPhOCD the stability of conformation A increases by ca. 4 kcal/mol. This is likely an effect of the strong dipole moment introduced by the electron attractive groups, due to which the change of the relative population of conformers can occur. Furthermore the conformation of tFPhOCD corresponding to the C surface conformation of PhOCD, where the phenyl moiety has the plane parallel to the metal surface and is partially adsorbed, is not feasible. It has already been shown that 3,5-trifluoromethylation of the phenyl ring

9356 J. Phys. Chem. C, Vol. 111, No. 26, 2007

Bonalumi et al.

Figure 6. Three relevant surface conformations of PhOCD, the differences being in a range of ca. 2 kcal/mol. Note that in conformers B and C the phenyl ring is close to the metal and can interfere in the interaction with the substrate, whereas in conformation A the chiral space is similar to that of CD. Calculations have been performed on a Pt 38 cluster (see Theoretical Calculations section), and are graphically represented on an extended surface.

Figure 7. (D) tFPhOCD shows the presence of a phenyl ring oriented away from the metal surface. (E) Surface quinuclidine bound (SQB1) of CD; the orientation of quinuclidine generates the same chiral space held by the conformation A. Calculations have been performed on a Pt 38 cluster (see Theoretical Calculations section) and are graphically represented on an extended surface.

greatly reduces its affinity for the Pt surface.52 Hence, according to both infrared data and computational results, the phenyl ring of tFPhOCD is mostly positioned in a different way compared to PhOCD and dMePhOCD, as conformation A shown in Figure 6. Discussion Performing enantioselective heterogeneous catalysis by means of modified transition metal surfaces requires control of the chiral sites which are formed after adsorption of the modifier molecules. CD has a uniquely shaped structure for this chemistry, and its ether derivatives show properties of great interest, being able to invert the enantioselectivity with respect to it, without changing stereogenic centers.21 In addition the insertion of an ether moiety to CD affects the adsorption potential of the alkaloid, thus allowing dynamic changes of surface coverage and catalytic activity.22 In particular O-phenyl derivatives of CD have the property of changing enantioselectivity by simply introducing substituents to the phenyl ring: although the 3,5-dimethyl substitution leads to the formation of an excess in the (S)-enantiomer of the alcohol in the hydrogenation of KPL (Scheme 1), in the same reaction the 3,5-bis-trifluoromethyl substitution leads to an excess in the (R)enantiomer. Changes of enantioselectivity could in principle be due to two distinct factors: i) a change in the anchoring mode of the modifier to the metal, or ii) a conformational rearrangement of the flexible parts of the modifier while maintaining the same

anchoring. The adsorption of CD on Pt has been thoroughly investigated using vibrational spectroscopy,26,53,54 and it seems now established that its catalytically active adsorption mode has the quinoline ring bound to the metal. ATR-IR spectroscopy delivers the fundamental information that also its O-phenyl derivatives exhibit the same adsorption pattern, identified by the quinoline fingerprints in the 1600-1500 cm-1 spectral range. Thus adsorption sites of these modifiers have the same anchoring to the metal as CD, based on the chemisorption of quinoline. The immediate consequence is that the O-phenyl moiety is responsible for the structural changes within the chiral site which is similar to that of CD and leads to an inversion of selectivity in the docking of the substrate. Computational studies on the conformations of the adsorbed modifiers help to define more accurately the position of the phenyl moiety. For PhOCD it emerges that the phenyl group can (i) be removed from the chiral space by rotation of τ3 thus leading to a similarly stable surface conformation as shown in Figure 6A; (ii) occupy the chiral space with the plane of the ring perpendicular to the surface (Figure 6B); (iii) be chemisorbed to the metal, with the plane of the ring parallel to the surface (Figure 6C). The spectroscopic investigation on the adsorption of PhOCD shows that there is evidence for the presence of three conformers: none of the conformations dominate, although all seem to be populated. The next step is to understand the reason for the inversion of enantioselectivity of PhOCD with respect to CD. It is therefore noted that surface conformations of CD seem to be dominated

Catalytic Chiral Metal Surfaces by the species depicted in Figure 7E.51 When confronted with the analogous conformations of PhOCD, it results that the phenyl ring can occupy the space adjacent to the quinuclidine moiety, where the reaction takes place. This occupation of the chiral pocket occurs for conformations B and C in Figure 6. Following this reasoning, the inversion of enantioselectivity obtained by PhOCD with respect to CD can be explained by a steric effect whereby the shape of the chiral space is altered by the phenyl ring. Furthermore, the reduced enantioselectivity (from 50% ee (R)-2 with CD to 21% ee (S)-2 with PhOCD under identical conditions to those described here)23 can be explained by a more difficult accessibility to the chiral space, which is partially occupied. The preceding agreement can only hold if it also explains the retention of enantioselectivity upon 3,5-dimethylation of the O-phenyl ring (dMePhOCD yields the (S)enantiomer as the PhOCD) and the inversion of enantioselectivity induced on the contrary by the 3,5-bis-trifluoromethyl substitution (tFPhOCD yields the (R)-enantiomer as for CD in the reference reaction). It has to be noted that methyl and CF3 groups have almost the same van der Waals radius, so that the two O-phenyl moieties are virtually identical from the point of view of the steric occupation. Relevant changes that are caused by the different ring substitution are the following: (i) the 3,5bis-trifluoromethyl substitution has been shown to decrease the adsorption potential of anisole, well below the 3,5-dimethyl substitution;52 thus, it follows that the O-phenyl moiety of tFPhOCD will not be able to adsorb with the phenyl ring parallel to the surface like in the structure of Figure 6C of PhOCD; (ii) the 3,5-bis-trifluoromethyl substitution stabilizes conformer A with respect to B to a greater extent than in the other modifiers; therefore, A is expected to increase its relative population. The preceding observations lead to a theoretical preference of tFPhOCD for the conformation in Figure 7D. The ATR-IR study confirms this picture, according to which tFPhOCD has a chiral space fundamentally similar to that of CD. This is consistent with the catalytic results that show the same enantioselectivity for both modifiers. On the other hand, dMePhOCD is found to adsorb similarly to PhOCD, in agreement with the similar catalytic behavior observed (both are (S)-inducing modifiers). To summarize the results, phenyl ring substitution on the O-phenyl ring within CD ethers can lead to conformational rearrangement of the carbon skeleton of the adsorbed modifier. This rearrangement is itself responsible for the opening or closing of the chiral space where the substrate binds to the surface modifier. It is important to note that several models have already been proposed for describing the interaction between substrate and modifier.10-16 All such models assume a mechanism where the substrate interacts with the nitrogen of the quinuclidine moiety, in the same region of the chiral space. Consequently, the opening-closing mechanism of the chiral pocket due to the O-phenyl group has a similar meaning for all proposed surface-modifier-substrate interactions. Computational studies are being performed to identify the details of such interaction which goes beyond the scope of the present work. The competition experiments provide useful complementary information to the previous discussion. The order of adsorption strength deduced from these results is

CD > PhOCD ≈ dMePhOCD . tFPhOCD The first relevant aspect of this energy order is that a rapid substitution of all O-phenyl ethers of CD can occur by means of CD, or in other words, the O-phenyl ether moiety causes in all cases a decrease of the adsorption potential of CD. Another relevant aspect is that such behavior indicates an active role of

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9357 the O-phenyl moiety in the surface binding. If this were not the case, no changes could be observed when passing from the 3,5-dimethyl to the 3,5-bis-trifluoromethyl substitution, while catalytic results show the contrary (Table 2). When the binding interaction of the O-phenyl moiety is lost due to ring fluorination, a large destabilization of the adsorption state of the modifier occurs. This is indeed an extreme case, since 3,5-bis-trifluoromethylation of the phenyl ring causes a large loss of adsorption potential. The important information is that such a mechanism is possible. The biomimetic nature of the chiral recognition proposed by this study should not be overlooked. Opening and closing of chiral spaces is typical for natural catalysts, which have typically a great enantioselective power and operate on similar principles of chiral recognition. The study here opens an interesting bridge between catalysis at inorganic surfaces and biological catalysts. Molecules with a complex conformational behavior are in both cases fundamental for allowing subtle structural changes that lead to selectively bias the binding properties. The mechanistic conclusions drawn here on the basis of combined analytical methods might trigger interesting ideas to researchers interested in the control of surface chemistry, in surface engineering and surface catalysis. Conclusions The investigations here presented on the adsorption of O-phenyl ethers of CD show that the tuning of the enantioselective properties of chiral surfaces generated by their adsorption can be understood by an approach based on different techniques, namely vibrational spectroscopy, electronic structure calculations and catalysis. The study of the adsorption mode of O-phenyl ethers of CD was performed, showing that the anchoring of all the modifiers is similar, and that major differences occur at the level of the conformation of the carbon skeleton of the molecule. A simple model of opening and closing of the chiral space adjacent to the molecule is proposed that can be fitted to the current views on the interaction between surface, modifier, and substrate. The reshaping of the chiral space that resulted critical for the understanding of the catalytic data opens interesting perspectives for the engineering and tuning of the properties of catalytic chiral surfaces. The conformational nature of the changes that are critical for enantiodifferentiation conceptually link chemistry at inorganic interfaces to that of biological catalytic systems. We anticipate that our findings will initiate attempts to gain a better mechanistic understanding of asymmetric catalytic surfaces and thereby pave the way to a rational design of catalytic chiral surfaces. Acknowledgment. The authors gratefully acknowledge the financial support by the Swiss National Science Foundation, the foundation Claude and Giuliana, and the Swiss Center for Scientific Computing (Manno) for computational resources. Dr. S. Diezi is thanked for performing the catalytic tests and Dr. T. Mallat for fruitful discussions. Supporting Information Available: Complete structure data in the form of the complete Cartesian matrix of the optimized structures of Figure 6 and 7 and full refs 24 and 28. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) ComprehensiVe asymmetric catalysis; Springer: Berlin, 1999; Vols. 1-3. (b) Blaser, H. U.; Pugin, B.; Spindler, F. J. Mol. Catal. A: Chem. 2005, 231, 1.

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